FAILURE ANALYSIS AND BOILER ACCIDENTS

Boilers can explode/fail to work if:

  1. the material used in the manufacture of a boiler is of inferior quality
  2. the design/manufacturing process and the welding process are not as per standards
  3. the boiler is not inspected periodically and defects observed are not rectified
  4. the boiler is not operated as per standard operating procedures
  5. it is not in charge of a properly competent person

Any negligence in the above can result in boiler accidents

Boiler accidents may involve loss of property, loss of production, injury or loss of lives to innocent people in the vicinity. Boiler accidents are highly catastrophic in nature because of their high explosive power and thus no stone should be left unturned to prevent them.

It is necessary to understand what are boiler accidents and what causes boiler accidents.

Once we identify the reasons why accidents occur, we shall definitely be able to prevent them.

Before going into details, let us see ‘What are boiler accidents?’. Section 2(a) of The Boilers Act, 1923 defines accidents as follows – “accident” means an explosion of a boiler or boiler component, which is calculated to weaken the strength or an uncontrolled release of water or steam there from, liable to cause death or injury to any person or damage to any property.

When we look at the definition of an accident, it is clear that not only is the explosion of a boiler an accident, but any damage to the boiler is also an accident under the definition of the Act.

Various causes of boiler accidents are as follows:

  1. The low water level in the boiler
  2. Flue gas explosion
  3. Faulty erection
  4. Fire hazards
  5. Flame impingement
  6. Failure of tubes
  7. Idle corrosion
  8. Improper feed water treatment
  9. Faulty operation
  10. Clogged flue gas passages
  11. Over steam pressure or H.T. pressure
  12. Faulty design
  1. The low water level in the boiler: This is the most common cause of the accident. The paucity of water in the boiler causes major accidents. Water shortness results in overheating of pressure parts such as furnaces, tubes in smoke tube boilers or headers and tubes in water tube boilers. Overheating results in loss of metal properties and causes the development of bulges to the furnace or headers and distortion of tubes. Shortness of water results in overheating and bulging of drum and headers, if not properly insulated. A fall in water level bares the tube ends and if expansion is weak, tubes may slip causing tube holes and tube ends to become oval in shape. Water tubes fitted with fins often crack at the junction of the fin and tube. During investigations, it was found that most boiler accidents have occurred by placing too much reliance on auto-level control and neglecting attention to the gauge glass.

During the investigation several reasons were found to have resulted in water shortness in the boiler, which is as follows:

