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It is called needle valve due to the shape of the closure member. It consists on a threaded stem with a conical end.
Stems with fine threaded have a slow linear movement when they turn, therefore a great number of turns are needed to have a full flow section. This makes the needle valve suitable for regulating flow, with a minimal waste and without cavitation at important differential pressures.
This valve is also placed in the bypass of the turbine inlet valve. That valve is normally butterfly or spherical type and not prepared to open against all column water pressure. The slow opening and regulated closure of the needle valve avoid cavitation and water hammer in the pipeline system.