Exhaust Gas Boilers:
- About 30% – 34% of Fuel Energy input to engine are discharged to Exhaust Gas, as Thermal Energy.
- This thermal energy is converted into useful work in Exhaust Gas Boiler.
Cochran Exhaust Gas Boiler:
1. A double-pass, vertical type, in which Exhaust gases from ME pass through 2 banks of tube.
2. Served as an efficient silencer, when the boiler is in use.
3. A separate Silencer, always fitted along with exhaust gas boiler, to be used when the boiler is generating more steam than required.
4. All or part of exhaust gases can be directed to the Silencer and atmosphere, without going through the boiler.
5. Working Pressure is around 7 bars.
Composite Boiler (Composite type Cochran boiler):
1. If Exhaust Gases and Oil fire can be used at the same time, it is termed Composite Boiler.
2. In double-pass, composite type Cochran Boiler, it provides a separate tube nest for exhaust gas passage, situated immediately above the return tube nest from Oil-fired Furnace.
3. Exhaust gases from Oil-fired Furnace and ME; pass through the tubes, which are surrounded by boiler water.
4. Separate Uptakes provided for Exhaust Gases and Oil-fired Smoke.
5. Heavy Changeover Valves are fitted, to divert the gases straight to the funnel, when desired.
Alternative Boiler: (Alternative type Cochran boiler)
1. If Exhaust Gases and Oil fire can be used only one at a time, it is termed Alternative Boiler.
2. Double-pass, Alternative Cochran Boiler, can be oil fired and heated by exhaust gases alternatively.
3. Since both systems use the same Combustion Chamber, one system required being blank, while the other is in operation.
4. Only one Uptake required.
Economiser as Exhaust Gas Boiler:
1. In this system, a separate Exhaust Gas Economiser EGE is connected to an Oil-fired Auxiliary Boiler (or an Accumulator) by means of piping and a set of Circulating Pumps.
2. Heat absorbed from exhaust gas in EGE is transmitted by working fluid, to Auxiliary Boiler or Accumulator, from which steam is drawn for use.
3. Economiser unit cannot deliver steam, directly to steam range.
4. Straight gas lead from ME Exhaust Manifold, pass through EGE under the Funnel, and this arrangement permits the Auxiliary Boiler or Accumulator, to be placed in any convenient position in ER.
5. Inlets and outlets of piping coils are connected to External Headers [Distributing and Collecting], that are simply inserted in Exhaust Trunk way.
6. Water from Auxiliary Boiler or Accumulator is fed by Circulating Pump through
a Non-Return Valve into Distributing Chest or header and from it, water passes into Coils.
7. Water and steam from outlets of these Coils pass into Collecting Header, and then to the steam space of Auxiliary Boiler or Accumulator.
8. EGE Safety Valve is adjusted at slightly higher pressure than Safety Valves of Auxiliary Boiler; in order to ensure that Economiser operates in flooded condition at all times.
EGE out of order:
1. Isolate the EGE.
2. Wash down the economiser tubes and completely dried.
3. Drain all water content.
4. Start the auxiliary boiler.
5. Maintain low steam consumption.
6. Proceed to next port with suitable speed.
7. Write down damage report.
Exhaust Gas Boiler Safety Valve setting:
1. EGE safety valves to be set under full load steaming condition, if Surveyor delegates the responsibility to CE.
2. Setting pressure ≯ 3% above max: working pressure.
3. In doing so, EGE steam pressure control is done as follows:
EGE steam pressure controls:
1) Steam control: By providing Dumping Valve in by-pass system, to Condenser.
Excess steam can be discharged into Condenser.
2) Water control: By shutting the inlet valve to boiler coils.
3) Exhaust gas control: By controlling the Exhaust Gas Damper.
Boiler Safety Devices as for UMS status:
1. Flame failure: (Photocell shut down combustion system and gives alarm.)
2. Low and high water level: (Level is maintained by feed pump, controlled by float
operated on/off switch.)
