Answer / carl_ellis

Pneumatic positioners equipped with a bypass option can be removed for maintenance without interrupting the valve service.

A position bypass is/was a common optional feature on pneumatic positioners that receive a pneumatic control signal (typically 3-15 psi, 20-100 kPa, or 0.2 - 1.0 bar) from a controller or I/P.

The bypass option is a pneumatic switch. The inlet is the control signal. The valve's outlet is either the positioner (normal mode) or the actuator (bypass mode).

If the positioner needs servicing, the bypass switch is changed from normal mode to bypass mode and the control signal will drive the valve actuator directly. The bypass assembly can typically be removed from the positioner with a couple bolts so that the bypass assembly remains connected to the process, while the positioner is removed for service.

Positioner Bypass operation does not work
- on valves with actuators whose input signal range does not match control signal range (a 1:1 output:input signal ratio). A valve actuator that requires 40-60 psi to actuate will not be actuated by a 3-15psi control signal.
- for split range valves
- for valves with control action (direct or reverse) the opposite of the control signal action.
- Electropneumatic positioners that use a 4-20mA input signal have no bypass option because there's no pneumatic signal to bypass to the actuator.

Carl Ellis
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