Relief Valve Fire Sizing - Pressure Vessel
Relief Valve Fire Sizing - Pressure Vessel
(OP)
In evaluating the pressure relieving requirements for a pressure vessel in the fire sizing case (UG-133(b)), the pressure is allowed to 21% above the vessel MAWP. There is no mention of vessel temperature, although Appendix M-14(a) suggests it should be considered.
If a high boiling point liquid is in the vessel, the case may arrise where the design temperature is exceeded well before the MAWP is reached. In this case, which of the following is correct?
1.) The vessel must stay below the design temperature and below 121% of the MAWP when relieving.
2.) The vessel must stay below 121% of the MAWP, and temperature is not considered.
3.) The vessel MAWP is derated for temperature, and the pressure must stay below 121% of the derated MAWP at the elevated temperature?
If a high boiling point liquid is in the vessel, the case may arrise where the design temperature is exceeded well before the MAWP is reached. In this case, which of the following is correct?
1.) The vessel must stay below the design temperature and below 121% of the MAWP when relieving.
2.) The vessel must stay below 121% of the MAWP, and temperature is not considered.
3.) The vessel MAWP is derated for temperature, and the pressure must stay below 121% of the derated MAWP at the elevated temperature?





RE: Relief Valve Fire Sizing - Pressure Vessel
RE: Relief Valve Fire Sizing - Pressure Vessel
As I recall, the author of the article (a PHd as I recall)suggested that the cooling effects of the liquid are considerable and should be considered in the vessel strength/RV settpoint calculations.
Contact Chemical Engineering magazine and request a reprint..... perhaps...???
Good Luck !!!................................MJC
RE: Relief Valve Fire Sizing - Pressure Vessel
If you determine that the yield strength of the vessel material will be exceeded before the contents begins to boil and generate vapor to be relieved you must protect the vessel from becoming overheated. This is usually done with either fireproof insulation & jacketing (i.e. stainless steel jacketing, not aluminum like is normally used) or with some sort of deluge system.
See API-520 & API-521 (available from www.api.org) for further guidance. Good Luck!
RE: Relief Valve Fire Sizing - Pressure Vessel
Generally, the API generates the formulas / correlations by simulating the worst scenario, and there is inherently a factor of safety considered, but, as you said, there can be situations that the yield strength of the vessel shell material may be exceeded in case no precaution is taken!
The point is, the onus is on the design engineer that such a situation does not arise. YOU must ensure that the vessel does not fail. Please consider any of the following while designing for a vessel exposed to fire (I am pulling it straight out of API, so no credit to me
1 Provide excellent drainage, and do not allow accumulation of any flammable material for a substantial period of time.
2 Provide External insulation to limit the heat transfer
3 Provide earth-covered storage.
4 Provide jacketing.
5 Limit the Fire areas with diversion walls
6 Last but not the least, in case you anticipate any major hazard, make suitable arrangements for depressurization of the vessel.
Hope this helps,
Best regards
RE: Relief Valve Fire Sizing - Pressure Vessel
RE: Relief Valve Fire Sizing - Pressure Vessel
RE: Relief Valve Fire Sizing - Pressure Vessel
I've also done it on a spreadsheet. Start off with the system volume to be depressured at the initial pressure and temperature (and mass). Calculate the flow through the valve and piping based on the valve Cv or Cg. After a few minutes, calculate the mass that has passed through the valve and recalculate the inventory in the system. Use the new mass in the system to adjust the pressure. Continue in steps until you reach the final pressure you want. You then can try different sized valves till you depressure the system in the required time.
Depending on the initial conditions, you may need to include some terms to adjust for the change in gas compressibility and/or temperature in the spreadsheet as the system depressures.
RE: Relief Valve Fire Sizing - Pressure Vessel
A restriction orifice (RO) is normally be installed to limit the flow, instead of the depressurisation valve. In this case, the RO will be installed together with the blowdown valve (which will be actuated automatically upon detection of fire, or during ESD system).
As mention by TD2K, both process simulation of PRO/II and HYSYS have its own depressurisation module to determine the maximum relieving rate and hence you can calculate the required orifice size taking into account the flow is critical. Please refer to API RP 520 Part 1 in determining the orifice size formulae for the critical flow regime.
A normal spreadsheet can be develop taking into account the isothermal behaviour of the gas system (but bear in mind that the depressurisation process within the vessel is near isentropic, say 60 to 70 % isentropic, while the process across the orifice plate is adiabatic). Grote has give some guideline on how to calculate the blowdown rate). Pls let me know if you need a copy of that paper and the spreadsheet.
Thanks
rusman
RE: Relief Valve Fire Sizing - Pressure Vessel
Can you please give the guideline of Grote & spreadsheet you have mentioned. Basically in the column for which the depressuring valve is to be provided, there is no liquid. Column contains molecular sieves through which ethylene gas will be flowing.
Regards
RE: Relief Valve Fire Sizing - Pressure Vessel
Thanks
RE: Relief Valve Fire Sizing - Pressure Vessel
The operating pressure is 15 kg/cm2g. Operating temp is 25 deg C. Vapor is ethylene with carbon dioxide which will be adsorbed by molecular sieves. Vapor flow rate is 40000 kg/hr. What does initial depressurisation pressure mean?
