9 replies to this topic

[Art Montemayor]

Distill:

A normal PSV works successfully in accordance to its design capacity and how close it discharges that same design rate. In other words, it wants to discharge the flow rate it was designed to handle. It is a dumb device in that if you don’t furnish enough fluid flow rate, it will open excessively and cause a relatively over-reaction to the need for relief. When it behaves in this manner, it “chatters” – i.e., it opens and shuts in rapid, successive responses because it has excess relieving capacity. Of itself, this is not a bad situation for making sure that relief is taking place. What is bad is that the repetitive opening and closing of the valve’s plug and orifice can cause physical damage to the valve itself and ruin it’s ability to control a relief situation. That is bad.

What is normally done to alleviate the situation is what you are contemplating: install TWO PSVs. However, the capacity of the PSVs combined is the total relief load. In other words, the capacity of each PSV is approximately ½ of the total load. Now, the second PSV will only kick in and relieve when the capacity of the first is used up and it can’t handle anymore fluid, causing the system pressure to start to increase. If sized appropriately, this arrangement will eliminate any chattering and still give you 100% protection for your worse scenario. However, the trade-off is that the PSV cost will be greater and so will their maintenance.

[djack77494]

If this is truly a problem, an alternative approach is to utilize a regulating PSV. Such a PSV is a bit more costly and functions more like a self contained control valve. It would open (relieve) in proportion to the degree that the pressure exceeds the design pressure. Such a valve should not be subject to "chattering".

[Qalander (Chem)]

Dear

I understand that the pop pressure should never exceed the design pressure,thus the 1.16*Design pressure should not be the VALUE

rather this might be Design Pressure/1.16 or some other appropriate figure. Other learned colleagues will jump in to guide indeed.

[jerald04]

Distill,

Apologies but may I digress from the intended topic for a while please?

You made a statement "Since the operating and set pressure is very close , I am selecting the valve type as Pilot Operated".

Can you explain further on this?

[maldini]

Dear jerald04 as the ratio between operating pressure and set pressure = or higher than 90 % it is recommended to use pilot safety valve as per API

Dear Distill
can u give me more details for your case and as I understood that your is blockage two phase (25% vap, and 75%liquid ) if u use dier method for sizing psv I don’t know from where chattering will occur

[Qalander (Chem)]

Thanks distill,

May I humbally suggest that if the contents to be depressurised are not very small in quantity then the consideration for keeping pop pressure lower is better

as otherwise the vessel (or any other containment enclosure in question) will remain subjected to higher than design pressure and
thus may yield stresses to a higher degree; sort of making the conditions dangerous.

Hope this is somewhat helpful.

[JoeWong]

Distill,
All discussion still remain qualitative and conceptual level.

Can you please provide detail information about the relieving flow, properties, set pressure, etc for both cases ? We may go a little bit detail to evaluate.