Cross Connection Practice Exam

When the low side control valve of a Reduced Pressure Backflow Assembly is opened and the gauge needle drops to zero with no water discharge, what is the likely cause?

1. Blocked inlet

2. Seized relief valve

3. Clogged pipe

4. Air lock in system

The correct answer is: Seized relief valve

The correct answer relates to the function of the relief valve in a Reduced Pressure Backflow Assembly. When the low side control valve is opened, the system is designed to maintain a certain pressure differential. If the gauge needle drops to zero and there is no water discharging, this indicates that the relief valve may be stuck or seized, preventing it from engaging properly. A properly functioning relief valve allows excess pressure to discharge when necessary to prevent backflow into the potable water supply. If the relief valve is not operational due to seizing or blockage, it will fail to release water even when the control valve is opened. Thus, the system will experience a series of pressure changes reflected in the gauge, leading to a drop to zero without any water exiting the assembly. While other options may also affect water flow and pressure, a blocked inlet may restrict incoming water, a clogged pipe would generally result in reduced flow rather than no flow, and an air lock could intermittently create pressure changes without leading to a total drop to zero in all situations. Therefore, in this specific scenario where the gauge drops to zero with no discharge, the most plausible cause is that the relief valve is seized.