What effect does a partially opened supply valve have during a main drain test?

During a main drain test, the purpose is to measure the available flow and pressure at the system’s discharge point to assess the performance of the fire protection system. If the supply valve is only partially opened, the flow provided to the system is restricted. This restriction will lead to a significant decrease in the full flow pressure, as the water cannot flow freely through the system due to the limited opening of the valve.

When the valve is partially opened, it reduces the volume of water that can enter the system, resulting in a bottleneck that affects both the flow rate and the pressure. Consequently, this situation creates a large drop in the pressure available at the test outlet, as the system cannot achieve its full flow potential. The main drain test relies on having maximum flow and pressure to accurately evaluate the system's capabilities; thus, maintaining the valve only partially opened hinders this assessment, leading to inaccurate and low pressure readings.

Understanding this phenomenon is crucial for ensuring that fire protection systems are adequately tested, as it directly affects system performance and effectiveness.