Differential valves are spring loaded valves on the outlet side of the meter. Most Liquid Controls meter systems with a vapor eliminator also have a differential valve. Vapor eliminators and differential valves work in conjunction to stop the flow of product through the meter until the vapor is eliminated from the system. In order to do this, the vapor eliminator and the valve are piped together at installation.
Differential valves are normally closed, but when the pump starts and pushes product into the system, the valve spring will give way to the flow pressure. In order to keep the valve closed when vapor is being expelled from the vapor eliminator, the vapor from the vapor eliminator is routed through the piping to the back side of the valve spring. The combined force of the expelled vapor and the strength of the spring keep the valve closed until the vapor is eliminated.
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