Subject : Rotary pumps. 12-3

Rotary pumps make up about 10% of the pumps we use in industry.

Unlike the more common centrifugal design, they are positive displacement (PD) pumps with several advantages:.

Did you notice I used the word "pressure" in the above paragraph? You will recall that centrifugal pump people substitute the word "head" because, unlike the PD pump, the discharge pressure in a centrifugal pump is determined by the specific gravity and volume of the fluid you are pumping.

The head of a centrifugal pump was limited by the diameter of the impeller and its speed. How much pressure will a rotary PD pump produce? It is limited only by:

In my schools I seldom talk about positive displacement pumps because, unlike centrifugal pumps, there is very little you can do to modify them to increase their performance. They are basically a spare parts business, where the performance of the pump is directly related to how well you maintain their internal clearances.

In other words, if you were an expert in rotary pumps it's still a parts replacing business, and you can learn how how to do that without having to go through any special training. However, if you're going to be called upon to solve a specific pumping problem, you're going to need a basic knowledge of these pumps because they represent a percentage of the pumps used by the process industry and present the same sealing problems as their centrifugal cousins.

Rotary pumps come in various configurations. In this paper we will leave out the reciprocating type of PD pump and address the rotary version only. You should know that there are several different rotary configurations being offered to industry. Among them :

If you are unfamiliar with some of these designs, almost any pump book has pictures of most of them. The following table will give you a feel for the capabilities of some of the above configurations:

ROTARY PUMP
GPM
PSI

Gear

1200 gpm

500psi

Lobe

1200 gpm

500 psi

Progressive cavity

1000 gpm

1000 psi

Three screw

1000 gpm

500 psi

Two screw

9000 gpm

1500 psi

Vane

1000 gpm

150 psi

In the following paragraphs we'll investigate the main differences between these designs and the centrifugal pump that dominates about 90% of the chemical process market.

The Pump Curve

Lets talk about the fluids you'll be pumping.

Pumping Slurries

The Effect Of Viscosity On The Pump And System Performance

The Head

Horsepower Requirements

NPSH Required

Rotary pumps present a few advantages over their centrifugal cousins. These advantages include:

In summary, PD pumps are great pumps and we would use a lot more of them if they could produce the volume of fluid most of our process applications require.

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