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8200 N. Austin AveMorton GroveMorton GroveIL 60053IL 60053USAUSATel 847 966-3700Tel 847 966-3700Fax 847 965-8379Fax 847 965-8379
Series AP Air Pressure Control Pilot

Operation / Characteristics / Hoffman Specialty Product Features / Primary Applications

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.
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