Regards
RE: Relief Valve Fire Sizing - Pressure Vessel
I also use the spreadsheet approach you describe above. One interesting thing I have noticed is that if you use the geometric mean of the starting and final pressures in the valve equations (P1 x P2)^0.5 you get an "effective mean" flowrate. This eliminates the need to iterate, which is good enough for many cases.
Regards,
Gunnar
RE: Relief Valve Fire Sizing - Pressure Vessel
Could I have a sample calculation for a column containing only vapor. Somehow I'm still confused & unable to proceed.
Regards
RE: Relief Valve Fire Sizing - Pressure Vessel
RE: Relief Valve Fire Sizing - Pressure Vessel
I've not seen your spreadsheet but I assume you account for the drop in pressure which will result in a gradual reduction in depressurisation flow. What I have done in my spreadsheet is to do a valve rating calculation in every iteration which ensures the installed Cv value of the valve is never exceeded. It reduces the flow thru the valve to match the valve Cv. What I am unable to do is to predict the drop in temp with the depressurisation ? Any hints?
Samiran
RE: Relief Valve Fire Sizing - Pressure Vessel
On the other hand, if you don't include the temperature effect (and it isn't enough to create problems with hydrates, freezing, metallurgy, etc), the density you will get with a constant temperature is lower than it will be if you included the temperature decrease of the gas, thus the flow rate through the valve will be lower than actual and you'll have a conservative sized depressuring valve.
RE: Relief Valve Fire Sizing - Pressure Vessel
Thanks,
G
RE: Relief Valve Fire Sizing - Pressure Vessel
To do what you are asking we use a simulation in HYSYS which applies the heat input to a series of flash vessels and over time the temeperature rises and boiling is predicted. By using this approach we get a better estimate of the relief temperature and a good idea of how the mixture in the vessel will boil with time.
RE: Relief Valve Fire Sizing - Pressure Vessel
My problem is that API 520 only allows for constant heat flux equations and is only concerned with the maximum (vapor) relief rate (Q/latent heat of vap). (I am not trying to model 2-phase flow with respect to time; yet.)
The way I believe heating of a liquid and how the boiling rate increases is as follows: (of a single component)
1. Heat goes into the fluid to heat up the liquid to the bubble point and the temperature rise is not uniform throughout the vessel. The liquid that stays in contact with the sides of the vessel the longest retains more energy and reaches the bubble point first.
2. Once some of the molecules near the sides of the vessel reach the bubble point, some to the heat input goes into boiling the liquid and some of the heat input goes into heating the other molecules that are not yet at the saturation temperature.
3. Eventually all of the liquid reaches the bubble point and all of the heat input goes into vaporizing the liquid, i.e. the maximum relief rate.
My question is, does anyone know of how I can model this mathematically. If there is any literature out there I would love to hear about it.
Thanks,
G
RE: Relief Valve Fire Sizing - Pressure Vessel
This is a favorite subject of mine, the situation arises when design pressures and temperatures for vessels are specified without a complete review of the relief systems to be installed early-on in the development of the scope of work. I think we should try to design vessels for expected relieving temperatures even if not sustained for long periods of time - but lots of engineers will disagree and their reasoning is valid as well. Bottom line is to build a safe plant within a reasonable cost; this aught to be possible.
As a design engineer, you must protect the plant from failure and as others in this thread have suggested - other measures beyond the relief valve need to be taken to prevent failure.
Technically, the ASME Code does not require the vessel relieving temperature to be less than the vessel design temperature. But you still need to look for other means of protection. If not yet built, perhaps the MAWP and temperature for the vessel can be increased without severe cost. Otherwise, look to interlocks, deluge systems, depressurization, etc, in addition to the relief valve. Also, don't forget to avaluate both the wetted and un-wetted walls of the vessel for over-heating and allowable stress reduction.
Charlie D.
The more you learn, the less you are certain of.
RE: Relief Valve Fire Sizing - Pressure Vessel
One persons favorite subject is anothers biggest headache.
I've posted two discussion threads on essentially the same issue - preventing vessel failure through exceeding design temperature (my cases however were not external fire)
Unfortunately almost all regulatory standards only address failure due to overpressurisation, not excessive temperature.
Varunpant posted a good summary of risk minimisation relating to external fire. For vessels that have a risk of runaway reactions (causing uncontrolled temperature excursions) often an automated system for vapor depressurising is the only reliable protection to excessive temperatures. Unfortunately regulatory standards give the impression that instrumented controls are not suitable for protecting vessels against failure. In any event, as Varunpant has already mentioned, it is our duty of care to ensure the safety of our plant and surroundings so measures should still be taken even if they are not defined in regulatory standards.
P.S. If anyone is interested, API RP521 addresses automated Vapor Depressuring in Section 3.19 as a guard to temperature excursions.
RE: Relief Valve Fire Sizing - Pressure Vessel
I really need help for calculating the flow rate and P-T evolution in a pressure vessel during depressuring. So I would thank each of you if you can let me have a copy of your spreadsheet and a copy of Grote's paper. Any other help and advice is welcome.
saitzaj@hotmail.com
Thanks a lot