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Level control valves are used in situations which require that the valve be able to shut tightly ("bubble tight") when the system is not operating.  This is always required on open top vacuum type filters that are located below the pool water level.  They are also required when an open surge tank is similarly located.  These valves are closed by powerful spring actuators.  They are opened by water pressure from the main circulating pump.  When no water pressure is present due to a shutdown of the circulation system, the spring actuator forces the butterfly valve closed.  A schematic system is shown below.  These valves are more accurately machined for close tolerances.  You will note that they give less resistance to flow, at any given flow rate, when comparing the flow charts for this type valve to those of a modulating valve.  A similar sized level control valve does not need to be set as deep as a modulating valve to achieve the same flow or pool turnover rate.  Also, the pilot valve allows one to hold a water level above the valve depth, as long as the operating level provides a difference between water levels sufficient to provide flow.  See table #2 for depth versus flow of level control valves. The level control valve actuator is a spring to close type that will open when pressure is applied to the underside of the diaphragm.  The pilot valve acts as a controller.  As water level in the tank drops, the pilot closes and bleeds off less water from the supply line, thereby increasing pressure on the actuator diaphragm, opening the LCV and allowing more water to flow into the tank.  Conversely, when the water level rises the pilot valve bleeds more water and less pressure is applied to the actuator allowing the spring to close the valve.  On system shut down no pressure is applied to the actuator and the spring closes the valve tightly.