# Fluid mechanics suction question

#### de\$per@do

In centrifugal pumps why diameter of delivery pipe is smaller than suction pipe????

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#### cragar

It might have to do with Bernoulli's principle , The smaller outlet will have a higher flow velocity and reduced pressure . But I am not 100% positive.

#### cjl

It's more to do with simple continuity. Centrifugal pumps deliver fluid at a higher velocity than they intake, so assuming incompressibility (a decent assumption for pretty much all applications with centrifugal pumps), the diameter at the outlet must be smaller than at the inlet since the volumetric flowrate is unchanged and the velocity has increased.

#### gmax137

... Centrifugal pumps deliver fluid at a higher velocity than they intake, ...
Well, pumps deliver at higher pressure than they intake. The outlet piping velocity will be dependent on the outlet pipe area (due to continuity), not the other way around.

The reason inlet piping is usually larger diameter than outlet piping is to minimize pressure losses upstream of the pump, in order to ensure the pump has adequate net positive suction head (to minimize cavitation in the pump).

#### cjl

Well, pumps deliver at higher pressure than they intake. The outlet piping velocity will be dependent on the outlet pipe area (due to continuity), not the other way around.

The reason inlet piping is usually larger diameter than outlet piping is to minimize pressure losses upstream of the pump, in order to ensure the pump has adequate net positive suction head (to minimize cavitation in the pump).
True, but some pumps directly increase the pressure of the flow. An example of this is pretty much any piston based pump - the outlet of the pump tends to be about the same velocity as the inlet, but at a significantly increased pressure (which of course can be traded for velocity easily enough). The pump does not inherently accelerate the flow, although it can be used to accelerate the flow if the pump is in conjunction with a nozzle. Centrifugal pumps accelerate the fluid significantly within the pump itself, so at the exit of the pumping mechanism, the flow is much faster than the inlet. You could indeed slow the flow down through a diffuser and then have an outlet that is the same size as the inlet, but the pump's mechanism inherently accelerates the flow. As a result, the outlet tends to be smaller than the inlet.

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