Pressure relief valve Selection question 1

[Curious3110]

Hello All,

This may be a very stupid question. But Why is the reason that we set pressure relief valve higher value than the Maximum allowable pressure value?

For example: I have a system which has a maximum pressure 75 PSI (limited by pump) but the pressure relief valve is set to 90 PSI? Shouldn't it be set lower than 90 PSI? or is There any thumb rule?

 

[LittleInch]

Not enough details and definitions matter.

If this is a pressure vessel then the relief valve set pressure should be no higher than the MaXimum Allowable Working Pressure (MAWP).

This is stamped on the vessel plate.

Setting a relief valve higher than that is not permitted.

You say "maximum pressure". Be clear about your definition. MAWP or some other pressure?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[The Obturator]

See also thread1203-499167


*** Per ISO-4126, the generic term
'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***

 

[Curious3110]

@LittleInch [highlight #FCE94F]
Thanks for your quick respond.

Please see attached Cut sheet of Pump. We use PG25 as a Fluid in this system.
And we have this Kingstone valve as a pressure relief valve.
KSV25CRE-2-090

Link

Do you think this is right choice?

 

[RVAmeche]

Is the relief valve protecting the system or pump? If the pump max pressure is 75 psig and its a 100 psig system, someone may have elected to put a 90 psig relief valve just in case.

If the system design pressure is 75 psig, a relief valve set pressure of 90 psig does not meet code. Relief valves require over-pressure (per Code) so non-fire cases may be 110% set pressure and fire cases may be 121% set pressure but the set pressure itself cannot be higher than MAWP or system design pressure.

 

[Curious3110]

ThanK @RVAMECHE

So if MAWP 90, for Non fire case My PSV should be 81 PSI and fire case 71 PSI? Is that a right understanding?

Could you please explain more about the "Code". Where could I find one? website?
I really need to read it thoroughly in order to understand this better.

 

[TiCl4]

Curious3110,

FIRST!
You must identify if that 75 psig "maximum pressure" is actually the MAWP of the pump or if it is the maximum deadhead pressure of the pump. I have found that pump manufacturers are often NOT knowledgeable in pressure relief and state the pumps maximum discharge (operating) pressure as just "maximum pressure". Most times the casing/seals/flanges can take a higher pressure (true MAWP), but you have to call the OEM and find the right person to discuss with in order to find out the answer.

 

[RVAmeche]

No offense but you need to talk to a senior engineer. Process safety isn't a good area to have these types of questions.

Code is the piping design code (ASME B31.1, ASME B31.3 typically for piping in the states). This document (which you have to buy) has rules and requirements you have to meet. Ideally somewhere there is a piping service index or something that says something like "XYZ Chemical System - Pipe Spec #6 - ASME B31.3 Normal Fluid Service - Typ Operating Pressure 35-50 psig - Design Pressure 100 psig" etc.

Relief valve set pressures can be <= the MAWP/design pressure of the system. If the system is designed for 100 psig, it can anything below, and including, 100 psig. Typically the design pressure is used as the set pressure to give yourself the biggest cushion between operating pressures and to keep things simple.

Pump max pressure =/= design pressure. Your system should have some sort of pipe spec that defines the design pressure/temperature which you use. These pipe specs may be across multiple processes depending on pressures, temps, materials, etc required.

 

[georgeverghese]

Took a look at the datasheets for this GRI pump on the net - if max permissible pressure is stated to be 75psig, then that is what the max setting of this RV ought to be. Preferably it should be lower if possible to suit system requirements.

 

[LittleInch]

It's not wildy clear and the GRI website isn't great, but as the material is some sort of plastic, it looks to me like the MAWP of the pump body is 75 psi. Therfore having a relief valve set at 90 psi is not correct.

In reality the pump will probably survive, but it isn't correct.

I think RVAs reply is a bit confusing. The set pressure should not exceed the design pressure or the MAWP. The max pressure in the system when the relief valve lifts can be higher,but not the set pressure.

Any system has one element which sets the MAWP. This could be the pump, the pipe or some other component like a flange or valve. That's what matters.

From your data your relief valve is set too high and should be no higher than 75psi, but depends on what the rest of the system looks like.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Curious3110]

@RVAmeche
NONE TAKEN. I do not hesitate to ask question If i dont know the things.
your help is much appreciated. This is a good learning platform.

 

[Curious3110]

Thanks!!
@TiCl4
@georgeverghese
@LittleInch

All your responses helping me understand more.

 

[FacEngrPE]

As physics limits pressure rise of a centrifugal pump to operating on it's pump curve, some arrangements of this system might not need the safety valve. However as indicated above it depends on the rest of the system. Some configurations would be dangerous without a safety valve.
The safety valve must be installed such that the pressure in the pump casing does not exceed 75 psi. The KSVCR SAFETY VALVE is (per the tables included) intended for use with compressed air. Using this safety valve on a liquid is not reasonable unless the OEM provides sizing tables for use with a liquid.