What valve type is actuated by a screw mechanism to control flow from hydrant side discharges?

The valve type that is actuated by a screw mechanism to control flow from hydrant side discharges is the gate valve. Gate valves are designed to provide unobstructed flow through the pipe when they are fully opened and effectively block the flow when fully closed. The operation of a gate valve involves a screw mechanism (often referred to as a stem) that raises or lowers the gate within the valve body. This allows for precise control over the flow rate, making gate valves particularly suitable for applications where it is important to have good flow management without significant turbulence.

In contrast, ball valves use a spherical disc to control flow and are typically operated by a quarter turn, which is more suited for applications where quick shut-off is needed. Butterfly valves utilize a rotating disc to regulate flow and are often used for larger systems, providing a compact solution but not relying on a screw mechanism. Check valves are designed to prevent backflow and do not facilitate flow control in the same manner, as they automatically close when flow reverses. Thus, the functionality of the gate valve with a screw mechanism clearly distinguishes it as the correct choice for controlling flow from hydrant side discharges.