Study Report of Superheater & Economiser Coil Failure Gas and Oil Fired Boiler
Super heater coil failure:
After going through various records and data available at site, we are of the opinion that
boiler was run with lesser feed water temperature (around 80 Deg. C) to economiser
inlet for prolonged period. Boiler steam loading was not carried out as per the standard
practice and unit was run at low steam load for long duration right from the start of the
plant. The above two factor would have led to too much of thermal stress to material of
superheater coils due to insufficient steam cooling of coils. We also understand from
site that during start up time the metal temperature of superheater coils were kept
beyond its safe working limit of 490 Deg. C (T 11 material) for longer duration, which is
also confirmed by the metallurgical report of Petroleum Corporation . With high total
dissolved solids (TDS) due to improper control of drum water chemistry (CBD) during
start up compounded with high water level operation in steam drum, we presume
priming of water into superheater coils has taken place which has led to excessive
thermal stress to tube metal and its subsequent failure as report from the site.
We are also of the opinion, after inspecting the quality of superheater bends, Ovality
percentage at bend has not been kept as per Standard specified by IBR and bends are
having visible knuckle at start and end of the bends. The stretching of material at bend
portion is very high resulting thinning of tube wall thickness as shown in the photograph
below:
We also understand from site that one crack has been observed in one of the header
stub welding due to poor quality of weld carried out at site as shown below:.
Various lugs provided in the superheater coils for maintaining the coil pitch is not
properly positioned and welded at site as shown below:
Economiser coil failures:
We understand from the site that four times economiser weld joints carried out at factory
has failed due to poor joint preparation and weld quality. This has in turn damaged
adjacent tubes in the bend sections of the economiser coils. All the above failures have
taken place at gas inlet side (Economiser water outlet side) which clearly indicates that
the initialization of reported economiser failure is not due to Sulphur dew point
corrosion to the outer surface. We have also seen the cut sample of failed samples at
weld location by the inspection department and we are of the opinion that welding of
coils were carried out without proper joint preparation as per standard engineering
practice. We have noticed weld joints were not provided with proper root gap leading to
insufficient penetration of the weld material as shown in the photograph below:
From the records we could find only one pin hole failure in the finned section of
economiser which is in the gas flow path of economiser assembly as shown in the
photograph given below:
The water jetting out of this pin hole has damaged the adjacent tube fins due to
extended running of the boiler in leaked conditions. Such type of pin hole failure is
normally attributed to oxygen corrosion and in this case also since the boiler was run for
quit some time without deaerated water we of the opinion that the pin hole developed in
the parent material of the finned section may be due to internal corrosion set in because
of presence of oxygen in the water. Since the boiler is running on furnace oil firing
during the above mentioned period, the SO3 present in the flue gas reacted with water
coming out of failed tube to form sulphuric acid over the surface of adjacent tube/fins
and aggravated the external corrosion.
We have noticed that from the cut bends of economiser section, quite a few weld joints
were without proper edge preparation/root gap provisions. We may have to investigate
at shop quality control, the procedure they have followed to pass such type of inferior
quality weld joints in economiser assembly. Since boiler design working pressure is low,
we keep our fingers crossed to observe that rest of the joints will not give further trouble
in the near future.
Client have also provided refractory materials to reduce gas leakage local to
economiser bends as shown in the photograph below restricting the thermal movement
for the economiser coils inadvertently. The same has to be removed before raising the
temperature in the boiler.
We have explained to clients how the gas leakage is kept to minimum level through
sleeve provided in the tube plate when the boiler is operation. To avoid acid due point
corrosion to the bends in the dead chamber of economiser assembly we have also
impressed upon clients the importance of maintaining feed water temperature to
economiser inlet header above 130 Deg. C. The insulation provide for the economiser
casing appears to be adequate to avoid corrosion to casing plates due to Sulphur dew
point. We have also requested clients to provide drain stub for all three sections of
economiser to easily identify coil leak during the boiler operation.