These are systems that are used to provide safety, such as Safety Instrumented System (SIS). The objective of SIS is to reduce the damage, SIS acts only when the error or damage has happened. The SIS does not increase the production output and there is no direct return on investment.
Partial Valve Stroke Testing (PVST):
Considering the fact that SIS act after the system got damaged, PVST is introduced. The only way one can be sure a valve is not stuck is to actively test the valve and see if there is any movement from the open position of the valve. This test can be performed as part of the periodical proof test of a Safety Instrumented System. However, proof testing generally only takes place once a year, which means that the valve will be tested only once in a twelve month period.
In order to ensure that the shutoff valves are not stuck and to avoid the frequent periodic proof tests, partial valve stroke testing can be of major assistance. The concept of partial valve stroke testing is relatively straightforward.
As its name already indicates, partial valve stroke testing involves the partial stroking of a valve to check for valve movement without fully stroking, or completely closing, the valve. This verifies that the valve is not stuck and still operational without shutting down the complete process.
By implementing partial valve stroke testing it is possible to detect stuck failures of valves. Therefore a certain diagnostic coverage of these dangerous stuck failures can be accounted for in reliability analyses. The reliability analyses will show that the SIS achieves an increased level of risk reduction.
Partial Valve Stroke Testing Options:
Instrumentation Based Solution:
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Complicated system with multiple potential failure causes. Safety loop usually disabled during PVST
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No real diagnostics information from the PVST
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Testing requires manual work - high testing costs.
Redundant Solenoids:
- PST not available or it requires difficult, manual configuration
- No diagnostics
- The solenoids can not be tested automatically
- Full stroke test needs to be done manually
Positioner - Solenoid:
- Higher nuisance trip rate
- Solenoid can not be tested automatically
- Manual testing and interpretation of test results
Control System Based Solution:
- Use DCS to initiate and Monitor PVST
- Solenoid and Limit Switch configuration
- Diagnostics Limited to timing of PVST
- Limited Safety Features
Intelligent PST Solution:
- Safety loop enabled during PVST
- Diagnostics information from PVST
- Possibility to optimize ESD valve maintenance
- Automated PVST - no testing costs
- Seamless Communication with DTM or EDDL
- Human interaction not required for PVST
- more effective use of maintenance resources
- Emergency trip captured
- Self-documentation