Brochure

112
Selection Guides
Pressure and/or Temperature Pilot Operated Steam Regulators (continued)
Temperature
Sensor
Balance
Tube
Signal
Temperature
Pilot
Outlet
Bleed
Orifice
Main Valve
Inlet
Pilot Inlet
Temperature Pilot Operation
Operation - Main valve with a Self
Contained Temperature Pilot
Self-contained temperature control pilots use a liquid-filled
bulb and bellows. 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.
The sensing bulb is completely inserted into a downstream
heated fluid to sense the fluid temperature. The sensing
bulb is connected to a bellows by a capillary tube. When
the bulb temperature is below the set point, a spring in the
pilot keeps the pilot valve open and allows steam to flow
from the pilot inlet through the signal line, and on 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.
Under constant steam demand, the pilot and main valve
remain relatively motionless. As the bulb temperature
increases and the liquid expands, the expansion is trans-
mitted 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 steam trapped under the main valve
diaphragm bleeds off through an orifice, allowing the main
valve to close.
Operation - Main Valve with a
Combination of Pilots
When a temperature pilot is installed in series with a pres-
sure pilot, the pilots perform their functions separately and
concurrently. Each pilot regulation cycle is exactly the
same as if used alone. When both pilots are open, the
main valve will open, if either pilot closes, the main valve
will close. The pressure pilot essentially acts to limit the
maximum pressure as the temperature pilot cycles to con-
trol temperature.
Spring
Pilot
Outlet
Balance Tube
Self-Contained
Temperature
Pilot
Bleed
Orifice
Main Valve
Inlet
Pilot Inlet
Main Valve with Temperature Pilot
Main Valve with Temperature and
Pressure Pilot
Series 2000
Hoffman Specialty
®
Selection Guidelines