Hello everyone,

I am a new process engineer. I would like to learn about relief device calculations and sizing PSV's. I am fresh out of school and I have to admit that these things were not taught to us back in college.

From some of my readings, I learned that a scenario has to be identified first then things would follow. I am lost with the number of equations to use in sizing PSV's. And is the objective of the calculations to determine the orifice area? Which should I read first, API 521 or API 520?

I hope you can help me where to start.

Thanks in advance.

Shadow:

I strongly recommend you research most (or all) of the threads you can on this Forum – for starters. You will discover that there is NO SUBSTITUTE for: (1) identifying ALL of the credible scenarios; and then (2) selecting the dominant (or design) scenario out of all the credible ones.

Doing the calculations is merely arithmetic – algebra, at worst. You can practically lay the actual calculations aside as far as importance is concerned. You have computer programs from manufacturers that do that for you – besides the actual PSV manufacturers who will check your calculations.

The MOST IMPORTANT thing to do when designing a pressure relief device is to IDENTIFY THE WORST CREDIBLE SCENARIO. To confirm what I state, all you have to do is read through API 520 and API 521 (which you should own). You will note that the majority of the "design" procedures are dedicated to identifying the worst scenario. Pure common sense tells you that if you don't do this first step successfully and accurately, the rest of the exercise is a horrible and tragic waste of time. An ultimate design based on the wrong scenario could be the root cause of a terrible industrial tragedy. You cannot afford to make this type of mistake.

Your reading should involve not only the API practices. You should also read Ludwig's "Applied Process Design for Chemical & Petrochemical Plants", the GPSA Engineering Data Book, and all the manufacturer's literature and books that you can.

Mr. Montemayor has given you a very good guidelines.
i) identification of credible scenaio
ii) determination of design scenario
iii) calculate and specify PSV

API Std 521 is focus on safety concept, scenario determination, relief load determination, whilst ;
API RP 520 Pt 1 is focus on PSV/RD faeture, selection and sizing
API RP 520 Pt 2 is focus on installation of PSV & RD.

All above shall be referred.

There are few handbook available online i.e Crosby handbook, Anderson & Greenwood technical manual, etc written by PSV supplier. It may helps you to have better understanding.

Check out in the following compilation :

• Useful Documents Related to Pressure Relief Valve (PRV) - Part 1
• Useful Documents Related to Pressure Relief Valve (PRV) - Part 2
• Useful Documents Related to Pressure Relief Valve (PRV) - Part 3

If you are dealing with chemical, foamy & reactive matierial, another reference would be good for your reference is Emergency Relief System Design Using DIERS Technology

Hey,

Start by determining your MAWP for the system under concern.

Identify all the variables that make up the sizing equation. You can use sizing equations shown in API RP520 Equation 2, 8 and 9 or ASME VIII.

1. Have your MAWP ready,
2. Determine what the required relief load is.....assuming all the upstream systems failed and gave out their maximum load output.
3. ASME VIII Div 1 (Ref. 52.4): This specifies set pressure limits for any relief device whether its in a fire case or any other case. Please refer to it. It will also guide you on how to determine allowable overpressure.
4. Sizing PSVs is not a difficult task when you have the facts. If you have only one psv on a vessel, then the set pressure must never exceed the MAWP and the allowable overpressure must never exceed 110% MAWP or 3 psi whichever is greater. Please see ASME VIII Div 1(Ref 52.4) I wish I could write more.