Boilers, Types, Defects, Safety Devices – Everything you need to Know

Boilers, Types, Defects, Safety Devices – Everything you need to Know

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‘Boiler’ is the most important machinery used onboard without which it’s almost impossible to run a ship. Marine Boiler is covered both in Function 4 and Function 6 of  MEO Class 4 Exam. To help you, we have compiled the most important topics that are been asked by surveyors every year.

What are the Boiler defects?

  1. Pitting,
  2. Corrosion fatigue,
  3. Stress Corrosion,
  4. Embrittlement,
  5. Overheating.
  6. Overpressure.

Types of Boiler Defects

1. Pitting Corrosion

Pitting is the most serious form of waterside corrosion and is the result of the formation of irregular pits in the metal, usually found in the boiler shell around the water level 

Caused by: 

  • Low alkaline conditions (produces localized deep pits) 
  • Oxygen presence (produces rounded isolated pits).

2. Corrosion fatigue

  • It occurs when a sufficiently high alternative stress level causes failure of the subjected material. 
  • It is the joint action of a corrosive environment and cyclic stressing and results in a series of fine cracks in the metal. 

Caused by: 

  • Poor circulation or irregular circulation 
  • Mechanical bending due to pressure effects. 
  • The fatigue cracks produced are aggravated by corrosion, which increases fatigue cracking and causes corrosion fatigue.

3. Stress Corrosion

  • “Stress corrosion” cracking is the process caused by the combined action of heavy stress and a corrosive environment.
  • This form of attack is often found around the ogee ring in vertical auxiliary boilers when undue stressing is set up by poor steam-raising procedures.

4. Caustic cracking or Embrittlement :

  • This is a form of inter-crystalline cracking [crack follows the grain boundaries] caused by the excess concentration of Sodium Hydroxide[Caustic Soda] in the boiler water, comes in contact with steel, under stresses and high temperature.
  • Caustic Soda is used for boiler water treatment, to maintain the correct pH value and required alkalinity, so excess concentration should be avoided.
  • Damage occurs to riveted seams, tube ends, and bolted flanges.

5. Overheating: 

  • If boiler tubes become overheated, either due to direct flame impingement, or lack of circulating water, then the high wall temperatures will lower the mechanical properties of the tube. This causes increased scale thickness build-up and causes the tube to bulge just before rapture. 

6. Overpressure: 

  • This could be classed as the failure of the tube by mechanical stresses, rather than thermal stresses. Thus, characteristic metal failure (plastic and brittle rapture) would be seen, but without the microstructure change to hard martensite material or scale build-up. The failure mode could be more sudden, thus less prominent tube bulging would occur.

What are the different types of Boiler corrosion?

Types of Boiler Corrosion

  1. Electrochemical Corrosion
  2. Caustic Cracking corrosion
  3. Corrosion by Oil
  4. Corrosion by Galvanic Action
  5. Gaseous Corrosion [O2, CO2, NH3]

 

What is ‘Accumulation of Pressure’?

  • The pressure is liable to rise after Safety Valves have lifted, caused by increased spring load due to increased compression.
  • This pressure rise in excess of Working Pressure is termed, “Accumulation of Pressure”.

What is the reason for Boiler accumulation test?

  • 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.
  • Tested when safety valves are new or boiler is new or safety valves and boiler are new ones.

Why Boiler water test carried out?

  • To know the boiler water condition.
  • To control chemical treatment and blowdown.

Why boiler water treatment carried out?

  • To prevent scale formation., corrosion, and impurities.
  • To prevent damage to steam-operated equipment and condensate lines.
  • To maintain alkalinity condition.
  • To improve boiler efficiency.

What are the different  Boiler Water Tests carried on Marine Boiler? 

