Fired Heater Minimum Operating Air Flow Requirements

[ianmcq26]

Dear All

I am currently performing a GAP analysis to determine whether there are deficiencies with our existing plant safety systems.

We have a fired heater with three forced draft burners. The fuel and combustion air flows are controlled using a burner management system that is within our DCS (BPCS).

An action has been raised to assess whether a hard-wired fuel-air ratio trip is required in the event that; should a deviation in combustion air flow fall below the stoichiometric requirements for a period of time due to a fault within the DCS, a hazardous scenario could occur as excess fuel enters the heater.

Currently we have hard-wired trips to shut-down the fuel supply in the event of loss of air fan or low flow (and the design and other trips generally conforms to guidance given in relevant API and NFPA standards).

Currently air is supplied with an excess of 15% (equivalent to 1.5 excess oxygen), however the low flow trip is specified at ~25% of the design flow and therefore there is potential that this would not prevent the hazardous scenario.

My first inclination would be to specify the new hard-wired trip set at the stiochiometric limit ie zero excess air, but I accept that this could introduce the risk of spurious trip scenarios.

Can anyone provide me with some guidance regarding a more practical solution or whether there is a margin of safety with a reduced air flow below stoichiometric requirements?

Why does this not appear to be addressed in the relevant codes?

Best Regards

Ian

 

[ash9144]

I've seen trips usually at 1.0%. If you get close to 0.5% you will start to see vocs in your stack and if you look in the fire box you can see the "ghosts".

 

[ianmcq26]

Thanks Ash

From a safety perspective this would make sense and we would also want to avoid releasing CO into the environment!

Just to clarify the limits and the mechanics of a hazardous scenario can someone comment on my theoretical assumption(!):

If incomplete combustion started to occur ie with some loss of stoichiometry; would I be correct in assuming that partial oxidation/smoking would occur but the flame would still be supported and accumulation of fuel gas would not occur?

In theory could the flame be supported until the O2 concentration depleted towards the LEL?

In NFPA there is a formula:

Minimum oxygen concentration to support a flame

MOC = LEL x(moles O2/moles fuel)

Below such a point (or at some point towards such), then the burner flame would go out and fuel would then accumulate.

My assumption for the hazardous scenario would be loss of burner flame, accumulation of fuel, air ingress and ignition - either auto ignition or from the pilots.

Although it is probably a sensible approach to set the trip near 0% stoichiometric limits, I would imagine that there is still a reasonable margin below this limit before a hazardous scenario could occur?

Thanks

Ian

 

[JLSeagull]

Perhaps the stoichiometry is not addressed in the heater standards such as API and NFPA because they use flame detectors to detect loss of flame.

In addition to oxygen content and lead lag controls, review the forced and induced draft pressure balance.

 

[ianmcq26]

We have flame detectors to detect loss of flame which activate hard-wired trips, but there is a concern regarding their reliability and whether a hazardous scenario could occur before they go out.

We have air flow lead control and air-fuel ratio alarms and software trips but this is all within the DCS. Should an upset occur with the DCS this could potentially lead to the hazardous scenario.

We have completed a preliminary LOPA study and assuming that loss of DCS could lead to the hazard and taking credit for other indicator alarms and flame detector trip etc we would still need a hardwired trip.

This could mean some substantial modifications ie 2oo3 voting inputs and 1oo2 outputs to ESD valves etc to meet the required PFD.

Ideally, the low combustion air flow trip could prevent a hazardous scenario.

 

[786392]

Dear
We did have our previous employer's one Steam boiler 'Low combustion air flow trip' over-riding the controls for safety reasons and this trip inter-lock prevented trials for firing the burners if any.
Hope this helps!

Best Regards
Qalander(Chem)