3 replies to this topic

[bobbobs]

I am a junior engineer that is very new to relief valves and I am wondering about the inlet and outlet sizes of relief valves. How are these determined? I am in the process of reading both API 520 and 521 but I don't believe that I have come across this topic yet. I have read about determine the orifice area and choosing the corresponding size (D, E, etc) but I have not seen the inlet/outlet sizing yet. This has piqued my interest because I noticed on one of out flowsheets that the inlet line to the PSV is sized for 6" and a reducer drops the size down to 1 1/2" for the inlet of the relief. Would someone be willing to help me out Thanks.

[herrani]

Hi bobbobs,

The size of the inlet line of a relief valve is calculated to minimise the pressure drop. The total pressure drop in this line has to be 3% of the set pressure, if I remember correctly from API 521.

The orifice area is defined by the calculations you may have read about in the API guidelines.

For each orifice area (D,E, etc.) there are standard inlet and outlet sizes, which you can get from API 526.

Because of this reason, you typically need reducers and expanders in connection with relief valves.

The size of the outlet line of a relief valve is calculated si its pressure drop is 10% max. of the set pressure.

Regards

Ferran

[JoeWong]

bobbobs,

PSV inlet - Size inlet line to avoid potential PSV chattering. Typical criteria is pressure drop (calculated based on rated flow of PSV) from protected equipment to PSV inlet limited to 3% of set pressure as recommended by API Std 521. However if you retrofitting existing facilities where line size already fixed, you may have problem meeting this 3% criteria. Then you may consider to increase PSV blowdown to avoid chattering. There are other concern and recommendation for inlet line. Read more in "Concerns & Recommendations on PSV INLET line"

PSV outlet - Size outlet line to avoid high back pressure which potentially affect PSV capacity, results relieving pressure exceeding MAWP and stability of PSV. Outlet line also shall be sized to avoid excessive noise & vibration. Typically Mach no of 0.7 for tail pipe and 0.5 for header. Read more in "Concerns & Recommendations on PSV OUTLET line"

From PSV outlet line, it will be connected to common collection header. More discussion in "Several Criteria and Constraints for Flare Network - PROCESS and PIPING"

Hope above help

[bobbobs]

Thanks for the help.

I took a look at API 526 to see what the standard sizes are and I came across another question. When choosing the inlet flange size what comes into play (other than making sure that the flange size is large enough to avoid high velocity)? Is it the ANSI flange class? Does the flange class have to match the inlet pipe class? On the project I am working, all of the inlet pipe uses a 300# pipe spec but some of the psv sizes have inlet flanges with an ANSI flange class of 150.