What is a valve that automatically opens whenever the controlling medium is lost called?

The term used to describe a valve that automatically opens whenever the controlling medium is lost is "Fail Open." This type of valve is designed to move to the open position upon loss of pressure or power, ensuring that the flow can continue in case of a system failure. This characteristic is particularly important in safety systems where maintaining flow is crucial to prevent hazardous situations, such as in cooling systems or in processes where pressure relief is necessary. By failing to the open position, the valve allows for an unobstructed flow, which can help in maintaining safe operating conditions.

In contrast, other types of valves do not operate in this manner. For example, a fail closed valve would automatically close, potentially leading to dangerous conditions if flow were necessary for safety. Spring-loaded valves rely on a mechanical spring to operate, which is different from the fail open mechanism that relies on the absence of the controlling medium. Manual valves require operator intervention to open or close, which does not align with the automatic function described in the question. Thus, "Fail Open" is the right answer as it best describes the functionality of the valve in question.