PSV lift and pressure relationship 2

[MortenA]

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

 

[Sharik]

Safety valves, safety relief valves and relief valves are supposed to open at the set pressure. According to ASME Section VIII, Div. 1, Paragraph UG-134(d)(1) there is a set pressure tolerance of plus/minus 2 psi for set pressures up to and including 70 psi. Above 70 psi, the set pressure tolerance is plus/minus 3%.

 

[PMarinshaw]

I worked for several SRV manufacturers for well over 20 years and I would certainly advise you to take the manufacturers claim of "full open at set pressure" with a grain of salt. Direct Spring SRV manufacturers have spent hundreds of thousdands of research dollars throughout the ages to try to get valves to open faster (or rather open with less overpressure). It is precisely because the valves do not open fully at set pressure, that the overpressure rules were written into the codes (this applies to gas applications as well!). The AGCO Series 80 and the equivalent model from Flow Safe Series have gas valves that open fully at set pressure, but no such animal exists for liquids. Special liquid trims and valve configurations have been introduced in the last 20 years to get better performance in valve opening vs set pressure. The Crosby JLT trim is such an example. But what it comes down to is this: It really doesn't matter what the SRV manufacturers' claim. Sizing of the relief valve is based on an allowable overpressure - 10% for most codes such as ASME VIII, API 520 etc. Even if the valve were to open at less overpressure or even at set point, you can NOT take credit for it in the sizing, so you get no advantage of any such claim. What the SRV manufacturers have found out, is that the liquid trims do improve or shorten the blowndown (difference between opening and closing pressures). The only way to get a liquid pressure relief valve full open at set point is to use a modulating pilot valve. The valve will throttle open with the pressure remaining constant until full lift is reached.

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.

 

[MortenA]

PMarinshaw what you write is more or less what I though myself. I also agree with your thoughs upon the relevance of this since the capacity as you write is calculated based on 10% (or whatever) overpressure.

You info re. the definition of set-point used by AGCO-Croby is also very interesting.

Best regards

Morten

 

[bljnv]

PMarinshaw has given a very good account on relief valves.
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.