Air pressure control pilots work by
sensing and balancing the downstream pressure against the force from an air pressure
source that has been regulated to a set point. This balancing action controls the position
of the pilot seat and, in turn, the main valve position. Variations in load or pressure
are immediately sensed on the air pressure pilot diaphragm, which acts to control the
position of the main valve, ensuring a constant downstream pressure.
Operation
of Air Pressure Control Pilots:
An external pressure-sensing line is attached to the downstream pressure piping.
The downstream pressure is fed back to the underside of the pilot diaphragm, balancing
against the force from the regulated air pressure in the pilot, causing the pilot valve to
move. Steam flows through the pilot seat, loading steam pressure on the main valve
diaphragm. The force from the steam pressure on the main valve diaphragm 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 pressure rises to the set point of the air pressure, 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 Air Pressure Control Pilots:
Accurate control of downstream pressure even with varying upstream pressure or load
conditions.
Required air regulator allows remote adjustment of outlet steam pressure.
No bias relay required.
Air pressure control pilots may be combined with pneumatic temperature pilots to provide
multiple functions.
Modulates to handle wide range of system loads.
Used on steam service only.
Normal failure mode of pilot is open, while failure mode of main valve is closed.
Features
of Hoffman Specialty Series AP Air Pressure Control
Pilots:
Lightweight, compact size prevents strain on mounting pipes.
Tight shut-off provided by hardened stainless steel pin and seat.
Packless construction eliminates seals that wear out and leak.
Travel stop and cover on diaphragm helps prevent over pressurization damage.
Removable strainer helps prevent debris from entering pilot.
Maximum temperature 450° F (232°
C)
Maximum operating pressure 250 psig (17.3 bar)
Primary Applications
of Air Pressure Control Pilots:
Any application requiring frequent outlet pressure changes.
Injection molding equipment.
Lab equipment.
General Catalog
Morton Grove
Tel 847 966-3700
Series AP Air Pressure Control Pilot