Which valve operates by adjusting an opening size to control flow and pressure in a water system?

The valve that operates by adjusting an opening size to control flow and pressure in a water system is the throttling valve. Throttling valves are designed specifically to regulate the flow rate and pressure of liquid or gas in a pipeline by changing the size of the fluid passage.

When the valve is partially closed, it creates a restriction in the flow path, leading to a drop in pressure on the downstream side and controlling the amount of fluid that can pass through. The ability to finely adjust the opening size allows for precise management of flow rates, making throttling valves ideal for applications that require balancing flow and pressure.

In contrast, isolation valves are primarily used to completely stop flow in a pipeline to perform maintenance or repairs, without the capacity for flow regulation. Check valves allow fluid to flow freely in one direction and prevent backflow but do not adjust flow rates. Gate valves, while capable of starting or stopping flow, are generally either fully open or closed, lacking the capability of controlling flow or pressure dynamically. Thus, the operational principles and intended applications make throttling valves the correct choice for controlling flow and pressure in a water system.