Author Archives: Mohammud Hanif Dewan

COC ORAL EXAM PREPARATION (PART – 13): ENTRIES OF IMPORTANT DOCUMENTS

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OIL RECORD BOOK:

Entries to be made in Oil Record Book as per MARPOL Annex I Requirements:

Oil Record Book: PART – 1 (MARPOL Annex I Regulation 17): 

Entries:

  1. Ballasting or Cleaning of fuel oil tanks
  2. Discharge of dirty ballast or cleaning water from fuel oil tanks
  3. Collection , Transfer and disposal of oil residues
  4. Non automatic starting of discharge overboard, transfer or disposal otherwise of bilge water accumulated in machinery spaces
  5. Automatic starting of discharge overboard, transfer or disposal otherwise of bilge water accumulated in machinery spaces
  6. Condition of the oil filtering equipment (Out of order or malfunction)
  7. Accidental or other exceptional discharges of oil
  8. Bunkering of all types of fuel oils or bulk lubricating oils
  9. Additional operational procedures and general remarks
  10. Recording of quantities retained in bilge water holding tanks (listed under section 3.3 of the IOPP Certificate) is voluntary.
  11. The recording of general maintenance of items pertaining to the OWS remains voluntary and is not required to be recorded in the ORB.

 

  • All Entries are to be made and signed by the officer or officers in charge of the operations concerned and each completed page shall be signed by the master of the ship.
  • This record book needs to be maintained onboard for 3 years.

(Source: MEPC 62/24, paragraph 7.23)

Cargo Oil record Book: PART – 2 (MARPOL Annex I Regulation 36)

  • Every oil tanker of 150 gross tonnage and above shall be provided with an Oil Record Book Part II (Cargo/Ballast Operations). The Oil Record Book Part II, whether as a part of the ship’s official log-book or otherwise, shall be in the form specified in appendix III to this Annex.
  • The Oil Record Book Part II shall be completed on each occasion, on a tank-to-tank basis if appropriate, whenever any of the following cargo/ ballast operations take place in the ship:

Entries:

  1. Loading of oil cargo
  2. Internal transfer of oil cargo while on voyage
  3. Discharging of oil cargo
  4. Ballasting of the cargo tanks and Ballasting of the dedicated clean ballast tanks
  5. Cleaning of cargo tanks including crude oil washing
  6. Discharge of ballast except from the segregated ballast tanks
  7. Discharge of water from the slop tanks
  8. Closing of all applicable valves after slop tanks have been discharged
  9. Closing of valves for isolation of dedicated clean ballast tanks from cargo and stripping lines after slop tank discharge operations
  10. Disposal of residues
  11. Accidental discharge of any oil content and the reasons for the same
  12. Conditions of the oil discharge monitoring system and the failures of the same to be recorded too
  • All Entries are to be made and signed by the officer or officers in charge of the operations concerned and each completed page shall be signed by the master of the ship.
  • This record book needs to be maintained onboard for 3 years.
  • For oil tankers of less than 150 gross tonnage operating in accordance with regulation 34.6 of this Annex, an appropriate Oil Record Book should be developed by the Administration.

MARPOL Annex V: Garbage

Description of the garbage:

Garbage is to be grouped into categories for the purposes of the Garbage Record Book (or ship’s official log-book) as follows:

  • A: Plastics
  • B: Food wastes
  • C: Domestic Wastes
  • D: Cooking Oil
  • E: Incinerator ashes
  • F: Operational wastes
  • G: Cargo residues
  • H: Animal Carcass(es)
  • I: Fishing Gear

Entries in the Garbage Record Book shall be made on each of the following occasions:

  1. When garbage is discharged to a reception facility ashore or to other ships
  2. When garbage is incinerated
  3. When garbage is discharged into the sea in accordance with garbage dischage regulations (4, 5 or 6 of MARPOL Annex V)
  4. Accidental or other exceptional discharges or loss of garbage into the sea

ODS (Ozone Depleting Substances) Record Book:

Vessels carrying ozone depleting substances (ODS) shall maintain a ozone record book. Entries to be made in terms of mass of substance under MARPOL Annex VI Regulation 12:

  1. Recharge (full or partial) of equipment containing ODS
  2. Repair or maintenance on equipments containing ODS
  3. Discharge of ODS deliberate or non delibrtae
  4. Discharge of ODS to land based reception facilities
  5. Supply of ODS to ships

COC ORAL EXAM PREPARATION (PART – 12): STERN TUBE

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1245Image Credit: www.libramar.net

Stern Tube:

The stern tube is a hollow tube-like structure at the stern or rear part of the ship. A ship needs the propeller to drive it forward in water. The propeller, located outside the ship, needs to be connected to the engine inside the ship’s engine room. The propeller shaft is used for connecting the ship’s engine and the propeller. The stern tube is a narrow hole in the hull structure at the rear end (aft peak) of the ship, through which the propeller shaft passes and connects the engine and propeller.

Stern tube bearings serve two main functions:
1. To properly connect the propeller to the ship
2. To keep water from leaking into the stern tube (and lubricant from leaking out)

There are three types of stern tube bearing arrangements that are used to accomplish these goals:
1. Stuffing boxes, which uses packing material to seal the area
2. Lip seals, which are connected to the tail shaft to prevent lubricant or water from passing through the shaft
3. Radial face seals, which are supported with specially fitted springs around the shaft, aft bulkheads and near the back end of the stern tube

Stresses in tail shafts:

1. Due to the considerable weight of the propeller, the tail shaft is subject to a bending stress.
2. There is a torsional stress due to the propeller resistance and the engine turning moment, and
3. Compressive stress due to the propeller thrust.

Stern Bearing Wear-down Measurement:

1. Use Poker Gauge or Wear down Gauge or Vernier Caliper.
2. Remove Rope Guard.
3. Take out Check Plug and Drain Plug.
4. Turn Tailshaft until ‘0’ marks on periphery of aft chrome steel liner flange and mating marks on Simplex seal and Sterntube are in line.
5. Measure at measuring plug (top) and drain plug (bottom) through 180°at same radial position as previous docking.
6. Compare with previous readings.

Stern Bearing Wear Measurement:

1. Measured at same radial position at every dry dock.
2. Look for reference mark left usually on propeller boss when measured during last docking.
3. The same mark should be used again, so that reading can be taken at same radial position.
4. Sometimes measurement had been taken when No. 1 Unit was at TDC, and important thing is that it should be at same radial position as last docking.
5. Measuring at top telltale hole or check plug is enough and bottom check plug is only used for counter check.
6. As a CE, witness the measuring procedure when vessel is on dock

Examining a tail shaft and stern tube:
1. Before the periodic inspection the bearing weardown should be measured.
2. After shaft removed given thorough examination.
3. On water lubricated shafts the integrity of the fit of the bronze liner should be checked by tapping with a hammer along its length listening for hollow noise indicating a separation.
4. Measure wear of shaft.
5. Examine key way for cracks especially the nut thread area.
6. Replace rubber rings.

How to check Stern tube wear down? (Lignum Vitae, water cooled stern tube)

1. Measurement can be taken by Wedge Gauge or Feeler.
2. If Wedge Gauge is used, the side of the Wedge contacting the bearing is chalked and inserted into the clearance space between top of the screw shaft and bearing.
3. Gauge is pressed home and withdrawn.
4. Clearance is measured on the Wedge at the point where the chalk marking is scrapped-of by bearing.

For Oil Lubricated bearing, 

1. 2 mm clearance.
2. When the ship is on Dry Dock, release system oil and remove plug on the end of the stern tube, and insert Poker Gauge to measure the distance from the datum to the top of the shaft.
3. The difference between new reading and original measurement is bearing wear down value.

All types of Stern Bearing:

1. Fit a Dial Gauge on the Rope Guard or Sterntube Nut so that the Gauge spindle is vertical and touching the Propeller Boss.
2. A hydraulic jack is placed on the Stern Frame Skeg at same point over a Keel Block so that the Skeg is supported.
3. A wood shoe is placed between the jack and the Propeller Boss.
4. The jack is then used to lift the Propeller until the Screw Shaft contact the upper parts of its Stern Bearing.
5. The lift recorded in Dial Gauge gives the bearing clearance.

Skeg: An arm extending to the rear of the Keel to support Rudder and protect Propeller.

Tail end shaft taper: ¾” to 1″ per foot length.

Stern tube bearing length:

  • Aft bearing 4 D Forward Bearing 1 – 2 D (Water-cooled)
  • Aft bearing 1.5 – 3 D Forward Bearing 0.6 – 1.25 D (Oil cooled)

How you check Stern tube Sealing in Dry dock?
–  LO header tank is topped-up and checked for leakage for 24 hours.

What action to be taken when SW leaks into Sterntube oil system?
–  Higher up the Stern tube oil tank level to counteract SW force.

Stern tube Leakage Test:

  1. Remove screw plug and fill up the stern tube oil (normally using main engine system oil). Oil pressure must amount to only 0.2 – 0.3 Kg/cm² more than SW pressure.
  2. Preheated to 60 – 70°C when viscose oil is used or low temperature prevails
  3. Remain in this condition for several hours (says overnight) and turn engine by Turning Gear to change the shaft position 3 – 4 times.
  4. Check oil leakage from Drain Plug and if it is all right, fit back Drain Plug and fill oil from venting and Filling Plug and close.
  5. Forward Seal is fitted.

Constant Air flow type Stern Tube: 

New Bitmap Image

Fig: Constant Air flow type Stern Tube (Source: KOBELCO Air Seal, Japan)

  • . A constant  quantity  of  air  supplied  from  the  air source, which then passes through the #2/3 seal chamber and is then spouted into sea. The air pressure is always maintained about 0.01 Mpa (0.1 bar)  greater  than  the  seawater  pressure  so  to slightly exceed the tightening force of the #1 and #2 sealing rings.
  • The air pressure is added to stern tube oil tank, which is installed at 3m (0.03 Mpa) above a shaft center.   The stern tube oil pressure becomes “air pressure in the #2/3 chamber + 0.03 Mpa and it also follows the draft change.
  • Any  draft  change  can  be  automatically  detected and both the air pressure in the #2/3 chamber and the stern tube oil pressure follow the draft change instantly. Accordingly all pressure differences on the aft sealing rings are always negligible.
  • Leaking oil and seawater can be drained from the #2/3 chamber.
  • In order to drain any leaking liquid smoothly, a small quantity of air is always blown through a flow controller on the drain tank.

There are two advantages of the constant air flow system:

  1. One is that seawater rarely comes into the #2/3 chamber because the air pressure is always greater than seawater pressure.
  2. The life of sealing rings, especially the #1 and #2 rings, increases because of the small pressure difference maintained.

IMO IS ON FULL SPEED AHEAD WITH COP21 AGREEMENT

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COP21_briefing_smallerImage Credit: www.imo.org

The Paris Climate Change conference (COP21) 2015 Agreement identifies a clear goal on two objectives:

  • holding the increase in the global average temperature to well below 2°C above pre-industrial levels and
  • to pursue efforts to limit the temperature increase to 1.5 °C above pre-industrial levels.

IMO has contributed over last decade and will continue to contribute to global GHG reduction goals. IMO and its Member States recognise the important need for international shipping, which accounts for 2.2% of CO2 anthropogenic emissions (Third IMO GHG Study 2014) to support global efforts to mitigate the impact of climate change.

To date, IMO is the only organization to have adopted energy-efficiency measures that are legally binding across an entire global industry and apply to all countries. IMO steps for reduction of CO2 emission:

  • Mandatory energy efficiency standards for new ships and mandatory operational measures to reduce emissions from existing ships, entered into force under an existing international convention (MARPOL Annex VI) in 2013.
  • By 2025, all new ships will be 30% more energy efficient than those built last year.

This is more than a target, it is a legal requirement, and demonstrates that IMO is the correct and only forum to identify solutions and an appropriate pathway for international shipping to de-carbonize with the rest of the globe.

Continuing efforts will include development of a global data collection system for ship’s fuel consumption to be discussed in detail at the next meeting of IMO’s Marine Environment Protection Committee(MEPC 69)  in 2016, further consideration of a total-sector reduction target for GHG emissions from international shipping as proposed by the Marshall Islands in 2015, and continued investigation of additional mechanisms for ships to support the implementation of the Paris Agreement.

MO Secretary-General Koji Sekimizu said,

“The Paris Agreement represents remarkable progress and builds on the 1992 Rio Earth Summit, which itself was a significant step forward. The absence of any specific mention of shipping in the final text will in no way diminish the strong commitment of IMO as the regulator of the shipping industry to continue work to address GHG emissions from ships engaged in international trade.”

During COP21, IMO highlighted the followings:

  • An update of its work to address GHG emissions from bunker fuels used for international shipping.
  • Report on its work on further developing guidelines to support the uniform implementation of the regulations on energy-efficiency for ships; and
  • Report on its efforts with regard to technical co-operation and capacity-building to ensure effective implementation and enforcement of the aforementioned new regulations worldwide and, importantly, activities to support promotion of technical co-operation and transfer of technology relating to the improvement of energy efficiency of ships.

