In both the flow control and needle valve a finely threaded stem allows gradual adjustment of the amount of controlled flow passing through the valve. Flow enters port 1, travels through an orifice sized by the tapered stem and out port 2. The flow control features a by-pass check which allows rapid free flow out port 1.

Although the basic function is flow restriction, the fundamental difference between the two is the needle valve controls flow in both directions (bi-directional) and the flow control valve controls flow in only one direction (allowing free flow in the opposite direction).