In most cases, to regulate the flow, the only thing one can do is to adjust/restrict the open section area of the passage in the valve, i.e. the flow area. Doing this is called throttling. It may sound easy, but in fact throttling is a very challenging task; reason being that the water will try to maintain the same flow rate through the throttled/restricted zone, with the result being increased flow velocity and a real risk of local water jets, even cavitation.

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