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Design pressure of PSV discharge piping

Design pressure of PSV discharge piping

(OP)
Our client is insisting that PSV discharge piping must have the same design pressure as the PSV set pressure of 1200 kPag (175 psig). Normally PSV discharge piping would have lower design pressure since it's open pipe to flare, ie pressure is limited to backpressure from hydraulic losses in the piping and flare system only. We are designing new pressure vessels with design pressure of 1200 kPag, with PSVs set at 1200 kPag, and flare discharge piping connects to an existing flare system. The existing flare system piping design pressure is 1200 kPag (ie unnecessarily high).

The client says the actual max backpressure is lower, at 500 kPag. We are checking the selected balanced bellows type PSVs as to whether they will operate with this backpressure as it's right on the 40% limit (but that's another issue). These PSVs will definitely not work if they ever needed to relieve against an actual backpressure of 1200 kPag.

My question is, should we accept a design with a PSV discharge piping design pressure of 1200 kPag? Given that, if this pressure actually existed, the PSVs would not work. We are under pressure to sign off and issue this design, and are currently being seen as the "bad guy" since I'm refusing to issue our package with piping design pressures of 1200 kPag to match the flare system we are connecting into.

RE: Design pressure of PSV discharge piping

If the client pays for 1,200 kPag design pressure, the client can have 1,200 kPag design pressure. There is no point in arguing against their preferences. It also does not make a lot of sense to install piping with lower pressure rating as this would effectively de-rate pressure rating of the entire flare system, and this again opens Pandora box with many potential issues. I have one painful experience of installing 300# B31.3 section in the existing B31.8 header designed for 64 barg (higher pressure rating).

Your only concern should be the second part of your question, i.e. whether any of the PSV's will see an actual backpressure in the ranges that could affect their performance. If yes, you need to ensure that the design will address those issues. If there is an existing flare network simulation model, you can see the calculated backpressure for the maximum combined simultaneous relief and ecide on which PSV type to use. If there is no such information, you can either charge the client for doing these calculations, or go for the design which ensures reliable PSV performance regardless of the actual backpressure.

Dejan IVANOVIC
Process Engineer, MSChE

RE: Design pressure of PSV discharge piping

Bob - You are correct. Standard PSVs are not mechanically designed for the same pressure capability on the inlet and outlet. This is clearly illustrated by the tables found in API 526. If needed, you can contact a manufacturer and they'll back you up on this argument.

However, in this case you're not talking about a high pressure. 1200 kPa = 174 psi, and that is easily within the limits of a 150# outlet connection (unless the temperature is very high). So, except for cases where the temperature is very high (check the tables in API526 to see the limits) you'll get valves with 150# inlet and 150# outlet connections. That's means you and your client can both be satisfied, despite it being wrong to say the outlet must always be able to tolerate the same pressure as the inlet. Perhaps this is just a miscommunication - possibly the client is saying they want 150# outlets and this has been morphed into a statement that the outlet must always be equal to the inlet.

RE: Design pressure of PSV discharge piping

I'd put a design pressure of 1200 kPag on the outlet piping. I agree and understand what you are saying but if the client's flare system is currently designed for 1200 kPag, I can understand their reluctance into "designing" an interconnecting piece of piping that is otherwise a 'weak' link. Maximum backpressure during relief and its effect on the PSV capacity is a separate issue. I do not automtically take the flare header design pressure = maximum backpressure.

As you've realized, you also need to check what the bellows limits are on the PSVs you are purchasing. If the bellows' backpressure is limited to less than 1200 kPag in a static or non-relieving state, I would want to make sure the client is aware of this 'weak link".

I know of at least one client that I've worked with that stipulates a piping minimum design pressure of 150 psig and that is what goes on the line list. Full stop be it process piping or the 3/4" section of piping downstream of the drain valve that you run to the sewer hub.

RE: Design pressure of PSV discharge piping

I have in the past, seen similar situations where the SRV outlet needed to be equal to the inlet rating, at a point when the SRV would not be required to open or not. In these cases the SRV supplier designed high back pressures bellows (and sometimes uprated the body pressure rating. Very often the limits greatly exceeded user standard API-526. Just saying that SRV's are available for such cases. Also consider pilot operated SRV where no bellows and higher outlet ratings are the norm. I do know as an example that 3" Cl.1500 x 4"Cl.1500 and many others were made to client specific requirements.

Per ISO, only the term Safety Valve is used for all overpressure eventualities regardless of design.

RE: Design pressure of PSV discharge piping

A mechanical design pressure of 1200kpag for the flare network does not necessarily mean that actual max backpressure (for the purposes of backpressure correction on balance bellows RV) is 1200kpag, so there is no conflict in design, from the information you've provided.

One reason for 12barg design pressure may be to withstand detonation pressure (due to flashback at the flare tip) in the event of loss of purge. Industry position on this is that connected RVs' are not expected to be in operation if there is a detonation within the flare network.

RE: Design pressure of PSV discharge piping

(OP)
Thanks folks for the useful replies. I have accepted the higher design pressure, provided the valves can operate with the stipulated max backpressure of 500 kPag.

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