At IMO, the Governments of the world come together to develop the regulatory framework for international shipping which forms the basis for investment decisions. There is a clear imperative now for IMO’s Member States to rise to the challenge set by the Paris Agreement. Secretary-General Sekimizu said, “I now encourage Governments to bring the spirit of the Paris Agreement to IMO and come forward with new, creative proposals and to approach them in a constructive and cooperative manner.” He said the challenge set by the Paris Agreement also extended,

  • to ship designers and marine engineers to develop the technological solutions,
  • to ship operators or ship managers, to seafarers and those who educate them and, importantly to the business of shipping, which needs to ensure that investment in innovative low carbon technologies is properly incentivised.

As the Paris Agreement once again highlighted, there is a clear imperative that development must now be truly sustainable. As a facilitator of global commerce international shipping is indispensable to the world, and IMO Member Governments, observer organizations and wider civil society will continue to drive the progress made in Paris.

The Key points of COP21:

The measures in the agreement included:

• To peak greenhouse gas emissions as soon as possible and achieve a balance between sources and sinks of greenhouse gases in the second half of this century

• To keep global temperature increase “well below” 2C (3.6F) and to pursue efforts to limit it to 1.5C

• To review progress every five years

• $100 billion a year in climate finance for developing countries by 2020, with a commitment to further finance in the future.

Source:  www.imo.org (Briefing: 55 Press released on 14/12/2015) and
www.bbc.com/news

COC ORAL EXAM PREPARATION (PART- 11): PROPULSION MACHINERY

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Marine-propulsion-systemImage Credit: Wikipedia.org

FUEL INJECTION TIMING

Checking of fuel injection pump timing:

  1. Turn 1 piston to TDC at the beginning of firing stroke.
  2. Turn backward to a point, a little earlier than fuel injection point.
  3. Shut fuel supply to engine, remove No. 1 fuel pump delivery valve assembly and put a bent pipe.
  4. Open fuel supply and when fuel lever is put to running position, oil will flow out at bent pipe.
  5. Turn engine towards TDC in its running direction slowly until fuel cease to flow.
  6. Check the marks on flywheel whether timing position is correct or not.

 Slight difference can be adjusted by:

For large engine:

    •  Timing can be altered by shifting the camshaft to the position relative to crankshaft, after removing the idler gear between crankshaft and camshaft.
    • Timing can be altered by individual fuel pump cam for adjustable cam type engine.

 

For small engine:

  • Adding or reducing shims on pump base.
  • Turning the plunger up & down adjustment screw on pump roller guide.
  • Shifting the coupling flanges between pump and drive side of the engine.

Crankshaft:

  1. Device for converting reciprocating motion of piston, driven by expansion of gases, to rotating motion.
  2. Power produced by engine is taken off the crankshaft by transmission.

Stresses in Crankshaft:

  1. Bending of crank pin, causes tensile, compressive and shear stresses. (Due to gas load)
  2. Twisting moment of journal, causes shear stress.
  3. Compressive stresses set up in journals and pins. ( Due to shrink-fit)
  4. Tensile stresses set up in webs. ( Due to shrink-fit)
  5. Torsional stresses due to power transmission fluctuate widely. ( In heavy sea)
  6. Shock loading on crank pin. ( Sudden fluctuation of engine speed )

Types of crankshaft:

  • Solid forged
  • Semi-built
  • Fully-built
  • Welded crankshaft.

In large marine engine which type is used and why?

Usually, Semi-built is used because:

  • Only one shrink-fit between web and journal as less chance of slippage.
  • Can get grain flow in way of web and pin.
  • Webs are smaller as there is no shrink-fit.
  • Can be repaired section by section when damage occurred.

Welded Crankshaft:

  • Die-forged crankthrow, consisting of thin webs and crankpin in one piece, having half a main journal on each side.
  • Welds are placed and welded at the middle of all main journals, to make complete crankshaft.
  • High technology Narrow gap welding process applied.

Fully-built Crankshaft manufacturing:

  1. Raw material melted in Cupola Furnace.
  2. Refined to remove impurities, by decarburising, controlling carbon amount and soaking time.
  3. Degassed in Vacuum Furnace, to remove H₂ and N₂.
  4. Molten metal is then poured into prepared mould.
  5. Removed from mould, after slow cooling, and casting is rough-machined.
  6. Normalised to improve grain structure, and tempered to remove stresses.
  7. Rough-machined to final dimensions.
  8. Cold roll the crank pin fillets, to increase bending and corrosion fatigue
  9. Finish machining.
  10. Shrink-fitting process [Shrinkage allowance: 1/570 to 1/660 of journal diameter.]
  11. Set upon a large lathe, and journals checked for throw, and throw errors machined out.

When to take Crankshaft Deflection:

  1. At initial installation and after 1000 running hours.
  2. At subsequent annual intervals if normal, ( 6000-8000 hrs. )
  3. At the time of main bearing overhaul or removal for survey.
  4. At foundation chock repair or renewal.
  5. Damage on bearing bracket, holding down bolt, chock.
  6. When major structure has been disturbed, such as: after fire breakout, propeller bending or impounding with something, ship grounding, before and after docking.

Causes of misalignment:

  1. Wear of main bearing lower shell.
  2. Wear and ovality of main journal pin.
  3. Main bearing damage.
  4. Main bearing pocket cracked.
  5. Bedplate deformed, transverse girder damaged.
  6. Foundation bolts loose or fractured.
  7. Foundation chocks broken, cracked or fretted.
  8. Slacked or broken tie bolts.
  9. Distortion of supporting ship’s structure.
  10. Defective structure due to corrosion.
  11. Defective propeller shaft bearing
  12. Lifting of flywheel side.
  13. Hull deformation due to: Improper loaded condition of vessel, grounding and fire.

Results of misalignment:

  1. Bending of crankshaft.
  2. Fatigue failure owing to cyclic stresses.
  3. Undue vibration within the engine.
  4. Damage to main bearing.

Why you measure crankshaft deflection?

  1. To ascertain whether or not, the axis of crankshaft journals deviates from theoretical shaft axis.
  2. Measuring is by a dial gauge, inserted between crank webs, and altered distances can be read, when turning the crankshaft.

How to know the amount?

  1. Difference between the values at TDC and BDC indicates the amount of crankshaft deflection, during one revolution.
  2. Interpretation of crankshaft deflections gives an indication of high and low bearings.

What will happen if a bearing is high or low?

  1. When a bearing between 2 cranks is higher than those on either side of it, both sets of crankwebs will tends to open out, when the cranks are on BDC, and close in when on TDC.
  2. Vice versa, if there is a low bearing between 2 cranks.

Requirements when taking crankshaft deflection:

  1. Hull deflection not excessive.
  2. Bed plate not distorted or bearing pockets not worn.

 Foundation Chock:

Purpose:

  1. To avoid misalignment on tank top surface.
  2. To carry out adjustments on individual chock.
  3. To correct any distortion.
  4. To absorb collision load by end chocks.
  5. To absorb side load, due to unbalanced reciprocating forces, by side chocks.

Advantages of Chockfast System: (Eposy Chock)

  1. Reliable and permanent alignment of machinery foundation.
  2. Resist degradation by fuel, LO and eliminate chock area corrosion.
  3. Give uniform and precise mounting.
  4. Non-fretting permanently.
  5. Reduce noise level.
  6. Can be used on all sizes and types of engines.
  7. Maintain C/S deflection, machinery alignment and even Hull fouling.
  8. Installation time is measured in hours, not in days.
  9. Withstand temperature up to 80°
  10. Give chock thickness up to 44 mm.

Chain Drive System:   

  1. Used for camshaft driving, on any length between shaft centres with very small friction loss.
  2. Fuel Pumps and Exhaust Valves are operated by Camshaft, driven from Crankshaft, by a roller chain [main] running over each sprocket wheel, being bolted to both shafts.
  3. Chain should wrap around at least 120°on both sprockets.
  4. Upward-running-side chain passes over an intermediate wheel, on which Tensioning Device is fitted.
  5. On another intermediate wheel’s shaft, there is another chain wheel and chain [smaller], to drive Start Air Distributor, Governor and Lubricators.
  6. Chain is lubricated by oil sprayer jets, with continuous stream of oil onto the chain.
  7. A roller chain consisting of side plates, bushing and rollers, and pin joints, which mesh with tooted sprockets.
  8. Shock-absorbing rubber clad guide bars, are provided to support the long chain, and to prevent transverse vibration.
  9. Renew cam chain after 15 years life.
  10. Factor of safety of chain: Never less than 25.

Slack chain: 

Symptoms:

  1. Excessive chain vibration and noise.
  2. Power loss in all units, indicated [by Power Card].
  3. Late injection, low Pmax, [by Draw Card].
  4. Late closing of Exhaust Valve, [by Light Spring Diagram].
  5. High exhaust temperature and smoke.

Effects:

  1. Impose heavy mechanical load, resulting fatigue failure.
  2. Damage to chain system and engine frame.
  3. Retardation of Fuel Pump and Exhaust Valve timings, resulting:
  4. Reduced Scavenge Efficiency, due to late closing of Exht: V/v.
  5. High exhaust temperature and smoke, due to after burning.
  6. Low Pmax, due to late injection.
  7. Reduced engine power.

Chain Casing Inspection: 

  1. Before 4000 running hrs and after lengthy voyage, chain tension is checked at mid span of slack side, in transverse direction.
  •    Limited transverse movement is ½ to one link pitch on slack side.
  •    Excessive tension may cause chain breakage.
  •    Excessive slackness may cause vibration and eventual failure.
  1. Elongation [chain wear] is checked between 3000–5000 running hours.
  •  Total length of 10 links drawn tight and measured, and chain-stretch calculated          in % by comparing with original length of 10 links.
  • Maximum elongation: ≯2%.  Over 2%, the whole chain must be renewed.
  • Due attention given when elongation reaches 1.5%
  • Stretching is the results of pin and bushing surface wearing out.
  • Chain length is measured in terms of number of links.
  1. Nozzle sprayers, LO pipes and oil flow and direction, checked.
  2. Loose bolts and pipe connections, checked.
  3. Every link checked for blemish, and bright marks due to misalignment of wheels.
  4. Sprocket teeth and wheel bearings checked for wear.
  5. Rubber clad guide bars, rollers and side plates, checked for crack or damage.

How to adjust chain tension: 

  1. Tensioning device [chain tightener] is used, and adjusting is limited to removing a maximum 2 chain links.
  2. Limited transverse movement is ½ to one link pitch on slack side.
  3. When tightening, engine is to be turned ahead, that the slackness of chain is on tightening side.

Advantages of Chain Drive over Gear Train: 

  1. Unaffected by foreign particles as gear trains.
  2. Class requires only a few links [6 links] for spares. The whole set required for gear train.
  3. Even if the chain breaks, engine can still be operated after repair.
  4. Accuracy of camshaft drive is very high, because chain tightener can adjust and compensate for inevitable mechanical wear. Gear train is non-adjustable.
  5. Enable camshaft position to be placed higher, thus shorten the hydraulic connections of fuel pumps and exhaust valves, and minimise timing error. 

Camshaft Timing Adjustment (By Pin Gauge):

  • As the chain stretches and re-tensioned camshaft is gradually retarded.
  • Thus camshaft must be repositioned relative to crankshaft to correct the timings of Fuel Pumps and Exhaust Valves.
  1. Engine must be in Ahead position.
  2. Bring cylinder no: 1 to TDC, and ‘0’ on flywheel.
  3. Check that cylinder no: 1 crank throw is in TDC; (with D-1 pin gauge)
  4. Check that camshaft position deviates from original marking; (with D-2 pin gauge) 

      If camshaft deviates:

  1. Connect high-pressure hydraulic pump oil connections to flanges next to chain drive, and pressurised until oil seep along the camshaft.
  2. Turn the whole camshaft using tackle until D-2 pin gauge mark is in line with original marking. Fixed markings are on roller guide housing [after removing cover] and on camshaft.
  3. D-3 pin gauge is for Lubricator Unit.

Methods of reversing:

  1. Direct reversal of engine: Propeller turns in opposite direction.
  2. CPP: Blade angle changes, as engine rotates in same direction.
  3. Diesel electric system: Engine and electric generator run in constant direction, supplying power to reversible electric motor.
  4. Reverse gears and clutches: Propeller turns in opposite direction.

Reversing Interlocks:

  1. Safety cut-out devices for JCW, Piston CW, and
  2. Reversing direction interlock.
  3. Hydraulic blocking device and mechanical blocking device for start air handle.
  4. Reversing Servomotor.
  5. Telegraph
  6. Turning Gear.
  7. Overspeed Trip.

Advantages of single cam and double cam:

  1. Single cam on camshaft is suitable for reversing of 2/S, large bore engine. But not suitable for 4/S engine, because reversing of 4/S engine requires turning of Inlet Valve cam, Exhaust Valve cam, Fuel cam and arrangement for Starting air Distributor, with their correct timings.
  2. Double cam on camshaft, is moved axially by means of servo system or manual system, so that all cams get their correct timings, in 4/S engine. (Used also for 2/S).

Lost motion:

Angular period between TDC points for Ahead and Astern running will be the “lost motion” required for Astern running.

  1. When reversing 2/S, exhaust ported engine, both Fuel Injection timing and Air  Starting timing must be changed.
  2. Retiming is carried out by altering camshaft position radially, relative to crankshaft. This is called “lost motion” of camshaft.

Why ‘lost motion’ necessary on some engine?

  1. Some 2/S, large bore, exhaust ported engines are Direct Reversing.
  2. Both Fuel Injection timing and Air Starting timing must be changed.
  3. Camshaft has single cam
  4. Retiming is carried out by altering camshaft position radially, (not axially), relative to crankshaft, by means of servo system.

