4 replies to this topic

[curious_cat]

I can reason out why excessive inlet piping pressure drop (say, >3% of set point) can make a SRV chatter: Under relieving conditions the valve sees a P lower than vessel and shuts then reopens and so on.  

 

But what's the explanation why an oversized SRV would chatter? If at all the inlet deltaP would be lower because of oversizing. 

 

I mean, I'm convinced by observation that they do chatter when grossly oversized but I'm blanking out what's the underlying explanation. 

Edited by curious_cat, 25 November 2013 - 01:14 PM.

[Lowflo]

_{All correctly sized PSVs are oversized to some degree. They'll cycle when the required flowrate is much lower than the rated flowrate. The more oversized, the higher frequency of that cycling. It's postulated that in extreme cases, that cycle frequency can mimic the frequency commonly referred to as chatter.}

[fallah]

Hi,

 

In an highly oversized PSV the actual relief area is much more than required area, then initial relieving pressure due to momentarily higher relief load cannot be sustained anymore in higher relief area condition even if overpressure source would be constantly sustained. It will lead to repeated PSV closing/opening cycles named chattering...    

Edited by fallah, 25 November 2013 - 02:23 PM.

[Bobby Strain]

CC,

     I think you knew the answer soon after you asked the question. You only need to know that a spring-loaded valve "pops" open to the fully open position and remains fully open until the blowdown pressure (lower than the set pressure) is achieved. At which point the valve closes. Then the pressure builds once again to open the valve and .......... If a modulating pilot operated valve is used, then this doesn't happen.

 

Bobby

[curious_cat]

I think I get it now. Thanks @Bobby & @fallah.

 

@fallah: Loved your very crisp & succinct "initial relieving pressure due to momentarily higher relief load cannot be sustained anymore in higher relief area condition even if overpressure source would be constantly sustained." 

 

@Bobby: Correct me if I'm wrong but it's like using a bang bang controller valve on the outlet of a liquid tank. If inlet & outlet pipe sizes are well matched the controller valve just stays open. But if outlet pipe's much larger it has no recourse but to open & shut quite often.  Modulation does solve this. 

Edited by curious_cat, 26 November 2013 - 02:19 AM.