Fire case scenarios 2

[Pfluid]

Hi All:
Need your help..
I'm sure that quite a few discussions has happened already on this topic. The topic is elimination of fire case when sizing the relief valves. I'm designing a small plant where there are few vessels such as reactor and storage vessels. All the vessels are protected by a PSV. I'm trying to gauge whether a fire case is a realistic scenario. There are no storage of flammable liquids around the plant and in the near vicinity.
My questions are:
1) What are the key things that need to be considered before a decision can be made for fire case elimination?
2) Whether the decision is dependent on the type of fluid stored inside the process vessel or fluid stored in the vicinity of the process vessels?
3) If there any guidelines in the NFPA which describes the process of fire case consideration in detail?
Thanks in advance for your help..
Pfluid

 

[RVAmeche]

If you're confident flammable materials (or heated combustibles) are not, and will not, ever be stored in the vicinity then you may be able to eliminate the case. I would make that a very clear design basis if you do eliminate the case. Some clients want to run the calcs and determine the requirements just to make sure they're covered in case their process/storage practices change in the future. The determination of whether or not you can eliminate it is clearly affected by the process fluids...if your process contains flammable fluids you have a the potential for fire; it's not just fluids stored nearby outside of the process.

 

[Latexman]

If the plant gets through a PHA without picking up any fire scenarios, that's a good indication. Find out who the PHA leader is, and start talking to them about it.

Chemicals that are NFPA Class I, II, IIIA, and IIIB-heated above it's flash point are "flammable fluids". Is there only non-flammable fluids flowing in piping through the area?

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[Pfluid]

Hi All:
Thanks for all your responses..
Few more questions:
1) I have a flammable fluid which comes out from the reactor and gets condensed and then collected into a storage drum. The flash point of this liquid is below 36.7 Deg C. The storage volume is 290 gallon. Since there are no flammable fluids storage in the vicinity, it is necessary to consider to the fire case for this vessel only. If the other vessels are less than 25 feet from this vessel then it is necessary to consider fire case for other process vessel as well? Or other process vessels can be exempted?
2) There are two flash points - Open cup and cleveland cup. Per industry stds, which shall be considered?
3) The term flash points - does it applies to liquid and as well as gas/vapors?

 

[RVAmeche]

In my opinion, since your process contains flammables and you're discharging them, you have the potential for a fire case. You could have a leak, spill, or overflow during discharge and create a fire. If there are other pressure vessels/tanks within 25 ft of vessels containing flammables, you have fire cases for them as well.

 

[dcasto]

Run the fire case scenario along with all credible scenarios. 99% of the time in process vessels and piping have other cases where the PSV for the system has one of the other scenarios controlling the size. The fire case is usually the controlling factor for storage vessels and with spill containment a mandate, you have a chance for a fire directly around that storage vessel.

 

[georgeverghese]

Use the closed cup flash point to be conservative. Flash point applies only to liquids. If it is listed for a gas, I would interpret that as being valid for the gas at below bubble point temp at atmospheric pressure. Leaks / loss of containment could occur through gasketed flanges, connected valve stem packings, ruptured instrumentation tubing, pinhole leaks on the vessel due to corrosion / poor fabrication, accidentally mechanical sheared off small bore instrumentation nozzles etc.

 

[vladrath]

You should consider getting the following paper.

'Vent sizing for fire considerations: external fire duration, jacketed
vessels and heat flux variations owing to fuel composition'