  1. Loss of water in the feed water tank: In many industries, water is stored in a big storage tank. This storage tank receives water from either the softener, D.M. plant, or return condensate of the plant. This water is then transferred by pump to a small water tank called ‘day tank’ in the boiler house. If the capacity of this tank is very small i.e., for one hour or so, and if the water level is not maintained, there will be no water in the tank and BFP will run dry resulting in a low water level in the boiler. Leakages in this tank also result in its level dropping and thus the supply of water to the boiler may get affected. In the case of deaerators, if the level of the deaerator is not maintained continuously, then as the capacity of the deaerator is usually small there is an immediate chance of water shortness in the boiler.
  1. High feed water temperature: This is usually where condensate is returned to a storage tank or when the feed water is preheated. If the feed water temperature is more than 70°C, BFP may lose suction due to vapour lock or due to low NPSH and therefore can result in water shortness in the boiler.
  1. Failure of BFP or leakages in the feed line or economizer: Failure of BFP(boiler feed pump) or leakages in the feed line or in the economizer, in a case where economizer is used, results in water shortness. Defective check valves may also restrict the supply of water to the boiler and may result in low water levels in the boiler.
  1. Mobrey/Level controller does not work: Failure of the Mobrey/Level controller results in water shortness in a boiler. In the investigation, it was found many times that mobrey has failed because either its float is punctured or electrical switches have become inoperative or the float rod is bent or jammed to move. In many cases, it was found that inlet pipes from the boiler to the mobrey are choked with scale. The photograph shows (refer to image 14) the bottom connection of mobrey fully choked with scale.
  1. The water level in gauge glass is not visible: The water level in G.G. may not be visible if it is not cleaned properly periodically or if it is full. In sugar industries, steam drums and G.G. are located at large heights and when level control is manual, boiler attendants may not be able to see the water level in gauge glass properly. Such instances cause confusion in the mind of boiler attendants and may result in water shortness in the boiler. The photograph shows (refer to image 15) a fully choked bottom standpipe for the level gauge.
  1. The eyesight of boiler attendant: If the eyesight of the boiler attendant is such that he cannot see the water level in G.G., it is likely, he may feel that there is water in the boiler when actually there is no level in the gauge glass. This is especially true of aged boiler attendants.
  1. Human error: The boiler attendant may get dizzy after lunch or after dinner and when level control is manual, naturally, he will be unable to maintain the accurate water level in the boiler.
  1. Flue gas explosions: Flue gas explosions cause accidents in the boiler. A mixture of unburnt fuel with air in explosive proportion and the application of sufficient heat to raise the temperature of this mixture to the ignition point causes flue gas explosions. Loss of ignition, fuel valve leakages, insufficient purging, failure of ID fan and secondary combustion of fuel result in such types of explosions. These explosions are highly catastrophic in nature.
  1. Faulty erection: Faulty erection may result in accidents to the boiler. If the condensate tank or day water tank for the boiler is erected just above the boiler due to space constraints, it is likely that water from the tank may overflow or leaks from the tank may fall over the boiler causing thermal stresses, thereby causing damage to several tubes.
  1. Fire hazards: Oil tanks are sometimes installed just above the boiler due to space constraints. Overflow or leaks from this tank may fall on the boiler catching fire. If such spillage is on the boiler shell, and if it catches fire, the shell of the boiler may bulge or the boiler may even explode. Such spillage on the floor or in cable ducts can cause a fire in the entire boiler house.
  1. Flame impingements: If the burner flame is not uniformly spread due to a partially clogged burner or damaged diffuser and if it hits the furnace wall, localised overheating occurs resulting in loss of metal properties thereby developing bulges in the furnace or headers. This causes accidents.
  1. Failure of tubes: Tubes in the boiler fail due to various reasons causing accidents. A sudden release of steam due to the failure of the tube can result in distortion or secondary damage to several tubes in the vicinity. Some of the reasons for the failure of tubes are:
  2. Excessive length or weight of tube – this causes hogging or sagging of tubes which can pull out tubes from the header, drum, or endplate.
  3. Excessive stress on the tube end or frequent expansion – this causes leakage through expansion. This, along with an inadequate cross-section of the plate at the tube hole can cause ligament cracks and bending stresses.
  4. Improper feed water causes oxygen pittings, wasting, corrosion, scale formation, cracks, grooving, etc.
  5. Temperature stresses, overheating, high heat flux rates, poor circulation, thinning, blockage of expansion, carry-over, high drum level, and heavy steam draw cause distortion or sudden rupture of tubes or SH coils. The following photograph shows (refer to image 16) several tube defects.
  6. Low-Temperature corrosion – causes fast wasting/pitting and rupture of tubes.
  7. Erosion of tubes – is caused by soot blowing, the abrasive action of flue gas/bed material, in FBC boilers and heavy draft.
  1. Idle corrosion: Boilers when not in use are liable to deterioration from corrosion; unless well cared for and made rust proof; they may depreciate more rapidly than when in use. This may cause general smooth wasting, pitting of shell/furnace, and grooving of stay bars and gussets. Reduction in thickness of pressure parts makes it unfit to be used at the originally intended working pressure (WP) and if used, causes boiler accidents.
  1. Improper feed water treatment: causes oxygen pittings, wasting of plates of shell/furnace, scale formation, cracks, corrosion and grooving of stay bars or gussets, which can lead to boiler accidents.
  1. Faulty operation: Overloading, high fluctuating loads, the sudden heavy draw of steam, very high or low water levels, and raising steam very fast can drastically reduce the boiler life due to thermal and mechanical fatigue.
  1. Clogged flue gas passages: Recently, the tendency of industries has been towards the conversion of firing for boilers, from oil to coal or agro waste fuels due to economic considerations. However, if flue gas passages are not of adequate cross section or get choked frequently, it causes overheating of pressure parts and thereby develops bulges in furnaces or headers. This can also cause bulging and rupture of tubes.
  1. Over steam pressure Or Hydro test (H.T.) pressure: Tendency to meet increased steam demand may tempt users to work the boiler at a pressure higher than the pressure at which the boiler is certified for use. If the boiler is worked at such high pressure, it can result in an explosion of the boiler/development of bulge to shell due to tension or can even collapse the furnace due to additional compressive load causing an accident to the boiler. It is also observed that negligence towards the pressure gauge during the H.T. of the boiler can cause permanent damage to the furnace/shell due to excessively high H.T. pressure, especially when positive displacement pumps are used to raise the H.T. pressure.
  1. Faulty design: Faulty design usually results in overheating of the furnace, development of bulges to a furnace, cracking of tubes, sagging and pull out of tubes from tube hole, bulges to the shell, distortion of end plates, bulges to end plate and development of ligament cracks in tube plates. In water tube boilers it can result in overheating of tubes and headers, distortion of tubes, bulges to the tubes or headers, sudden rupture of tubes, erosion of tubes, development of ligament cracks drum or header tube holes and damage to SH coils. These usually occur due to poor circulation, high heat transfer rates and carryover.

