Fire Case Piping Design
Fire Case Piping Design
(OP)
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?
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?





RE: Fire Case Piping Design
RE: Fire Case Piping Design
Does that change the situation any for you?
RE: Fire Case Piping Design
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
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RE: Fire Case Piping Design
RE: Fire Case Piping Design
RE: Fire Case Piping Design
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.
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.
RE: Fire Case Piping Design
System has failed when design conditions and allowable stresses have been exceeded.