In an earlier post I discussed the critical role the final control element plays in a safety loop or safety instrumented function (SIF) in safety parlance. This equipment mostly stays in one position until called upon to move should an emergency situation arise. Digital valve controllers like the Fieldvue DVC6000 SIS provide partial stroking of the valve to process manufacturers design their safety instrumented functions to reduce the Probability of Failure on Demand (PFD).
Even with the advancement of intelligence in digital valve controllers to do this partial stroke testing, a problem remained in testing the solenoid valves used in the safety instrumented function. These solenoid valves are installed to quickly bleed the air supply to the valve actuator that is holding the SIS valve open or closed. The only real way to test this solenoid valve has been to trip it causing the safety function to occur. These spurious trips can be quite strenuous on the plant piping and process equipment.
Riyaz Ali, a development manager in Emerson’s Fisher division showed me the latest advancements to the DVC6000 SIS to test the solenoid without causing safety valve movement. What the technology team found through extensive research and development is that the solenoid valve can be pulsed for a split second by smart SIS logic solvers like the DeltaV SIS system.
This time window of the pulse is long enough for the solenoid valve to vent which provides verification that it is functional. But the time window is short enough so that the actuator does not bleed off enough pressure to make the SIS valve move. Diagnostics in the DVC6000 SIS can sense and capture the data for the momentary pressure blip across the solenoid valve during the test. It also records pressures, travel information, and other diagnostic information.
Beyond solenoid testing, Riyaz mentioned the DVC6000 SIS is capable of collecting data during a trip event, much like an airline’s “black box” flight recorder. This data collection can be triggered upon a change in actuator pressure, valve travel, input current, pressure differential, travel deviation, travel cutoff, or an externally defined trigger event. This data can be helpful when reviewing the causes of a safety trip as well as having the data available for regulatory reporting.
One final point Riyaz emphasized is the DVC6000 SIS spurious trip protection which provides maximum output pressure to the solenoid at minimum input signal in a case where the 4-20mA signal between the smart logic solver and digital valve controller is lost or severed.
Together, these technologies give process manufacturers an end-to-end way of checking the safety instrumented functions including the solenoid valves, to assist their design, implementation, and ongoing testing phases of the IEC 61511 safety lifecycle.