thePetroStreet

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

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,

Thanks Dear,

You are good in your terms. My email ID is

engrusmanamin@gmail.com

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