Brochure

111
Selection Guides
Pressure and/or Temperature Pilot Operated Steam Regulators
Series 2000
Operation - Main Valve with a
Spring or Air Pilot
Pressure may be controlled by use of either a spring
pilot or an air pilot. The only functional difference is that
a spring pilot uses a spring to apply loading force to the
pilot diaphragm and the air pilot uses air pressure.
Downstream pressure is sensed and fed back to the pilot
through the feedback line to the underside of the pilot
diaphragm. The downstream pressure balances against
the spring (or air pressure) force in the pilot, causing the
pilot valve to move. This movement opens or closes the
pilot valve. When the downstream pressure is below the
pilot set point, the force from the spring or air opens the
pilot valve and inlet steam flows through the pilot. The
open pilot valve allows the flow of steam through the
pilot seat and signal line, and on to the underside of the
main valve diaphragm. The force from the steam pres-
sure 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 steam demand
decreases, the downstream pressure will rise. When the
downstream pressure rises, the pilot valve senses the
change relative to the spring or air loading force and the
pilot begins to close. Less steam flows through the 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.
Air Supply
Feedback
Control Line
Balance
Tube
Outlet
Bleed
Orifice
Main Valve
Inlet
Pilot Inlet
Signal
AP-4A
Air Pilot
Main Valve with Air Pilot
Outlet
Balance
Tube
Bleed
Orifice
Signal
Feedback Line
Main Valve
Inlet
Pilot Inlet
Spring
Pilot
Main Valve with Spring Pilot
Hoffman Specialty
®
Selection Guidelines