The Good, the Bad and the Ugly

Abstract
Operational characteristics of static ESD valves impose design and testing requirements that are very different from those required for a control valve operating in a fully dynamic mode. Partial stroking of ESD valves can be a good complement to full stroke testing, as long as we have a clear understanding of the implications of the assumptions in diagnostic coverage and the credit taken for this type of test.This paper reviews the pros and cons of PST in the processing industries, from an independent and objective viewpoint, with absolutely no vested interest from either vendors or end-users.

On-Line Proof TestingThe objective of on-line proof testing is to detect covert undetected dangerous failures, which automatic diagnostics cannot pick-up. This is the only purpose. On-line proof testing does not detect random hardware failures related to spurious trips. You could proof test today and have a spurious trip tomorrow.Safety standards’ metrics for meeting a design SIL are solely concerned with the probability of a dangerous undetected failure appearing in the presence of a process demand on the SIS. Therefore, on-line proof testing is critical to the safety availability of an SIS. Increasing the proof test frequency is directly proportional to an improvement in lowering the average PFD. Consequently, increasing the SIL.To place λDU failure rates in to perspective, the following chart shows the typical distribution across the subsystems of an SIF in the COG process industry.

PARTIAL STROKE TESTING (PST) ESD valve partial stroke testing (PST) is a method whereby a portion of the valve assembly is tested at a more frequent interval than the full test rate. In simple words: an accelerated (partial) proof test. Perceived Advantages of PST: · May provide an improvement to the SIL of the SIF. · Provides predictive maintenance data. · May allow extension of the full stroke test (FST). · May overcome IEC 61511 architectural constraints (Questionable). · May reduce the need for valve bypasses. · Valve is always available to respond to a process demand during the test period (when properly designed).

Perceived Disadvantages of PST: · Tests only a portion of the valve DU failures (30% to 70%) · Not applicable to tight shut-off valves. · May increase spurious trip rate. · Incorporates additional equipment with its own testing requirements (Safe and dangerous failures). · Potentially converts the valve/PST smart equipment assembly toa type “B” complex subcomponent, per IEC 61508-2. · If PST always strokes 10%, buildup forms at 10% of stroke. · Makes the plant manager nervous! (Oscillating ESD valve). ESD on-line partial stroke testing should be considered as a complement to full stroke testing and not “in lieu of ”. The following table shows an overview of valve failures detected by PST and FST.

Valve Failure Modes
Mode Effect Test
Valve Body Leak Pressure Test at TA
Valve plug/seat Fail to close FST / Pressure Test
Stem packing seized Valve stuck PST
Airline to actuator crimped Sluggish response FST or PST w/speed of travel test
Airline blocked Fail to close PST or FST
Valve stem buildup Valve stuck PST or FST
Debris retained in seat Fail to close FST / Pressure Test
TA: Turn around FST: Full stroke test PST: Partial stroke test

PST Technology Partial stroke testing is not a new concept. It has been performed in different applications for many years. Traditionally, using mechanical devices, such as jammers, collars or engagement pins. What is new is the emerging technology that allows PST to be performed with limited human intervention in a semi-automatic way. Smart microprocessor based devices have b