Larner–Johnson valve

Each valve is capable of passing a flow rate of 19 cubic metres/second, which is the equivalent of 19 tons of water per second.

The valves control the flow of water out of the Ashokan Reservoir into the Catskill Aqueduct down to New York City.

When it moves downstream, it seals against a conical surface at the outlet of the valve, closing off the flow.

When closed, the valve is held shut by the supply water pressure, providing a good seal.

A servomechanism is used to generate the large forces needed for these huge valves, power being derived from the pressure of the water itself.

The piston is double-acting and splits the internal valve body in two, with water pressure on both sides.

If the pilot valve is opened, water drains from the upper chamber and its pressure is reduced.

This depends on the piston size, the pressure reduction downstream and the flows through the two control valves.