Partial stroke testing topic of Emerson blog
Jim Cahill, author of the Emerson Process Experts blog, discusses many topics that affect the process industries. Frequent postings focus on safety instrumented system topics, which typically garner readers’ responses.
According to Emerson, partial stroke testing “checks for valve movement without fully stroking the valve. Many applications will allow 10% movements to verify valve response without upsetting the critical process line. Diagnostic data is collected and an alert is given if the valve is stuck.”
Partial stroke testing (PST) is the topic of a February 4, 2008 post Emerson’s Jim Cahill made in the Emerson Process Experts blog . The post, titled, “Positioners and Partial Stroke Tests in Safety Applications” comments on an InTech magazine Web excusive article, “Valve failure: Not an Option ,” in which the author describes methods of implementing PST to reduce the probability of failure upon demand, average (PFDavg).
According to Cahill, “the author enumerates four methods of performing the PST: by the emergency shutdown system (ESD); by a positioner-based device; by a 2-out-of-2 (2oo2) or 2-out-of-3 (2oo3) redundant device; and by a 2-out-of-4-doubled (2oo4D) redundant device.”
However, when the author described the use of positioner-based devices, Cahill consulted with Emerson’s Riyaz Ali, who offered some counterpoints. “Advanced positioners or digital valve controllers…have been designed specifically to operate safety shutdown valves and (have) gone through the rigorous design, testing and certification process defined in the IEC 61508 international safety standard for use up to SIL 3 applications. This design, testing and certification process was developed toensure the applicability of the technology for this process safety application.”
Cahill also wrote, “Riyaz notes that it is true that a very few applications do require shorter process safety times. He points out that it is not necessary to use a solenoid valve (SOV) to improve the stroking speed. Positioners can use pneumatic devices to achieve faster stroking time. For process manufacturers who still would like to use an SOV in the SIF loop, these SOVs have different capacities to meet the stroking speed requirements. Also, some of the more modern positioners like the DVC6000 SIS can also monitor the health of the SOV when it's used with a single-acting actuator. It performs checks for the dangerous failures of SOVs on-line without affecting the process.
“When [digital valve controllers are] used in standalone mode or in pneumatic series with SOV or other pneumatic accessories, it continuously checks the pneumatic integrity (functioning of I/P and pneumatic relay) to ensure that these components are working and ready to drive the valves upon a safety demand.”
Plant Engineering magazine managing editor Jack Smith posted this comment to Cahill’s blog: “…PST also does not account for the integrity of the valve seat, or lack thereof. Also, when testing during a shutdown or turnaround, many times shutoff valves are not tested under normal operating pressure. PST is all about statistics…”
Cahill again consulted Riyaz for a response to Smith’s comment. According to Cahill, Riyaz pointed out that “it is true that a partial stroke test, as its name indicates, does not check seat integrity for Normally Open valves (FAIL Close during Demand), which is the normal operating conditions for the majority of safety shutdown valves. However, from past incidents, the major concern is that the shut down valves that don’t move periodically are susceptible to sticking or remaining stuck upon a safety demand.
“…a partial stroke test helps to break the initial static friction of the final control element in an SIF loop. This helps ensure that the safety shutdown valve is not jammed and is available on a safety demand. So, in addition to the diagnostic benefits, partial stroke testing also provides some failure prevention benefits.
“Your point raises a valid concern that the PST does not check the seating part of the valve stroke. However, a key part of the process safety design stage is that the selected valve be correctly sized based on process fluid, process parameters, metallurgy, actuator shut off, etc. Generally, on-off valves are not used to throttle energy during normal operation. As a result, appreciable wear and tear on trim area is not expected. Therefore, it is expected that the valve will shut to the desired shutoff leakage classification per its design. The concerns in this case are:
• Whether the valve remains in one state for long time without movement
• Will it move or not due to process build up (until something is stuck between plug and seat at the closure end of valve)?
“This is where a partial stroke test is useful and has been proposed in the IEC 61511 part 2 section 11.4.5 as an‘additional diagnostic.’ A diagnostic coverage factor is applied in the SIF design calculations to account for partial diagnostics.”
Riyaz’s response also talks about seat leakage tightness testing, non-throttling safety shutdown valves, valve signature tests of digital valve controllers, algorithms inherent in microprocessor-based devices and the use of digital valve controllers for PST during normal operation to exercise movement and perform full stroke valve health tests during a turnaround or shutdown.
For Jim Cahill’s original “Positioners and Partial Stroke Tests in Safety Applications” post, click here .
For comments to this post, click here .
For the original InTech Web exclusive, click here .
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