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Liquid ring pump

Liquid ring pump

Liquid ring pump

In our plant we are using a liquid ring pump to compres a gas from 80 mbar up to 4 bar.
The pump is a Siemens Elmo, single-stage double acting pump. The complete pump is build of 2 pumphouses, a low-pressure and high-pressure house.

The low-pressure pump has a diameter of 50cm and rotates with 750rpm.

The high-pressure pump has a diameter of 30cm and rotates with 1500rpm.

We understand that the diameter of the high-pressure pump needs to be smaller, the volume of the gas at the inlet of the highpressure pump is smaller due to the compression.

Our question is:
Why is it that the high-pressure pump turns faster??
When we reason it, higher RPM has the complete opposite function of the smaller diameter. In fact, the capacity of liquid ring pumps can be controlled with the RPM's.

Who can give us answers?

Thanks in advance,

RE: Liquid ring pump


A Liquid Ring Pump (LRP) is a compressor - in the application that you are using it.  Like any positive displacement compressor (in which category it falls), it's displacement is what gives it its capacity.  This displacement depends on the design of the housing and the rpms.  Siemens Elmo is the expert here, since they fabricated the beast(s), and can easily answer your specific questions.  They should be consulted.

Since you have described 2 (two) LRPs with distinct rpms, the two machines' displacements are designed to coordinate and yield the desired delivery capacity as well as the intermediate, inter-stage pressure.  The overall design is critical for the efficient distribution of the compression work between both stages (or LRPs) and this is the function of Siemen's choice of housings.   The fact that they selected two different rpms for the two machines indicates to me that they selected the housings on an economical decision.  This often happens with fabricators who are forced (by the purchaser) to compete price-wise with other fabricators.  Often, they are forced to choose the cheapest way to resolve the problem - rather than supply what they would consider to be the best and most efficient manner of working the problem.  Utilizing existing engineering design that is "on the shelf" is much cheaper than starting a new, specific design for each application as it comes along.

RE: Liquid ring pump

Not the rotation speed but the tip speed is important for your sizing: 19.6 m/s for the first stage and 23.5 m/s for the compressor are perfect.

Liquid ring vacuum pumps require a tip speed in the range 16 - 20 m/s, LR-compressors are running in the range 20 - 30 m/s (depending of design and material).

The higher tip speed is necessary in the second stage in order to keep the ring stable  (because of the higher gas density).

The lowest possible speed is choosen in order to save energy (el. power absorbed).

A "Direct drive" with a 4-pole motor is also a good choice.

It is certainly better to control the suction capacity / suction presssure with a by-pass than trying to  control the rotating speed.

hope it helps,


RE: Liquid ring pump

Thanks for your reply's!
It was all very helpfull!
Especially the site "vacuum-guide.com" helped a lot, I found some very usefull downloads at the Stirling-SIHI site.


RE: Liquid ring pump

One more question.
I was thinking more about the tipspeed.

your saying:
"The higher tip speed is necessary in the second stage in order to keep the ring stable  (because of the higher gas density)."

Is that because of the fact that, if the density is higher, the force that's hold on the ring by the compresses gas will be larger? And that's the reason why the we need a larger tip speed, so the ring will be in place due to a larger spin force?
Is that correct?


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