Brochure

113
Selection Guides
Series 2000 (continued)
Operation - Main Valves with
Pneumatic Temperature Pilots
The air pilot and pneumatic temperature pilot combination
is used to control temperature in systems with rapid
changes in the required heat load. An air PRV is used to
limit the pressure of air supplied to the pneumatic temper-
ature pilot. Limiting this supply pressure limits the air pilot
loading force and hence the main valve downstream pres-
sure.
When the pneumatic temperature pilot senses a tempera-
ture below the set point, it delivers an air signal to the air
pilot based on the sensed temperature. This air signal
becomes the air pilot loading force. If the pressure down-
stream from the main valve is below the air pilot loading
force, the pilot valve diaphragm pressure is no longer bal-
anced. The pilot valve opens and inlet steam is passed
through the air pilot to the signal line. Steam flowing
through applies pressure on the lower side of the main
valve diaphragm. This force from the steam compresses
the main valve spring and the main valve opens.
Under constant steam demand, the pilot and main valve
remain relatively motionless. As temperature rises to the
pneumatic temperature pilot set point, the temperature
pilot lowers the loading force to the air pilot. When the
loading force decreases below the force produced by the
downstream pressure, the air pilot begins to close. Less
steam flows through the air pilot 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 with the GT610-IP Electro-Pneumatic
Transducer is similar with the exception that the sensed
temperature is represented by an electronic signal which
the transducer converts to a pneumatic control signal for
the air pilot.
These arrangements give rapid response for heat load
changes and it also limits main valve downstream pressure.
Air Supply
Feedback Line
Balance
Tube
Outlet
Bleed
Orifice
Main Valve
Inlet
Pilot Inlet
Signal
AP Air
Pilot
Temperature
Sensor
315 PNT
Temperature
Pilot
Air PRV
Regulator
High Pressure
Supply Air
Outlet
Inlet
Signal
AP Air
Pilot
GT-610-IP
Transducer
Feedback Line
Air PRV
Regulator
High Pressure
Supply Air
Bleed
Orifice
Main Valve with Pneumatic Temperature
Pilot and Air Pilot
Main Valve with Electro-Pneumatic Transducer and
Air Pilot
Hoffman Specialty
®
Selection Guidelines