Needle valves are used in instrumentation systems and work on an identical principle to screwed stem globe valves. The difference being that instead of a plug point, a needle valve has, as you would expect, a needle point. Operation is generally by a t-bar, rotated to push the needle into the seat. The seat to seal interface is the needle passing through and sealing on a conical area machined inside the body (or on a seat insert screwed / swaged into the body). Energisation of the seal is by the mechanical force applied through the stem.