Balanced Draft Furnaces and Trips

[Cokers]

Hello All,

We have a peculiar problem.

We have 2 furnaces sharing a common forced draft fan. The situation is as follows, operations are requesting that if one of the 2 furnaces trip to keep the FD fan on. The idea is to allow one of the 2 heaters operating and to avoid a total shutdown of the unit. It is understood however, that under specific scenarios like, hydrocarbon detection in the FD fan inlet or push button ESD, the FD fans will trip with the heater.

My concern with this is that it is still unsafe based on the following,

Maintaining the heater and FD fans online during a flame out:

The fuel gas mixture can/may ignite if it hits a hot spot on the refractory, resultiing in an explosion, fire. Feeding combustion air to this scenario will exacerbate the situation.

Maintaining the heater and FD fans online during a tube leak:

A tube leak due to a line rupture, can result in a fire. Adding combustion air to a fire will worsen the situation.

The obvious solution is for each heater to have its own balanced draft system,

Your feedbacks on whether, FD fan can remain online after a furnace trip would be much appreciated.

 

[Chance17]

For both situations, the worse hazard is accumulations of unburned hydrocarbons.
If this happens,the reignition could be very bad indeed.
Maintaining the combustion air is a mitigation to prevent hydrocarbon accumulation.
The hydrocarbons either burn or are purged outside the furnace.

 

[Cokers]

Thanks Chance17.
Ok I may have missed some other critical info.

The pilot burner will trip the FG to the heater on loss of flame (flame scanners installed), this means that accumulation as such is very limited since if there is a flame out of the main burner, it will be re-ignited by the pilots before any hazardous accumulation, otherwise, there is no FG entering the system (base on flame detection alarm).

I came up with at least one scenario where the combustion air could be a risk. This is when we have a coil leak and the feed catches on fire. In this instance, there is no trip and the only safeguard would be the operator observing a smoke plume or waiting for a secondary alarm of some kind. In both cases, combustion air would still be flowing feeding the fire.

I recognise that this scenario is applicable to all heaters with combustion air.

We are likely are going to allow the fans to remain online but for specific cases where the risks are limited. Examples would be, low flow trips, low/high FG/PG pressure trips, High temperature trips as in all these cases FG is prevented from entering the firebox.

All other trips shall trip both heater and fans together. This will mean that the fans will have to have dedicted trip logic block.

Thanks for your feedbacks!