Why ‘lost motion’ not necessary on some engine?

  1. Some 2/S and 4/S engines are Direct Reversing.
  2. Inlet Valve cam, Exhaust Valve cam, Fuel cam and arrangement for Starting air Distributor, with their correct timings, must be changed.
  3. Camshaft has double cam
  4. Retiming is carried out by altering camshaft position axially, from Ahead cams to Astern cams, by means of servo and manual systems.

Lost Motion Camshaft:

  1. When reversing 2/S Exhaust Ported Engine, both Fuel Injection and Air Starting timings must be changed.
  2. Lost Motion Clutch cam design can be used to alter reversing direction.
  3. Camshaft position is altered radially relative to crankshaft.
  4. Same cam is used for ahead and astern running.
  5. Reversing Servomotor, operated by Engine Reversing Controls, is fitted to camshaft drive mechanism to do this.
  6. Camshaft will lose motion or be retarded, through required angle (about 98°) by oil operated Lost Motion Clutch, causing the Reversing Servomotor to rotate the camshaft.
  7. Fuel Pump cam and Air Start cam will now operate the Engine in reversed direction.
  8. Lost motion is carried out while the Engine is at rest.
  9. For Uniflow Scavenge Engine, the second Servomotor is fitted to Exhaust Valve drive.

4 Stroke Engine Reversing Systems: 

  1. By means of camshaft, shifting axially. (Direct Reversing)
  2. By CPP.
  3. By gearing and clutch.

Tacho Generator:

  • AC or DC generator that provides an output voltage proportional to rotational speed, to remote rpm counter (tachometer).
  • It may be used to measure speed, or as part of automatic control system, to regulate speed.
  • In Sulzer RTA, also used for overspeed trip, using output current.
  • Fitted on ME intermediate shaft, for remote rpm counter.
  • Fitted on housing of Reversing Servomotor, and driven by gear wheel on Cam Shaft for overspeed cut-out.

V-Type Connecting Rod:

1)  Side by side   2)  Articulated    3)  Fork and Blade.

 Cross-head bearing is prone to failure, because of:

  1. High sudden load: Full effect of combustion, directly to the bearing.
  2. High bearing pressure: Bearing is placed high and the whole assembly reciprocates full length of stroke. So, limited bearing area results in high specific load.
  3. Distortion: Bending moment and deflection are maximum at centre, where pin is often bored to carry piston rod.
  4. Poor lubrication: Due to following factors:
  • Slow oscillating movement: Connecting rod swings through 25~30°, hence it is difficult to build up full fluid film.
  • Reciprocating movement: Vertical movement of pin and bearing disturbs oil supply. It is difficult to get smooth, uninterrupted oil flow.

5.  Two-stroke engine: No load reversal takes place, which does not help the oil flow into loaded part of bearing.

Different approaches adopted to overcome cross-head bearing problems:

  1. Conjugate deflection: Bearing deflection follows that of crosshead pin. Natural deflections of pin and bearing remain in line, resulting in lower specific load.
  2. Crosshead mounted LO pump: Attached high-pressure pump, operated by connecting rod movement, press oil into bearing gap when bearing load is lowest.
  3. Large diameter stiff crosshead pin: Reduced Length/Diameter ratio, but pin deflection is minimum for uniform distribution of oil films over the whole bearing
  4. Continuous full length bearing face under pin:
  • Low specific load on bearing.
  • Load is transmitted directly downwards.
  1. Large diameter pin and smaller “Connecting rod : Crank throw” ratio: Obtained higher sliding velocity of the bearing, with better LO oil film, to carry high loads.
  2. Hardened crosshead pin with high degree of surface finish: Surface finish is preferably better than  0.1 µm.
  3. Eccentric bored bearing: One of the finest designs for crosshead, which gives the same effect of load reversal. (GMT engine)
  4. Thin shell bearing: Bearing is renewable and pin is detachable. Produces high load carrying capacity, and better resistance against fatigue failure. Thin shell gives true circular shape, which improves lubrication characteristics.

Thrust block:

  • To prevent axial movement of crankshaft, resulting from propeller thrust.

Measurement of Axial clearance of thrust pads:

  1. Thrust block is cleaned by draining oil. And lift the top cover
  2. Place screw jack between casing and the back of the coupling, and push the thrust shaft aft until the collar is hard up on the
  3. Check alignment of shaft and take feeler gauge reading of open pads by using long feeler. It is inserted at one corner and ease diagonally across to the other.
  4. Repeat this operation, moving the shaft
  5. Difference between two readings is total axial clearance.
  6. Axial clearance is 1 ~ 2 mm. (0.5 ~ 1.0 mm for new engine and for engine in service, it must not exceed 2.0 mm.) 

Alternative method:

  • Bear the thrust collar on foremost thrust bearing segment, by pressing the crankshaft
  • Set dial gauge (zero position) to flywheel.
  • Bear the thrust collar on aftmost thrust bearing segment, by pressing the crankshaft
  • Check clearance by reading the indication of dial gauge.

Radial clearance of journal bearing:

  1. Remove end cover with oil seal.
  2. Radial clearance measured, by taking lead reading, or roughly by means of feeler gauge.
  3. Radial Clearance is 0.5 ~ 0.8 mm for 440 mm dia.shaft.

Advantages of Tilting Pad Bearing:

  1. Have ability to absorb, change in direction of load, more readily.
  2. Have greater flexibility to absorb shaft deflection or misalignment.
  3. Tilting of pads, allow oil to form wedge shaped film, between faces of collar and pads.
  4. Wedge shaped oil film prevents metallic friction and enables the thrust pads to carry loads.

Disadvantages:

  • Each pad in a set must be exactly the same thickness, and even a ‘thou’ difference might result in a single pad carrying the entire load, thus increasing the risks of failure.

Plumber block renewal:  (during heavy weather)

Practically it should be done in calm weather, but following ways can reduce overheating of plumber block bearing.

  1. By applying maximum lubrication.
  2. By applying maximum cooling after opening out the cooling coil out into bilge.
  3. By reducing to suitable speed.
  4. Then the ship proceeds to the sheltered sea and renew the plumber block bearing.

Removal of plumber block bearing.

  1. Take immobilisation permit.
  2. Mark the relative positions between each bearing halves and between the lower bearing halve and the
  3. Remove the upper bearing halve.
  4. Lift the shaft at the place close to bearing by jackscrew.
  5. Remove the lower bearing halve with chocks from the stool.
  6. Sent both bearing halves for repair.

Refitting Procedure:

  1. After repairing, place back the lower halve with chocks on the stool. But foundation bolts should not be placed.
  2. Remove the shaft-lifting device.
  3. Boxed back upper half.
  4. Remove all coupling bolts of intermediate shaft flange close to the bearing.
  5. Alignment checked by gap and sag method.
  6. After ensuring that the alignment is satisfactory, tightened foundation bolts.
  7. Refit and tighten the coupling flange bolts.

Allowances:

Gap method:   Equal to or less than  0.10 mm per meter for  1 to 2 pieces of shafts.

  • ≤  0.15 mm / m  for  3 to 4 pieces of shafts.
  • ≤  0.2 mm/m  for > 5 pieces.

Checked with a feeler gauge between the two coupling flange faces, at least at four places to check whether the bearing is in line with shaft or not.

 Sag method:

  • ≤  0.10 mm for 1 to 4 pieces of shafts.
  • ≤  0.15 mm for more than  4 pieces shafts.

Place a straight edge over the two flanges, at least four places around, to check whether the bearing is in line with shaft or not , or out of the shafting vertically and horizontally.

CPP:

Two main types:  Hub Servo and External Servo.

Hub Servo Type:

  1. Pitch altering mechanism, enclosed in propeller hub is most popular type, and used for higher power above 1000-Bhp.
  2. Propeller mechanism consists of 4 main parts:
  • Propeller hub incorporating servomotor, crank pin ring for turning blades, and necessary seals.
  • Oil distribution box (transfer box), mounted at forward end of tailshaft.
  • Control system; either pneumatic or
  • Hydraulic system; motor or shaft driven pumps, cooler, filter, and tank. etc.

Functioning Principle:

Movements of piston effect blade pitch:

  1. Servomotor in propeller hub consists of a piston rod with piston, which moves axially fore and aft when pressure oil is led to either side of piston.
  2. Piston rod is equipped with 4 or 5 “ears”, depending on number of propeller blades.
  3. Each ‘ear’ has a transverse slot in which a shoe slides.
  4. Eccentric crank pin fits into the hole of sliding shoe.
  5. Crank pin ring is supported on a bearing, which is built-in into hub body.
  6. When piston rod moves axially by pressure oil, crank pin ring rotates in circular motion, transmitted via piston, piston rod, slot, sliding shoe, and crank pin.
  7. Propeller blades, which are bolted to crank pin rings, turn.

Failure Arrangements:

  1. Hydraulic system failure:  
  • Safety springs, fitted in main servo, push the servo piston forward, to allow propeller pitch to full ahead position, in the event of hydraulic system failure. The springs are powerful enough to overcome friction, but RPM of 70% maximum should not be exceeded.
  1. Telemotor system failure:
  • Hand-operated control valve is used, in the event of telemotor failure.
  1. Main hub servo failure: 
  • If main servo fails, the system has either Emergency Servo or Mechanical Link.

CPP Bridge Control:

  1. CPP in large vessels are usually fitted with Combinator Control on the Bridge.
  2. A single lever controlling both propeller pitch and engine speed, either through pneumatic or electronic means.
  3. In either case, closed loop circuits are employed, so that feedback of propeller position and engine speed, balance off the control signal.
  4. In electronic control system, ME load is kept at desired value, by automatically changing the propeller pitch, irrespective of variation in external conditions; e.g. change in resistance in propulsion caused by wind and sea.
  5. Main panel receives, converts and transmits signals, and a potentiometer for adjusting ME load, and an instrument showing fuel pump setting, is provided.
  6. Control panel on Bridge contains instrumentation corresponding to that of Main panel.

Pilgrim Nut: 

  1. Pilgrim nut is a threaded hydraulic jack, screwed onto tailshaft, provided with hydraulic oil connection, steel jacking ring and nitrile rubber tyre.
  2. It gives predetermined frictional grip between tailshaft and propeller boss.
  3. Spherical graphite cast iron tapered sleeve is bedded onto shaft cone, before mounting the boss, to achieve better fit.
  4. When combined with Pilgrim Nut pushing up, it ensures a good frictional grip.
  5. No key is required; friction is sufficient to prevent slip.

Propeller mounting procedure:

  1. Tapered sleeve is bedded onto shaft cone, propeller boss is mounted, and pilgrim nut is run-down the shaft threads.
  2. Steel jacking ring on landing face of the nut, is loaded with hydraulic pump to predetermined pressure, and this forces the propeller hard on its cone.
  3. Pressure is released on jacking ring and air release plug
  4. Nut is hardened-up with spanner, and locked in normal way.

Propeller removing procedure:

  1. Pilgrim nut is taken-off the end of the shaft, reversed so that jacking ring is facing outward, and screw back the nut onto shaft, leaving some clearance between it and propeller boss.
  2. Studs are screwed into aft face of the boss and a “strong back” plate is fitted over the studs.
  3. Stud nuts are fitted so that the plate contacts with jacking ring.
  4. When hydraulic pressure is applied to jacking ring, propeller is pulled-off the conical end of the shaft.

Shaft Generator:

Shaft Generators are fitted on diesel engine propulsion ships, especially those sailing for long period at a constant ship speed.

Lloyd’s Requirements:

  1. Lloyd’s register would regard a shaft generator as a service main generator, if ME is intended to operate at constant speed. (CPP).
  2. If ME does not operate at constant speed, shaft generator would be disregarded as a service main generator, and at least 2 other independent generators would be required.

Running condition:

  1. Full generator capacity is available at within 60~100% of normal speed.
  2. More suitable for shaft with CPP, [constant shaft speed and variable blade pitch].

Advantages of shaft generator:

  1. Saving in fuel cost is main advantage.
  2. Saving in LO consumption, repair and maintenance cost due to reduced main generators’ running hours.
  3. Reduction in noise, space and weight, capital saving by reduction of numbers and ratings of main generators.

Disadvantages:

  1. Reduction in ship speed.
  2. Problems can arise to maintain electrical supply, during emergency manoeuvring astern.
  3. Increase in capital cost.

ME driven Generator:

  1. Fuel consumption is saved.
  2. Lower running and maintenance cost.
  3. Lower noise level in ER.
  4. Simple and most compact installation.

Varying speed of ME, driving a fixed pitch propeller, can be converted by variable gear ratio, to provide constant Generator speed.

