Self-contained temperature control pilots use a liquid-filled bellows and bulb. The actuating force for the pilot results from the volumetric expansion of the liquid as the bulb temperature increases. The expansion or contraction of the liquid controls the position of the pilot seat. Variations in temperature sensed by the pilot sensing bulb act to control the position of the main valve, ensuring a constant temperature of the heated fluid or gas.

Operation of Self-Contained Temperature Control Pilots:

Self-contained temperature control pilots operate on the liquid expansion principle. A sensing bulb is completely inserted into a downstream heated fluid to sense the fluid temperature. The liquid-filled sensing bulb is connected to a bellows by a capillary tube. As the bulb temperature increases and the liquid expands, the expansion is transmitted through the capillary tube, creating an actuating force on the bellows. The bellows expand to close the pilot valve, shutting down the flow of steam through the pilot seat and signal line to the underside of the main valve diaphragm. The force from the steam pressure pushes against the main valve spring to control the main valve position. The main valve opens or closes in response to its diaphragm movement.

When downstream temperature rises to the set point of the temperature pilot, the pilot valve closes, shutting off steam flow to the main valve diaphragm. The steam trapped under the main valve diaphragm bleeds off through an orifice, allowing the main valve to close.

Characteristics of Self-Contained Temperature Control Pilots:

• Self-contained temperature control pilots may be combined with pressure pilots or solenoid pilots to provide multiple functions or safety overrides.
• Modulates to handle wide range of system loads.
• Response time is somewhat faster than vapor tension type regulators due to smaller bulb.
• Operate against high differential pressures.
• Require higher pressure drops to open main valve than self-contained temperature regulators.
• Pilot failure mode is open, while main valve failure mode is closed.

Features of Hoffman Specialty Series STPA Self-Contained Temperature Control Pilots:

• Lightweight, compact size prevents strain on mounting pipes.
• Liquid expansion pilot has a wide range temperature adjustment.
• Easy temperature adjustment with calibrated dial.
• Non-metallic, lockable temperature adjustment knob.
• Accuracy of control ± 10° F (± 5.6° C) with system recirculation.
• Bulb overheat protection up to 100° F (55.6° C).
• Tight shut-off provided by hardened stainless steel pin and seat.
• Packless construction eliminates seals that wear out and leak.
• Removable strainer helps prevent debris from entering pilot.
• Used on steam service applications.
• Maximum temperature 450° F (232° C)
• Maximum operating pressure 250 psig (17.3 bar)
• Optional Wells:
• Copper
• Stainless Steel

Primary Applications of Self-Contained Temperature Control Pilots:

• Applications requiring accurate temperature control of heated fluids.
• Storage tanks.
• Shell and tube heaters with constant loads or if loads are smoothed out by buffer tanks.
• Aftercoolers.
• Applications requiring solenoid pilots to override for automatic shutdown and start-up.
• Where safety overrides are required.