Predicting chattering
Predicting chattering
(OP)
We are analyzing a system at a plant for a postulated accident and are predicting transient conditions. This is a water system and in the transient two phase conditions are predicted. Predicted seal leakage is an input to the system and the transients show peaks of high leakage flow into the system, but the leakage drops off after a minute or so and we are concerned both with predicting peak pressures as the peak flow likely overwhelms the valve capacity if mostly steam; and also the potential to start chattering. Can someone point me to references which address quanitifying conditions that lead to chatter? We will want to either show that chatter is not expected, or if we can't avoid it with procedures, then attempt to evaluated it. Understand evaluating chatter is not expected to be accurate... but there it is.
Any pointers or references would be appreciated.
VGARZANI
Any pointers or references would be appreciated.
VGARZANI





RE: Predicting chattering
Have you tried talking to the PSV vendors to see if they have equations/tools to predict chattering/simmering?
Tyco, Farris are two that may have the information, or know where to get it.
"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
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RE: Predicting chattering
If the inlet frictional losses exceed the blowdown setting.
In the above case, once the PSV opens, the frictional losses will cause the PSV to close because the pressure at the PSV inlet will have dropped below the closing pressure (determined by the blowdown setting). The pressure will quickly rise again, and the PSV will open again, only to close once flow has started.
This is my understanding of chattering. It is fairly simple. I did see one reference to a paper that talked about chattering being a resonance frequency effect, but i dont have the reference handy.
Perhaps others can comment on my idea of chattering.
Regards,
Jonathan
RE: Predicting chattering
(2) Inlet pressure drop too high. 3% of set pressure in hydraulic losses for a conventional or bellows valves. Inlet pressure drop does not affect remote sensed pilot valves.
(3) Valve capacity is too large. The rule of thumb is required flow / rated capacity should be greater than 25% for vapor and 10% for liquid.
Try API 520.
RE: Predicting chattering
1. Tolerate high back pressure (well over 50%)
2. Can be set up with remote sense to overcome inlet losses.
3. Modulate to relieve only the capacity required, this means that oversized modulating pilot operated valves are not an issue.
Hope that this helps.
RE: Predicting chattering
RE: Predicting chattering