The field pneumatic pressure controller (pressure controller) compares the measured process pressure to a setpoint and sends an output signal to a final control element (control valve) which acts to hold the process pressure at the setpoint. Field pressure controllers are used when local control of pressure is required and the following conditions hold:
• It is not necessary to change the pressure setpoint from a central control house
• It is not necessary to record the pressure from a central control house
• It is not necessary to change the controller tuning from a central control house
• The controlled pressure is not part of a cascade loop
Field pressure controllers may be either indicating controllers or recording controllers. Blind controllers are rarely used.
Specification of Field Pressure Controllers
Field pneumatic pressure controllers should have weatherproof cases of fiberglass reinforced plastic, or aluminum. Colored epoxy or baked vinyl is used to provide a textured finish. If the case contains electrical components, it should meet the electrical classification requirements for the area. The pressure element should be suitable for the process pressure and fluid. The controller should include an indicator for the process pressure and setpoint. The setpoint should be easily adjusted using a knob either inside or outside the case, depending on the need to make adjustments.
The controller should operate on an instrument air pressure of 18 to 22 psig. The output should be 3 to 15 psig. (Although some locations have safely used other supply mediums such as natural gas, nitrogen, and carbon dioxide, caution is advised since some instrument components may be incompatible with the medium selected. When available, instrument quality air is preferable.)
The controller should include a two-position, bumpless, auto/manual switch that is internally mounted to avoid accidental switching. The controller action should be specified as either direct or reverse acting, and it should be possible to change the action in the field. The controller should include supply and output pressure gages. The process connection should be ½-inch female NPT. The case should include a socket or yoke for mounting on a 2-inch pipe.
The control mode is normally selected from the following options:
• On/off control. For alarms, protective devices, and startup and shutdown of equipment
• Proportional only control. For simple pressure control where small variations from the setpoint are unimportant. The proportional band should be adjustable at least from 0% to 200%
• Proportional plus reset control. This is the usual control mode for field pressure controllers. The proportional band should be adjustable at least between 0% and 200%. For applications where the controller occasionally operates above or below setpoint or in intermittent service, it should be specified to include anti-reset windup