It's essentially a flow calculation. Be sure to use the valve's capacity, not the required relieving capacity used to size the valve.
If it's a liquid, it's relatively straightforward. You determine the outlet piping system and the components in it and convert that to equivalent feet. What is the pressure where the PSV outlet line is discharging to (if not atmospheric) and therefore, what inlet pressure (eg. PSV outlet) you need to get that flow rate through the line.
If it's a gas, you'll likely need to do a compressible flow calculation using the same approach as above. You might get away with an incompressible flow calculation IF the pressure drop through the PSV outlet line is small enough (less than about 10% of the absolute inlet pressure).
If it's 2 phase flow, it's more difficult, a lot more difficult.
There are lots of other considerations to what I've gone through here, you need to look at your entire system to decide what needs to be calculated or verified and what can be taken as a 'constant'. In some cases the back pressure in a flare system is taken as a constant number, in other cases, you need to calculate it. It depends on the case being considered.
It's liquid in the system. The inlet of the RV is connected to the discharge of a pump and its outlet is connected back to the pump suction. I think the pump suction pressure is about 20 psi. Other operating condition are:
- liquid: seawater
- discharge flow rate required: 10gpm
- temperature: 20-25 degree C
The set pressure for this RV is 275 psi.
What's your background? If you haven't taken an fluid flow courses, I can't really teach you that here. And I hesitate to hand off equations to someone who doesn't understand them, especially for PSV calculations.
I am just looking for a fast and easy way to determine the back pressure as required by the supplier. Ok I must admit I have not taken any fluid flow courses but I still have to find out the answer coz it's part of my job. Anyway if u are reluctant to help, just forget about it.
Perhaps you should keep your cool... TD2K is one of the most helpful and knowledgeable member in the fori... may be his wording was not the best politically speaking... but we are engineers not politicians. The intent of the forum is to give guidance... and not to perform detailed engineering calculations unless somebody willingly agrees to it...that's called consulting.
The way you posted your initial question was too broad.
Regarding your specific question, sounds like what you are looking for is a thermal relief valve (check with your vendor)... look for them in the internet they are usually 1/2".
good luck.
a.
Freshguy, if you haven't taken any fluid flow courses, you should not be filling in data on PSV data sheets without someone experienced and qualified reviewing your numbers. That person would be the best person to give you the calculation methods and some guidelines when to use them and when not to use them. The problem is that there are lots of things to consider in setting that number and while it's usually straightforward, it's not always. I'm currently overseeing the work of several new engineers who have got the training but still 'make mistakes' because of lack of experience in what to consider.
One of the best references is a recent book "Guidelines for Pressure Relief and Effluent Handling Systems" published by the American Institute of Chemical Engineers in 1998. There is substantial material on two-phase flow. A CD is included which contains two-phase flow sizing methods.
