Instrumentation Questions Part One

Introduction

Valves are critical components in various industries, including power plants, chemical plants, and oil and gas operations. Ensuring the reliability and safety of valves is paramount to prevent costly breakdowns, environmental hazards, and injuries.

The Partial Stroke Test (PST) is a non-intrusive method employed to assess the functionality of valves, providing valuable insights into their operational readiness.

What is a PST Test?

The principle of the partial stroke test is based on the fact that valves that have been in a static position for extended periods may become stiff and unresponsive. This can lead to serious safety hazards and downtime in crucial processes.

A PST test involves moving the valve stem or actuator a predetermined distance, typically between 10% and 25%, and then returning it to its original position. 

The purpose is to verify if the valve can move smoothly and reliably within a limited range without fully opening or closing.

PST Test Method

The partial stroke test method involves the following steps:

1. Preparation: Disconnect any electrical or pneumatic connections to the valve. Ensure the valve is isolated from the process and depressurized.
2. PST Execution: Manually or using a motorized actuator, move the valve stem or actuator the specified distance. Record the time taken for the movement and the position of the valve stem. Return the valve to its original position and repeat the process several times.
3. Data Collection: Observe and record any irregularities in valve movement, such as sticking, hesitation, or abnormal noises. Perform additional tests, such as an ultrasonic test, if necessary.

PST Test Type

A PST test is a non-destructive test (NDT) that does not cause any permanent damage to the valve. It falls under the category of dynamic testing, which involves evaluating the valve's performance under actual operating conditions.

Importance of PST Tests

PST tests provide valuable information about the valve's condition, including:

• Stroke Time: PST test has the ability to verify the valve's stroke time. The stroke time is the time taken for the valve to move from a fully open to a fully closed position. It is a critical factor in ensuring the proper functioning of emergency and safety valves.
• Valve Reliability: PST tests ensure that the valve can move smoothly and promptly, minimizing the risk of valve failure under critical operating conditions.
• Predictive Maintenance: Regular PST tests help detect potential problems early, allowing for proactive maintenance and preventing costly repairs or downtime.
• Cost-effective Solution: The PST offers a cost-effective solution for testing valves. Unlike traditional full stroke tests, where the entire process has to be shut down for testing, the PST only requires partial valve movement. This results in significant cost savings in terms of material and labor, making it a preferred choice for valve testing in industrial plants.
• Safety Enhancement: PST tests help ensure that valves can respond appropriately to emergency situations, reducing the risk of accidents or environmental hazards.

Limitations of PST Tests

While PST tests are valuable, they have certain limitations:

• Limited Stroke: PST tests only evaluate a portion of the valve's stroke, and may not detect issues related to fully open or closed positions.
• Influence of Lubrication: Valve lubrication can affect the results of a PST test, so it is crucial to consider the lubrication condition during testing.
• False Positives: PST tests may indicate issues that are not related to valve performance, such as actuator or stem problems.

PST Applications

One of the main applications of the partial stroke test is for emergency shutdown (ESD) and safety valves. These valves play a crucial role in controlling the flow of hazardous substances and preventing catastrophic accidents. The PST ensures that these valves are fully operational and capable of closing or opening when necessary.

The PST is also used for testing control valves in critical processes. In control systems, these valves regulate the flow of material to achieve desired process parameters such as pressure, flow rate, and temperature.

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