Why PSVs are Placed So High?

IMO,

PSV are used to relieve excess pressure in system. PSV in real terms is a rupture disc not a valve. PSV, while releiving excess pressure, if it is adjacent or at lower elevation than other components, high pressure fluid may cause damage to neighbouring system components or personnel.

this may be the logical reason without getting into hydraulics.

I don't know anything about this sort of thing, but, these seem like logical suggestions.

If it's a pressure relief valve in a water/steam system then you will vent steam instead of water. Also this venting will be above any vulnerable personnel.

If it's a rupture valve in a system carrying something other than water then you will again vent gas before liquid and not lose all your liquid on the floor.

http://www.spiraxsarco.com/resources/steam-engineering-tutorials/safety-valves/safety-valve-installation.asp

thats where the gas normally migrates to

One important factor is that the relief valve code specifies that the outlet piping should slope away from the relief valve and that there should be not liquid pockets. This is important for several reasons, First, if the outlet piping is liquid filled then the relief valve must relieve against the backpressure created by the liquid head. If the pressure is relatively low and the liquid head relatively high the protection is compromised. Secondly is the temperature is below the freeze point of the liquid, disaster looms. Thirdly if the liquid is corrosive, well you will rot out the tail pipe. Lastly as others have mentioned, you want the relieving stream away from people and other equipment.

does it mean, PSV (pressure safety valve) rules are applicable to PRV (pressure relief valve)

Yes; PSV and PRV are two scarcely different terms for the same thing.

You are right in say that, but why you are rated 'off topic'

If your comment is not related to the original Forum Thread, post 8 and 9 are as well so

There are some worrying comments in this thread.

I'll just put my history out - i have worked in Oil & Gas contracting for 20 yrs and am currently working on a project for a well known oil major reviewing all of its relief devices on an FPSO.

PSVs almost always discharge into a flare header so the comments about discharging to personnel are a red herring. For those PSVs that discharge to atmosphere personnel are protected by routing the discharge piping to a safe location.

Relief valve codes (API 510 & 521) specify that inlet and outlet piping must have no pockets and be free draining.

For the discharge the ultimate destination of the flare header is a flare stack where all relief streams are combusted. Upstream of the stack is a vessel to remove any liquids. The flare header must slope down to this vessel. Also there must be no pockets so you cannot jump the flare header over or under another pipe. For this reason it is usually easiest to have a dedicated top level on the pipe rack for the flare header with no other piping. The PSV discharge must be downhill into this header so this puts the PSV as the high point.

The reasons for this are the back pressure, corrosion and freezing issues already mentioned.

On the inlet side the design, calculation etc for PSVs is easiest when considering vapour phase and relief nozzles are almost always in the vapour space of a vessel. In most relief cases you are most concerned with relieving a large vapour stream so the take off is usually from the top of the vessel or the overhead line. This also tends to put the PSV at a high point.

Comments 1 & 10 seem to confuse the two key pressure relieving devices.

A PSV (also called PRV) which is a spring loaded device which opens at a set pressure and then closes if the pressure falls and allows continued operation. These are by far the most common devices. (See API 526 for sizings as well as 520 / 521 for operation) in use. There may once have been a distinction in names between liquid and vapour service but API 520 calls them all PRV (just to be silly on my current project the items are tagged PSV-XXXX but called relief valves or PRVs)

A bursting disc. This is a circular piece of metal or graphite of various designs (flat, concave, convex) which open at a set pressure and then do not close. If one of these is used then once it has burst the process has to be shut down and the item replaced. The most common usage of these items is on the low pressure liquid side of a heat exchanger which has a high pressure gas in the tubes, because they respond far more quickly than PSVs to the sudden pressure surge caused by a burst tube

At one time I would have agreed with your comment about almost all relief devices discharging into a flare header but as I get older and travel to more locations, there are more and more that disharge directly to the atmosphere.

I have never known PSVs for steam to discharge into a flare header, and rarely for ammonia PSVs.

The term PRV sometimes means "pressure regulating valve" and sometimes means "pressure relief valve"; the latter is the same as a PSV, and neither is a rupture disk.

Unfortunately, this thread is laden with OT confusions, and correct answers that have been incorrectly voted OT.

Fair point on the steam relief valve (it also applies to air, N2 and other non hydro carbons)and pressure regulating valve.

Ammonia reactors in my (limited and ancient) experience do go to a collection header and a discharge vent, but liquid refrigerant ones would be a different kettle of fish

However the point I made for those that discharge to atmosphere holds. Operators can be protected from the discharge of steam and air relief by tail pipes.

The OP has talking about PSVs above the piperack. Because of code it is easiest to build a flare header above all other lines on a pipe rack. because of code PSVs that discharge into a flare header must be above this line.

PSVs that do not go into the header are often located with those that go into the header to keep them all together, e.g. to prevent buggering about

PSV's are used in many industries other than O&G.

In water industry a PSV venting to atmosphere is often considered a safe location - depending on 1. Safety Aspects, 2. Environmental Concerns, 3. Process Considerations.

Australian standards require PSV's to be mounted with the spindle vertical in most cases - I suspect it is the same elsewhere.

Although I'm not a designer, I believe it is simpler and cheaper to mount the PSV on the top of a vessel where the connection is simpler e.g. screwed straight into a socket in the vessel as opposed to adding an elbow (and holding the elbow with a spanner/wrench while you do it).

We have a minority of PSV's on the side of vertical vessels.

Thank you for all your views and opinions.

I have understood the following by reading all the above comments.

PSV/PRV has to be located above flare header. So at whatever elevation the flare header is running, PSV/PRV has to be above it. This is to avoid accumulation of liquids in the downstream piping which is detrimental to both valve and piping operation.

If the PRV/PSV is not connecting to flare and venting out to atmosphere, the discharge has to be routed to a safe location for personnel's and/or assets safety.