Fire Case Piping Design

[ConstantEffort]

Is it your practice to omit a fire case from the design of a process plant pipe's pressure rating, pipe supports, pipe flexibility?

This line I'm questioning is downstream of a PSV protecting a heat medium oil expansion vessel. Normal operation is in the neighborhood of 295F, the design temp at 400F. For these conditions, carbon steel with CL150 flanges is just fine. In a fire case, the oil will boil off at 950F and generate enough backpressure through the small-diameter vent line to pop the larger fire-sized PSV... however, the pressure exceeds the pressure rating of the CL150 carbon steel flanges and valves by a wide margin.

This is not the only system in the plant subject to a general plant fire, but it is the only line which has no other operating scenario except the fire case. So, our group responsible for making our line list has included the fire case (exceeding the flange & valve pressure rating) in the line list as the operating scenario. No other line has a P&T combo published on the line list which exceeds the limits of the pipe class.

I requested the case be removed from the line list to be consistent with the rest of the plant and my prior experience. I would then design the pipe for pressure and pipe stress by the real normal operating case rather than the fire case. But I got more pushback than expected from some pretty darn smart engineers... making me think that maybe I'm off base and have been "doing it wrong" all this time. Thoughts?

 

[BigInch]

I can't say that I believe in a PSV system that protects equipment which has already failed.

 

[ConstantEffort]

The equipment hasn't failed though... the flanged joints may not be tight anymore, but the predicted stress in the vessel is not yet approach yield at 950F. The oil in the tank regulates the temperature of the vessel as it boils off. The PSV and line are intended to get as much hydrocarbon out safely to the flare before the fire does overcome the vessel.

Does that change the situation any for you?

 

[LittleInch]

If I read this right the line in question is the vent line?

This line is a small diameter line which as you say "it is the only line which has no other operating scenario except the fire case." so presumably this tank is normally at atmospheric pressure and releases very small quantities of gas - correct??

However in a fire case and the oil boils, you then get large amounts of vapour going down this ??m long line sufficient to cause a back pressure of X at a temperature of 950F??. clearly then the pipe is then exposed to pressure and temperature.

how much does it exceed the notional flange and equipment rating?

B 31.3 does allow some considerable margin over ratings for short periods, but if this is something like 200% or more then your line has a high potential to fail catastrophically.

to include in the line list a case where the stated pressure and temperature is greater than the design flange rating is, IMHO, not correct and should not be included as the normal max operating T & P.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[BigInch]

I was trying to say that pressure and temperature greater than their ratings means essentially that the system has failed, at least in the normal engineering design sense, and you're down to only having to calculate "survival time" if anybody's life still depends on it.

 

[georgeverghese]

The hot oil drum itself has a mechanical design temp which does not cover fire case relief (probably 400degF + 20?), and the same approach applies for the piping. The PSV mechanical design temp itself excludes firecase relieving temp of 950degF being the basis for RV selection - it is limited only to handling this relief capacity.

 

[ConstantEffort]

The 1" vent line is going to choke off pretty quickly once we start producing vapor. The 4" PSV line that goes ~30 feet before hitting a flare header is the larger concern.

The PSV is set at 150psig, the "notional" pressure rating is 35psig for the valves and flanges. I fully expect leakage, but think a majority of the vapor to continue on down the line. The pipe and vessel are beyond allowable stresses, but below the expected yield stress at that temperature. The system should hold pressure enough to force a decent share of the oil vapor into the flare.

to include in the line list a case where the stated pressure and temperature is greater than the design flange rating is, IMHO, not correct and should not be included as the normal max operating T & P.

Thanks for the validation. This is the crux of my disagreement with the line list owner. Every other line in the plant is subject to a fire and some incredible temperatures, but this is the only one treated as if the fire case is an operating scenario.

 

[BigInch]

Yield stress means nothing in this context.
System has failed when design conditions and allowable stresses have been exceeded.