A shut down valve (also referred to as SDV or Emergency Shutdown Valve, ESV, ESD, or ESDV) is an actuated valve designed to stop the flow of a hazardous fluid upon the detection of a dangerous event. This provides protection against possible harm to people, equipment or the environment.
Shutdown valves form part of a Safety Instrumented System. The process of providing automated safety protection upon the detection of a hazardous event is called Functional Safety
Types of actuation
As shutdown valves form part of a SIS it is necessary to operate the valve by means of an actuator. These actuators are normally fail safe fluid power type.
Typical examples of these are:
- Pneumatic cylinder
- Hydraulic cylinder
- Electro-hydraulic actuator
In addition to the fluid type, actuators also vary in the manner in which the energy is stored to operate the valve on demand as follows:
- single-acting cylinder - Or spring return where the energy is stored by means of a compressed spring
- double-acting cylinder - Energy is stored using a volume of compressed fluid
The type of actuation required depends upon the application, site facilities and also the physical space available although the majority of actuators used for shutdown valves are of the spring return type due to the fail safe nature of spring return systems.
For shutdown valves used in safety instrumented systems it is essential to know that the valve is capable of providing the required level of safety performance and that the valve will operate on demand. The required level of performance is dicated by the Safety Integrity Level (SIL). In order to adhere to this level of performance it is necessary to test the valve. There are 2 types of testing methods available being
Proof test - A manual test that allows the operator to determine whether the valve is in the “as good as new” condition by testing for all possible failure modes and requires a plant shutdown
Diagnostic Test - An automated on-line test that will detect a percentage of the possible failure modes of the shutdown valve. An example of this for a shutdown valve would be a partial stroke test. An example of a mechanical partial stroke test device