Self-contained or self-operated temperature control valves
Self-contained or self-operated temperature control valves do
not require an outside energy source such as compressed air or
electricity (Figure 26). They depend on a temperature-sensing
capillary tube filled with either an oil or a volatile liquid.
In an oil
filled system, the oil expands as the sensing bulb is heated.
expansion is transmitted through the capillary tube to an actuator
bellows in the valve top, which causes the valve to close. The
opens as the sensing bulb cools and the oil contracts; a spring
provides a return force on the valve stem.
A volatile liquid control system is known as a vapor pressure
vapor tension system. When the sensing bulb is warmed, some of
the volatile liquid vaporizes, causing an increase in the sealed
system pressure. The pressure rise is transmitted through the
tube to expand the bellows, which then moves the valve stem and
closes the valve. Thermal systems actuate the control valve either
directly or through a pilot valve.
In a direct-actuated design, the control directly moves the valve
stem and plug to close or open the valve. These valves must compensate
for the steam pressure force acting on the valve seat by generating
a greater force in the bellows to close the valve. An
adjustable spring adjusts the temperature set point and provides
return force to move the valve stem upward as the temperature
A pilot-operated valve uses a much smaller intermediate pilot
valve that controls the flow of steam to a large diaphragm that
then acts on the valve stem. This allows the control
system to work against high steam pressures due to the smaller
of the pilot valve.
For self-contained temperature valves to operate as proportional
controls, the bulb must sense a change in the temperature of the
process fluid. The difference in temperature from no-load to maximum
controllable load is known as the proportional band. Because the
size of this proportional band can be varied depending on valve
the accuracy is variable. Depending on the application, proportional
bands of 2 to 18°F may be selected, as shown in the following