The 'Needle Valve' controls flow in both directions, whereas the 'Flow Control' valve controls flow in one direction and provides full-flow in the reverse direction. Both are typically used in flow meter applications, primarily when a consistent, calibrated, low flow rate must be maintained. They typically have a relatively small orifice with an extended, tapered seat, and a needle-shaped plunger on the end of a threaded stem, which exactly fits the seat. A check valve is used to prevent backflow. Some check valves only work when installed in a horizontal manner. Check valves do not have operating mechanisms because the valve operates automatically, depending on the pressure of the flow. In sensitive systems, "cracking pressure" is the back pressure at which the check valve becomes engaged.

A bidirectional valve is simply a valve that’s rated to function from inlet to outlet, or outlet to inlet. It’s like you don’t even need to label “outlet” or “inlet.” The main benefit of using a bidirectional valve is the ability to control flows and pressures in both directions.