Which type of valve restricts flow in both directions?

A needle valve is specifically designed to provide precise control over the flow of fluid in a system. Its construction includes a long, tapered point that fits into a seat, allowing for fine adjustments to the flow rate. This design feature gives needle valves the ability to restrict flow more effectively than many other types of valves, and importantly, they can restrict flow in both directions. This characteristic is significant in various applications where controlling fluid flow is critical, as it can help maintain consistent pressure and prevent backflow.

The other types of valves mentioned have different purposes. For instance, a pressure relief valve is primarily designed to open at a predetermined pressure to prevent system overload, and a flow control valve regulates flow in one direction and usually allows free flow in the opposite direction. Therefore, while they have their roles in fluid management, they do not restrict flow in both directions like a needle valve does.