Hello.

Why are in general, water pumps outlet made smaller diameter than their inlet ?

This is not asking why inlets are larger; but you can also elaborate on that if you want.

Volume of flow, when you increase pressure you are moving a greater volume of material compared to a lower pressure inlet. In short its to make sure you have enough inlet material to prevent pump cavitation, which can damage a pump

Thanks. ¿? The volume in is the same as volume out. The speed changes. Why is faster outlet speed preferred ?

Its not necessarily speed but change in pressure. Though velocity can come into play. It depends on what the pumps application is.

 

Not all pumps have difference inlet/outlets. Transfer pumps have identical inlet outlets. Those type of pumps doesn't care about pressure.

Their function is to move a fluid from a to b.

 

Pressure pumps are designed to increase the pressure, placing the same volume of material inlet to outlet side and reducing the area of piping from one side to the other increases the pressure.

 

Hydraulics is based on pressure/volume relations.

Not all pumps have difference inlet/outlets. Transfer pumps have identical inlet outlets. Those type of pumps doesn't care about pressure.

Their function is to move a fluid from a to b.

Thanks.

¿? It is about the ones with different in-out diameters. I would say pressures do matter to all pumps.

All pumps move fluid volume from a to b. If b is confined/restricted in flow, pressure rises. Still, why is the outlet made usually smaller ?

A transfer pump cares about volume not pressure. Ie move contents of tank a to tank b. Line size just needs to be sufficient for the volume rating. Pressure pump applications are different line diameter is critical

.. Still, why is the outlet made usually smaller ?

Back pressure to take out any possible slack in local pressure along the feed-line, which could cause air pockets, and result in flow anomalies.

Edited May 12, 201510 yr by StringJunky

Take a 2" line at 20 psi if you reduce contents of the 2" line to a 1" Line you get greater pressure. Pressure means force. So for instance in Hydraulics greater pressure is greater lifting capacity

Back pressure to take out any possible slack in local pressure along the feed-line, which could cause air pockets, and result in flow anomalies.

Thanks I couldn't remember the technical term lol.

Constant volume pumps have similar inlets and outlets as they do not rely on the larger pressure of the larger diameter to move fluids at a higher speed in the lesser diameter.

Think of a piston in a cylinder. With the inlet valve open, the cylinder fills up, then the inlet valve closes and the outlet valve opens. The upward motion of the piston then forces the fluid out, such that ( downstream of the outlet valve ) the fluid is at the same pressure a9 and hence volume ) as upstream of the inlet valve.

Since water is practically incompressible, the volume flows in and out are pretty much the same.

At the outlet, much pressure+KE is available, and some pumps let adjust the proportion by design. A big speed reduces the pipe diameter, and if the use demands it, it can convert to pure pressure (imperfectly in this direction).

 

At the inlet, the designer has usually no choice. The pump is needed because the pressure is small, so the achievable speed is limited and it demands wider pipes.

 

Take water at 1atm and RT: it was little vapour pressure so one could convert nearly all to speed, or 14m/s - but the impeller still needs pressure! Its blades have a speed mismatch versus the water, and the blades must accelerate the water too, so only a fraction of 14m/s are available for the inlet, 5m/s or slightly more.

 

Near-boiling liquids are more difficult.Oxygen at 90K and 1.5atm has only 0.5atm convertible to speed before it boils, that would be 9.4m/s.

 

Discharge pressures like 700bar in one stage are easy for a centrifugal pump: count with 350m/s impeller speed. Problem, the inlet prefiribly has 1/3 the impeller diameter hence 120m/s, letting the liquid cavitate at the inlet.

- For milder situations, the inducer suffices. Over several turns, this helix just in front of the impeller brings the liquid from a small axial speed into a big azimutal one and raises the pressure too, so the impeller produces no cavitation.

- For difficult cases, a booster pump may raise the inlet pressure to reduce the size of the main pump.

 

Very cute example there: http://www.lpre.de/energomash/RD-170/index.htm

- On Рис.10 (Fig. 10, at about 55% of the document), the inducer has nearly the diameter of the impeller for liquid oxygen.

- On Рис.11в (60% of the document), the booster pump, powered by its own turbine receiving hot oxygen.

This pumps 1792kg/s of liquid oxygen to 602bar within a D=409mm impeller. Burnt in four ID=380m chambers (Рис.3, 35% of the document), it lifts ten engine railways.

 

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The situation is reversed at water turbines, and the thrive for efficiency brings its difficulties, demanding the least possible outlet speed. Cavitation imposes to put the wheel below the outlet altitude and build huge smooth outlet channels:

http://en.wikipedia.org/wiki/File:Hydroelectric_dam.svg

from http://en.wikipedia.org/wiki/Hydroelectric_dam

I remember having a pump that had same size inlet and outlet. Some design which could handle both water and air, but did slowly move its tubing along in the outlet direction.

Edited May 13, 201510 yr by Endy0816

There is much confusion about the workings of centrifugal pumps and this thread shows that even normally accurate experts are confused by this machine.

 

About the only completely correct posts were the original question about inlet and outlet sizes and John Cuthber's observation that volume out must equal volume in for an incompressible fluid like water.

 

It was also correctly observed that some pumps have equal inlet and outlet pipes sizes, particularly positive displacement types that operate in an entirely different manner and have no need of the change of size.

 

The plain fact is that the size change is an essential part of the workings of a centrifugal pump and must occur. The actual outlet size will depend upon where the expansion occurs, it is often largely included in the casing.

 

Needless to say the operation of a centrifugal pump has nothing to do with 'centrifugal force' and in fact the centripetal force the fluid is subject to impedes the pump action and must be overcome by the mechanics of the pump.

I think I can add another couple of correct statements;

Re

 

Take a 2" line at 20 psi if you reduce contents of the 2" line to a 1" Line you get greater pressure. Pressure means force.

Mr Bernoulli doesn't think so: and no it doesn't.

The Bernoulli family were certainly your friends as far as centrifugal pumps and pipelines are concerned, although they never explained the workings of such pumps, which were first introduced in the 15th century.

 

http://en.wikipedia.org/wiki/Bernoulli_family

Wasn't one of them also quite crazy ?

Or am I thinking of someone else ( Boltzman perhaps )

Mad or not you need Bernoulli to understand how the difference between the operation of a positive displacement and a centrifugal pump.

 

In particular you need to be able to pick the correct answer in this question.

 

https://www.youtube.com/watch?v=Dvs_AV4JYdw

 

 

 

 

Edited May 21, 201510 yr by studiot

Needless to say the operation of a centrifugal pump has nothing to do with 'centrifugal force'

 

Doesn't it?

 

If you count only 0.5*density*speed² you compute less than the pressure achievable by a centrifugal pump. That would be the pressure resulting from the liquid being as fast as the impeller and converting this speed to pressure. A centrifugal pump achieves up to twice that pressure gain.

 

Doesn't it?

 

If you count only 0.5*density*speed² you compute less than the pressure achievable by a centrifugal pump. That would be the pressure resulting from the liquid being as fast as the impeller and converting this speed to pressure. A centrifugal pump achieves up to twice that pressure gain.

 

 

Perhaps you mean multistage pumps?

 

Here is an extract from the Grundfoss centrifugal water pump design manual.

This speaks for itself