PSV functional diagrams 2

[SmilerFDL]

Hi all,

I'm comparing different sources on various subjects of safety relief valves and found different diagrams showing a "typical" valve opening/closing characteristic.

Of course there is API520 (8th ed.) which provides figure 6 (page 14), the Consolidated Catalogue for Safety Relief Valves (dated Oct. 2008), the LESER handbook (

http://frame.leser.com/engineering/file/EHB_en_engineer_handbook.zip)

and SpiraxSarco (

http://www.spiraxsarco.com/resource...ety-valves/introduction-to-safety-valves.asp)

describe PSV operation. Of course there are more sources.
The curve from API, is similar to the diagram in the Consolidated catalogue, where the PRV lifts to 50%-60% right at set pressure and then continously lifts with further pressure increase. Closing is proportional to the pressure decrease, API starts closing at ~105% of set pressure, Consolidated at 98% of set. Valve reseats at the required blowdown of 93% set pressure.
In contradiction the diagrams of LESER and SpiraxSarco show proportional opening to ~104% set pressure, then (nearly) full opening at ~105% set pressure. Closing starts at 99% of set pressure and is rather parabolic then proportional, also reseating within the blowdown limit.
I assume the latter show curves for pop-action ("safety") valves, while API and Consolidated show "relief" valves? Anyhow I wonder, why API and Consolidated instantly open at virtually set pressure, while others need huddling chamber etc. to overcome spring force.

Which leads me to another question bothering me: What range is the additional force required to compress the spring to 100% disc lift? I expect the force is linear with the lift and should not exceed 105% of CDTP? Or even less?

CU, Smiler

 

[don1980]

All of these curves are intended to show the generalized lift characteristice of pop valves. There's wide variation for any partuicular valve, but they follow this general trend. In words, these diagrams are all saying the same thing:

[ul]Pop valves don't immediately open to full capacity (full lift). They pop open but not fully open. Overpressure is required in order for a pop valve to reach full lift.[/ul]
[ul]Pop valves close along a different path, different from the opening path. The pressure must decrease below the set pressure in order for the valve to close. The amount of pressure decrease below the set pressure (blowdown) will vary, depending on the fluid state, the type of valve trim, and the mechanical characteristics of the specific valve.[/ul]

That is all that these diagrams are trying to illustrate, and that's all that one can take away from them. Note that these drawings don't increment the X and Y axes with specific valves.

Regarding your question about CDTP in this post and in the previous one, understand that CDTP has no relevance at all for the relief designer. CDTP is only relevant to the maintenance shop, when testing a PSV on a test stand.

 

[MortenA]

DOn, started out the same way as you, but then i took a closer look at the Leser book, and it actually shows the opposit (and claims its typical for a spring loaded valve: It opens proportinally from approx 100% SP to aprox 104% and then jumps to 100% open (and the Leser diagram is actually quite specific wrt axis.

 

[don1980]

Morten - I see what you're talking about. Leser drew their diagram differently. Regardless, one shouldn't assume that their API 526 pop valves don't require overpressure in order to reach the full-open position. The two descriptive points I listed above apply to Leser valves too. I can't say why they chose to represent the lift curve the way they did.