A safety instrumented function (SIF) within a safety instrumented system (SIS) consists of a sensor, logic solver, and final element. As part of the IEC 61511 global safety standard, these elements within the SIF must be tested during the operational phase of the safety lifecycle. Of the 3 elements, the final control element, typically a safety valve, has mechanical movement which must be tested.
In a Processing magazine article, How Digital Valve Positioners can Simplify Safety Life Cycle Phases, Emerson’s Riyaz Ali describes how digital valve positioners & associated diagnostics help extend the time intervals between safety valve full movement proof testing.
Riyaz opens the article describing challenges with safety valves:
Emergency valves are not continually opening, closing or throttling like a typical control valve — but instead normally remain in one static position for long periods of time, and are expected to operate reliably when an emergency occurs. Valves that remain in one position for long periods are subject to becoming stuck and may not operate when needed.
How often this testing is performed is a function of the hazard it is designed to mitigate and failure modes and failure frequencies of the elements within the SIF. Testing full movement requires either a shutdown of the surrounding process or bypassing the safety valve with the process running if these bypass paths exist. Riyaz explains the problem with bypassing safety valves:
Although bypassing the safety valve during testing is done to improve the probability of failure on demand (PFD), not all parts of this testing approach contribute to this goal because the time the system remains in bypass must be considered in the PFD calculation. For long bypass periods or frequent testing, the negative impact on PFD can be significant.
Partial proof testing, which means partially moving the safety valve to verify it is not stuck in the fully closed or fully open position, can extend the time required between full movement proof testing. He notes:
This partial-stroke technique can detect a large percentage of potential valve failures. Furthermore, performing this type of test on line without shutting down the process can improve the PFD without a loss of production.
Digital valve controllers, such as the Fisher FIELDVUE DVC6200 SIS, can perform these partial-stroke tests—manually as well as automatically from HART protocol-enabled systems. A digital valve controller:
…receives feedback about valve travel position, along with existing supply and actuator pneumatic pressures. This allows it to diagnose not only itself, but also the valve and actuator to which it is mounted.
These diagnostics can uncover issues associated with:
- Valve stuck
- Solenoid stuck
- Low air supply or pressure droop
- Dirty air supply
Read the article for Riyaz’s full list of conditions that digital valve controllers can find during partial-stroke testing and additional benefits that these devices provide in terms of predictive maintenance and regulatory reporting.