Reduced Pressure Principle Backflow Prevention Device Assembly Check Valve Springs

The first check valve spring shall have a tensile strength of 5.0 pounds per square inch or greater. The second check valve is spring loaded in the direction of flow and is independent of the pressure required to open the relief valve. In the event that the pressure increases downstream of the RPZ, the second check valve closes to control the backflow. The second check valve spring shall have a tensile strength 1.0 pounds per square inch or greater. The relief valve spring shall have a tensile strength of 2.0 pounds per square inch or greater.

There shall be a 3-pound buffer between the first check valve spring and relief valve spring. The Buffer Value is the difference in pressure between the force of the first check valve spring and the relief valve spring. When the buffer value is exceeded, the relief valve will open, discharge water sufficient enough to cause the zone pressure to decrease to at least 2 PSI less than supply pressure and then close. The relief valve will discharge water when the supply pressure decreases by the buffer value amount. The less the buffer value the more likely the relief valve will discharge due to pressure fluctuations.

For example; if the supply pressure for device A drops by 3.5 PSI, the relief will discharge. For Device B, the supply pressure would need to drop by 4.0 PSI before the relief would open. Therefore, for water systems that have a drastic pressure change, a device with a higher buffer value would less likely discharge water from the relief valve. As you can see in this chart, that device B has a higher buffer value than device C. Therefore, device C would be more sensitive to minor pressure fluctuations than device B. For water systems that have a significant frequency of pressure fluctuations, a soft-seated check valve or a pressure-reducing valve upstream of the device may eliminate or reduce the fluctuating pressure situation and the occurrence of relief valve discharge to some extent.