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Psv Inlet Pressure Drop


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#1 asantosa

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Posted 05 July 2018 - 04:32 AM

Dear sirs,

When a pressure drop higher than 3 % is reached at the inlet of a PSV, it migh cause chattering; Chattering is a cause of instability of the flow to be relief, and might cause also damages in the seat of the PSV.

But, appart from this, API 520 part II mention that chattering also migh cause lower capacity of the PSV.; Could you please explain why ? I mean, I understand chattering as a cause of inestability of the flow, but actually I do not understand how this directly recrease the PSV capacity...If I consider that the inlet pressure of the PSV remains constant , higher delta P at the inlet pipe would cause higher pressure in the protected vessel, but the inlet pressure of the PSV ( better said, relieving pressure would remain the same), so at the end, no matter the inestability, the  rated flow thourgh the orifice of the PSV woudl be the same or not...?

thanks



#2 fallah

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Posted 05 July 2018 - 06:25 AM

 

When a pressure drop higher than 3 % is reached at the inlet of a PSV, it migh cause chattering; Chattering is a cause of instability of the flow to be relief, and might cause also damages in the seat of the PSV.

But, appart from this, API 520 part II mention that chattering also migh cause lower capacity of the PSV.; Could you please explain why ? 

 

asantosa,

 

PSV relief load well enough lower then the PSV rated capacity (or on the other perspective oversized PSV), might lead to the PSV instability and subsequent chattering; because the low relief load might not be able to maintain the PSV in open position continuously hence the PSV may become closed and opened repeatedly which is nominated as PSV chattering...



#3 latexman

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Posted 05 July 2018 - 06:25 AM

Chattering is the rapid, repeated opening and closing of the PSV.  If the PSV is closed part of the time, of course the capacity is less than 100% open.



#4 asantosa

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Posted 05 July 2018 - 06:40 AM

Dear Fallah, Latexman,

Thanks for your kind responses, now I understand the reason why chattering causes a decrease in capacity.

Following with this situation: for a existing PSV, in case inlet delta P is higher than 3% (recommended by API ,or 5%) I have read that one option ( appart form installing a pilot operated valve ,or increasing inlet pipe size), is increasing blowdown-since increasing blowdown ( being blowdown the difference between PSV set pressure and closing pressure) would lead to higher flow to be released during closing time of the PSV-, so a earn in capacity could be reached in this situation.

Do you gree with that?. I mena, what is the reationship between blowdown and inlet pressure drop losses ? ( unless the  only relationship passes through understanding that higher than 3 % losses lead to chattering and capacity reduction, and increasing blowdown recovers that  PSV capacity)

 

Thanks !!






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