COC ORAL EXAM PREPARATION (PART-10): SHIP CONSTRUCTION

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Ship_size_(side_view)Image Credit: wikipedia.org

Definitions and Ship’s Dimensions

Hull:
The structural body of a ship including shell plating, framing, decks and bulkheads.
 Afterbody :
That portion of a ship’s hull abaft midships.
 Forebody:
That portion of a ship’s hull forward midships.
 Bow :
The forward of the ship
 Stern :
The after end of the ship
Port :
The left side of the ship when looking forward
Starboard :
The right side of the ship when looking forward
Amidships:
point midway between the after and forward perpendiculars

Length Overall (L.O.A.):
Length of the vessel taken over all extremities.
Ship Construction- Ship Dimensions
Base line:
A horizontal line drawn at the top of the keel plate. All vertical moulded dimensions are measured relative to this line
Moulded beam:
Measured at the midship section is the maximum moulded breadth of the ship
Moulded Draft/ Draught:
The distance from the bottom of the keel to the waterline. The load draft is the maximum draft to which a vessel may be loaded
Moulded Depth:
Measured from the base line to the heel of the upper deck beam at the ship’s side amidships.
 Sheer:
Curvature of decks in the longitudinal direction. Measured as the height of deck at side at any point above the height of deck at side amidships
Camber / Round of Beam:
Curvature of decks in the transverse direction. Measured as the height of deck above the height of deck at side
Rise of floor / Deadrise:
The rise of the bottom shell plating line above the base line. This rise is measured at the line of moulded beam
Half siding of keel:
The horizontal flat portion of the bottom shell measured to port or starboard of the ship’s longitudinal centre line. This is useful dimension to know when dry-docking.
Tumble home:
The inward curvature of the side shell above the summer load line.
Freeboard:
The vertical distance measured  from the waterline to the top of the deck plating at the side of the deck amidships. Normally exposed to weather and sea.
Flare:
The outward curvature of the side shell above the waterline. It promotes dryness and is therefore associated with the fore end of ship
Extreme Beam:
The maximum  beam  taken over all extremities.
Extreme Draft:
Taken  from the lowest point of keel to the summer load line. Draft  marks  represent extreme drafts.
Extreme  Depth:
Depth of vessel  at  ship’s side  from  upper deck  to  lowest point  of keel.
Half  Breadth: 
Since  a  ship’s  hull  is  symmetrical about  the  longitudinal centre line, often  only the half beam  or half breadth at any section  is given.
SCANTLING
The dimensions of the structural items of a ship, e.g. frames, girders, plating , etc.

strong>INTERCOSTAL
Composed of separate parts, non-continuous
CENTER OF FLOATATION
It is the center of the waterplane area and is the axis about which a ship changes trim.

CENTER OF BUOYANCY
It is the center of the underwater volume of the ship where the force of buoyancy acts.

CENTER OF GRAVITY
It is the point at which the whole weight of the object may be regarded as acting. If  the object is suspended from this point, it will remain balanced and not tilt.

TONNAGE MEASUREMENT

  • This is often referred to when the size of the vessel is discussed, and the gross tonnage is quoted from Lloyd’s register.
  • Tonnage is a measure of the enclosed internal volume of the vessel, 100 cubic feet representing one ton
  • Its normally divided into categories as follow:
  1. DISPLACEMENT TONNAGE
  • A ship’s displacement is the sum of the ship’s actual weight (lightweight) and it’s contents (deadweight).
  • The metric unit of measurement is 1 tonne (= 1000 Kg).
  • The displacement represents the amount of water displaced by the ship expressed in tonnes.
  • The weight of water displaced therefore equals the weight of the ship

TONNE PER CENTIMETRE (TPC)
It is the mass required to increase the mean draught by 1 centimetre.

LOAD DISPLACEMENT
The weight of the ship and its content, measured in tonne. The value will vary according to the ship’s draught.

DEADWEIGHT SCALE
It is a scale diagram indicating the deadweight of the ship at various draughts.

FORM COEFFICIENT
It is devised to show the relationship between the form of  the ship and the dimension of the ship.

2. Lightweight Tonnage (LWT)

  • The lightweight is the weight of the ship as built (hull, machinery) including boiler water, lubricating oil and the cooling water system.
  • Lightweight like displacement is expressed in units of tones.
  • It assumes importance in a commercial sense only when considering the value of the vessel which is to be broken up for scrape.

3. Deadweight tonnage (DWT)

  • Deadweight is the weight of the cargo which a ship carries plus weights of fuel, stores, water ballast, fresh water, crew and passengers and baggage.
  • It is the difference between the loaded ship displacement and the lightweight.

4. Gross Tonnage (GT)

  • Measurement of total internal volume of a vessel and includes all under deck tonnage and all enclosed spaces above tonnage deck.
  • 100 cubic feet of space being considered as 1 ton

5. Nett Tonnage (NT)

  • Ship measurement derived from gross tonnage by deducting spaces allowed for crew and propelling power.
  • 100 cubic feet of space being reckoned as 1 ton

LOAD LINE

The marking on the ship side that relate to the loading condition of the ship termed as the load line mark.

Load line mark

  • consists of a ring 300 mm in outside diameter and 25 mm thick which is
  • intersected by a horizontal line 450 mm in length and 25 mm thick, the upper edge of which passes
  • through the centre of the ring. The centre of the ring is placed amidships and at a distance equal to the assigned summer freeboard measured vertically below the upper edge of the deck line.

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Margin Plate: 

  1. The outboard strake of the inner bottom.
  2. Knuckle down to the shell by means of Margin Plate at angle of 45°to tank top, meeting the shell almost at right angle.
  3. It can form a bilge space.

Keel plate:  

Keel is a horizontal plating of increased thickness, which runs along the centre line, for complete length of bottom shell plating.

Types of keel:   (1) Bar keel  (2) Flat plate keel  (3) Duct keel.

 Bar keel:  

  • The first type, used from wood to iron ship building.
  • Do not provide sufficient strength for larger ship.
  • No direct connection between the keel and floor.

Flat plate keel: 

  • A keel of welded ship. The centre girder is attached to the keel and inner bottom plating by continuous welds.
  • Keel plate width is about 1 to 2 meter
  • It must be full thickness, for 3/5 of length amidship and then thickness may reduce towards the ends of ship.

Duct keel:  

  1. An internal passage of watertight construction, running same distance along the length of ship, often from fore peak to forward machinery space bulkhead.
  2. It is to carry pipeworks, and entrance is at forward machinery space bulkhead through a watertight manhole.

Bulkhead

Class A bulkhead

  • Constructed to prevent passage of flame for 1 hour standard fire test at 927°C
  • It must be insulated so that the unexposed sides will not rise more than 139°C above the original temperature within the time, as follows:

Class A- 60 ,  1 hour:       Class A- 30 ,  30 minutes.

Class B bulkhead:

  • Constructed to prevent passage of flame for ½ hour standard fire test
  • It must be insulated so that the unexposed sides will not rise more than 139°C above the original temperature within the time, as follows.

Class B- 15 ,  15 minutes:       Class B- 0 ,  0 minute.

Class C bulkhead:   

  • They are constructed of non-combustible material.

 Standard fire test:     

  • The exposure of a material specimen in a test furnace, to a particular temperature for a certain period of time.

Collision Bulkhead:

  • Foremost major watertight bulkhead, which extends from bottom to main deck (upper deck).
  • It is at a distance of L/20  from forward perpendicular.

Corrugated bulkhead:   

  • Used on transverse bulkhead, thus improves transverse strength.

Non-watertight bulkhead:   

  • Any bulkhead, which does not form, part of a tank or part of a watertight subdivision of a ship, may be non-watertight.

Wash bulkhead: 

  • A perforated bulkhead fitted into a cargo tank or deep tank, to reduce sloshing or movement of liquid through the tank.

After peak bulkhead:

  • Provided to enclose the stern tube in watertight compartment.
  • Aft peak bulkhead needs only to extend to first deck above load water line.
  • Plating must be doubled to resist vibration around stern tube.

Minimum required bulkhead:

  1. One collision bulkhead.
  2. An after peak bulkhead.
  3. One bulkhead at each end of machinery space.
  4. Total no: of bulkheads depends upon the ship and position of machinery space

Functions of bulkhead:

  1. To increase transverse strength of ship, particularly against racking
  2. To divide the ship into watertight compartments.
  3. To give protection against fire.
  4. To prevent undue distortion of side shell.
  5. To restrict volume of water, which may enter the ship, if shell plating is damaged.

Construction of bulkhead:

  • Collision bulkhead must extend from bottom to upper deck.
  • Aft peak bulkhead needs only extend to first deck above load water line.
  • All others must extend to uppermost continuous deck.
  • Plating usually fitted vertically, and thickness gradually increases from the top downward.
  • Stiffeners are fitted at 750mm apart, but collision bulkhead and deep tanks have 600mm spacing.

Why Collision Bulkhead kept at L/20 of the ship?

  • In the events of collision and grounding, standard of subdivision has to give good chance, that the ship remains afloat under such emergencies.
  • Longitudinal Bulkheads are avoided, as far as possible, as they might cause dangerous angles of heel, in the event of flooding of large compartment through damage.
  • Transverse Bulkheads are reliable in this case, and Classification Society requires a watertight Collision Bulkhead within reasonable distance from forward.
  • If the ship is supposed to have wave trough amidships, there will be excess weight amidships and excess buoyancy at the ends, hence the ship will be (Assuming wave length = length of ship)
  • If the ship is supposed to have wave crest amidships, there will be excess weight at the ends, and excess buoyancy amidships; hence the ship will be
  • By “Trochoidal Theory”, wave height from trough to crest is 1/20 of the wave length, therefore maximum shearing force usually occurs at about L/20 of ship from each end.
  • For this reason, Collision Bulkhead is located at L/20 of the ship, so that it is not so far forward, as to be damaged on impact. Neither should it be too far aft, so that the compartment flooded forward causes excessive trim by bow.

Panting:

  • As wave passes along the ship, they cause water pressure fluctuation, which tends to create in and out movement of the shell plating, especially at forward end.
  • This in and out movement is called panting.
  • Resisting structures against panting are beams, brackets, stringer plates, etc.

Racking:

  • When a ship rolls, there is a tendency for the ship to distort transversely.
  • This is known as racking.
  • Resisting structures are beam knee, tank side bracket, and especially transverse

Slamming or Pounding:

  • When ship is heaving and pitching, the fore end emerges from water and re-enter with a slamming effect.
  • It is called pounding.
  • Resisting structure: extra stiffening at the fore end.

Hogging:

  • When buoyancy amidships exceeds the weight due to loading, or when the wave crest is amidships, the ship will hog.

Sagging:

  • When the weight amidships exceeds the buoyancy, or when the wave trough is amidships the ship will sag.

Function of port hole:
1) For light    2) For ventilation    3) For escape for emergency.

Transverse stresses:

  • Transverse section of a ship is subjected to transverse stresses, i.e. static pressure due to surrounding water, as well as internal loading due to weight of structure, cargo, etc.
  • Structures or parts, that resist transverse stresses:
  • Transverse bulkhead
  • Floors in double bottom
  • Brackets between deck beams and side frame
  • Brackets between side frame and tank top plating
  • Margin plates
  • Pillars in holds and tween deck.

Local stresses:

Causes:

  • Heavy concentrated loads like engine, boiler.
  • Deck cargo such as timber.
  • Hull vibration.
  • Ship, resting on blocks in dry dock.

Dynamic forces:

  • Caused by the motion of the ship itself
  • A ship among waves has three linear motions:
  1. Vertical movement: heaving 
  2. Horizontal transverse movement: swaying
  3. Fore and aft movement: surging And
  • three rotational motions:
  1. Rolling about longitudinal axis
  2. Pitching about transverse axis
  3. Yawing about vertical axis.
  • A ship among waves has three linear motions:
    1. Vertical movement: heaving
    2. Horizontal transverse movement: swaying
    3. Fore and aft movement: surging  AND 
  • three rotational motions:
    1. Rolling about longitudinal axis
    2. Pitching about transverse axis
    3. Yawing about vertical axis.

The difference between Timber Load Line and Load Line:

  • When ship is carrying timber, the deck cargo gives additional buoyancy and a greater degree of protection against the sea.
  • The ship has smaller freeboard than normal (type-B) vessel.

Bulbous Bow:

  • It is a bulb shaped underwater bow.
  • Reduce wave making resistance, and pitching motion of the ship
  • Increase buoyancy forward, and hence reduce pitching of the ship
  • Outer plating of bulbous bow is thicker than normal shell plating, to resist high water pressure and possible damage cause by anchor and cables.
  • Due to reduction in wave making resistance, it can reduce SFOC under full speed and loaded condition.

Bow Thruster:

  • Lateral Bow Thrusters are particularly useful, for manoeuvring in confined water at low speed.
  • For large vessel, used at channel crossing, and docking.
  • For research vessels and drilling platform, etc. very accurate positioning
  • Bow Thruster consists of: (As a Rule)
  • A controllable pitch or reversible impeller, in athwartship watertight tunnels.
  • Bridge controlled and driven by
  • Thrust provided is a low thrust, about 16 tons.
  • Greatest thrust is obtained, when ship speed is zero.
  • Less effective, when ship gets underway.
  • Athwartship tunnels appreciably increases hull resistance.
  • Close the tunnels at either end, when not in use, by butterfly valve or hydraulic valve.

Cofferdam:

  • A narrow void space between two bulkheads or floors that prevents leakage between the adjoining compartments.
  • In tankers, between cargo tanks: In ER, between DB LO tank (sump tank) and adjacent tanks. Maximum width =  760 mm.

Double Bottom:

  • The double bottom consists of outer shell and inner skin, 1m and 1.5 m above the keel and internally supported by

Double Bottom Tank:

  • Double bottom space is subdivided longitudinally and transversely, into large tank, by means of watertight structures. Its functions are:
  1. Protection of shell in the events of damage to bottom shell.
  2. Tank top being continuous increases the longitudinal strength.
  3. To act as platform for cargo and machinery.
  4. Can be used for storage of fuel, fresh water, ballast, and for correcting list, trim and draught.
  5. Diminish oil pollution, in the event of collision.

