Reduced Pressure Principle Backflow Prevention Device Assembly Relief Valve Spring and Operation
During normal flow with a demand downstream, the relief valve will be in a close position since the supply pressure is greater than the pressure in the zone, which is downstream of the relief valve. If the inlet or supply pressure should become less than atmospheric pressure, the relief valve will open to the atmosphere and discharge any water that is in the zone. If the inlet or supply pressure decreases to a pressure less than the zone pressure (the pressure downstream of the first check valve), the relief valve will open and stay open until the pressure in the zone is less than the supply or inlet pressure.
Since the design criteria states that the first check valve spring shall be no less than 5 PSI, the pressure loss downstream of the first check valve within the body of the RPZ will be 5 PSI less than the supply side pressure during a static state. For example, with a RPZ in a static state, the supply pressure is equal to 100 PSI. The pressure reading taken at test cock number three downstream of the first check valve is 95 PSI. Therefore, the difference in pressure or differential pressure across the first check valve it 5.0 PSI. This would indicate that there is a 5 PSI spring in the first check valve, which is greater than 2 PSI needed to keep the relief valve closed.
