PSV lift and pressure relationship
PSV lift and pressure relationship
(OP)
I API 520 part 1 fig 20 (7 ed.) shows a typical relationship for a standard spring operated PSV between lift and pressure (lifting and closing).
It says "typical". I have now had a client who says that his PSV vendors claims that the valve open 100% as soon as set pressure is reached.
We are talking liquid service.
Would anybode here like to comment on wether it is as displayed in API 520 or if normal spring operated valves (not pilot) exists that opens 100% as soon as the set pressure is reached?
Best regards
Morten
It says "typical". I have now had a client who says that his PSV vendors claims that the valve open 100% as soon as set pressure is reached.
We are talking liquid service.
Would anybode here like to comment on wether it is as displayed in API 520 or if normal spring operated valves (not pilot) exists that opens 100% as soon as the set pressure is reached?
Best regards
Morten





RE: PSV lift and pressure relationship
RE: PSV lift and pressure relationship
One other idea to pass along. The definition of set point on liquid service varies from manufacturer to manufacturer. It is generally not defined as the first onset of liquid flow but at some sustained flow. For example, AGCO-Crosby define liquid set point as an uninterupted flow stream the diameter of a pencil. Why do they all do this? - there is a simple explanation. Because the ASME code criteria for valve capacity certification is based on 10% overpressure (single valve, no fire), or 16% overpressure (multiple valves, no fire), or 21% (fire case). Since thwe valves have to be full open at the 10% overpressure to acheive consistent, guaranteed flow rates, delaying the "liquid set point" definition makes it easier to acheive.
RE: PSV lift and pressure relationship
You info re. the definition of set-point used by AGCO-Croby is also very interesting.
Best regards
Morten
RE: PSV lift and pressure relationship
On liquids (incompressibles) the set point is the start of continuous flow from the valve outlet. The lift of the valve on incompressible is proportional to the increase in set pressure. In general the valves will attain full lift at pressures equal to or less than the stated over pressure.
For liquid trim, some manufacturers have two types - coded valve which have over pressure at 10 percent and non-coded trims will have over pressure 25 percent.