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why pipe diameter increases after pump discharge

why pipe diameter increases after pump discharge

(OP)
i have noticed on most,if not all, situations that there is a reducer pipe installed immediately after centrifugal pump discharge to increase the diameter of the discharge pipe. why?

RE: why pipe diameter increases after pump discharge

The outlet flange and size of the pump is optimized to reduce cost and velocities can be quite high. It makes no appreciable difference if the outlet flange were say 8" diam vs 4" diameter to the process, but a big difference to the cost of the casting / pump body.

However economic sizing of the downstream pipe usually requires a larger pipe.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: why pipe diameter increases after pump discharge

When comparing the differences in diameter on the suction and discharge pump nozzles, a number of considerations are taken into account.

The discharge nozzle size of a pump is sized to relate to the velocity of the flow for which the pump is designed.

The suction nozzle size is usually sized one size larger than the discharge nozzle. Some exceptions are noted in the case of solids-handling pumps where the suction nozzle size is made the same size as the discharge nozzle so that particles that enter the pump can also exit the pump. This assumes that the impeller design in that pump will also pass the same particle size.

The reducer on the inlet side of a pump ensures that the velocity of liquid in the suction line is slowed sufficiently to provide a smooth flow of liquid with minimized friction losses in the pipeline to ensure as high as possible an NPSH available from the system.

The pipe increaser (reducer) on the discharge side of the pump increases the diameter of the discharge pipeline at the pump nozzle to reduce the velocity of the liquid in the discharge pipeline. The lower velocity reduces the friction head which allows the pumping system to operate at a lower pressure.

The pump nozzle size is related to the design of the pump. The discharge pipeline size is related to the friction head of the pumped fluid.

RE: why pipe diameter increases after pump discharge

For the discharge side, the practical reason is that in order to reduce losses the discharge velocity in the pipe should be in the order of 5-10 ft/s inside a facility. Anything higher will result in excessive friction loss and a lot of noise emanating from the pipe because the velocity is so high. If you are sending water out into a municipal water transmission pipe the velocity in the pipe should be 5 ft/s or less. Remember that with anything you do there is a minimum velocity that is required to overcome the check valve to ensure it does not chatter

For the suction side, the practical reason is that the maximum velocity in the pipeline should be limted to around 5 ft/s. Anything higher will result in excessive friction loss which could affect the NPSHa

We as designers don't care why the pump manufacturer size their pump nozzles the way they do. They have their own reasons. We just care about the piping connecting to the pump nozzle.

RE: why pipe diameter increases after pump discharge

(OP)
thank you for all your responses, that's great education you have just given me

RE: why pipe diameter increases after pump discharge

I'm wondering how these reducers apply to "straight run" requirements on each side of the pumps.
For example we can't put an elbow less than something like 5 pipe diameters from the pump.
Is it ok to put a reducer right next to the pump?
(I see them that way all the time, just double checking whether it makes sense)

=====================================
(2B)+(2B)' ?

RE: why pipe diameter increases after pump discharge

2
For me, it kind of depends on the orientation and relative size. In general though a reducer, even an eccentric one ( top flat or bottom flat) going down one or two pipe sizes, doesn't induce a lot of swirl or turbulence, unlike and elbow or tee.

If you have a vertical flange then a concentric reducer should be fine if connected directly to the flange, IMHO.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: why pipe diameter increases after pump discharge

electricpete

Depends somewhat on the type of pump and application.

Split case pumps require a long radius elbow on the inlet. Different straight run recommendations would be made for lower and higher fluid velocities.

https://blog.craneengineering.net/5-basic-rules-of...

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