The operator for the metering valve is as important as the valve itself. Several types of operators are available to provide compatibility with the system in which they are installed. The operator must be able to position the valve in very nearly identical relationship to the seat on both opening and closing strokes. This is essential to provide consistent, accurate metering of steam discharged by the humidifier.
Electric motor modulating actuators, by their design, provide true linear positioning characteristics on both opening and closing cycles. Pneumatic operators may or may not be able to provide the precise positioning and holding characteristics critical to accurate control.
Rolling diaphragm-type pneumatic operators are recommended, provided that they meet the following criteria:
- A large diaphragm area—12 square inches or more (30 cm)—to provide ample lifting force. This permits the use of a spring heavy enough to stabilize both the hysteresis effect and the flow velocity effect on the positioning of the valve stem versus air pressure to the operator.
- A diaphragm material that is highly resistant to wear or weakening from continuous cycling and high temperatures.
- An operator stroke that is long enough, in conjunction with the valve’s plug and seat design, to provide rangeability ratios.
- Easy serviceability.
All modulating operators, whether electric or pneumatic, should incorporate a spring return. This is necessary to ensure closing of the valve if there is an interruption of power or control air to the unit.
For industrial in-plant operation and for certain very limited duct applications, a solenoid operator may be used to provide simple on/off operation. This type of operator should not be specified for duct applications without a detailed analysis of the system.