Issues with the control valve?
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On the PID diagram, the control valve is marked with FL/SC – fault lockout // spring-close. So, in the event of a gas supply failure, will this control valve operate in the open or closed position?
In the spring-close mode, when there is no air in the cylinder, the spring expands and the valve closes.
FL means fault retention; it is double-acting. However, the spring-operated type is air-open, meaning it closes in the event of an air supply failure. It’s confusing.
So how should this fault lock be understood?
Fault locking refers to the valve remaining in its original position when the air supply is cut off
Single-acting actuator: when the air supply is functional, the spring closes the valve; when the air supply fails, the lock valve assumes a LOCK position to maintain it in that state! ! !
When the air supply is stopped, thanks to the check valve, the air in the cylinder remains trapped there for a certain period of time. However, due to some leakage in the cylinder, it will return to its fault position after a while. The length of time during which the air can be retained depends on the amount of leakage in the cylinder
FO: on in case of fault, FC: off in case of fault, FL: maintains its position in case of fault
Fail Locked Spring to Close/Open (FLSC/FLSO): These terms are used to indicate on which side of the diaphragm actuator the spring should be installed. In practice, the pneumatic setup should include a pneumatic relay set at a pressure level that corresponds to an ‘Instrument Air’ failure condition (for example, 0.3 MPag). When there is a loss of air pressure, it causes the air to remain trapped within the actuator, thereby keeping the control valve in its current position. Over time, the air will leak out of the actuator, and under such conditions the spring inside the actuator will tend to close (or open), as indicated by FLSC (or FLSO). These terms are used to determine in which direction the spring should be installed within the diaphragm actuator. In practice, the pneumatic connection design should include a pneumatic relay whose setting is set to the \"instrument air failure value\" (for example, 0.3 MPag). A gas fault causes the gas to become trapped inside the actuator, thereby locking the position of the control valve in case of a fault. Over time, air will leak out of the actuator; under such conditions, the spring inside the actuator will tend to close (or open) according to FLSC (or FLSO), replicating someone else’s 0.0!