3. Low and high Steam pressure: (If steam demand drops, high steam pressure will
shut down burner and/or ME speed reduced. Low steam pressure
alarm, will be given if there is fault in combustion condition)
4. Fuel temperature: (Deviation from set temperature range, cause burner to be shut
off and alarms given for both low and high temperature.)
5. Fuel pressure: (Low fuel pressure cause automatic controller to shut down
burner and alarms given.)
6. Smoke density: (Emitted smoke through uptake, being monitored and if deviate
from normal limit, shut down the system and alarm given.)
7. Air / fuel ratio: (Air register damper controller keeps correct ratio, and shut down
the system and alarm given on deviation.)
8. Draught fan failure: (Air supply fan failure operate audible and visual alarms.)
9. Very low water level: (Burner stopped and alarms given.)
10. Very high water level: (Burner stopped or ME slow down and alarms given to
avoid foaming and carry over.)
11. High flue gas temperature: (Burner stopped and alarms given.)
Safety Devices on Boiler:
1. Flame failure alarm
2. Low water level alarm.
3. Very low water level alarm and cut-off.
4. High water level alarm
5. Low steam pressure alarm
6. Low oil temperature alarm and cut-off
7. High oil temperature alarm and cut-off
8. Low oil pressure alarm and cut-off.
9. Force Draught Fan failure alarm and cut-off.
10. Power failure alarm.
11. Safety Valves.
12. Gauge Glass.
13. Easing Gear.
Mountings on Boiler:
1. Safety Valves
2. Easing Gear.
3. Gauge Glass.
4. Pressure Gauge.
5. Feed Check Valve.
6. Flame Eye.
Open-up Procedure:
1. Stop firing and cool down.
2. All steam valves tight shut.
3. Blow down until empty. [ Ship-side Cock opened first, then gradually open Blow-down Valve. When loud noise dies down and blow-down pipe becomes cold, boiler is about empty. Blow-down Valve shut and then shut Ship-side Cock.]
4. Easing Gear lifted.
5. Open Air Vent Cock, Salinometer Cock and Drain Cock of water Gauge Glass, to let air enter. Ensure no vacuum and only atmospheric pressure inside, before knocking in the manholes.
6. Slacken dog-holding nut of Top Manhole door, break the joint, from the place safe from blast as a safeguard against scalding, and then nut removed and door taken out.
7. When knocking in the Bottom Manhole door, use crowbar and stand back when breaking the joint, as there may be hot water left.
8. Mud holes and all other doors open-up for cleaning, both smoke side and waterside.
9. Allow the boiler to be ventilated before entry.
Boiler Internal Inspection: [For Survey]
1. After normal open-up procedure, allow the boiler to ventilate.
2. Initial internal inspection is done before cleaning, for general condition and
any special deposited area.
3. Plugged Blow-down hole to prevent choking.
4. Cover Manhole door’s landing surface to prevent damage.
5. Final internal inspection done after thorough cleaning:
a) Check level gauge connections for blockage.
b) Check securing system of internal pipes and fitting.
c) Hammer-test furnace, fire and stay tubes.
d) Check firebrick, casing, baffles and welding seams.
e) Tubes checked for leak, crack, distortion and bulging
f) Check fireside is clean, without soot.
g) Cleaning and inspection of Manhole doors, joint landing surfaces.
h) Use new joints.
Refitting Procedure:
After Internal Survey:
1. Remove plug at blow-down pipe.
2. Box back all manholes and mud doors with new joints, and refit all mountings.
3. Open Air Vent Cock, and boiler filled-up with water up to ¼ of Gauge Glass level. ( If hydraulic test is required, fill-up completely.)
4. Normal flash-up procedure follows.
5. Pressure setting of Safety Valves, under steaming condition, with Surveyor’s presence.
Safety Valves:
Function:
1. Must open fully at definite pressure, without preliminary simmering.
2. Must be still opening until pressure in boiler has dropped to a certain definite value, not more than 4% under set value.
3. Must close tight without chattering.
4. Must close tightly without leaking.
Setting of Safety Valves:
1. Take Standard Pressure Gauge for accuracy.
2. Fill up water up to ¼ of Gauge Glass level, and shut Main Stop Valve, Feed Check Valve.
3. Without Compression Rings, Hoods and Easing Gears, reassembled the Safety Valves with spring compression less than previous setting.
