6 replies to this topic

[kasri]

Dear all,
(1)In ApI 520,it is stated as " the difference between the set pressure and the closing pressure is blowdown".
For example: If PSV set pressure is 10 barg, then its relieving pressure( fire case)=12.1 barg.
In this what they mean blowdown, difference between ( set pressure) 10 barg and ( relieving Pressure) 12.1 barg they termed as blowdown.Please clarify.
(2)Normally the valve get start to lift when the set pressure achieves then whats does the relieving pressure role.Is it they adjust the spring for set pressure means what about the relieving pressure is it the pressure developed to fully lift the disc?Please clarify.
regards,kasri

[fallah]

kasri,

1- Don't confuse relieving pressure with closing pressure. In your case closing pressure should be lower than 10 barg, e.g., 9.5 barg. Then the blowdown would be 0.5 barg or 5% (with respect to the set pressure).

2- The relieving pressure (set pressure plus overpressure) is the point that the PSV is going to be in full lift position.

Fallah

Edited by fallah, 08 May 2012 - 04:36 AM.

[jrtailor09]

Fallah ,

How can we find the closing pressure of PSV?

Regads,

Jatin

[fallah]

How can we find the closing pressure of PSV?

jatin,

By subtracting blowdown from set pressure. The blowdown value to be specified by PSV vendor and could be adjusted within a specified range by blowdown ring.

Fallah

[Shivshankar]

Fallah,

Can you calrify following definition for blowdown ?

Blowdown (reseating pressure difference) -
difference between actual popping pressure and actual reseating pressure, usually expressed as a percentage of set pressure or in pressure units.

With below 2 different links there are different terms used for relief valves, they used different terms and different definitions.

Glossary for releif valve terms

http://www.controlan...ary/relief.html

http://www.wermac.or...ure_relief.html

Regards
Shivshankar

[fallah]

Shivshankar,

They are not so different definitions, just using different terms with the same concept...

Actually, the main point is that a PSV is normally reseat at a pressure (reseating pressure) a little bit lower than the set pressure (popping pressure) and the difference between these two pressure values is called "blowdown". It could be expressed as an absolute value (set pressure - reseating pressure) or a percentage of set pressure [(set pressure - reseating pressure)*100/set pressure].

Fallah

[Shivshankar]

Fallah.

Thanks to clear my confusion.

Regards
Shivshankar