Wing Tank:

Purpose:

  • To carry water ballast or liquid cargo.
  • Protection of shell in the events of damage to side shell.
  • To locate oil cargo tank
  • To correct list of the ship.

Deep Tank:

  • When ship is underway in light condition, it is necessary to carry certain amount of water ballast.
  • If DB tanks alone are used for this purpose, the ship might be unduly “stiff”.
  • So it becomes a practice to arrange one of the lower holds, so that it can be filled with water when necessary.
  • This permits a large amount of ballast to be carried without unduly lowering the centre of Gravity of the ship.
  • Such a hold is called a Deep Tank.
  • This tank is usually designed to carry dry cargo, and in some cases may carry vegetable oil or oil fuel as cargo.
  • If the tank extends full breadth of the ship, a middle line bulkhead, called Wash Plate must be fitted to reduce free surface effect.
  • Strength of Deep Tank structure is greater than that required for dry cargo hold bulkhead.

Freeboard:

  • Vertical distance from water load line, up to the main deck [freeboard deck], measured at the shipside amidships.
  • Main deck is the highest deck that is water sealed. Water falling on upper decks may run down companion ways, but it cannot go any further down into the ship than the main deck.
  • Freeboard has considerable influence on seaworthiness of the ship. The greater the freeboard, the larger is the above water volume of the ship and this provides reserved buoyancy, assisting the ship to remain afloat in the event of damage.

Reserved buoyancy:

  • Watertight volume of a ship above the water line is called the reserved buoyancy.
  • It can be defined as the buoyancy, a ship can call upon, to meet losses of buoyancy in case of damage to main hull. [Water plane area, multiplied by freeboard.]

 Purpose:

  • To meet loss of buoyancy, in case of hull damage.
  • To provide sufficiency of freeboard, to make the vessel seaworthy.

Marking of freeboard:

Marking of minimum allowable freeboard, in conjunction with an overall seaworthiness evaluation, is to ascertain that the vessel:

  1. is structurally adequate for its intended voyages,
  2. has adequate stability for its intended voyages,
  3. has a hull that is essentially watertight from keel to freeboard deck, and watertight above these decks,
  4. has a working platform that is high enough from water surface, to allow safe movement on exposed deck, in the heavy seas,
  5. has enough reserved buoyancy above the water line, so that vessel will not be in danger of foundering and plunging when in heavy seas.

Hatchways: 

These constructions must be in accordance with standards, such as heights of coamings, covers, and fittings exposed. They have standard of strength and protection.

Machinery Casing:  

Machinery space openings on exposed portion of freeboard deck (superstructure deck), must be provided with Steel Casing, with any opening fitted with Steel Doors. Fiddley Opening is to have permanently attached Steel Covers.

Tonnage:  

  • Tonnage is a measure of cubic capacity, where one ton represents  100 ft³ or  2.83 m³. It is a measure of the ship’s internal capacity.

Gross Tonnage:

  • Gross tonnage is the total of the Underdeck tonnage & the tonnage of the following spaces:
  1. Any Tweendeck space , between second and upper deck.
  2. Any excess of hatchways over ½ % of vessel’s Gross Tonnage.
  3. Any permanently closed-in spaces, on or above the upper deck.
  4. Any engine light and air space on or above upper deck, at shipowner’s option and with Surveyor’s approval.
  5. Certain closed-in spaces, on or above the upper deck are not included in gross tonnage, and these are known as Exempted Spaces.

Exempted spaces:

  • Dry cargo space.
  • Space fitted with machinery or condensers.
  • Wheelhouse, chartroom and radio room.
  • Galley and bakery.
  • Washing and sanitary spaces in crew accommodation.
  • Light and air spaces.
  • Water ballast tanks not appropriated for any other use.

 Net or Registered Tonnage:

  • It is obtained by making “deductions” from the Gross Tonnage.
  • Principal “Deducted Space”, which already have been included in Gross Tonnage are:
  1. Master’s and crew accommodation.
  2. Chain lockers and space for working anchor and steering gear.
  3. Propelling Power Allowance.
  4. Ballast tank, capacity ≯ 90%.
  • Port and Harbour dues are assessed on Net Tonnage.

Where Tonnage value is used?

  1. To determine port and canal dues.
  2. To determine Safety Equipment.
  3. To measure the size of fleet.

Propelling Power Allowance:

The largest “Deduction” and is determined according to certain criteria, as follow:

  1. If machinery space tonnage is between 13% and 20% of gross tonnage, PPA is 32% of gross tonnage.
  2. If machinery space tonnage is less than 13% of gross tonnage, PPA is the amount expressed as a proportion of 32% of gross tonnage.
  3. If machinery space tonnage is more than 20% of gross tonnage, PPA is 1.75 times the machinery space tonnage.
  4. There is a maximum deduction for propelling power of 55% of gross tonnage, remaining after all other deductions have been made.

Tonnage Deck:  The tonnage deck is the second deck, except in single deck ships.

Water tightness of steel hatch cover:

Rubber jointing is used, and the hatch being pulled down by cleats and cross joint wedges.  Cleats are placed about  2 m apart with minimum of two cleats per panel. Cross joint wedges should be 1.5 m apart.

Hose test and chalk test:

  1. To check the water tightness of hatch covers and watertight doors :
      • By using water jet pressure of 2 kg/cm² and a distance of  5 m, and jet diameter  ½”.
      • If hose test cannot carried out, chalk test can be done.

2. Cover or door seals, painted with chalk powder, and close the cover or door tightly.
3.  Open the cover or door, and check whether the chalk painted is cut off or not.

COC ORAL EXAM PREPARATION (PART – 9): REFRIGERATION & AIR CONDITIONER

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Image Credit: almaco.cc

Refrigeration:

 What is a Refrigerant?

  • Refrigerants are used as working substances in a Refrigeration systems.
  • Fluids suitable for refrigeration purposes can be classified into primary and secondary refrigerants.
  • Primary refrigerants are those fluids, which are used directly as working fluids, for example in vapour compression and vapour absorption refrigeration systems.
  • These fluids provide refrigeration by undergoing a phase change process in the evaporator.
  • Secondary refrigerants are those liquids, which are used for transporting thermal energy from one location to other. Secondary refrigerants are also known under the name brines or antifreezes

 

Desirable properties of a refrigerant

  1. Low boiling point (otherwise operation at high vacua becomes a necessity)
  2. Low condensing pressure (to avoid heavy machine plant scantling and reduce the leakage risk)
  3. High specific enthalpy of vaporisation ( to reduce the quatity of refrigerants in circulation and lower machine speeds, sizes etc.)
  4. Low specific volume in vapour state (reduces size and increases efficiency)
  5. High critical temperature (temperature above which vapour cannot be condensed by isothermal compression)
  6. Non corrosive and non solvent (pure and mixed)
  7. Stable under working conditions
  8. Non flammable and non explosive
  9. No action with oil ( the fact that most refrigerants are miscible may be advantageous e.e. the removal of oil films, lowering pour points etc, provided separators are fitted
  10. Easy leak detect
  11. Non toxic
  12. cheap, easily stored and obtained

Refrigeration Working cycle:

The refrigeration cycle is shown in the Figure and can be broken down into the following stages:

234

Fig: Vapor Compression Refrigeration working cycle

Cycle 1-2: 

Low-pressure liquid refrigerant in the evaporator absorbs heat from its surroundings, usually air, water or some other process liquid. During this process it changes its state from a liquid to a gas, and at the evaporator exit is slightly superheated.

Cycle 2-3: 

The superheated vapour enters the compressor where its pressure is raised. The temperature will also increase, because a proportion of the energy put into the compression process is transferred to the refrigerant

Cycle 3-4: 

The high pressure superheated gas passes from the compressor into the condenser. The initial part of the cooling process (3-3a) de-superheats the gas before it is then turned back into liquid (3a-3b). The cooling for this process is usually achieved by using air or water. A further reduction in temperature happens in the pipe work and liquid receiver (3b – 4), so that the refrigerant liquid is sub-cooled as it enters the expansion device

Cycle 4-1:

The high-pressure sub-cooled liquid passes through the expansion device, which both reduces its pressure and controls the flow into the evaporator

Thermostatic Expansion Valve (TEV)

The thermostatic expansion valve performs following functions:
1) Reduce the pressure of the refrigerant: The first and the foremost function of the thermostatic expansion valve is to reduce the pressure of the refrigerant from the condenser pressure to the evaporator pressure. In the condenser the refrigerant is at very high pressure. The thermostatic expansion valve has an orifice due to which the pressure of the refrigerant passing through it drops down suddenly to the level of the evaporator pressure. Due this the temperature of the refrigerant also drops down suddenly and it produces cooling effect inside the evaporator.
2) Keep the evaporator active: The thermostatic expansion valve allows the flow of the refrigerant as per the cooling load inside it. At higher load the flow of the refrigerant is increased and at the lower loads the flow is reduced. It won’t happen that the load on the evaporator is high and the flow of the refrigerant is low thereby reducing the capacity of the evaporator. The thermostatic expansion valve allows the evaporator to run as per the requirements and there won’t be any wastage of the capacity of the evaporator. The TEV constantly modulates the flow to maintain the superheat for which it has been adjusted.
3) Allow the flow of the refrigerant as per the requirements: This is another important function of the thermostatic expansion valve. It allows the flow of the refrigerant to the evaporator as per the load on it. This prevents the flooding of the liquid refrigerant to the compressor and efficient working of the evaporator and the compressor and the whole refrigeration plant.

TEV construction:

  1. Small quantity of Vapour Refrigerant is sealed in a bulb or phial, and attached to Compressor suction pipe, just coming out from Evaporator.
  2. Other end is connected by Capillary Tube to the chamber above Flexible Bellow in valve body.
  3. The space below the Bellow is in communication with Evaporator outlet pressure (this is called Equalising Line)
  4. If no further action is taken, pressure above and below the Bellow will be equalised and hence no superheat is obtained.
  5. This is overcome by providing adjustable Bias Spring under the Bellow, and Bias Spring pressure is proportional to required superheat.

 Operation:

  1. Refrigerant Liquid from Condenser enters into TEV via Dryer, it expands to Evaporation Pressure, and some flash gas is formed.
  2. Flash Gas amount varies between 25 – 35%, depending on refrigerant type, plant capacity and ambient temperature.
  3. Mixture of this expanded gases and some part of liquid, passed into Evaporator, where complete Evaporation takes place.
  4. Evaporator outlet pressure plus Spring pressure tends to close the valve, and is opposed by the pressure above the Bellow, trying to open it.
  5. This pressure above the Bellow is in relation to temperature in Compressor suction pipe.
  6. Equilibrium condition is reached, when Superheat is correct at phial attachment point.
  7. Starved condition in Evaporator will result greater Superheat, so expansion of Vapour Refrigerant in phial will tend to open the valve further, to increase the flow.
  8. Flooded condition in Evaporator will result lower Superheat, so contraction of Vapour Refrigerant in phial will tend to close the valve further, so decrease the flow.
  9. Superheat Temperature adjusted at: 3 – 6°C, by Bias Spring pressure.

 Why Equalising Connection is fitted? 

  1. In some plant having large Evaporator or Multi-circuit Evaporator, excessive pressure drop across Evaporator occurs, and always tend to starve the Evaporator and increase the Superheat.
  2. To counteract this, if pressure drop across Evaporator, exceeds 3 bar, an Equalising Connection must be provided at TEV.
  3. A direct connection between underside of Bellow and suction piping of Compressor, preferably between phial and Compressor.

Safety devices on Refrigeration Plant:

  1. LP cut-out switch: Set at a pressure corresponding to 5°C below the lowest expected evaporating gauge reading.
  2. HP cut-out switch: Set at a pressure corresponding to 5°C above the highest expected condensing gauge reading.
  3. Lub Oil LP cut-out: Oil pressure usually set at   2 bar  above crankcase pressure.
  4. Condenser cooling water LP cut-out.
  5. Liquid shock valve on Compressor cylinder head.
  6. Bursting disc on cylinder head, between inlet and discharge manifold.
  7. Bursting disc on Condenser, [if fitted].
  8. Relief valve on Condenser.
  9. Master solenoid valve: To prevent liquid being entered into Compressor, when the plant is standstill, especially in Large Plant.

Refer plant survey:

  1. General examination of machinery and testing under working condition.
  2. The log examined, to ascertain successful operation during voyages.
  3. Compressor and prime mover to be open-up and examined.
  4. Primary system to be leak-tested to their w. p. and brine cooling coils are to be hydraulically tested to 3  kg/cm².
  5. Survey is done at 1 year from the date of installation, and special periodical surveys are to be carried out at 5 years intervals. ( 1+ 5 )

CFC: Chlorofluorocarbon

  • Due to damaging effects on OZONE layer and causing Global Warming, most CFCs are now replaced by HFCs,
  • HFC 134a has Ozone Depletion Potential, ODP ‘0’ and Global Warming Potential, GWP ‘0.28’.
  • Banned from 19 May 2005

 Defrosting: 

A method of removal of frost, built-up on Evaporator coils. Defrosting should be done before snow thickness exceeds ¼”.

