Partial stroke testing (PST) is an important tool for oil and gas operators [and other in-process industries and applications], providing a method of proving the safety and performance of emergency shutdown valves (ESVs). For many years the technique has helped prove the functionality of final element assemblies by testing a percentage of the failure modes of the final element assembly. The two most well used tools for PST do exhibit drawbacks.

PST was originally made possible through the use of simple mechanical systems that could only offer testing. As the need for diagnostic capabilities grew, smart systems evolved to advance PST capabilities. The two main types of systems, positioners and electronic, offer different approaches, and each makes compromises to achieve the original goal of demonstrating safe ESV performance.

Positioners are currently the most common form of smart PST systems available. They can be a complex solution as although the device can easily be configured to give any desired percentage of partial stroke, other components may need to be added to maintain the desired stroking speed. Derived from control valve position technology, PST positioners were the obvious choice for an immediate replacement for mechanical testing, with a built-in capability to reliably move a valve to a given position.

Improved with the addition of an emergency shutdown (ESD) function, positioners became a fundamental part of the control system for an actuator. PST positioners can create an effective redundant ESD when used in conjunction with a solenoid valve. 

Compromises

Positioners do not test the valve at the full designated operating speed, meaning a question mark exists over the results they produce. Their use requires a reconfiguration of the pneumatic control, they can only operate across a limited flow rate, and they are not compatible with all valve types. They also have high safe failure rates, which can result in spurious trips of the safety function. Difficult to commission, install, and service, they also reduce diagnostic coverage, one of the main misgivings of the mechanical systems they replaced.

Many manufacturers and oil and gas operators turned to electronic systems to alleviate shortcomings. Electronic systems can perform a partial stroke adequately for an emergency situation, deliver the required level of diagnostics, and address issues associated with positioners. Most systems use electronic control boxes, which connect the power supply to the solenoid valve and are fitted in close proximity to the actuator. They de-energize the solenoid valve to perform the partial stroke test, monitoring either the valve position or instrument pressure or both to determine the success of the test.

Despite solving some issues associated with positioners, electronic systems also have limitations. For example, additional equipment is usually required (such as an electronic control box), making the system more costly and harder to service. The safety integrity level (SIL) performance also can be compromised through solenoid valve selection while electronic systems can create a spurious trip risk.

Despite benefits of positioners and electronic systems, neither system could offer a holistic, fully reliable solution given the inherent safety risks of oil and gas operations.

Hybrid PST

In developing a hybrid technology, solenoid valves served as a starting point. Electronic systems provided simpler operation as no extra pneumatic components were required, and positioners offered the advantage of the diagnostic system being built into one of the control elements. The logical conclusion was to integrate the diagnostic system into one of the final element components and install the diagnostic system into a high integrity solenoid valve. This led to the creation of the first fully integrated PST solution with low safe and dangerous failure rates. The integrated unit combines a high-integrity solenoid valve with an electronic control system to provide high-level diagnostics, simple pass/fail reporting, and reduced engineering requirements. It eliminates the possibility of over-stroke.

Richard Harvey is business development manager at IMI Precision Engineering. Edited by Mark T. Hoske, content manager, Control Engineering, CFE Media, mhoske@cfemedia.com.

MORE ADVICE

Key concepts

• Partial stroke testing provides a method of proving the safety and performance of emergency shutdown valves.
• A hybrid approach resolves some drawback with two existing technologies.

Consider this

When looking at replacement PST technology, do costs beyond capital cost factor into the analysis?

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www.controlengeurope.com 

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