If a fail as positioned valve is 50% open and pneumatic pressure is lost, the valve will:

When a fail-as-positioned valve is at 50% open and pneumatic pressure is lost, it is designed to maintain its current position rather than move to an open or closed state. This behavior is characteristic of fail-as-positioned technology, which ensures that in the event of a failure, the valve does not inadvertently move to a fully closed or fully open position that could create unsafe conditions or disrupt process control.

In this scenario, since the valve was initially at 50% open, losing pneumatic pressure will not change that position; the valve will simply remain at the 50% open setting. This feature is crucial in processes where maintaining a specific flow or pressure is essential to overall system stability and safety.