Direct and Pilot Actuated

In the direct-actuated type, the power unit (bellows, diaphragm, and so on) of the thermal actuator is directly connected to the valve plug and develops the force and travel necessary to fully open and close the valve. Direct-actuated regulators are generally simpler, lower in cost, and more truly proportional in action (with somewhat better stability). In the pilot-actuated design, the thermal actuator moves a pilot valve, internal or external. This pilot controls the amount of pressure energy from the control agent (fluid through valve) to a piston or diaphragm, which in turn develops power and thrust to position the main valve plug. The pilot may be internal or external. When external, independently acting multiple pilots are also available. Compared to direct-actuated TCVs, pilot-actuated regulators have smaller bulbs, faster response, and narrower proportional band, and they can handle higher pressures through the valve. Pilot-actuated regulators can also handle interrelated functions through use of multiple pilots, such as temperature plus pressure plus electric interlocks.