The non-return valve can be equipped wirh a counterweight and may have an optional hydraulic damper cylinder with a undirectional oil flow metering valve.
The weight (used only on horizontal line) can be adjusted continuously along the lever and permanently fixed to provide the minimum torque necessaryto properly close the valve.
The closing time of the valve can be controlled, within limits, by metering the hydraulic flow in a damper circuit
The unidirectional oil flow control valve hydraulic/damp-er cylinder can be set from full open to full closed by adjusting the external coupling. If required it is possible to furnish an auxiliary unidirectional oil flow metering valve which be used to abjust the opening time.
The hydraulic cylinder is complete with a by-pass device and safety valve set permit the free of the oil when the closing time is set too long or the hydraulic flow metering valve is not funcrtioning.
This abnormally high pressure can reached only if the closing time is set too long or the hydraulic flow metering valve is not functioning.
The high return flow in the butterfly valve caused by the valve closing too slowly can generate very high dynamic torque which could exceed allowable stresses if protected by the relief valve.