In Safety Instrumented System (SIS) Instrument, installing a Partial stroke testing device (PSD) in the SIS final element viz. in Automated on/off valve helps to improve the Proof test requirements.
The PSD is an intelligent electronic field instrument, configured to initiate, control, and monitor in an automatic way the partial movement of the on/off valve, which moves from the normal operating point towards the safe position. The valve travels partially in a safe manner not to disrupt the running system.
Stores the gathered data and communicates in a digital way with remote data management systems.
Design considerations to be taken into account with Partial Stroke Testing Device (PSD).
Below are a few essential points to be considered while designing a valve with a Partial Stroke testing device.
Where,
λd = dangerous failure rate
TIa = Partial stroke test interval
TIm = Test interval manual testing
DC = Diagnostic Coverage Factor
Below is narrated an example by a specific manufacturer for understanding the partial stroke testing device PSD installation concepts, it may vary with different manufacturers.
Wiring and installation scheme for installation of Partial stroking device is typical with many installations as detailed below.
Thus installing a Partial stroke testing device (PSD) in the SIS final element, helps to improve the Proof test requirements. It is recommended to go for PSD wherever the SIS valves can’t be tested under scheduled proof test intervals by taking careful consideration of the process needs.
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