Top-14 reasons for Boiler Tubes failure

 

        What are the potential reasons for Boiler Tube failure??

Following are the direct causes for Boiler tubes or pressure parts failure.

1-Operation of the Boiler at higher pressure:

Boiler pressure parts are designed to operate at particular pressure for continuous operation. If they are being operated at higher pressure than recommended, leads to failure of pressure parts may be at welding joints or even at plain surfaces.

Boiler pressure parts lose their strength as they become older and older.

As per IBR-7, Boiler operating pressure should be reduced by 5% after 25 years of operation. That is A boiler operating at 109 kg/cm2 pressure should be operated at 103.55 kg/cm2 after 25 years.

Boiler operating pressure reduction chart

Age of the Boiler (From the date of first use in years	25	35	45	50	60	70	80	90	100
% of reduction in working pressure	5	10	15	20	30	40	50	60	70

 

2-Operation of the Boiler at higher temperature:

Operation of the Boilers at Higher temperatures can cause metallurgical weakening of the tubes, leading to deformation, cracking, or rupture. Continuous exposure to high temperatures causes creep, where the metal elongates and loses strength over time.

Rapid or excessive temperature variations cause thermal expansion and contraction, leading to cracks. Operation of the Boilers at higher temperature can also increase internal scaling.

3-Pressure parts or tubes internal scaling:

Poor quality of DM water can lead to internal scaling, this internal scaling of tubes results into poor heat transfer subsequently overheating and tube failure.

Calcium, Magnesium & silica present in water if not treated well can form hard scales on pressure parts internal surfaces.

Operation of the High Boiler at lower pH (< 8) for long time eventually leads filure of pressure parts.

In sugar-based cogeneration plants, there is the chance of entering sugar traces in process return condensate. Due to this, pH of feed water drops ( up to 6) quickly and result into tube failures.

Operation of the high-pressure Boiler at pH around 5 to 6 results into tubes failure in just 3 to 5 hours.

4-Over heating of tubes:

Over heating of pressure parts due to wrongly set burners, air and internal scaling, poor water circulation & soot build up leads into tubes failure.

5-Erosion of tubes:

Erosion of pressure parts is mainly due to high flue gas velocity, bed materials in case of AFBC & CFBC boilers and even due to high velocity of steam and water.

So, ensuring right flue gas velocity at various locations of the Boiler is Vital to take care of erosion of pressure parts.

In case of AFBC & CFBC Boilers, bed coils (Evaporator & Super heaters) and water wall tubes erode due to bubbling beds.

Wrongly designed economiser, evaporators tubes and super heaters coils may suffer from erosion due to high velocity of water and steam. Usually, steam/ water velocity increases due to under sized tubes/coil or operation of the Boiler at over load.

Velocity of the fluid increases as the size of the tubes decreases.

Velocity (m/sec) = Flow (m3/sec) / Area (m2)

Based on above relation, velocity of the fluid increases as the Area of the tubes/pressure parts decreases and Flow increases.

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6-Fatigue Failure:

Repeated thermal and mechanical stress due to frequent start up and shutdown cycles, frequent load variation leads to thermal shocks & failure of pressure parts.

7-Chemical cleaning of The Boilers internals:

Excessive use of chemicals especially acids during Boiler chemicals cleaning leads to Pressure parts failure during operation.

8-Stress & corrosion cracking:

Tubes fail due to corrosion, corrosion is mainly due to low pH, dissolved oxygen and other impurities in water which lead to pitting and weakening of the tubes. If Boilers are being operated in such conditions pressure parts go into failure.

9-Poor welding joints:

If welding joints quality is poor i.e if No standard welding procedure was followed during installation stages the joints fails upon small variation load or during abnormal operations.

Joints fail in following cases;

1-Poor welding techniques followed

Read>>>>> What is the significance of post weld and Pre-weld heat treatments (PWHTs)?

2-Poor skill of welder

3-Selecting wrong electrodes etc

4-Wet atmospheric condition

10-Poor operations:

What are the poor operation methods which lead to failure of pressure parts>>>>>??

Boiler pressure parts failure largely depends on Boiler operation skills of the operators.

Pressure parts may fail in following poor operation techniques

1-Not following Boiler start up procedures

2- Not following Boiler shut down procedures

3-Quick start-up of the Boiler, not following the Boiler start up curve

4-Not operating the vents & drains as per requirement or SOP

5-Sudden load variation

6-Operating the Boiler by ‘Bypassing the Boiler protections & Interlocks’.

7-Not providing minimum circulation steam to Boiler during start-ups, shut downs or even in case of ab normal operations.

Not knowing the function of ‘start up vent’ is the main reason for super heater coils failure during Boiler shut downs or low load operations.

Start up vent is used to provide minimum required flow to the Boilers during start-ups, which helps in cooling the super heater coils during No load operations

8-Operating the Boiler on low drum levels or not taking the quick shut downs if there is no water flow into the Boiler.

9-Wrong operation of attemperator. Sudden opening and closing of attemperator valves lead to thermal shocks in super heaters.

10-Operating the Boiler at higher drum levels. Operating the boiler at higher drum levels leads to carryover of wet steam or water into super heaters, which leads to thermal shock & tubes failure.

11-Giving water wall bottom headers blow down at high steam rate: This leads the starvation of water wall tubes.

12-Wrongly set safety valves & higher blow down percentage of safety valves:

In a Boiler, if drum safety valves pop up frequently before popping up of super heater safety valves, then it leads to the starvation of super heater coils and even failure.

12-Hydrogen damage:

Reaction between steel and water at high temperatures produces atomic hydrogen, which diffuses into the metal & leads to embrittlement and cracking, resulting in a fish mouth rupture.

13-Un even thermal expansion of pressure parts:

Indian Boiler Regulations Book-

Un even of thermal expansion of Boiler tubes due to not following Boiler start up curves & start-up/Shut down procedures or restriction to thermal expansion lead to cracks or failure of joints of pressure parts.

 

Read>>>Why does the thermal expansion occur in Boilers??

14-Metallurgical defects:

Poor quality of metals used for manufacturing of Boiler pressure parts lead to failure of pressure parts during operation.

What are the main reasons for Boiler tubes ‘fish mouth’ opening??

Fish mouth openings are catastrophic failures in Boilers, It’s a type of rupture where the tube bursts open, resembling a fish's mouth. This failure is highly dangerous and usually indicates severe operational or maintenance issues. The main reasons for a fish mouth opening in boiler tubes are:

Following are the major reasons for Boiler tubes ‘fish mouth’ opening.

1-Short term over heating: Occurs due to localized overheating when tubes are exposed to direct flame or high temperatures without sufficient cooling

2-Internal scaling

3-Operation of the Boiler with low water level

4-Starvation of super heater coils & water wall tubes

5-Hydrogen damage

6-Erosion of pressure parts, mostly in case of AFBC & CFBC Boilers.

7-Operation of the Boiler at higher pressure & Temperature than recommended.

8-Over heating.

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