2 replies to this topic

[abhi_agrawa]

Hi!
In different projects, I have seen two different locations of PSV's. In some projects PSV's are located on the drum and in some on the vapor outlet line from the drum. Can someone please clarify under what conditions do we put the PSV on the drum and when do we put them on the vapor outlet line. Does one of these offer a clear advantage over the other?

Thanks,
Abhishek

[pleckner]

It is always preferable to put the PSV right on the nozzle of the vessel it is protecting. This is done to minimize the inlet pressure drop (the 3% Rule if you will). However, this is sometimes impossible or just very difficult to do.

During the design phase of a project, the PSV sizing is typically done next to last whereas the vessel design is typically done early on. Thus we don't know the nozzle size the PSV really requires. We do a quick PSV size and make the nozzle one-size larger but the actual nozzle size won't be confirmed until later. So as to not have to issue a change order to the manufacturer while the vessel is in fabricaiton (and incurr a substantail economic penalty), we just put the PSV on the vapor line instead. Note the vapor line should be big in comparison to the PSV size, if not, we have a lot more issues to contend with.

Another reason would be access. We always need access to the PSV and sometimes layout is such that it is much better to put the PSV on the vapor line rather than directly on the vessel. Then there is the need, sometimes, to install dual PSV's where one is a spare. This lends itself more to the vapor line installation; again a matter of layout and access.

[JoeWong]

Phil has mentioned 3 main and most important points :

i) Layout constraint
ii) Accessibility
iii) Economic (avoid penalty for late changes)


I would like to add 2 minor points :

iv) Some drum may have internal such as mistmesh and vanepack demister, under this condition you may needs to locate you PSV outlet upstream of these internal (on the vessel)

v) PSV with inlet & outlet piping may induced additional loads on the vessel (especially thin wall vessel). It is easier to handle if you put on the vapor line.


regards,

JoeWong