When variations in capacity and pressure are required beyond those that can conveniently be obtained from a single impeller, then multi-stage series/parallel pumps are used. These pumps have two or more impellers, each enclosed in its own volute, that are usually part of a common body. With a two-stage pump, a transfer valve is located at the outlet of the first-stage volute. It directs the water either to pump discharge VOLUME (parallel) position, or to second-stage intake PRESSURE (series) position, depending on the pump performance desired.
When the transfer valve is in VOLUME (parallel) position, the water enters each impeller eye from a common intake and leaves through a common pump discharge.
If the transfer valve is moved to PRESSURE (series) position, the first-stage pumps its full volume and pressure directly to the second-stage intake instead of to pump discharge. The second-stage then pumps this same volume of water to pump discharge, but at approximately twice the first-stage pressure. With the transfer valve in this position, first-stage discharge pressure closes the flap valves in the side intake passageways that prevent water from bypassing back to the first-stage intake passageway.
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