Boiler Water Tests

  • Chloride Test.
  • Excess Phosphate Test.
  • Total Dissolved Solid Test (Conductivity Test).
  • pH value Test.
  • Hydrazine Test.
  • Alkalinity Test: [‘P’ Alkalinity (Phenolphthalein), ‘M’ Alkalinity (Methyl-orange) and Total Alkalinity]

Alkalinity Tests:

  • ‘P’ Alkalinity finds the presence of Hydroxyl, Phosphate and half of Carbonates, excluding Bicarbonates.
  • ‘T’ Alkalinity gives the total quantity of all Alkaline Dissolved Salts in the boiler water.
  • ‘M’ Alkalinity finds the presence of remaining Carbonates and Bicarbonates. 
  • Total Alkalinity is   < 2 x ‘P’ Alkalinity. 
  • The desired value is  150 – 300 ppm for ‘P’ Alkalinity.

Increase of Alkalinity Level:

Causes:
  • Alkalinity treatment has been done recently.
  • Using of Alkaline rich makeup feed water.
  • Incorrect strength of reagent used.

Effect:    Caustic Embrittlement.

Decrease of Alkaline Level:

Causes:
  • Feedwater is contaminated with acid.
  • Direct water loss from the boiler.
  • Incorrect strength of reagent used.
Effect:   Corrosion.

Chloride Test:

  • Gives the quickest indication of any salt-water leakage into the boiler.
  • Must be carried out daily. 
  • Chlorides of Calcium, Magnesium and Sodium are extremely soluble.
  • Chloride level is proportional to the TDS level in the boiler water.
  • Rate of blow-down is governed by chloride level.
  • Chloride Level should be  0 – 300 ppm, and blow-down if > 300 ppm.

Increase of Chloride Level:

Causes:
  • SW leaking into the system.
  • Incorrect strength of reagent used. (Silver Nitrate and Potassium Chromate).
  • Due to the treatment of chemical and hardness salt reaction.
Effects:
  • Increase in TDS level causes Foaming/Priming.
  • Drop-in Alkalinity causes Corrosion.

Phosphate Test:

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

pH value Test:

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

Condensate pH:

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

Hydrazine Test: (for Dissolved Oxygen)

  • Hydrazine ppm maintained at 0.1 – 0.2 ppm.
  • If < 0.1 ppm, increase the dosage by 25% hydrazine.
  • If > 0.2 ppm, decrease dosage by 25% hydrazine. 

How to take proper boiler water sample?

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

Safety device fitted on automatic boiler[or] Safety devices on Boiler on UMS statues

Boiler Safety Devices

  • Flame failure alarm
  • Low water level alarm.
  • Very low water level alarm and cut-off.
  • High water level alarm.
  • Low steam pressure alarm
  • Low oil temperature alarm and cut-off
  • High oil temperature alarm and cut-off
  • Low oil pressure alarm and cut-off
  • Forced Draught Fan failure alarm and cut-off
  • Power failure alarm.
  • Safety Valves with Easing Gear.
  • Gauge Glass
  • Smoke density alarm
  • Air/fuel ratio alarm
  • High flue gas temperature alarm

What is Caustic Cracking or Embrittlement?

This is a form of inter-crystalline cracking;

  • Caused by an excess concentration of Sodium Hydroxide[Caustic Soda] in the boiler water, and the material under stresses and high temperature.
  • Caustic Soda is used for boiler water treatment, to maintain correct pH value and required alkalinity, so excess concentration should be avoided.
  • Damage occurs to riveted seams, tube ends and bolted flanges.

To prevent Caustic Embrittlement:-

  • Sodium Sulphate, Na2SO4 should be dosed, give protective layer.
  • The ratio of Na2SO4: NaOH should be maintained at 2: 5

What is furnace blow-back (boiler backfire) & How to prevent?

Furnace blowback occurs due to:- 

  • Insufficient purging of the Furnace, during burner firing.
  • An accumulation of oil and its vapour within the furnace, which is ignited by a re-lit burner
  • It can also occur when a sudden admission of air occurs into a fuel-rich flame.

To prevent this occurrence:

  • There should be an adequate period of time (of air purge) with the air registers (flaps) fully open, and the forced draught fan ‘On’.
  • While lighting up, minimum fuel should be admitted, to maintain a ‘minimum flame’, before ignition occurs.
  • Proper maintenance of burners should be carried out, to prevent oil dribbling, while the burner is off.
  • Always maintain the correct air/fuel ratio.

