Valves used in safety applications tend to stay in opened or closed positions for very long, time intervals. As Emerson’s Joe McHugh explained in a Plant Engineering article, Double up on solenoid safety:
…all valves experience stiction if they remain inactive for too long. (Note: Stiction is the friction that tends to prevent stationary surfaces from being set in motion.)
These valves must be moved periodically, especially a solenoid valve, since:
…if this valve experiences stiction-related failure, the emergency shutdown valve won’t be able to stop the flow of a flammable or even explosive fluid — leading to possible catastrophe.
In a safety instrumented function, solenoid valves are part of the final element which are designed to move the process to a safe state.
In addition to the ESD [emergency shutdown] valve, they can include a pneumatic, electric or hydraulic actuator and solenoid valve.
To address the stiction concerns, the solenoid valve must be regularly tested.
Periodically testing solenoid valves serves two mechanical purposes: It prevents stiction, and it proves it isn’t there to begin with. Bringing the valve through a single cycle is all that’s necessary. De-energize the solenoid coil so the valve fully closes, and then reenergize the coil to return the valve to the open position.
The problem with testing by this process is that it shuts down the process. But, by:
…adding a second solenoid valve to the design of the ESD valve, each solenoid can be tested individually.
Emerson’s ASCO Redundant Control System (RCS):
…is a proven pilot valve arrangement that has no single point of failure and provides built-in redundancy and diagnostics to optimize plant safety and reliability while maximizing uptime…
Joe describes some key advantages of this redundancy which enables automated online testing and online maintenance, avoids nuisance trips, made with 316L stainless steel for corrosive environments, and provides high safety reliability for applications up to SIL 3.
Read the article for more on these advantages as well as safety instrumented system architectural considerations. Visit the Redundant Control Systems page on Emerson’s ASCO website for more and connect & interact with other solenoid valve experts in the Fluid Control & Pneumatics group in the Emerson Exchange 365 community.