A partial Stroke Test is done for On-Off valves which are used in critical services and do not operate for a longer period of time.
Every plant has equipment or instruments whose regular preventive maintenance is needed. Preventive Maintenance is done to ensure that the equipment or the instrument is running properly.
Why is a Partial Stroke Test (PST) needed?
Almost all plants have some or the other critical on-off valves. These on-off valves can be of Emergency Shut Down, or product supply or feed inlet to the plant or flare system. Now, these valves cannot be operated when the plant is running.
A Full Stroke Test cannot be done for emergency valves in a running plant. Operating these valves will disturb the process and will have an adverse effect. So how will we do Preventive Maintenance of these valves to confirm their healthiness? Whether these on-off valves will operate properly when required or not? Will these valves operate during an emergency scenario? The answer to all these questions is the Partial Stroke Test of the on-off valve.
The main ideology behind the Partial Stroke Test is to know whether the emergency on-off valves will operate when required during the time of emergency or not. A partial Stroke Test for critical on-off valves is also a requirement of the Safety Instrumented System.
Generally, the on-off valves used for the Emergency Shut Down, or product supply or feed inlet to the plant or flare system are not operated for a long period of time. Here long period can be a few months or even a few years in some cases.
We know that an on-off valve has many components like a solenoid valve, actuator, body, and other valve accessories like a trip valve, air volume booster, quick exhaust valve, and many more.
Therefore, over a long period of time, the solenoid valve’s coil might fail, minor or major passing from the actuator, the body might get jammed, or any other accessory might fail due to varying environmental conditions. Hence testing them periodically is much needed to ensure that these valves will work during times of emergency.
How to do a Partial Stroke Test (PST)?
A Partial Stroke Test (PST) is a test in which stroke to the Emergency Shutdown (ESD) valve is given partially. This method is also called a Partial Valve Stroke Test (PVST). In a PST, typically a valve is given a command to move 10 % to 20 % from the system like DCS.
The command from the DCS is given to the digital valve controller (DVC) or Intelligent Valve Positioners (IVP). The set point for the PST totally depends on how many operations of the valve is permissible as per process and as per vendor recommendations also. Analog command is used to operate the valve as per configurations.
The system captures all the diagnostic data as well as captures the command vs feedback of the valve to come upon a conclusion whether the valve will be operated as and when required or not. Although the PST cannot detect 100% of problems, PST detects almost 70% to 80% of problems in a valve.
The DVC or IVP which is used for PST diagnoses the potential issues and communicates test results. The DVC or IVP provides documentation of any emergency event which can serve as proof test documentation. The DVC or IVP also calculates the stroke time which is compared to the required stroke time of the valve.
Benefits of Partial Stroke Test (PST)
- Actual problems can be diagnosed without shutting down the plant
- Reduces the probability of failure on demand
- Helps to increase the system reliability
Standards for Partial Stroke Test (PST)?
Some standards to be followed for doing the Partial stroke testing (PST) of a valve. These standards are governed and regulated by the International Electro-technical Commission (IEC) and the Instrument Society of America (ISA). Below three are the standards that define the requirements for the safety-related systems and also how to quantify the performance of the PST systems.
- IEC61508 – Functional safety of electrical, electronic, and programmable electronic safety-related systems
- IEC61511 – Functional Safety: Safety Instrumented Systems (SIS) for all the process industry sector
- ANSI/ISA-84.00.01 – Functional Safety: Safety instrumented systems for all the process industry sector
Drawbacks of Partial Stroke Test
Despite all these advantages, PST has some drawbacks also. In some processes, the PST cannot be done. Because doing a PST will operate the vale by a few % like 10% to 20%. At 10% to 20% opening also, some processes have adverse effects. And thus, PST disturbs the plant in this condition. Also, the
PST is not able to capture all the faults and diagnostics. PST can only capture 70% to 80% of faults. For a full diagnosis of the valve, an FST needs to be done.
Thus, in the services where Emergency Shut Down, product supply or feed inlet to the plant or flare system valves are used, PST can be used. PST will help to diagnose the majority of problems without operating the valve fully. This way PST will increase the reliability of the system. PST will also decrease the probability of failure on demand.
