What device is used to isolate the water supply on the mainline during maintenance?

• A. Mainline isolation valve
• B. Drain valves
• C. Unions
• D. Sleeving

Answer: (A)

Isolating the water supply on the mainline during maintenance relies on a device that can completely stop flow in the main line. The mainline isolation valve is designed for this purpose, providing a positive shutoff that allows you to work downstream without water moving through the rest of the system. With the valve closed, you can repair, replace, or service components and then reopen to restore service quickly. Drain valves, while useful for removing water from a line after isolation, do not shut off the mainline by themselves. Unions are fittings that make it easier to disconnect sections for service, but they don’t stop flow. Sleeving is a repair method for damaged pipe, not an isolation device.