Foaming:  

  • Formation of a thick layer of steam bubbles, on top of water surface inside the boiler.

Causes:

  • Higher water level than normal.
  • High amount of TDS, total dissolved solids.
  • High amount of suspended solids.
  • Contamination by oil and other organic substances.

Priming:  

  • Rapid carry-over of a large amount of water, in steam, as it leaves the boiler. 

Carry-over:  

  • Carry-over of a small amount of water, in steam, as it leaves the boiler.

Causes:

  • Excessive foaming, 
  • Improper amount of steam space [Higher water level than normal]  
  • By a sudden rush of steam, such as when the steam stop valve is suddenly opened.
  • Forcing the boiler.

Effects:

  • Water hammer
  • Contamination and scaling
  • Fluctuation of working water level

To prevent Foaming:-

  • No lube oil should be allowed to enter the boiler 
  • Surface or scum blowdown should be used frequently to expel any floating impurities from the boiler.

To prevent Priming:- 

  • Never keep the water level too high 
  • Open steam stop valve slowly

Feedwater contamination by oil

  • If oil enters the boiler, it will coat the heating surfaces, and significantly raise the metal temperatures. 

Indication:- 

  • The indication of oil in the boiler water can be seen in the gauge glass, or as an increase in fuel consumption, for the same steam delivery. 

Causes:- 

  • Causes are over lubrication of machinery, leakage of heating coils & in-efficient filtering of feed water.

Prevention:- 

  • To prevent this from occurring, a Weir system is used in the Observation tank of the hot well. 

Boiler mountings [Classification society’s minimum requirements] 

Boiler Mountings

  • Two Safety valves.
  • One Steam stop valve.
  • Two independent Feed check valves.
  • Two independent Water gauge glasses (or equivalent).
  • One Pressure gauge.
  • One Salinometer cock or valve.
  • One Blowdown/scum valve.
  • One independent Low boiler water level [with fuel shut-off device and alarm]

What is boiler water contamination?

Boiler contamination are:-

  • Oil
  • Seawater
  • Dissolved gases; O2, CO2 & H2S
  • Suspended solid;  mud, clay, sand 

What is the cause of contamination?

  • Oil:- over lubrication of steam machinery, leakage of heating coils, rupture of fuel oil heaters & in-efficient filtering of feed water.
  • Seawater:- condenser leakage, feed water tank leakage, feed water already contains chloride, evaporator carryover.
  • Dissolved gases:- Air which can enter any opening such as drain, makeup, cascade tanks and system under vacuum such as turbine seal & condenser.
  • Suspended solid:- solids result from chemical conversion of salt and impurities in feed water.

What are the effects of contamination with the following?

Oil: 

  • promote foaming /priming
  • high insulation on the heating surface
  • convert to acid and directly attack boiler metal

Seawater:

  • Corrosion and acid attack
  • Scale and sludge formation
  • Foaming & priming due to high TDS
  • Jump in chloride level
  • Drop-in alkalinity level
  • Dissolves gases:
  • Pitting corrosion
  • Reduced ph value [Alkalinity] of feed water
  • Accelerates general corrosion
  • Suspended solid:
  • Foaming
  • Promoting different corrosion cell.

Types of Safety Valve:

Type

Construction

Valve Lift

Ordinary spring-loaded safety valve

Wing valve, no waste steam piston, no floating ring.

D/24

High lift safety valve

Wing valve, has a waste steam piston, no floating ring

D/12

Improved high lift safety valve

Wing valve, has a waste steam piston and floating ring.

D/12

Full lift safety valve

For high-pressure boiler, no waste steam piston.

D/4

Full bore safety valve

For high-pressure boiler up to 21 bar, operated by relay

D/4

Types of Boiler Gauge Glass:

  • Fitted directly.
  • Fitted to a large bored bent pipe.
  • Mounted on a hollow column.
  • Fitted to a column with its centre part solid.
 

 

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