What is a Pneumatic Temperature Controller?

Close up of a pneumatic temperature controller

Oil and gas producers use a Pneumatic Temperature Controller to control a set temperature in production vessels.  Its most common use is in indirect heaters, emulsion treaters, reboilers, steam generators, heat exchangers, cooler shutter controls, and salt bath heaters.

How Does It Work?

Kimray's Pneumatic Temperature Controller

Pneumatic Temperature Controllers have a stainless tube to monitor changing temperatures. This tube connects to a diaphragm or a bellows assembly.

A pilot plug seat is throttled from the combination of differential pressure across the diaphragm or bellows. This changes in the length of the stainless tube.

The pilot plug consists of two stainless balls fused together. One of these balls is the supply pressure inlet. The other ball is the pressure vent.

Indirect vs. Direct Output Pressure

line art of a pneumatic temperature controller comparing direct acting to indirect action

Pneumatic Temperature Controllers use both indirect and direct output pressure during operation.

Any pilot or control valve can receive indirect output pressure. Furthermore, the stainless tube grows in length in response to higher temperatures in the system. As a result, the tube is able to move the thermostat diaphragm.

Then the output pressure decreases, causing the intended pilot or control valve action.

Venting direct output pressure ensures the pilot or control valve does not receive a signal. Furthermore, the direct output pressure lengthens the stainless tube allowing the thermostat diaphragm assembly to move toward the seat.

This much like the indirect output pressure does. Also, the thermostat has a semi-throttle action. This accounts for output pressure increases and acts on the diaphragm.

Lastly, when the temperature begins to decrease, the action of the Pneumatic Temperature Controller reversed to increase output pressure.