Emergency Shutdown Valves (ESDVs) are designed to operate only under critical conditions. In many systems, they remain static for months — or even years — before being required to function.

This creates a hidden risk: a valve that appears operational may fail when it is actually needed.

Partial Stroke Testing (PST) was developed to address this exact problem — verifying valve functionality without interrupting the process.

Why PST is critical in safety systems

In safety-critical applications such as oil & gas, LNG, petrochemical plants, and power generation, ESD valves are part of Safety Instrumented Systems (SIS).

Their reliability directly affects system safety and compliance with standards such as IEC 61508 and IEC 61511.

Without periodic verification, failure modes such as:

• Valve sticking
• Actuator degradation
• Seal wear or friction increase

may remain undetected until a shutdown event occurs.

What Partial Stroke Testing actually does

Partial Stroke Testing verifies that the valve and actuator can move — without performing a full closure that would interrupt the process.

During a PST cycle:

• The valve moves partially (typically 10–20% of its stroke)
• Movement, torque, and response are monitored
• The valve returns to its original position

This allows verification of mechanical and actuator performance while the system remains online.

How PST improves system reliability

The main benefit of PST is reducing the Probability of Failure on Demand (PFD), which is a key factor in Safety Integrity Level (SIL) calculations.

By identifying hidden failures early, PST helps ensure that valves will operate correctly when required.

From an operational perspective, PST also:

• Reduces the need for full shutdown testing
• Minimizes production interruptions
• Supports predictive maintenance strategies

Technologies used for PST

Several technologies are used to implement Partial Stroke Testing:

• Smart valve positioners with built-in PST functionality
• Dedicated PST controllers for ESD and blowdown valves (BDVs)
• Integration with Safety Instrumented Systems (SIS)

These systems allow controlled testing and data collection for performance analysis.

Common challenges in PST implementation

While PST provides significant benefits, improper implementation can reduce its effectiveness.

• Incorrect calibration can lead to false results
• Excessive stroke movement may impact process stability
• Lack of data analysis limits predictive maintenance value

PST should be implemented as part of a broader testing and maintenance strategy.

Best practice for reliable testing

Partial Stroke Testing should not replace full functional testing, but complement it.

A balanced approach includes:

• Regular PST during operation
• Full stroke testing during planned shutdowns
• Continuous monitoring of actuator and valve performance

This combination provides both operational flexibility and safety assurance.

What this means for your system

If your ESD valves remain inactive for long periods, there is a risk that hidden mechanical or actuator issues will go undetected.

Implementing PST allows early identification of these issues, improving both safety and operational reliability.

Key takeaway

Partial Stroke Testing is not just a maintenance tool — it is a critical part of ensuring functional safety in valve automation systems.

By enabling verification without shutdown, PST provides a practical balance between safety requirements and operational continuity.

Next step

If you are evaluating PST implementation or experiencing uncertainty in valve performance, start by reviewing how testing is currently performed and whether partial stroke testing can improve system reliability.