RPZ in a Flow or Dynamic State – First Check Valve Opened
The relief valve is hydraulically connected to the differential pressure across the first check. After the relief valve is closed tight, the supply pressure continues to increase causing the first check valve to open. The relief valve will close before the first check valve, since the force necessary to close the relief valve is less than the force necessary to open the first check valve. As the check valve opens, water fills the zone between the first and second check valves and downstream of the relief valve. A loss of pressure is created across the first check valve due to the force loading of the first check valve spring. The water pressure in the “zone” (the area between the first and second check valves) is now a minimum of 5 PSI less than the supply pressure. The pressure loss must be equal to or greater than 5 PSI across the first check valve.