4. Raise boiler pressure to desired blow-off pressure.
5. Screw-down Spring Compression Nuts of any lifting valves, until all are quiet.
6. Adjust each valve in turn:
a) Slacken Compression Nut until valve lifts.
b) Screw-down Compression Nut sufficiently enough, so that when valve spindle is lightly tapped, valve returns to its seat and remain seated.
c) Measure gap between Compression Nut and spring casing.
d) Make a Compression Ring equal to this gap, and insert under Compression Nut.
e) Gag the Spindle of this Safety Valve, to prevent opening, while remaining valve is being set.
7. Remaining valve is again set and insert Compression Ring.
8. Remove gag and retest both valves to lift and close together.
9. Caps and Cotter Pins padlocked.
10. When the Surveyor satisfied the setting pressure, Easing Gear should be tested.
11. All Safety Valves set to lift at ≯ 3% above approved working pressure (design pressure).
Accumulation of Pressure:
1. Pressure is liable to rise after Safety Valves have lifted, caused by increased spring load due to increased compression.
2. This rise in pressure is known as “Accumulation of Pressure”. {OR}
3. Pressure rise in excess of Working Pressure is termed, “Accumulation of Pressure”.
4. Accumulation of pressure test is carried out to see whether this safety valve is suitable or not for this boiler. Pressure rise after safety valves have lifted, must not exceed 10% of working pressure.
5. Tested when safety valves are new or boiler is new or safety valves and boiler are new ones.
Safety Valve on EGE and Economiser:
1. Slightly higher set pressure than drum Safety Valves.
2. It is to ensure operation under flooded condition at all times.
Hydraulic Testing of Boiler:
Necessary condition:
1. Boiler internal inspection is not satisfactory.
2. Surveyor demanded.
3. After structural repairs of boiler.
Requirement:
1. Surveyor must be present.
2. Gag the Safety Valves.
3. Close all opening.
4. Blanks inserted at Main Steam Stop Valve and Gauge Glass.
5. Measuring tape placed around boiler to check bulging.
6. Deflection gauge placed in the furnace.
7. Remove lagging to check leak points.
Procedure:
1. Open vent cock, fill boiler with warm water completely, until water overflows from vent cock, and close the vent cock.
2. Attach force pump and test pressure gauge.
3. Apply water pressure, 1.25 times of approved working pressure, for not more than 10 minutes.
4. If satisfied, Surveyor will stamp on bottom front plate near the furnace.
Chemical Treatment:
Two ways of treating the water for boiler use:
1. External Feed Water treatments.
2. Internal Boiler Water treatment.
External Feed Water Treatment:
Purpose:
1. To reduce TDS in feed water.
2. To arrest Suspended Solid Particles.
3. To reduce Dissolved Gases.
4. To prevent feed water system corrosion, by maintaining correct pH value of feed water.
Treatments:
1. To reduce TDS, the best way is to use evaporated feed water.
2. To arrest Suspended Solids, use feed line filters.
3. To reduce Dissolved Gases, inject Hydrazine and maintain Hot Well temperature between 60~70°C to promote O₂ deaeration through hot well vent.
4. To prevent feed water system corrosion, use Salinometers on feed line or Evaporator outlet. Maintain pH value by dosing Hydrazine or Amine.
Internal Boiler Water Treatment:
Advantages:
1. Precipitate Calcium and Magnesium salts, into non-adherent, harmless sludge.
2. Prevent these salts, from baking on boiler heating surfaces.
3. Sludge is blown-down from boiler.
4. Treatments also remove Dissolved Oxygen, Dissolved Gases, and CO₂, to avoid their corrosive actions.
Treatments for Moderately Rated Auxiliary Boiler:
[Medium and Low Pressure Boilers]
(1) Phosphate Treatment:
