Need of Pressure Safety valve

[liberoSimulation]

I'd like to understand the need of PSV any each equipment assuming that each equipment is considered and treated individually.
Is this correct and is it based on CODE or STANDRAD requirements ?

Regards

 

[JimCasey]

A pressure Safety valve is used to protect life and property. A fired pressure vessel contains as much energy as many sticks of dynamite, and after some tragic accidents in the 19th century, governing agencies mandated safety valves.A safety valve must pop to full open and pass its nameplate capacity at no greater than 3% greater than the set pressure.

Most states have adopted ASME section 1 as law with respect to pressure safety valve installations on boilers. Similarly they have adopted Section VIII which governs pressure vessels that are un-fired, such as storage tanks. Valves on unfired vessels are referred to as "Safety-relief valves" SRVs are allowed 10% overpressure, 16% if used in a staggered setpoint system, and 21% in a fire.

Even the safety valve on your lowly water heater must conform to code. If your water heater overheats and overpressurizes it can take out your house, and most of the neighbors.

 

[liberoSimulation]

Good history
But do you still need individual PSVs for each suction drums of a centrifugal compressor with 4 stages each stage has a suction drum.
Or can somebody argue that no need to install PSV on each suction drum where the PSVs installed at the last stage discharge put for blocked discharge can relief the overpressure from fire would happen around suction drums

Regards

 

[Homayun]

Libero,

If an equipment can be isolated and there is a possibility of over-pressure, then yes, it should be protected for that over-pressure case by installing a PSV.

If in your case the high pressure gas of your compressor can go to suction vessel, which usually will have a lower design pressure, by malfunctioning the anti surge valve, then you need to protect your vessel in such a case.

I think that each stage suction drum, must be protected by PSV's.

 

[BenThayer]

you have to look at the discharge pressure of each section of the compressor (or stage as you are calling it) and see what pressure that section can develop in the deadheaded state.

the case of the anti-surge opening is quite peculiar based on where the piping is and relative to a discharge check valve. typically, the anti-surge is dumping back and causing recirculation but not added pressure (it is not adding mass into the closed system). you may develop suction pressure because the supply source is continuing to dump to the suction. or you may develop suction pressure because you are taking a large pressurized source at the discharge and dumping back.

but you may want to look at the recirculation case in light of where any intercooling stages are because you can get very cold depending on what you are compressing.

if each drum has a mawp that would require protection, it is not necessary to vent the full flow, just the flow needed per the "stage's" curve to get the discharge pressure below the mawp.

 

[liberoSimulation]

You guys talked about the protection of the compressor interstage equipment but the question is general for all equipment connected by piping and operating in a plant and that just an example.
My question again, do we need to study all relieving scenarios (fire, thermal, blocked discharge, utility failure, failure of control valves..etc)of each equipment in the plant wherever it is applicable even though with no valves between them or group some and study such scenarios??

Regards

 

[jte]

libero-

For ASME VIII-1 you may have one pressure relieving device (valve, rupture disc) protect several vessels provided that the pressures are relieved in all vessels below the limits mentioned by JimCasey (ref: VIII-1 UG-125). For example, you may have a shell side of an exchanger connected to a drum which has a relief valve on it. As long as the relief path cannot be blocked in (and is adequately sized, etc), it is adequate for protecting the shell side of the exchanger. There may be a block valve between the exchanger and drum, but it must have adequate controls (e.g. car seal open) to prevent it from being closed while the exchanger is in operation. Read up on UG-125 and related paragraphs, especially UG-135.

See also Appendix M. I'll highlight the first part of the definition of pressure relief path from App. M: "consists of all equipment, pipe, fittings, and valves in the flow path between any protected equipment and its pressure relieving device..."

jt