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.
Read>>>Boiler interview questions & Answers
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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