What is a holdback valve?
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When a control valve requires FL, it is necessary to have a hold-down valve; what is the principle behind a hold-down valve? Is it just a one-way valve?
The function of the hold-down valve is to ensure that the air pressure inside the diaphragm of the control valve is not lost in the event of a failure in the air supply, thereby keeping the valve position unchanged for a certain period of time. Unlike check valves, check valves cannot achieve this function.
A hold-down valve is a valve that keeps the valve in its position during an accident, and it has nothing to do with check valves.