Valve World Americas: PST Workshop Raises More Questions than Answers

• Leo Minervini, Tyco Westlock, USA
• Angela Summers, SIS‐TECH Solutions, USA
• Ryohei Furuta, Kitz, Japan
• Kees Meliefste, Dow Chemical, the Netherlands
• Stan Hale, Score, USA
• Steve Farmer, Emerson Valve Automation, USA

Hinssen, who previously worked with Exxon Mobil, began by saying that “for quite some time now there has been a disconnect between the end‐users—struggling and resisting to implement PST—and suppliers/contractors, who are moving ahead at a rapid pace.” To help resolve this disconnect, Hinssen and the rest of the panelists selected six debate topics, with the ultimate goal of identifying, clarifying and maybe even reducing the PST gap.

First up, a discussion on the impact of IEC 61508 (+IEC 61511/ISA S84/TR95.05.01) on applying PST. Comments from the panel and the audience yielded more questions than answers, including:

• What do we do with the data?
• Why can’t there be a simple go/no-go approach as preferred by end users?
• The new/proposed standards make SIL ratings more complex and not simpler. Who requested these highly complex PST systems anyway? End Users? Suppliers? EPCs?
• What is the benefit of PST if it doesn’t prove out the full functionality of the product being tested (partial vs. full stroke testing)?

Next, panelists discussed what is lacking in the current international standards to address terminology, functionality and responsibility. The following points were made:

• The IEC standards are electrical/controls standards and do not address the final control element, which is the mechanical component (the valve).
• Perhaps some other organization should address a mechanical standard, but the participants were unclear which organization this might be.
• The IEC standard is dedicated to collecting data and loses sight of the objective of providing a safe mechanical system.

Still more questions were raised as the group discussed the criteria for evaluating PST results. Someone must set parameters, but whom? What about setting up an alarm for non-functionality? A number of participants suggested operators could overlay PST data on the original footprint data and look for deviations that can be related to system functionality and failure prediction, though they acknowledged this is “easier said than done.”

What diagnostic coverage factor can be achieved and what safety credits can be claimed when applying PST? Valve assembly failure scenarios, failure distribution, fail action and functional requirements must be taken into account when determining diagnostic coverage. But workshop participants still speculated about whether there is sufficient data in the market to use PST data to predict system operation in a given application and relate it back to system safety. Ultimately, the group did not come up with a good answer to this question. Also asked: Is it possible to set a robust alarm using PST without being too narrow (nuisance alarms) or too wide (inappropriate safety credits)? Again, the participants did not reach a conclusion.

Can PST provide a realistic reduction in dangerous failure probability? It may be possible to use industry failure rate databases, but will each company need to develop its own failure history database? Participants did acknowledge that improvements have been made in using PST to make predictions and through robust diagnostics. Still, acceptance criteria must be developed for PST and more vigorous diagnostics. Some suggested the industry should follow the nuclear industry model of knowing the critical valves, critical characteristics and critical evaluation criteria, and then using PST to measure the trends.

Participants speculated there is too much complexity in PST technology for most users and that could lead to dangerous data misinterpretation. Also important is the need for operating training and for automatic processing of complex data. Hinssen said “the best method to provide useful information from the PST system to the operator is to automatically process the complex diagnostic data through advanced software algorithms and provide a simple ‘good, fair or poor’ health message to the operator.”

This Partial Stroke Testing workshop panelists concluded that PST should have been recognized as a conditioned-based monitoring system instead of a safety process system and, while still somewhat confusing, has a place in plant safety-related system functionality.

To help continue the debate, Hinssen has established a Partial Stroke Testing blog at: http://www.valveworld-partialstroking.blogspot.com.

Gary J. Grauerholz is president of LTP International LLC. He is also a contributing editor to Valve Magazine. Reach him at garyg@leantoperfection.com.