Blowdown Calculations of PSV

[Usman Amin]

Can somebody tell, how to calculate blowdown of conventional PSV?

[Black Onyx]

Dear Usman,

Blowdown is "the difference between set pressure and re-seating pressure of a PSV expressed as a percentage of set pressure (or sometimes in pressure units)". Ideally a PSV should always pop-up at Set Point to release pressure and similarly it should always reseat (close) at Set Point.
However, a PSV is a mechanical device operated by spring, therefore, depending of the stresses and characteristic of spring, it always re-seats at pressure slightly lower than actual set point.

Blow down is normally supplied by the PSV vendors, it can be tested in workshop on a bench test by actually popping the PSV and then reclosing it to determine the pressure at which it reseats. API-520 Part -1 refer to ASME Section VIII Div-1 for guidance on blow downs. A figure has been attached for your reference

Hope i have answered your query, for any further clarification please provide explicit problem. If you can give me you mailing address, i have an excellent presentation on PSV basics covering almost all the definitions and design aspects.

regards,

[Usman Amin]

Thanks Dear,

You are good in your terms. My email ID is

engrusmanamin@gmail.com

[ibrahim]

usman, if you still stuck with some questions and calculation, you can also use following link to ask some technical guys to make if easy for you to select the size.

http://www.thepetrostreet.com/thepetrostreet_003.htm