 Reasons for defrosting:

  1. Affecting heat transfer properties.
  2. Affecting air flow and circulation.
  3. Liquid back to Compressor. 

Defrosting Systems:

  1. Water wash defrosting
  2. Hot gas defrosting
  3. Electric defrosting
  4. Manual shut down defrosting
  5. Warm brine defrosting

Various methods to defrost Brine System: 

  1. Hot brine thawing: Best and fastest method, used powerful brine heater with separate thawing system. Watertight trays under the pipes, collected the dripping water.
  2. Hot air from atmosphere: It is important that isolating doors in air trunks are perfectly tight, so as to prevent hot air going into cargo spaces.
  3. By shutting off brine : Allow the snows to be melted by the heat of the air in circulation. Very slow operation and tends to throw back great deal of moisture into cargo space.

 

  • Direct expansion grid system:   Hot gas defrosting.
  • Battery cooling system:    Water spray, electrical or steam heater.
  • Brine cooling:    Hot brine thawing.

Cargo Fridge Defrosting:

  1. In Battery System, hot brine passing brine heater is used.
  2. Steam is released to brine heater and brine flow is restricted by brine inlet valve, until brine temperature has risen above 0°
  3. Brine temperature of 43°C is suitable for defrosting.

Why Cold Room is defrosted and how many methods of defrosting?

  • Coil Room is required to defrost to gain more Heat Transfer Efficiency.
  • Methods of Defrosting are: ( i)  Plant stopped and manual watering    (ii)  Hot gas circulating   (iii)  Electric Heater.

Faults in Shipboard Refrigeration Systems

  1. Undercharging of Refrigeration System

 Indication:

  • Compressor is running hot and performance of the compressor falls off due to high superheat temperature at the suction side of compressor.
  • Suction and discharge pressure of the compressor is low.
  • Large vapor bubbles in the liquid sight glass.
  • Low gauge readings in the condenser.
  • Ammeter reading for the compressor motor is lower than normal.
  • Rise in room temperature which is to be cooled.
  • Compressor is running for extended period of time.

Causes:

  • Leakage of refrigerant at the shaft seal, flange couplings, valve gland etc.
  • Expansion valve may be blocked at the strainer.
  • Partial blockage of refrigerant at the filter or drier or evaporator may cause undercharging.

Action:

  • Identify and rectify the leakage of refrigerant from the system.
  • Clean the filter and drier.
  • Charge the system with fresh refrigerant as required.

2.  Overcharge of Refrigeration System

Indication:
•           The liquid level in the condenser is too high (high condenser gauge reading). This will reduce the available condensing surface, with corresponding increase in the saturation temperature and pressure.
•           High pressure switch of the refrigerant compressor activates and stops the compressor.
•           The suction and the discharge pressures are high.
Causes:
•           It may be due to the reason that excessive refrigerant has been charged in the system.
•           Air in the system may also cause over charging indication.
•           It may also be due to the formation office on the regulator.
Action:
•           Remove the refrigerant from the system. This is done by connecting a cylinder to the liquid line charging valve, starting the compressor, and then operating the charging valve.
•           Purge the air from the system and maintain effective cooling.
•           Remove ice from the regulator by using any of the defrosting methods.

3.  Moisture in the System
This normally comes with the ingress of air in the system. Moisture may freeze at the expansion valve, giving some of the indication of under charging. It will contribute to the corrosion in the system. It may cause lubrication problems and breakdown of the lubricating oil in the refrigerant compressor.

Action:
–  Renew silica gel in case of minor moisture.
–  collect refrigenant and remove all air and moisture by vacuum pump if the amount is huge.

4.  Air in the System
Indication:
•  This may cause the refrigeration compressor to overheat, with a high discharge pressure and normal condensing temperature.
•  There are possibilities of small air bubbles in the liquid sight glass of the condenser.
•  Condensing pressure of the refrigerant in the condenser may be high.
•  If there is excessive air, it may reduce the cooling capacity of the system, making the compressor to run for the extended period of time.
•  It may cause the gauge pointer of the condenser to jump indefinitely.

Causes:
•  During charging, air may enter in to the system.
•  If Freon-12 is used air may leaks in to the suction line because the working pressure of the Freon-12 refrigerant is less than the atmospheric pressure.
Action:
•  Air in the system can be removed by collecting the system gas in the condenser, leaving the condenser cooling water on and venting out the air from the top of the condenser because air will not be condensed in the condenser but remains on top of the condenser above the liquid refrigerant.
•  Connect the collecting cylinder to the purging line of the condenser, open the valve, and collect air in the cylinder.
•  After purging the air from the system don’t forget to shut the purging valve.
•  Check the level of the refrigerant in the system. If required, charge the system with fresh refrigerant.
•  Restart the compressor with all safety precautions.

5.  Oil in the Refrigeration System
Indication:
•  Temperature is not dropping in the cold rooms as normal, due to fact that oil act as insulation in the evaporator.
•  It may cause excessive frost on the suction line.
•  Refrigerant compressor runs for the extended period of time.
•  Lubricating oil level in the compressor will drop.
•  Refrigerant level will fall if oil has caused blockage.

Causes:
•  This may happen if the oil separator is not working properly.
•  Oil may carry over from the compressor and may not come back to the compressor due to blockage in the system.
•  Defective piston rings or worn out liner of the compressor may cause the oil to carry over along with the refrigerant.
•  Compressor may take high capacity current during starting.

Action:
•  Check the oil separator for proper functioning.
•  Check the drier for proper cleaning and if its require cleaning clean it
•  Evaporator coil should be drained to remove any trace of oil.
•  If there is oil in the cooling coils, increase the condenser and evaporator temperature differentials and remove excess frost on the suction pipe.
•  Heat pipes with blow torch.

6.  Flooding of Refrigerant in the System
This is seen as liquid getting back to the suction of the refrigerant compressor. It may be due to a faulty or incorrectly adjusted expansion valve and also due to solenoid valve leakage. It may also result from overcharging of the refrigeration system. Flooding may lead to an iced up evaporator.

7.  Evaporator Coil Icing:
Icing of the evaporation coils which may happen due to:
1. Cause:Too low temperature setting
Action: Increase the coil temperature by adjusting TEV or it’s sensor.
2. Cause: The coil capacity is less
Action: Install large capacity evaporator coils
3. Cause: Defrost is not operational
Action: Check if the defrost system is functioning at regular intervals.

8.  Compressor Start and Stops Frequently:
If while maintaining the correct temperature of the ship’s provision room or reefer cargo, the reefer compressor is frequently cutting-in and out, then such problem needs to be sorted out immediately. The most normal causes for such operation are:
1. Cause: Wrong Setting of Cutouts: It may be because the high pressure (HP) cutout is set too high or LP cutout is set too low
Action: Check and change the setting to advisable limit
2. Cause: Differential Setting Span is Small: The low pressure (LP) cut out is provided with starting and stopping pressure setting. If the setting span is too small, it will lead to frequent cut-in and cut-out of the compressor
Action: Change the setting and increase the span between starting and stopping compressor pressures.
3. Cause: Defective Valves: If the compressor discharge valve is leaky or the line solenoid valve is not closing properly, this will lead to variation in sensor pressure and result in frequent cut-in and cut-out of compressor
Action: Replace all the defective valves
4. Cause: Clogged Suction Filters: Compressor is provided with a filter in the suction line. If that is clogged, it will lead to frequent LP cut out
Action: clean the filter.

 9.  Compressor Starts But Stops immediately
When the compressor in the reefer circuit starts and suddenly stops, it can be because of the following reasons:
1. Cause: Low pressure cut out gets activated
Action: Ensure that all the suction line valves are in open condition, the refrigeration is properly charged and the low pressure cut out is not defective.
2. Cause: Defective oil pressure cut out
Action: Check for proper functioning of oil pressure cutout and replace the defective cutout.
3. Cause: Defrosting timer is getting activated frequently
Action: If the defrost timer is getting activated frequently, leading to cutout of compressor, check and repair defrost timer.
4. Cause: The lube oil level is below required level
Action: This can be because of leakage of lube oil from seal or carry over of oil. Rectify the leakage and refill the oil level.
5. Cause: Foaming of oil leading to reduced oil pressure
Action: Ensure no foaming takes place, renew the oil if required.
6. Cause: Motor overload cutouts are activating
Action: Ensure that electrical motor trips are working properly.

10.  Excessive icing up at Compressor suction:

Causes:

  1. Abnormal operation of TEV.
  2. Overcharge of the system.
  3. Moisture in the system owing to dirty Dryer.
  4. Defective Suction valve:

    Indication:

  1. Continuous running of Compressor.
  2. Insufficient cooling effects.
  3. Noisy operation.
  4. High suction pressure.

11.   Defective Discharge valve:

    Indication:

  1. Continuous running of Compressor.
  2. Insufficient cooling effects.
  3. Noisy operation.
  4. High suction pressure during running.
  5. Low discharge pressure during running.
  6. Suction pressure rises faster after Compressor is shut-down.
  7. Warm cylinder head.

12.  Choked Expansion valve:

Causes:  

Due to dirt and freeze-up of water present in system.

  Effects:    

  1. Starved Evaporator
  2. High superheat temperature.
  3. Rapid Condenser pressure rise can cause stopping of Compressor,

 Remedy: 

  1. Clean Expansion valve and filter
  2. Renew Dehydrator.

Secondary Refrigerant:

  • Calcium Chloride Brine ( 3 ½ lb. of Ca Cl₂  +   1 gal. of water ) with density of 1.25 is widely Sodium Dichromate or lime added to maintain
  • pH values of 8.0 – 8.5.
  • Sodium Chloride Brine.

Why LP Cut-off fitted?

Fitted as safety control and it protect against:    (a) Extreme compression ratio.   (b) Freezing up of Evaporator.  (c)  Entrance of air and water vapor resulting from LP side leakage.

Fridge compressor sump oil filling:

  1. Stop condition:  (i) Tight shut both inlet and outlet valves of compressor.  (ii) Open filling plug and fill to required level. (iii) Air purge to be done when plant resume.
  2. During running:   (i)  Make vacuum pressure in crankcase and suck oil itself.                                       (ii)  Ensure oil pipe immersed in oil to prevent air ingress.

What is made of fridge filter dryer?     

Ans:     (i)  Activated Alumina   (Aluminium Oxide)    (ii)  Silica Gel    (Thorzone)

Safety Devices fitted on Fridge Compressor:

  1. Safety Head or Unloader.
  2. Bursting Disc in Compressor.
  3. LP and HP Gauges and Cut-out.
  4. LO Low Pressure Cut-out.
  5. Condenser cooling water Low Pressure Cut-out

Charging of Refrigeration Plant:

There are two methods for charging reefer plants:

  1. Liquid charging and
  2. Gas charging.
    Now a day’s gas charging is preferred over liquid charging because it is more safe and simple.

Gas Charging of Refrigeration Plant:
For gas charging, a special T piece valve block with mounted pressure gauge is provided to combine three connectors inter-connecting:
-Vacuum pump
-Charging Cylinder
-Charging Point

Following steps are to be taken for charging gas into the reefer plant:
New Bitmap Image
1. Connect gas bottle or charging cylinder, vacuum pump and charging point in the reefer system to the valve block.
2. The discharge of the vacuum pump is to be connected in the empty recovery bottle
3. First open the valve between vacuum pump and charging bottle located in the valve block without opening the main valve of the charging cylinder. This will remove all the air inside the pipe. Once vacuum is reached, close the valve of charge cylinder in the valve block
4. Now open the valve of the charging point pipe in the valve block and run the vacuum pump until the vacuum is reached. This will remove the trapped air from this pipe. Then shut the valve in the valve block
5. Now keep the system idle for 5 minutes to check there is no pressure drop. This will ensure there are no leakages in the system
6. Now open charging bottle pipe valve and the charging point pipe valve located in the valve block. This will set the line for charging. Ensure that the vacuum pump valve is shut7. Now open the main valves in the charging cylinder and charging point of the reefer system
8. Do not overfill the system. Make sure the receiver has 5 % space for expansion
Ensure that no refrigerant is leaked out in the environment as these effects the ozone layer in the atmosphere.
Gas bottle is kept on weighing scale for measuring the amount of charged supplied to the system.

Air Conditioning:

Relative Humidity:

Ratio of amount of water vapour in given volume of air, to maximum amount of water vapour that can be present before precipitation occurs.

Control of temperature:

Comfortable temperature range is about 22°C and RH about 60%, (usually 40 ~ 70%).

 All zone temperature:   

  1. Controlled by Compressor suction pressure, via solenoid valve as step controlling.

Thermostat, placed at some accommodation space actuates the Master Solenoid

Valve of the plant, which will stop the Compressor, when pre-set temperature

is reached.

  1. Capacity Unloader of Compressor units, does last step controlling, as required.

Particular zone temperature:

  1. Controlled by flap valve fitted in each zone loop.
  2. Local cabin temperature can be adjusted by volume control at delivery point of air duct controller.

Ozone Depletion: 

  1. Ozone gas layer is a region of the atmosphere, 12 – 30 miles above Earth’s surface.
  2. This layer moderates the climate, and protects life on Earth from ultraviolet rediation
  3. Release of industrial waste and other process breakdown ozone layer and so disturb natural balance.
  4. Chlorofluorocarbons, CFCs, at ground level, rise and broken down by sunlight, whereupon chlorine reacts with and destroys ozone molecules.
  5. Single chlorine atom may destroy 10 – 100,0000 ozone molecules.

