Chattering of Safety Valve based on oversizing

[mandarkardekar]

We are procuring PSV from one of reputed vendor where there is a remarkable difference between effective and actual discharge of orifice.

Vendor has selected "R" type orifice as per API520, with effective discharge area as 10322mm2.

However actual discharge area for the same is specified as 12668mm2.

How we can check and ensure that there will be no chattering due to this difference. Vendor is silent on this. Is it not vendor's area of responsibility?

Can anyone provide guidelines about Chattering of valve for above reason.

 

[moltenmetal]

Relief valves are sized on a worst case scenario. It stands to reason that all relief cases encountered in practice will require a SMALLER valve than the one selected based on the worst case relieving rate. Therefore, the notion that a relief valve can be oversized is totally wrong-headed. If you expect to encounter relief events frequently enough to worry about whether an "oversized" relief valve will chatter, you must mitigate those cases by means of a modulating pressure control device, i.e. a back pressure regulator or pressure control loop with control valve. The alternative would be to set up a scheme with multiple relief valves in parallel, with the smaller valves set at lower pressures. That is sometimes done to separate process relief contingencies from catastrophe contingencies such as fire.

 

[mandarkardekar]

Thank you sir.

 

[CKchi]

In my opinion, First, you have to understand the "effective orifice" and " actual orifice" area.
Per API 520, the effective orifice area is used for initially sizing the PSVs, However, many
PSVs' certified area are presented in ASME actual orifice area.

There are some correlation between API effective discharge and ASME actual discharge:

A(api)= K(ASME) x A(ASME) / K(API)
A(api): effective orifice
K(ASME): actual discharge coefficient
A(ASME): actual discharge area
K(API): effective discharge coefficient

SO you cant compare effective discharge (R orifice) with actual discharge (12668mm2) in this way. you should ask the PSV manufacturer to provide you their equivalent effective discharge area. Generally the equivalent effective discharge area by PSV manufacturer will be slighter bigger than API calculated result.