When shutting a large valve, what will help to reduce high pressure?

• A. A bypass valve
• B. A relief valve
• C. Open the downstream valve
• D. Close the upstream valve first

Answer: (A)

Reducing high pressure when shutting a large valve comes from creating a controlled path for the water to continue flowing, so the flow doesn’t slam to a halt. A bypass valve provides that alternate route around the valve being closed, letting water keep moving and pressure ease off gradually. This smooths the transition and minimizes the surge (water hammer) that can occur when a large valve stops flow suddenly.

Opening a downstream valve doesn’t provide the same dedicated escape around the closing valve, so it’s less effective at preventing the surge. A relief valve is designed to vent excess pressure, but it doesn’t create the temporary bypass needed to control the surge during isolation. Closing the upstream valve first tends to trap the pressure and actually increase the peak surge rather than reduce it.