There are many industrial control valve applications that require the valve to stay at the last set point as a failure state. This is frequently called out as Fail Last Position, Fail in Place, or Fail Freeze. These terms are interchangeable, so we will use Fail Freeze for the rest of this article.
When specifying a positioner for these applications it is important to understand that with a positioner there are two separate failure modes.
1. Loss of Signal is when the signal that provides the set point from the control system is interrupted.
2. Loss of Air Supply is when the instrument air supply that provides the motive power for the pneumatic actuator is interrupted.
These must be treated as independent conditions. With Fail Freeze it is imperative that you understand which failure mode the end user needs. Is the application “Fail Freeze on Loss of Signal”, “Loss of Air Supply”, or both? The answer will let you specify the correct positioner and/or accessories.
Fail Freeze on loss of Signal:
This is accomplished using a positioner configured with an I/P module specifically designed to hold the air pressure in the actuator when the voltage of the signal drops below the minimum required to operate the I/P. This is typically in the range of 9-12 VDC. When the signal is lost, the valve will be held at or near the set point and will not drift. When the signal is restored, the positioner will resume normal operation without operator intervention.
Fail Freeze Positioners or I/P’s cannot be converted from Fail Freeze to Fail Safe or vice versa. You must purchase the correct positioner for your application.