Ozone Depletion Substance (ODS):

CFC 11     1.0                Halon1211     3.0     (Used in portable extinguishers)

CFC 12     1.0                Halon1301     10.0   ( Used in fixed installation)

CFC 115   0.6                HCFC 22        0.05

CFCs: Chlorofluorocarbon Refrigerant:

Chlorofluorocarbon Refrigerants includes:  

CFC11,  CFC12,  CFC22,  CFC 115,

CFC500,  CFC502,  CFC 503 and  CFC 504.  (8 Types)

Difference between Air Cond. and Fridge:

  • Air Cond. controls Humidity, Temperature and Flow Rate of fresh air.
  • Fridge cools down the provisions.

Air Reducing Valve:

  1. Fitted on compressed Air Bottle outlet.
  2. Reduced compressed air is used for control of Reversing Mechanism in unidirectional gear drive engines, ship whistle, automatic controls and air motors.
  3. High-pressure air enters under the valve.
  4. The spring, acting on the valve spindle, opens the valve and the air passes to the reduced pressure
  5. Compression given to the spring controls the amount of opening of the valve.
  6. If the opening increases, the higher pressure obtained on other side, acts to close down the valve to normal lift, and hence correct reduced pressure maintained.
  7. A Relief Valve is fitted on low-pressure side to prevent excessive pressure rise on reduced air system.

Where fitted Dehumidifier and its function.

  1. Fitted at discharge side of Reducing Valve on control air line.
  2. Main function is to prevent oil and condensate water passes through control air line.

Psychrometric chart

This chart is used for finding the relative humidity of air which has been measured using a ‘wet and dry bulb’ thermometer. This is a pair of thermometers, one of which has its bulb wrapped in a damp cloth. The drier the air,the greater the evaporation of water off the cloth and therefore the lower the reading on the ‘wet bulb’ thermometer.

124

Dew point:
When a mixture of dry air and water vapour has a saturation temperature corresponding to the partial pressure of the water vapour it is said to be saturated. Any further reduction of temperature (at constant pressure) will result in some vapour condensing. This temperature is called the dew point, air at dew point contains all the moisture it can hold at that temperature, as the amount of water vapour varies in air then the partial pressure varies, so the dew point varies.

COC ORAL EXAM PREPARATION (PART – 8): AIR COMPRESSOR

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Air Compressor

H-274
Image Credit: leelloyds.com

Effects of leaking valves in Air Compressor:

First Stage Suction:

  • Reduce air delivery
  • Reduce 2nd stage suction pressure
  • Unload the compressor
  • Increase running time.

First Stage Delivery:

  1. Reduce air delivery
  2. Increase discharge temperature
  3. Less air drawn in, due to high-pressure air leaking back into cylinder.

 

Second Stage Suction:

  1. Reduce air delivery
  2. High temperature & pressure in 2nd stage suction line
  3. Increase running time

Second Stage Delivery:

  1. Reduce air delivery
  2. Increase suction pressure in 2nd stage
  3. Increase delivery pressure from 1st stage.
  4. Back pressure from air bottle.

How to check Air Compressor Efficiency?

  1. Regular overhauling of valves done or not.
  2. Check Air Bottle filling time.
  3. Compare test results and records.

How to check Air Compressor capacity is sufficient?

  • Total no. of Air Compressors must be sufficient to fill the empty Air Bottle to maximum pressure within 1 hour.
  • Must be sufficient to start at least 12 times for Reversible Engine, and at least 6 times for Non-Reversible Engine.

Bursting Disc:

  1. Fitted on the shell of Intercooler at waterside.
  2. Relieves pressure if the tubes burst.
  3. Rolled Copper Alloy and relief pressure is set while the disc is at softest condition.
  4. Material tends to harden due to time and surrounding temperature, and set pressure also increased.
  5. Bursting Disc needed to be annealed, to regain correct relief pressure.

Compound Valves, why used in Air Compressor?

  1. Give large area of opening and small amount of valve lift.
  2. Improve Volumetric Efficiency, as valve open and close in minimum time.
  3. Reduce bumping clearance.
  4. Reduce wear and tear.

LP Relief Valve opening:

Causes:

  1. HP suction valve
  2. Intercooler choked.
  3. Relief valve, jammed by foreign particles, in open position.

HP Relief Valve opening:

Causes:

  1. HP discharge valve, in closed position.
  2. After cooler choked.
  3. Relief valve, jammed by foreign particles, or spring sticking in open position.

(Relief Valves opening pressure are set at  ≯ 10% above stage pressure.)

Why Intercooler is fitted?

  1. Reduce air temperature and volume, and increase air density for next stage.
  2. Increase Compressor Capacity and Volumetric Efficiency.
  3. Better lubrication for cylinder and rings.
  4. Water and excess oil can be drained out, preventing fouling of Intercooler and pipes, Air Bottle corrosion, and starting airline explosion.
  5. Work done is saved.
  6. Metal stresses reduced, due to control of temperature.

Volumetric Efficiency:

VE  = (Volume of Air drawn into Cylinder) / (Stroke Volume of LP Piston)

and

VE = (Volume of Air discharged as ‘free air’) / (Stroke Volume of LP Piston)

Why Multistage Air Compressor is built?

  1. To obtain near to ideal isothermal compression, compressor is to be constructed of multistage with inter-stage cooling.
  2. Inter-stage cooling reduces air temperature and volume after 1st stage compression, thus increase mass of air for 2nd
  3. Workdone is saved and air compressor efficiency increased.

Other advantages are:

  1. Easy to get high final air pressure.
  2. Easy to control air temperature.
  3. Easy to maintain correct lubrication.
  4. Better compressor balancing.
  5. Reduction in size.
  6. reduction in clearance volume loss.

Capacity of air compressor:

  1. Capacity is checked upon how much filling time is lowered.
  2. Compressor should have enough filling capacity so that sufficient stopping time should be provided between fillings.
  • 12 consecutive starts in reversible engine  and
  • 6 consecutive starts in non-reversible engine.

What is Free Air Delivery, FAD?

  1. Capacity of Air Compressor is stated in terms of [ m³/ hr ].
  2. Volume of air actually discharged in 1 hour, that would occupy if expanded down to atmospheric pressure and cooled to atmospheric temperature.

Safety devices on Main Air Compressor:

  1. Bursting Disc on Intercooler: (At water side)
  2. Bursting Disc and Fusible Plug (121°C) on Aftercooler
  3. Relief valves on LP and HP stages. (Set to lift at 10% rise above normal stage pressure.)
  4. Automatic Moisture Drain Valve.
  5. Cooling water supply failure alarm.
  6. Low LO pressure alarm.
  7. Relief valve on crankcase LO pump.
  8. Delivery air HT cut out and alarm on Aftercooler outlet. (Max. 93°C)
  •   LP discharge pressure 4 bars and HP discharge pressure 30 bar
  • Intercooler inlet air 130°C and intercooler outlet air 35°C
  • Aftercooler inlet air 130°C and Aftercooler outlet air 35°C:

Intercooler is single pass type:  Aftercooler, double pass U-tube type:}

Uses of Compressed Air: 

  • Engine Starting 20 to 25 bar
  • Boiler Soot Blowing 20 to 25 bar
  • General Service (Whistle, Pneumatic Tools, Lifeboat , service air for cleaning and Pilot ladder) 7 to 10 bar
  • Instrumentation and Control 5 bar

Air Filter: 

  1. Material: Felt, Metal gauze or Nylon strands
  2. Removes contaminants from air. Dirt and dust act as abrasives and increase wear.
  3. Contaminant deposits on valves can become hot and source of ignition.

Hazard of Dirty Filter: 

  1. Very dirty filter or muffling a filter results in large pressure drop.
  2. Air has to be compressed over higher range.
  3. In extreme case, discharge air temperature may exceed flash point, or auto-ignition temperature resulting in an
  4. As a safety against explosion, air temperature is limited to 93° Fusible Plug (121°C) or a High Temperature cut out (93°C) is provided on Compressor.

Pressure Test on Air Compressor:

  • Cylinders, cylinder cover, Inter & After coolers are hydraulically tested to:

Air Side:  1.5 x max. Working Pressure.
Water Side: 4 bar or 1.5 x max. WP (whichever is greater)

Air receiver:

  • Total capacity of air receivers must be sufficient to give at least 12 starts for reversible engine, and at least 6 starts for non-reversible engine, without refilling the receivers.
  • There must be two identical main air receivers and one emergency bottle for every vessel.

 Mountings of Air receiver:  

  1. Fusible plug: 
  • composition – Bismuth 50%, Tin 30%, Lead 20%,
  • Melting point: 220΄F (104.4΄C).    Fitted at the reservoir’s bottom or on reservoir at ship side, when relief valve (safety valve) is not directly fitted on the reservoir.
  1. Atmospheric relief valve: provided for back-up of fusible plug. In case of ER fire when CO₂ flooding is required, this valve is opened before evacuating ER.
  2. Spring loaded safety valve: setting pressure: 32 bar (for 30 bar working pressure), with ≯ 10% rise in accumulation of pressure. May be fitted directly or with extension.
  3. Compensation ring: when a hole is cut or machined in pressure vessel, higher stresses will subject to the material around the hole, and to reduce this, compensation rings are fitted. It is a flange on which a valve or fitting is usually mounted.
  4. Manual Drain valve or Automatic Drain valve.
  5. Pressure gauges.
  6. Access doors.
  7. Main starting air valve, auxiliary starting air valve, filling valve, service air or whistle air valve.

Internal surface coating:

Graphite suspension in water, Linseed oil, Copal vanish or Epoxy coating having basic properties of anti-corrosive, anti-toxic or anti-oxidation.

Safety devices on Main Air Bottle:

  1. Fusible plug.
  2. Pressure Relief Valve
  3. Atmospheric Relief Valve.
  4. Low Air Pressure alarm.
  5. Automatic or remote control Moisture Drain Valve.

VENTILATION ARRANGEMENT FOR HAZARDOUS SPACES IN ENGINE ROOM

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The following engine room spaces are very hazardous on-board the ship and therefore, a special ventilation arrangement should be provided for safe entry:

(a) CO2 storage compartment
(b) Battery room
(c) Purifier flat
(d) Refrigerating machinery space
(e) Shaft tunnel

Ventilation Arrangement for Confined Spaces of Ships:

(a) CO2 storage compartment
– CO 2 compartment is situated on open deck , well ventilated and easily accessible .
– Its compartment door is opened outward.
– Boundaries between CO2 compartment should be watertight.
– Ventilation louver is fitted at the bottom near the floor , for the leaky CO2 to release to atmosphere because CO2 is more heavier than normal air.
– In some ships , exhaust fan is provided . Its suction is taken from bottom of the compartment and lead to open air deck . Exhaust system is separately provided from others.
– In some modern built ships , CO2 leakage warning alarm is provided

(b) Battery room
– Battery room is provided good ventilation, to prevent accumulation of Hydrogen gas and risk of battery explosion
– In the room , batteries should not be placed on the floor . It must be put on the level racks.
– Ventilation inlets are arranged below the battery level.
– Ventilation outlets are arranged at the top of a battery compartment.
– If the vent is other than direct to the outside , an exhaust fan is required. The fan is in the air stream from the compartment.
– The exhaust fan must be independent from other ventilation systems.
– All ventilation ducts should be made with corrosion resistant material.

(c) Purifier Flat
– Purifier flat is usually located at the E/R bottom floor. In some ships , purifiers are provided with separate compartment.
– Normally purifiers are operating with heaters to obtain efficient purification. Therefore , inflammable gases and some leakage of heated oil are usually accumulated in the Purifier flat . By this reason purifier space or room is provided with an exhaust blower to achieve better ventilation. , in addition to the fresh air supplied by E/R ventilation
– Out let form the purifier room blower lead to the open deck . Its louver is fitted with cover , normally open , but to close in case of fire in engine room , for CO2 release .
– The exhaust blower can be stopped from emergency remote station outside engine room .

(d) Refrigerating machinery space
– Ventilation system for Refrigerating plant is depend upon the type of refrigerant used.
– In Freon Gas refrigeration system, normal E/R ventilation is sufficient for the refrigerating plant space. If the plant is provided with separate compartment normal ventilated louver is provided.
– But for Ammonia and CO2 refrigeration system ,being they are harmful to human life , positive
ventilation system i.e , exhaust fan must be used to repel any leakage of CO2 and NH3 gas to outside
atmosphere. This reduce the risk of danger for explosion or suffocation.

(e) Shaft Tunnel
– In shaft tunnel , natural draught fan is used. The force ventilation , supplied air for Engine Room is
passing through shaft tunnel via opened water tight door. The hot & foul air from shaft tunnel may
escape through emergency exit from shaft tunnel to open deck.
– In some ships , shaft tunnel ventilation is provided.

COC ORAL EXAM PREPARATION (PART-7) – MARITIME SECURITY

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isps1
Image Credit: www.msecurity.ru

When do the security measures come into force ?

– 1st July, 2004.

SOLAS amendments adopted in December 2002 :

Chapter V:
– Automated Identifications Systems(AIS)

Chapter XI-1:
-Ship identification number
-Continuous Synopsis Record (CSR)

Chapter XI-2:
-Measures to enhance maritime security
-International Ship and Port Facility Security (ISPS) Code (Parts A & B)

Automatic identification systems (AIS)

– The requirement became effective for all ships by 31 December 2004.
– In 2000, IMO adopted a new requirement (as part of a revised new chapter V) for all ships to carry automatic identification systems (AIS) capable of providing information about the ship to other ships and to coastal authorities automatically.
– AIS to be fitted aboard all ships≥300 gross tonnage engaged on international voyages,
– cargo ships≥ 500 gross tonnage not engaged on international voyages
– all passenger ships irrespective of size.

