one SV protects 3 or more equipment

[gelsi]

Hi,
an other question.

Normally, several plants have one relief valve that protects more than one equipment.
I think it's a good design criteria to minimize the number of valves but if there is the fire contingency, maybe it's better to install more valves:

[ul]when the equipment group is located in more than one fire area, a valve each fire area sub-group of equipment seems a better solution[/ul]
[ul]when there is a two-phase line, for example an overhead line from column, through a cooling water condenser to separator, it is possible a dangerous surge during the relief[/ul]
[ul]when there are many meters of line between the equipments, because of pressure drops, the overpressure can become too high (more than 121% of design pressure) [/ul]

what do you think about?
when is this practice not recommended?

 

[mk3223]

All the concerns as you mentioned are legitimate for the process overpressure protection.
As discussed in the previous forums, the process relief cases should be reviewed according to the Codes, such as API-521, and the proper and adequate PSV valves should be sized and installed as necessary by the Code and local Laws and Regulations.

 

[gelsi]

ok, but there are so many SV that protects 4, 5 equipments and more...

 

[mk3223]

As you have seen, one PSV is used to protect the system of several equipment if the system is a single unit and can't be blocked. Typically, the plant should have detail docs of the PHA or HAZOP analysis regarding the over pressuring protection of the process system. For example, if two equipments could be over pressured caused by blocking the pipe in between, the individual PSV for each equipment should be considered.

 

[gelsi]

mk3223, I'm sorry if I was unclear, I was referring to an interconnected group of equipments without the possibility of intermediate interceptions

 

[mk3223]

To be clear if you have a sketch of the system can be shared.
Typically, it's acceptable once the PSV is adequate and proper to protect the weakest piece of the several equipments which are connected.

 

[MortenA]

if there are no (automated) valves between the equipment a single PSV is OK.

 

[gwalkerb]

I think you need to just work your way through the various relieving scenarios, to decide what the appropriate number of relief valves is.

You mentioned that sometimes the equipment is far away from the relief valve. Selection and installation of the PSVs should be considering the distance from source of over-pressure to PSV - as you mentioned, there may be a large enough pressure drop that the overpressure exceeds it's allowable limit. At that point, that PSV wouldn't be a valid source of protection for that equipment, and that seems like a pretty clear scenario where you might need multiple PSVs in the same continuous system, simply due to the pressure drop over distance.

Then, for the fire scenarios, you would need to review the PSV capacity vs the various fire relieving rates. Unless your vessels are extremely large, a single PSV will often have enough capacity to relief multiple vessels that are in a fire, but again, you would need to determine what the relieving rates are, and as above, consider the distance from overpressure to the PSV. Just work your way through each combination of fire scenarios (i.e. all fire areas at once, just one, or some other combination), and decide if that single relief valve would operate properly under all those conditions.

I don't typically deal with multi-phase flows, so I'm afraid I can't provide any feedback to that consideration.

But overall, I don't think there's necessarily a right answer. I would agree that having the minimum number of relief valves is the best approach, you just need to ensure that whatever minimum you've determined has properly been evaluated vs all relieving scenarios.