Self-contained valves are suitable for pressure and temperature control. Valves are mechanical in operation, rugged, simple to install, and lowest in initial cost. Valves of this type are the least accurate and cannot be interfaced with single or multiple loop controllers.

Better accuracy and more flexible control options are possible using an electronic control system. Instruments are offered to measure pressure, temperature, flow, level, and a number of other indications. The instrumentation is tied to a controller which provides a voltage or current signal which is spanned to reflect the control variable. This signal has to then be converted through a valve to perform the final control. See sketch 1.

Air operated valves use a diaphragm to generate a force to stroke a valve open and closed. A transducer is required to change the voltage or current signal to a corresponding air signal to stroke the valve. Adding a pilot positioner to the control valve improves control response as well as providing feedback if required. Transducers are vulnerable to moisture in the control air and are somewhat fragile devices. The valve uses a packing to seal the actuator shaft and is vulnerable to leaking and binding.

Call us if you have a control valve application question - 1-800-752-0556.