AFBC Boiler - Bed Coils Frequent Failure

bad selection of coil materials

the most suitable material for coil pipes is SA- 335 GR. p22

SA-335 Seamless Ferritic Alloy Steel Pipe for High-Temperature Service

Hi blackpanic73 sir,

Thank you verymuch for replying.

Please send any other causes for such bed coil failure.

with regards,

roopesh

How old is your Boiler?

Hi ducon sir,

Thank you verymuch for responding to my question.

Boiler commissioned 6 years before.

Please send causes and any other reasons regarding this bed coil failure.

Thanking you,

with regards,

roopesh

If it is that early then you are knocking at the wrong doors. Get in touch with the Boiler manufacturer to advice you.

The major reason is caustic gouging in bed tubes of these high pressure AFBC Boilers.

Corroision in the boiler is prevented by maintaining the boiler water condition in such a way that the magnetic layer is maintained.Boiler water has to be at Ph 9 to 10.2 in order to preserve the magnetic layer of steel. This is done by dozing sodium phospate in the boiler through HP dozing system. in case there is a mechanism of alkalinity in the boiler water anywhere in the evaporative circuit caustic gouging or massive caustic attacks occur which are leading to your bed coil frequent failures. Get this tested and checked for high caustic levels.

Roopesh!

What is the failure mechanism in bed coils? Means, is it failing due to out side erosion or due to internal grooving? How frequently you are able to inspect your bed coils from out side? Please also see if you are using coal quality in your boiler for which it is designed?

After coal price policy changes, all users are buying relatively poor quality coal which is needed to be fed at higher rate and operators have to be vigilant in maintaining the bed height etc. Coal feed velocity increases and all these changes cause heavy erosion in the bed coils particularly over the coal nozzle plates areas. You might be observing failures in those areas?

If the boiler load varies rigarously and if boiler tripping is frequent and if black-out happens frequently, all these reasons contribute in braking the magnetite layer inside the bed coil tubes and tubes become suspetible for internal erosion and hence failure. During all these happening, bed coil tubes are exposed to relatively high temperaure and comparatively in dry conditions because of Feed pumping failure etc. All these reasons are beyond water chemistry of the boiler feed water.