Criteria to say a PSV is oversized 1

[pine05]

Hi

I want to ask, what is the criteria to say that a selected PSV is oversized ?

For instance, I have a calculated relief rate of 100%. When I calculate the orifice size I found that the orifice size is 1.877 in2 (larger than 1.838 in2 for K size but much lower than 2.853 in2 for L size)

If I choose the L type of orifice, it will make the rated flow rate 150% of calculated.

In this case, is it safe to have selection like that ? Or it can be considered as oversized PSV ? thus it may lead to chattering problem

Thanks

 

[DSB123]

pine05,
Normal to size the RV orifice for the worst case relief scenario and then provide an orifice size above this. If you are unsure as to the sizing of the RV or selection of the orifice I would pass you calculations under the eye of a suitably qualified mentor.

 

[psafety]

If a compressible the next size larger will typically beat the valve, chattering. Liquid is different. I never oversize for compressibles.

 

[pine05]

thanks for the input

if i decide to stagger the PSV, is it okay if I use different size of PSV, let say the 1st one is 1.5F2 and the 2nd one is 3K4 ?

Thanks

 

[don1980]

pine05,the criteria for saying that a PSV is oversized is instability (chatter). As long as the over-capacity doesn't cause the valve to chatter, then it's OK.

Remenber, any time that you have more than one credible scenario, one will be the controlling case and the PSV will be oversized for the other scenario. Even when you only have one credible scenario, the PSV is oversized. The point I'm making is that all properly designed PSVs are inherently oversized. That is normal and perfectly OK, as long as the installation is designed properly to prevent chatter.

 

[ChEMatt]

don1980,

What would change in the installation to prevent chatter? Can you give some examples?

Onwards,

Matt

 

[don1980]

Matt, I'm saying that nearly all PSVs have potential scenarios that require less capacity than the valve itself has. Therefore, one could say that nearly all valves are oversized. As long as you calculate the inlet losses based on the capacity of the valve, and those losses are acceptably low, then the valve is unlikely to chatter. When a pop-acting valve opens, it flows at its capacity, regardless of the required capacity.

 

[sheiko]

In order to know if your PSV is oversized, you should also check that all credible scenario generate a relieving flowrate at least equal to 25% of PSV´s capacity (shown in vendor data sheet). I suggest you consult API STD 521.
See also:

http://www.sache.org/workshop/2003Faculty/PressureReliefDevicesScottOstrowski.ppt

"We don't believe things because they are true, things are true because we believe them."

 

[moltenmetal]

I think there's some misunderstanding of the function of a relief valve. If there are credible operational scenarios for overpressure they should be addressed by design. A relief valve is there for emergencies.

A PRV should be seen as part of multiple layers of safety protection on a plant. PRVs are the last, passive layer of protection against overpressure.

Indeed a valve sized for fire or reactive relief will be over-sized for most other contingencies, and it may chatter if the fire isn't as bad as imagined- but who cares? The valve will be replaced after the fire. If you expect FREQUENT challenging of a PRV you've selected the wrong type of protection.

Remember too that a relief event is usually not without hazard of an operational or environmental or economic nature. Relief has to go somewhere...

 

[tjpengineer]

You need to concern yourself with non recoverable pressure loss on the inlet and outlets, based on Rated Flows. Try a balanced bellows valve if the outlet drop is greater than 10% of valve set.