What is typically the part of a valve that moves to start or stop the flow of fluid?

The part of a valve that moves to start or stop the flow of fluid is the stem. The stem is a crucial component because it connects the actuator (or handle) to the part of the valve that actually controls the flow, such as a disc or ball. When the stem is turned or pulled, it moves the closure element (like a disc) into or out of the valve seat, effectively opening or closing the valve.

In various types of valves, the design of the stem allows for precise control of the flow of fluid, and it’s typically connected directly to a control mechanism, such as a handle or automated actuator. This mechanical action is essential for the valve's operation and ensures that fluid flow can be managed efficiently and safely.

The other parts mentioned serve different functions. For example, the disc or core may act as the actual barrier to flow, and the seat is the surface against which the disc or closure element seals. However, they do not move by themselves to initiate or stop fluid flow; that action is the responsibility of the stem. Understanding the function of these components is vital for effective troubleshooting and maintenance of valves in various systems.