What is the need for air lock relay
- To achieve the fail last position in valves.
- To retain the position of the instrument on supply fail condition
- If air supply fails valve will remain in the last position
Air lockup relay is basically a 3 port pneumatically operated equipment that’s used for control valves in critical areas. Say during the failure of air supply, valve stem travels in the direction of spring tension. Basically, an air to open valve get closed and air to close valve get opens during air failure. This can create process offset…in order to prevent such conditions, airlock up relays are used that has three ports, an IN, OUT and a supply port… supply port will be given from an air header that supply/distribute air to various instruments near the valve location. This supply pressure opposes the tension of a spring that separates the inlet port from the outlet port of the air lock-up relay… In port will be connected to the output of a positioner or an I to P converter or an SOV, depending upon the valve setup as arranged. The output port of the air lock-up relay will be connected to the inlet of the valve actuator… so, as long as the lock-up relay receives the supply pressure, its inlet port will be connected to the outlet port. there is a spring tension adjustment in the lock-up relay where you can set the set point of the supply pressure as required while most of them come with a pre-adjusted set point… so, as long as the lock-up relay receives supply pressure, it keeps the inlet port connected to the outlet port and once the supply pressure fails or falls below the lock-up relay spring setpoint, the outlet port gets closed from the inlet port considering there is an air supply failure. Once this condition happens, the backflow of air due to valve spring tension gets blocked by the lock-up relay and the valve will remain in the present position or the actual position during failure and we can avoid process offset. ROTEX and SAGA are some of the lock-up relay manufacturers.