In Safety Instrumented System (SIS), Solenoid valves (SOV) which are used to Open / Close the Automated On / Off valves and Emergency Block valves play a critical role. These solenoid valves need to be designed and operated with care.
Solenoid valves are the most frequently used control elements in process plants. They are commonly used to shut off, release, dose, distribute or mix any liquids/gases.
For that reason, solenoid valves are found in many application areas with different kinds of requirements.
The three general types of solenoid valves are as follows
The electrically actuated solenoid is the most commonly fitted with pneumatically operated valves.
When used as a component in a safety system, the probability of failure on demand average (PFDavg) and spurious trip rate (STR) of a solenoid assembly are key design parameters.
The simplified equation versions of the low-demand PFDavg equations for common solenoid arrangements, which show the significant contributors to PFDavg and STR for solenoids, are given as
For (1oo1), PFDavg = λDU x Ti/2
Spurious trip rate (STR) = λSU
λDU = Dangerous Undetected failure rate
λSU = Safe Undetected failure rate
Generally operate with 24V DC, and 120V AC.
Useful life is a key parameter for any device. Emerson’s (Asco) manual on solenoids in SIS lists solenoid useful life as 3 to 10 years.
As per Failure Mode Effect and Diagnostic Analysis (FMEDA) for solenoids gives the useful life for a solenoid is 30,000 hours for the coil and 10 years for the solenoid valve assembly
ASCO Series 316 solenoid valves
ASCO series 551 Solenoid valves
For improving reliability SIS valves that are integrated with solenoid technology shall be designed to reduce thermal degradation
SOV pilot valve systems to be constructed with built-in redundancy to eliminate single points of failure.
Redundant coil SOVs improve valve availability and allow online maintenance and automated testing to prevent spurious trips that cause process interruptions.
Below are possible failures of Solenoid valves, proper care is to be taken to overcome this.
SIS solenoid valves must be carefully chosen taking into consideration several factors described above to have trouble-free plant operations.
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