The regulation requires that AIS shall:
1. Provide information – including the ship’s identity, type, position, course, speed, navigational status and other safety-related information – automatically to appropriately equipped shore stations, other ships and aircraft;
2. Receive automatically such information from similarly fitted ships; monitor and track ship
3. Exchange data with shore-based facilities.

Ship Identification Number

In December 2002, the Diplomatic Conference on Maritime Security adopted a number of measures aimed at enhancing security of ships and port facilities. This included a modification to SOLAS Regulation XI-1/3 to require ships’ identification numbers to be permanently marked in a visible place either on the ship’s hull or superstructure. Passenger ships should carry the marking on a horizontal surface visible from the air. Ships should also be marked with their ID numbers internally.

  • The IMO ship identification number is made of the three letters “IMO” followed by the seven-digit number assigned to all ships by IHS Fairplay (formerly known as Lloyd’s Register-Fairplay) when constructed.
  • The IMO number is never reassigned to another vessel and is shown on the ship’s certificates.
  • This number is assigned to the total portion of the hull enclosing the machinery space and is the determining factor should additional sections be added.
  • The number should Permanently marked and visible on:

* stern or side of hull or superstructure
* horizontal surface for passenger vessels

  • Contrasting color with the alphabet size not less than 200 mm; width proportionate to height and raised lettering or by cutting it into or center punching (or other equivalent)
  • This is a unique seven digit number that is assigned to propelled, sea-going merchant ships of 100 GT and above upon keel laying with the exception of the following:

– Vessels solely engaged in fishing
– Ships without mechanical means of propulsion
– Pleasure yachts
– Ships engaged on special service (e.g. lightships, SAR vessels)
– Hopper barges
– Hydrofoils, air cushion vehicles
– Floating docks and structures classified in a similar manner
– Ships of war and troopships
– Wooden ships

Continuous Synopsis Record (CSR)

  • To be issued by the Administration to each ship under its flag
  • In case of changes, Administration to issue amended record
  • To be kept on board and available for inspection at all times
  • Basic diary of ship or historical record of ship – Issued by Administration, but must be maintained and updated onboard and left onboard with change of ownership or registry.
  • Information should be included in CSR:

* name of flag state
* date of registry
* ship’s ID number
* name of owners
* name of registered demise charterers
* name of shipping company
* name of classification society(ies)
* name of authorities or associations issuing certifications

Ship Security Alert System

  • Required to be provided on all ships constructed on or after 1 July 2004 and ships constructed before 1 July 2004 :

– Oil & chemical tankers, gas & bulk carriers, and cargo HS craft, of equal or more than 500 GT; Passenger ships including HS passenger craft; not later than the 1st survey of radio installation after 1 July 2004

–  other cargo ships of equal or more than 500 GT and mobile offshore drilling units; not later than the 1st survey of radio installation after 1 July 2006

  • Shall initiate & transmit alert to a competent authority designated by administration
  • Shall identify ship, its location and indicate that ship’s security is under threat
  • Shall not send the alert to any other ship nor raise alarm on board
  • Shall continue the alert until deactivated or reset
  • Have activation possible from bridge & at least one other location
  • Performance standard; not necessarily requires new
    equipment installation (eg. Piracy alarm)
  • It’s a security silent ship-to-shore alert
    * activated from bridge and at least one other location
    * not heard by other ships
    * include name and location of ship
  • Protected from inadvertent initiation
  • Administration or coastal state notification requirements

ISPS Code:

The Code provides for considerable flexibility to allow for required security measures to be adjusted to meet the assessed risks facing particular ships or port facilities.

It has two Parts:

  • Part A containing mandatory provisions covering the appointment of security officers for shipping companies, individual ships and port facilities. It also includes security matters to be covered in security plans to be prepared in respect of ships and port facilities; and
  • Part B containing guidance and recommendations on preparing ship and port facility security plans.

Maritime Security (MarSec) Levels:

The ISPS Code contains three security levels. The security level will be set by the Contracting Government (CG).

The three levels are:

  • Security Level 1, normal; the level at which ships and port facilities normally operate.

–  This will mean that minimum protective security measures shall be maintained at all times.

  • Security Level 2, heightened; the level applying for as long as there is a heightened risk of a security incident.

–  This means the level for which appropriate additional protective security measures shall be maintained for a period of time as a result of heightened risk of a security incident.

  • Security Level 3, exceptional; the level applying for the period of time when there is a probable or imminent risk of a security incident.

–  This means the level for which further specific protective security measures shall be maintained for a limited period of time when a security incident is probable or imminent, although it may not be possible to identify the specific target.

COC ORAL EXAM PREPARATION (PART-6) – BOILER OPERATION & BW TREATMENT

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Boiler :

Boiler Automatic Burning System:

1. With correct water level, steam pressure transmitter initiates cut-in at about 1.0 bar below working pressure.
2. Steam pressure transmitter initiates Master Relay to allow ‘Air On’ signal to force draught fan.
3. Air feedback signal confirms ‘Air On’ and allows 30-sec. delay for purge period.
4. Then Master Relay allows Electrode to strike ‘Arc’.
5. Arc striking feedback signal confirms through electrode relay and allows 3-sec. delay.
6. Then Master Relay allows burner solenoid valve for ‘Fuel On’ operation.
7. Fuel On feedback signal allows 5-sec. delay to proceed.
8. As soon as receiving Fuel On feedback signal, Master Relay checks ‘Photocell’, which is electrically balanced when light scatter continuously on it.
9. Result is OK and cycle is completed.
10. If not, fuel is shut-off, Alarm rings and cycle is repeated.
11. Steam pressure transmitter initiates cut out automatically at about 1/15 bar above W.P.

Accumulation Pressure Test:

1. Required for new boiler or new safety valve.
2. Tested under full firing condition.
3. Feed Check valve and Main Stop valve shut.
4. Test is to be continued as long as water in the boiler permits, but 7 minutes for Water tube Boiler and 15 minutes for Cylindrical Boiler.
5. With Safety Valve opening, Boiler pressure must not accumulate to exceed 10% of Working Pressure.

Difference Between Safety Valve and Relief Valve:

1. Safety valve fully open at set pressure. Start open at set pressure. But relief valve fully open at 15 – 20% above set pressure.
2. Safety valve close at set pressure. But the relief valve close below set pressure.
3. Safety valve relieve excess mass. But the relief valve relieve excess pressure.
4. Safety valve can open manually but relief valve cannot open manually.
5.  Safety valve set to open 3 % above WP  but the relief valve set to open 10 % above WP.

Waste Heat Recovery System

  • The use of exhaust gas from main engine to generate steam is means of heat energy recovery and improved plant efficiency.
  • In ME ≯ 40 % of fuel consumed is converted into useful work, and 30 – 34 % of remaining energy contain in Exhaust Gas.

Waste Heat Recovery System is employed as:

1. Composite boiler system.
2. Two separate boiler system ( One oil fire and one ordinary coil type exhaust gas boiler)
3. Tubular type heat exchanger system ( One oil fire and one tubular economizer)
4. Separate steam receiver system ( Two duel pressure boiler and one economizer)
5. Advanced waste heat system (Exhaust gas economizer with 3 separate sections).

Why boiler water test carried out?

  1. To know boiler water condition.
  2. To control chemical treatment and blow down.
  3. To prevent scale formation, corrosion and impurities.
  4. To prevent damage to steam operated equipment and condensate line.
  5. To maintain alkaline condition.
  6. To improve boiler efficiency.

Boiler Water Test:

1. Chloride Test.
2. Excess Phosphate Test.
3. Total Dissolved Solid Test (Conductivity Test).
4. pH value Test.
5. Hydrazine Test.
6. Alkalinity Test: ‘P’ Alkalinity (Phenolphthalein) and ‘M’ Alkalinity (Methyl-orange) and Total Alkalinity

Proper Sample:

1. Sample line is usually located in steam drum, just above the tubes and as far as possible from chemical feed line.
2. Sample water is taken at water surface, since highest concentration is at this point.
3. Sample water is cooled down to about 25°C.
4. Flush out sample stream for 5 minutes before taking.
5. Test apparatus should be cleaned with sample water.
6. Sample water is tested as soon as possible after drawing.

Alkalinity Tests:

1. ‘P’ Alkalinity finds presence of Hydroxyl, Phosphate and half of Carbonates, excluding Bicarbonates.
2. ‘T’ Alkalinity gives total quantity of all Alkaline Dissolved Salts in boiler water.
3. ‘M’ Alkalinity finds presence of remaining Carbonates and Bicarbonates.
4. Total Alkalinity is < 2 x ‘P’ Alkalinity. 5. Desired value is 150 – 300 ppm for ‘P’ Alkalinity. Increase of Alkalinity Level: Causes: 1. Alkalinity treatment has been done recently. 2. Using of Alkaline rich makeup feed water. 3. Incorrect strength of reagent used. Effect: Caustic Embrittlement. Decrease of Alkaline Level: Causes: 1. Feed water is contaminated with acid. 2. Direct water loss from boiler. 3. Incorrect strength of reagent used. Effect: Corrosion. Chloride Test: 1. Gives quickest indication of any salt-water leakage into boiler. 2. Must be carried out daily. 3. Chlorides of Calcium, Magnesium and Sodium are extremely soluble. 4. Chloride level is proportional to TDS level in boiler water. 5. Rate of blow-down is governed by chloride level. 6. Chloride Level should be 0 – 300 ppm, and blow-down if > 300 ppm.

Increase of Chloride Level:
Causes:
1. SW leaking into system.
2. Incorrect strength of reagent used. (Silver Nitrite and Potassium Chromate).
3. Due to treatment chemical and hardness salt reaction.

Effects:
1. Increase in TDS level causes Foaming/Priming.
2. Drop in Alkalinity causes Corrosion.

Phosphate Test:

1. Presence of Phosphate in sample means no hardness salts.
2. Na₃PO₄ added to boiler water, precipitate all scale forming hardness salts of Calcium and Magnesium.
3. With Phosphate Test done, no need to do Hardness Test.
4. Phosphate ppm of 20 – 40 is satisfactory, and blow-down if > 40 ppm.

pH value Test:

1. Once Alkalinity Test is done, no need to check pH value, since Alkalinity and pH value are proportional.
2. Litmus Strip is used for quick reference however.
3. pH value maintained at 10.5 – 11.5.

Condensate pH:

1. Condensate pH is measured at Condenser outlet.
2. By theory, it should not be acidic, i.e. (9.5 – 11.5) but practically it is always less than 9.5.
3. (8.3 – 8.6) is satisfactory.
4. If < 8.3, increase dosage by 25% Condensate Corrosion Inhibitor. 5. If > 8.6, decrease dosage by 25% Condensate Corrosion Inhibitor.

Hydrazine Test: (for Dissolved Oxygen)

1. Hydrazine ppm maintained at 0.1 – 0.2 ppm.
2. If < 0.1 ppm, increase dosage by 25% hydrazine. 3. If > 0.2 ppm, decrease dosage by 25% hydrazine.

Types of Boiler Gauge Glass:

1. Fitted directly.
2. Fitted to a large bored bent pipe.
3. Mounted on a hollow column.
4. Fitted to a column with its centre part solid.

 

Boiler Corrosion:

(1) Electro-chemical Corrosion:

  1. Hydrogen ions (H⁺) are generated by acid concentration under hard dense deposits and can penetrate grain boundaries of tube metal,
  2. Hydrogen attack can occur very rapidly, causing the tubes cracked, failed and ruptured.
  3. General wastage occurs when pH value is < 6.5.
  4. Pitting (Air Bubble pitting and Scab pitting) occur when pH value is between 6–10 in the presence of dissolved Oxygen.

(2) Caustic Cracking corrosion:

  1. Inter-crystalline cracking occurs when excess concentration of Caustic Soda (NaOH) in boiler water, comes in contact with steel, under stresses and high temperature.
  2. Metal becomes brittle and weak.
  3. Damage occurs to riveted seams, tube ends and bolted flanges.
  4. 4. Prevented by dosing Sodium Sulphate (Na₂SO₄) to give protective layer.

5. Ratio of Na₂SO₄ to NaOH should be maintained 2 : 5.

(3) Corrosion by Oil:

1. Animal or vegetable oil decomposed to fatty acid and causes corrosion. 2. Causes are over lubrication of machinery, leakage of heating coils & inefficient filtering of feed water. 3. Prevented by Liquid Coagulant Treatment, which coagulates oil droplets & suspended solids and settle them at low points, and they can be blown-down.

(4) Corrosion by Galvanic Action:

1. With dissimilar metals in a saline solution, galvanic action results and more anodic metal corrodes. 2. Corrosion occurs when feed water is contaminated with salt-water. 3. Prevented by carrying out Chloride Test daily. 4. Chloride Level should be 0 – 300 ppm, and blow-down if > 300 ppm.

CO₂:
1. Reacts with H₂O to form Carbonic Acid (H₂CO₃) which reduces pH value (Alkalinity) of feed water and accelerates general type of corrosion.
2. Groovings along the pipe’s bottom, bends & threaded section.

NH₃:
1. Attacks Copper based Alloy, in the presence of O₂.