3 replies to this topic

[jprocess]

Dear All,
Blocked outlet scenario often is a probable and sometimes is the governing case in relief study of plants.
In some cases it can release high flowrates to flare network for example in compressor stations.
I think it has a high dependency to isolation philosophy of equipments using isolating valves.
How can we modify or optimize the isolation philosophy?Is it possible to omit or minimize the number of isolating valves?
My second question is about centrifugal pumps.Rather than reciprocating pumps,usually we do not consider a PSV at discahrge of centrifugal pumps because design pressure is set above the maximum operating pressure: the shut-off pressure.
As you all know we should always consider a check valve and a kind of on/off valve at discharge of centrifugal pumps.Malfunction of check valve or on/off valve at off condition can lead to blocked outlet scenario.Why we do not consider it?
Thanks in advance.
Cheers.

[djack77494]

j,
You are always welcome to design your system with a minimum number of isolation valves installed, provided your installation satisfies all code requirements. With a very reliable instrumented system, you may sometimes use instrumentation as an alternate means of dealing with potential overpressure scenarios. This could reduce your relief requirements.

Concerning centrifugal pumps, I'm not sure I understand your question. In my mind, the best way of "dodging" large PSV's for blocked outlet scenarios is to design the entire discharge system for full pump shutoff pressure. If you can economically do that, your problem is solved. It doesn't matter what check valves or isolation valves you have downstream of the pump. So to answer your question, "we do consider it, and we design the discharge system so that it does not result in pressures above design. Of course, there are situations where this approach is not practical. Then, it may be necessary to add a PSV on the pump discharge sized for a blocked discharge scenario.
hth,
Doug.

Dear All,
Blocked outlet scenario often is a probable and sometimes is the governing case in relief study of plants.
In some cases it can release high flowrates to flare network for example in compressor stations.
I think it has a high dependency to isolation philosophy of equipments using isolating valves.
How can we modify or optimize the isolation philosophy?Is it possible to omit or minimize the number of isolating valves?
My second question is about centrifugal pumps.Rather than reciprocating pumps,usually we do not consider a PSV at discahrge of centrifugal pumps because design pressure is set above the maximum operating pressure: the shut-off pressure.
As you all know we should always consider a check valve and a kind of on/off valve at discharge of centrifugal pumps.Malfunction of check valve or on/off valve at off condition can lead to blocked outlet scenario.Why we do not consider it?
Thanks in advance.
Cheers.

[pleckner]

@j:

I'm not sure what you are getting at with your last question,

"As you all know we should always consider a check valve and a kind of on/off valve at discharge of centrifugal pumps.Malfunction of check valve or on/off valve at off condition can lead to blocked outlet scenario.Why we do not consider it?"

The check valve and block valve I believe you're referring to are typically installed right at the pump discharge so other than the pump casing and a very short run of pipe, there is no PSV blocked-in scenario for downstream equipment. One should always try to specify the pump casing design pressure for shut off containment.

[Wouter]

Indeed minimizing the number of block valves at the discharge of compressors or pumps is the way to go.
Even though they don't take away the overpressure scenario, consider sealing the valves or mechanically/electrically locking the valves.
I have never heard of failure of checkvalves causing a blocked outlet scenario. Most scenarios I encounter are high pressure backflow from checkvalve failure.