Now, if we can eliminate the above-said causes of accidents, then we can definitely prevent accidents. The following guidelines will help prevent accidents:

  1. Proper feed water treatment is essential for the smooth working of boilers and most of the causes of boiler accidents related to feeding water can be fully eliminated with proper feed water treatment. Energy stored in the boiler is enormous and accidents are catastrophic in nature therefore water quality and water level protection devices are of paramount importance.
  1. Strict adherence to operating and maintenance controls is essential for ensuring safety through discipline.
  1. Immediate action must be taken and recorded whenever there is a deviation from the operating parameters, which must be documented in the Boiler Log Book.
  1. Continuous training of boiler operators is a must to upgrade their skills.
  1. Expert supervision by people with adequate knowledge of boiler operations is required at the supervisory and managerial levels. The National Board of Boilers and Pressure Vessels Inspectors U.S., in their incident report, has mentioned that the primary cause of boiler accidents is low water condition and operator error calculated to @80%.
  1. The day water tank should be adequate, preferably to supply feed water for at least 2 hours at MCR (Maximum continuous rating). The tank should be periodically inspected and any defects or leakages should be attended immediately. This tank should have two independent audible alarms for low level, for the boiler attendant’s attention. Deaerators must be provided with automatic water level controllers and the level in the deaerator should be maintained.

The temperature of feed water should be controlled and the selection of BFP (boiler feed pump) should be such that there shall not be any vapour lock. The day water tank should be installed at a sufficient height so that adequate NPSH (Net positive suction head) is available for BFP. Leakages in the feed line/economizer should be attended immediately. Maintenance of electric motors, pumps and switchboards and feed check valves should be carried out periodically.

At least two water level controllers should be provided to each boiler and their impulse lines should be taken out from two different positions on the boiler shell. Mobrey level controllers should be periodically cleaned and checked for their fitness, rather than alertness. It must be ensured that inlet pipes from the boiler to the mobery/level controllers are always clean. G.G. should be frequently drained and true water level must be maintained at all times during the working of the boiler. G.G. standpipes should be of adequate size.

The level column should have a facility for cleaning and should be drained to ensure that there is no accumulation of scale. It must be ensured that inlet pipes from the boiler to G.G. are always clean. Where G.G. are located at a large height, a remote water level indicator should be provided. Boiler attendants should be asked to get their eyes checked by a doctor, in case of any doubt regarding eyesight. This is especially necessary for aged boiler attendants. It should be ensured that the boiler attendant does not sleep on duty either during the day or at night.

  1. An adequate number of explosion doors should be provided to prevent accidents from flue gas explosions. Proper purging should be carried out before firing the boiler each time. Ignitors, fuel valves, and ID/FD fans should be serviced periodically.
  1. A condensate tank or day water tank should never be installed just above the boiler.
  1. Oil tanks should never be installed just above the boiler. Spillage from the tank should be cleaned immediately. Electrical cables should be kept dry and free from oil at all times. A sufficient number of fire extinguishers should be kept in the boiler house.
  1. Burners play a crucial role in combustion processes. It is imperative to take proper precautions to guarantee that the diffuser is functioning correctly, the burner nozzles are unclogged, and the flame is evenly distributed without making contact with the furnace walls or tubes.
  1. Excessive long tubes should be avoided. Proper hangers/baffles for appropriate support should be used to avoid sagging or hagging. Tubes in steam or mud drum should not be excessively and frequently expanded. More attention should be paid to periodical cleaning and feed water treatment to enhance the life of tubes. Conditions that cause temperature stresses, overheating, high heat flux rates, poor circulation, thinning, blockage of expansion, carry-over, high drum level and heavy steam draw should be avoided through proper operation and maintenance.

