CoC Oral Exam Preparation (Part-6) – Boiler Operation & BW Treatment

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.

Written by Marine Study

Marine Study

“For Maritime Education and Knowledge”

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Munez

Good job sir. Pleased to see more types of questions with answers for steam coc.