Two-Solenoid Safety Instrumented Functions

 The practice of process safety has its own language, including concepts such as PFDavg, 1oo2 (one out of two), 2oo2, 2oo3, etc. A whitepaper, Doubling Up On Safety (signup required for download), explores a two-solenoid safety methodology that lets you exercise your solenoid valve online—protecting your plant, improving uptime, and enabling you to better adhere to functional safety requirements.

The solenoid valves are associated with the emergency shutdown (ESD) block valves in a safety instrumented function (SIF).

In this typical Emergency Shutdown solution, the shutdown valve stops the flow of hazardous fluids upon detecting dangerous conditions. The solenoid valve responds to the ESD controller to vent the actuator to a fail state.

These valves are rarely exercised except during actual shutdown events or periodic testing.

But despite its critical role in ensuring the safety of the facility, equipment and nearby personnel, the solenoid valve rarely gets used. In fact, it can remain in the same position for months or even years— increasing the chances that it will fail to close on command. The biggest perpetrator of this failure is stiction.

Stiction is the sum of all static friction causing the valve to stick in place, usually due to inactivity. If:

…this valve experiences stiction-related failure, the Emergency Shutdown valve won’t be able to stop the flow of toxic, flammable or even explosive fluids—leading to possible catastrophe. Though critical for valve maintenance, the problem with exercising the solenoid valve is that it may introduce some downtime. Fortunately, there’s a simple and easy solution.

Proof testing a solenoid valve:

…serves two mechanical purposes: it prevents stiction and, at the same time, proves it isn’t there to begin with. Bringing the valve through a single cycle is all you need to do: de-energize the solenoid coil so the valve fully closes, and then re-energize the coil again to return the valve to the open position.

With a single solenoid for your safety valve, this causes a shutdown. By adding a second solenoid:

…to the design of your ESD valve, you can test each solenoid individually. You can even program your controller to run periodic tests automatically, preventing valve stiction, lowering the average PFD and avoiding plant accidents—all without ever taking your system offline.

Read the whitepaper on installation practices or using pre-packaged dual-solenoid solutions such as the ASCO Series Redundant Control System (RCS). It is a pilot valve system with no single point of failure that can result in unwanted closure of the process valve. It is suitable for up to SIL 3 applications and greatly reduces spurious trip rates.

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