Low-temperature corrosion not only causes fast wasting/pitting of tubes but also reduces the life of the chimney drastically and therefore should be prevented by controlling flue gas temperature. Flue gas temperature is a good indicator of what is happening to a boiler and therefore it should be closely monitored. Erosion of tubes, in some cases, cannot be controlled fully. However, proper periodic/predictive maintenance can help prevent the unexpected breakdown of a boiler.

  1. Idle corrosion can be prevented by dry or wet preservation methods. Boilers, which remain idle for a long time, should be thoroughly inspected and non-destructive tests should be conducted before using such boilers.
  1. Proper feed water treatment is the most essential ingredient for the reliability of boilers in industries. Proper feed water treatment automatically solves all the problems related to feeding water such as pitting, wasting, scale formation, cracks, corrosion, grooving, etc.
  1. It must be ensured that flue gas passages are free and their movement is seamless.
  1. Periodical cleaning of the boiler and overhaul of all the valves and mountings not only increase the reliability of the boiler but also gives us an opportunity to locate hidden defects if any. Proper and safe operation of the boiler not only prevents accidents but also enhances the life of the boiler. Boilers should be inspected with extreme care to avert boiler accidents/explosions.
  1. A boiler should never be worked at a higher pressure than its certified pressure. Periodic servicing, setting of SVs at certified pressure and testing of safety valves can help prevent accidents due to over-steam pressure. The boiler should never be hydraulically tested to a pressure higher than prescribed by IBR, 1950. Particular attention should be given to the pressure gauge and the pressure parts during hydraulic tests. It should be ensured that in no case H.T. pressure exceeds the required. H.T. pressure, especially when positive displacement pumps are used to raise the H.T. pressure.
  1. Faulty design many times makes so-called good boilers unreliable and unsafe for use. Safety, reliability, efficiency and economy are the four keywords and important aspects of good boiler design. In today’s world of competition, often, the economy is given more consideration which may result in a loss of reliability and safety aspects of the boiler. A well-designed boiler can achieve three important aspects of boiler design safety, reliability and efficiency, which may be at a slightly extra cost but also can fetch good fame, distinct recognition and prosperity in long term for the boiler manufacturer.

Statutory Instructions:

  1. The boiler shall not be used unless it is registered and certified
  2. The boiler shall not be used after the expiry of the period mentioned in the Certificate/P.O
  3. Boilers shall not be used unless the transfer is reported to the Director of Boilers/CIB
  4. The boiler shall not be used at a higher pressure than the maximum pressure for which it is certified
  5. The boiler shall not be used unless it is in charge of the person holding the certificate as required by the Rules
  6. The explosion of boiler/boiler components shall be reported to the Director of Boilers/CIB within 24 hrs. Such a report shall contain a true description of the nature of the accident and of the damage, if any, caused to the boiler/boiler component, the injury/death caused to a person or damage to any property
  7. Works, such as repairs, welding, tube replacement, etc shall not be carried out to boiler/boiler components without the sanction of the Director of Boilers/CIB

Photographs of some of the boiler accidents and defects are shown on the NEXT PAGE. These photographs are numbered from 1 to 24.

The description of the accident/defect is as follows:

  1. The explosion of the shell was due to undercutting at the long seam
  2. Bulge to the furnace due to flame impingement
  3. Shell damaged to 50 mm depth due to mishandling
  4. Final Conv. SH header, 70mm thick cracked due to creep
  5. The boiler turned upside down due to the explosion
  6. This boiler got damaged due to the explosion of another boiler in the vicinity
  7. Badly scaled smoke tubes
  8. Tube failure due to hydrogen damage
  9. Dissimilar weld failure due to the use of improper electrodes
  10. Fully choked feed standpipe of a boiler due to scale
  11. Header bulged due to the scale
  12. Collapsed furnace due to fatigue crack at FEP
  13. Bulged furnace due to low water level
  14. The bottom connection of mobrey choked with scale
  15. The bottom standpipe of G.G. choked due to the scale
  16. Tube failure due to erosion
  17. Tubes bulged due to long-term overheating
  18. Tubes ruptured due to short-term overheating
  19. SH coil bend choked due to scale
  20. Dissimilar weld failure due to improper workmanship
  21. 76.2 mm OD tube bulged due to scale-elevation
  22. 76.2 mm OD tube bulged due to the scale-side view of 21
  23. Lamination of furnace plate
  24. Shell ruptured due to undercut
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Author:

Sanjay Sarode

Joint Director of Boilers (Retd.), Maharashtra State

Creative Engineering Consultants, Thane

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