Reduced-Pressure Assembly on a Municipal Makeup Line With Seasonal Pressure Swings
A food-processing plant draws potable water from a 6-inch municipal main to feed a boiler makeup tank. The service line also supplies a hose station used to wash down a floor drain area that occasionally backs up with process water. The utility reports static pressure of 95 psi in winter but only 48 psi during summer peak demand, and the plant's booster pump holds downstream pressure at 70 psi. The plant installed a 4-inch reduced-pressure principle backflow prevention assembly (RPBA) on the makeup line. During a summer inspection, the RPBA relief port discharges continuously whenever the booster pump runs. The assembly is installed 24 inches above the floor, with the relief port piped to a floor drain through a 2-inch line containing a 90-degree elbow and 10 feet of horizontal run. The plant wants to remove the RPBA and replace it with a double check valve assembly to stop the discharge.
1. Identify the most likely cause of the continuous relief discharge and explain why the proposed double check valve substitution is or is not appropriate.
Read the worked answer
The continuous discharge is a symptom of a pressure differential reversal across the first check, not a failed device. With only 48 psi upstream in summer and 70 psi held downstream by the booster pump, the downstream pressure exceeds the reduced upstream pressure, so the differential relief valve opens to protect the zone between the checks. This is exactly the condition an RPBA is designed to relieve. The proposed double check valve assembly would not relieve this pressure and would not provide the same level of protection. Because the line serves a boiler makeup tank and a hose station near a floor drain subject to process backup, the hazard includes both backpressure and backsiphonage potential with a contaminant that could be hazardous. A double check is suitable only for low-hazard, non-health conditions where backpressure is controlled. The correct fix is to address the hydraulic imbalance, for example by relocating the assembly, adding a pressure-regulating valve, or reconfiguring the booster pump control so it does not pressurize the assembly outlet above the supply pressure. The drain line also needs review: a 2-inch line with an elbow and 10 feet of run may not provide adequate air gap or drainage capacity, and the relief port should discharge through an air gap or properly sized drain.
2. The plant argues the RPBA is oversized at 4 inches and that a smaller 2-inch RPBA would solve the problem. Evaluate this argument using the given pressures and flows.
Read the worked answer
Oversizing can contribute to relief valve chatter, but the stated symptom is tied to pressure, not flow. The relief valve opens because downstream pressure of 70 psi exceeds upstream pressure of 48 psi in summer. A smaller assembly does not change that pressure relationship; it only changes capacity and pressure loss. If the 2-inch assembly is undersized for the actual peak flow, the increased pressure loss across the first check would make the differential worse and could increase discharge. If it is correctly sized, the relief may still open whenever the booster pump raises downstream pressure above the reduced upstream pressure. The judgment call is to measure actual flow and pressure at the assembly inlet and outlet before resizing. The plant should also verify that the booster pump is not drawing from the assembly outlet in a way that creates backpressure, and confirm the assembly is installed in the correct orientation and that the relief port is not obstructed. Resizing is not a substitute for correcting the pressure imbalance or for maintaining the required level of protection.