Furnace Pressure dropped and rise to HH limit during an over pressurisation event on an Aux Boil

[JimMoriarity]

I am looking for some help here with some combustion expert/ boiler air flow design expert, who can direct me to calculate relationship b/w boiler furnace pressure and Stack damper area.

We have a Force Draft package auxiliary boiler with steam output capacity of 280 t/hr . The boiler is fitted with 2 IGV controlled fans with total max fan flow of around 400 T/hr. The fan is connected with Recirculation flow which recirculates some flow for boiler efficiency. The stack damper area is 110 sq ft. At high loads output ( >65% of boiler load) our boiler furnace pressure is around 15 mBar. The control system trips the boiler at 30 mbar to conserve integrity of casing etc.

We have had an event on site where the boiler casing had over pressurised (gas side) and we are not sure whether it was due to back pressure rise or due combustion issue. I am inclined to think that boiler damper closed somehow and that over pressurised the casing and eventually the casing gave away a small hole to release the pressure.

1) how do we calculate the minimum area of stack opening required to keep the boiler furnace pressure within required working condition,
2) We have strangely observed furnace pressure , stack outlet pressure gas side economiser pressure to drop to zero. Would this happen at the point of trip?(i.e at the exact instance of pop) before it rises back again?

I am trying to work out whether its possible for boiler casing to pop at < 28 mbar That is well within the design capability, instrumentation on site isn't really great and I am struggling to find whether what we believe happened, actually happened?

I have also looked at Fan flow, fan IGVs, recirculation flow O2 , CO and NOx reading out of the boiler and they haven't moved from the norm, I would also like to mention that, before the event on the boiler in the last few weeks we have observed the boiler super heater temperatures and metal temp raised by 30 degC and we have been putting more deSup water to cool down the steam. What other parameters I should look to investigate this issue further?
I would really appreciate any useful pointers on this matter.

Regards
Jim

 

[JimMoriarity]

Can anyone help me with this query or point me in the right direction?
Thanks

 

[Duwe6]

Most probable cause of boiler Casing being bulged is 'woom' deflagration burn when relighting the boiler. You are correct that the combustion air fan cannot generate enough pressure to bulge the casing; just look at the clearance between the fan blades and the fan casing. The chances of getting an Operator to admit to shortchanging the Interlock Relight Purge interval - paperclip jumper on the timer interlock terminals - is slightly less than ZERO.

Fairly common occurrence.