1. AGK 100 or Adjunct B is used.
2. Combat scale-forming salts to form non-adherent sludge.
3. Give Alkalinity to reduce corrosion.
4. Form Iron Phosphate Film on internal surfaces, as protection against corrosion.
(2) Caustic Soda Treatment: [NaOH]
1. Maintain correct pH value and required Alkalinity.
2. Precipitate scale-forming Permanent Hardness Salts. [Chlorides and Sulphates of Calcium and Magnesium: They are in Acid nature. ]
3. Remove Temporary Hardness Salts. [Bicarbonates of Calcium and Magnesium: They are slightly in Alkaline nature.]
4. Excess concentration of NaOH may cause Caustic Cracking of metal.
(3) Soda Ash Treatment: [Na₂CO₃]
1. Precipitate scale-forming Permanent Hardness Salts, (Non-Alkaline Hardness Salts) as Calcium Carbonate, CaCO₃.
2. Produce NaOH, to give required Alkalinity.
(4) Dissolved Oxygen Treatment:
1. Two chemicals, Hydrazine N₂H₄, and Sodium Sulphite Na₂SO₃ are used to remove dissolve O₂.
(5) Liquid Coagulant Treatment:
1. High molecular weight, colourless solution, likes Sodium Aluminate or Starch is used.
2. Coagulate oil droplets and Suspended Solids, and settle them at low points.
3. They can be Blown-down.
(6) Blow-Down Treatment:
1. Integral part of water treatment program, as it removes solids, which are results of chemical conversion of salts and impurities in water.
2. Surface Blow-down or Scumming is quick removal of solids, without wasting feed water.
3. Bottom Blow-down is vital, when solids become dense, heavy and remain at boiler bottom, despite circulation.
4. Daily short blows of top and bottom are necessary.
Boiler Laying up Procedure:
(Boiler may be laid up wet or dry.)
Wet Method:
1. When laid up in warm climate, boiler is filled with water until it comes out from air vent.
2. Then the boiler is sealed off.
Dry Method:
1. Boiler is emptied and cleaned thoroughly on both fire and waterside.
2. Corroded parts, wire brushed and coated with anti-corrosive paint.
3. Shallow metal trays filled with quicklime should be placed in both water and fire space.
4. Then boiler is closed up airtight.
Proper Maintenance of Boiler:
Water Side:
- Daily boiler water test.
- Boiler water treatment.
Gas Side:
- Regular cleaning.
Combustion System:
- Fuel pressure, temperature and viscosity correct values maintained.
- Burner maintenance.
- Air register, Air damper and forced draught fan.
Caustic Cracking or Embrittlement of metal:
1. Caused by excess concentration of Sodium Hydroxide, NaOH [Caustic Soda] in boiler water, and the material under stresses.
2. Ratio of Na₂SO₄ to NaOH should be maintained 2 : 5.
3. Caustic Soda is used for boiler water treatment, to maintain correct pH value and required alkalinity, so excess concentration should be avoided.
4. Excess concentration of NaOH may be from Overdosed Chemical and Leakage.
5. Damage occurs to riveted seams, tube ends and bolted flanges.
To prevent Caustic Embrittlement:
1. Sodium Sulphate, Na₂SO₄ should be dosed, to give protective layer.
2. Ratio of Na₂SO₄ : NaOH should be maintained at 2 : 5
Turn Down Ratio of Burner:
1. The ratio of maximum to minimum oil throughput of the burner.
2. In case of pressure jet burner, this can be stated in terms of square root of the ratio of maximum to minimum oil supply pressure.
3. Large Turn Down Ratio of up to (20: 1) is available with blast jet burner, without having resort to unduly high pressure.
Foaming:
Formation of thick layer of steam bubbles, on top of water surface inside boiler.
Priming: Rapid carry-over of large amount of water, in steam as it leaves the boiler.
Carry-over: Carry-over of small amount of water, in steam as it leaves the boiler.
Causes:
1. Higher water level than normal
2. High amount of TDS, total dissolved solids
3. High amount of suspended solids
4. Contamination by oil and other organic substances
5. Forcing the boiler
Effects:
1. Water hammer
2. Contamination and scaling
3. Fluctuation of working water level