Flow through a failed control valve at RELIEVING CONDITIONS
Flow through a failed control valve at RELIEVING CONDITIONS
(OP)
I've just been looking at API 521 (2007) and discovered that Sections 5.10.3 and 5.10.4 (on page 28) specifically state that for estimating the relieving flow through a failed inlet and outlet control valves "All flows should be calculated at relieving conditions", i.e. the set-pressure plus over-pressure margin (e.g. 10%).
I'm used to taking the PAHH trip condition as the basis for upstream conditions (and based on other threads in this forum, so are others) and I'm concerned that I may have missed this significant 'change' to the standard.
Does anyone know of an acceptable mitigating basis to enable me to retain use of the trip pressure rather than relieving pressure?
I'm used to taking the PAHH trip condition as the basis for upstream conditions (and based on other threads in this forum, so are others) and I'm concerned that I may have missed this significant 'change' to the standard.
Does anyone know of an acceptable mitigating basis to enable me to retain use of the trip pressure rather than relieving pressure?





RE: Flow through a failed control valve at RELIEVING CONDITIONS
RE: Flow through a failed control valve at RELIEVING CONDITIONS
For example, say you have a vessel with a MAWP of 150 psig, and you're evaluating the failure of a control valve feeding that vessel. When calculating the flow across that failed control valve, use the relieving pressure (165 psig) on the downstream side of the control valve. That's what this sentence is saying. Of course, that's to your favor because it reduces the flowrate across the failed valve.
The next question is, what value should be used for pressure on the upstream side of the failed control valve. That's an important decision, and the answer will vary from case to case. In most cases, it's appropriate to use the normal pressure - the upper end of the normal pressure range. Otherwise, you're calculation is based on multiple independent failures - double jeopardy.
RE: Flow through a failed control valve at RELIEVING CONDITIONS
The issue will be the independence of any failure mechanism and if there ia common mode of failure (e.g. loss of instrument air) that causes the upstream vessel to relieve AND the control valve to fail open, then the scenario could reasonably be required to be caterd for in the downstream vessel. No commmon mode failure, then double jeapordary as Don1980 says IMO.
In short, no easy answer.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way