Operating positions for two main-port valves could be either shut
(closed) to ensure that no flow in any way undergoes, fully open for max
flow, or sometimes partially open to any degree involving. Such valves
are already called by various names for instance regulating, throttling,
metering, or control valves. As one
example, needle valves have elongated conically-tapered discs and
matching seats for fine flow control. For a lot of valves, there might
be a mechanism to suggest because when much the valve is open, but on
many occasions other indications of flow rate are employed, including
separate flow meters.