Sunday, April 12, 2009
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Many real life experiences have shown that if valves are not work or stroke for substantial period, they may failed to work (stuck at preset position) when it is on demand. This is particular critical to safety related valves i.e. Emergency shutdown (ESD) valve, blowdown (BDV) valves, pressure dump valves, etc and potentially lead catastrophe event. Valve reliability and availability is extremely critical and important in a safety system. Functional test of these valves during operation is required to main valves reliability and availability while maintaining zero / minimum production downtime. One of the testing method is on-line Partial Stroke Testing (PST), which is particular suitable for ESD valves, as well as other critical valves. PST—supplemental testing offers a method of testing the valve by moving it, typically 15-25%, and back to the original position in a short period of time in order to confirm the valve’s ability to move (not stuck in place) and its suitability for continued safety instrumented system (SIS) service.
A PST with dedicated testing interval improve a Safety Integrity Level (SIL) level of a particular SIS system. For example, a typical one-out-of-one (1oo1) ball valve with 1oo1 solenoid (Proof test, PT interval of 1 year, the average Probability of Failure on Demand (PFDavg) is about 2.25 x 10-2, this lead to SIL of 1.65. With a PST, it improve the PFDavg to 7.36 x 10-3, equivalent to SIL of 2.13 with typical PFDavg Reduction of 67%. Detail refer to "Valve Failure :Not an Option". This sometime is important and useful during plant revamping and modification. For example, present SIS system demand only SIL 1 . Plant revamping and modification results SIL 2 demand. Inclusion of PST would make SIS system sufficient.