VALVE MAGAZINE Spring 2025

VALVE AUTOMATION FOR EMERGENCY SHUTDOWNS

Example of a digital transmitter. All images courtesy of Westlock Controls.

Positioner Safety Instrumented System Installation

SIS Logic Solver DO

Supply pressure

24V SIS

24V SIS

Low power solenoid

LC340 HART line conditioner (one per valve)

PST does not test solenoid 24V SIS

SIS Valve and actuator

Position feedback via HART only when SIS is high

Digital Smart Positioner Controller

Valve Link

Position feedback through Valve Link only when SIS is high

DCS

CON: Positioners perform PST by bleeding air from the system via its own pneumatic relay. By performing in this manner, the solenoid valve remains untested by the positioner’s PST function. In order to test the solenoid valve, an additional test is required from the SIS system to “pulse” the signal to the SIS solenoid valve. With this method, there is no safeguard to prevent against over-travel and creates an increased chance for an unintended spurious trip.

CON: Positioner needs a line conditioner to communicate HART on the DO wires increasing overall cost. CON: In an ESD event, the positioner becomes unpowered and cannot provide confirmation of valve closure and requires the addition of limit switches for feedback.

CON: Positioner is an extra piece of equipment interjected into the pneumatic system, greatly increasing the chance of spurious trips.

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SPRING 2025

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