I received an email with a great question to an earlier post, Improving Local Control around Safety Shutdown Valves. The question was:
Can you provide more information on your Local Control panel with Safety Shutdown Valve that haves a quick exhaust systems; can the partial stoke test not close the valve do to differential pressure on the quick exhaust system. Also if a shutdown signal is given during a test will it close the valve?
I spoke to Riyaz Ali, who shared his expertise in the earlier post. Here’s his great answer in its entirety with picture and hyperlink added by me:
It is true that use of quick exhaust valve (QEV) on large valve with DVC6000 SIS for partial stroke test may dump large air during test. Generally, QEV operates on water column pressure differential and are sensitive. However, we recommend using volume booster on those applications where stroking speed is concern. One may argue that Volume Booster is for “Fill” time and not for “Exhaust”. However, we have done test in the lab and established that volume booster will be much better pneumatic accessories, specifically when used with DVC6000 SIS for partial stroke test without causing instability in the operation during partial stroke test and as well meeting stroking speed requirements.
The picture illustrates a schematic of a large spring-return piston actuator with a DVC6000 and a volume booster to achieve a stroking speed requirement of less than two seconds.
During PST test, if demand arises DVC6000 SIS will take valve to safe state.
Here is more information about the LCP100 (Local Control Panel) for your perusal.
I hope that pulling this process safety-related question out of the realm of email into the open might help someone else with similar questions.
Update: I’d like to thank commenter JM for pointing out my LCP100 hyperlink going to the wrong spot. I’ve fixed. Thanks, JM!