SCRUTINIZE THE ACTUATORS Start by taking a hard look at the valve and actuator specifications for the two valves. Actuators often are left to the vendor or project instrument engineer to specify. They're actually (pun intended) pretty complicated devices. There are three types of actuators: air-to-close, spring-open; air-to- open, spring-close; and no spring, fail-last position. It sounds like the ingredient valve could be air-to-close, which is a bad choice because a bad instrument air supply could cause the spring to allow the valve to open, especially if there is a lot of back-pressure against the air diaphragm inside the actuator. If you find out the actuator was specified wrongly, replace it with one that has a higher inlet pressure and change the actuator to air-to-open; often this only requires a re-orientation of the actuator, as is the case for some clam-shell types.
展开▼
