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.