The Science Behind Partial Stroke Testing 
 

Critical valves in a process plant are vital components that control the flow of fluids, gases, or other materials within the system. Their failure to operate correctly can lead to severe consequences, including process disruptions and safety hazards. Ensuring the proper functioning of these valves is essential for maintaining operational efficiency, safety, and overall integrity of the process plant. Partial stroke testing (PST) is an essential tool that operators can leverage to reduce the risk of critical valve failure.

Partial Stroke Testing is a crucial aspect of ensuring the reliability and safety of valve monitoring and control systems. At its core, PST involves the controlled movement of a valve to a predetermined position within its operational range, typically limited to a fraction of its full stroke. This controlled movement allows for testing of critical components such as the valve, actuator, and solenoid valve while keeping it in a safe operational state.

The key benefits of PST include the ability to assess the functionality of the entire valve system without interrupting the normal operation of the valve. By performing partial strokes at regular intervals, operators can verify the accuracy and reliability of the valve’s output signals while the valve remains in service. This proactive approach to testing helps identify potential issues or deviations in valve positioning before they escalate into larger problems, ultimately enhancing the overall safety and performance of the system.

Safety valves for emergency shutdown must be periodically tested to maintain their functionality and SIL rating. PST is a cost-effective method where the valve is partially closed without interrupting the process.

While PST doesn’t replace mandatory full stroke tests, which require a plant shutdown, it can extend the time between Full Stroke Test (FST), leading to increased production and cost savings. Intelligent PST helps differentiate between “safe failures” for scheduled maintenance and “dangerous failures” needing immediate attention.

Adding a PST capable transmitter-based valve controller to the actuator offers an optimal balance of cost and functionality with minimal moving parts, analog input, digital output, and reliable operation even without power. Offering a distinct advantage over the conventional methods used via a positioner, as positioners are optimal for continuous usage. In the case of critical valves, many stay dormant for long periods of time.

 

Digital EPIC D500 Position Transmitter 

Westlock Controls’ Digital EPIC D500 is a second-generation transmitter-based intelligent device that offers advanced diagnostic functionality. Among its state-of-the-art diagnostic features is Partial Stroke Testing (PST). During PST, the valve moves to a preset travel limit within a configured time and then returns to its original position. Additionally, the D500 captures and stores PST signatures, leveraging the valve signature produced at commissioning as a baseline for future comparisons. Any deviation from this baseline by a preset hysteresis limit triggers an alarm, indicating and identifying potential issues.

 

In valve monitoring and control applications, the accuracy and reliability of valve operation is paramount. Partial Stroke Testing emerges as a vital tool for ensuring operational integrity and regulatory compliance. By incorporating PST into maintenance routines, operators can mitigate risks, optimize system performance, and uphold the highest standards of safety and reliability in industrial environments.

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