Subject : Rotary pumps.
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:.
- They will put out a constant volume of liquid regardless of
the pressure they encounter. They put out the constant volume with
each rotation of the shaft.
- They do not impart velocity to the liquid they are
- The discharge pressure is determined by resistance, and not
affected by the specific gravity of the fluid.
- There is no radial thrust transferred to the shaft as you move
on the pump curve.
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:
- The strength of the pump casing and the internal
- The power available from the pump driver (normally an electric
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.
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 :
- External gear, Internal gear, Lobe, Progressive cavity, Three
screw, Two screw, One screw, Flexible tube, Sliding vane, Flexible
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:
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
- The centrifugal pump curve shape is determined by the specific
speed or shape of the impeller. Although there are a number of
head/capacity combinations possible, there is only one best
efficiency point. If you want to match the best efficiency point
(bep) of a given size pump to your application, you're going to
have to change the impeller diameter or speed of the pump.
- The centrifugal pump application engineer is charged with the
responsibility of matching the system curve requirements with the
pump curve. This problem does not exist with rotary pumps. They
will supply what ever head is needed to move the fluid, but no
- Rotary pumps do not have a best efficiency point. They pump a
given capacity against any pressure the system requires. If you
want to change the capacity you have to change the speed of the
pump. You do not have the option of trimming or changing a
component inside the pump.
- If I wanted to fill a tank with a centrifugal pump I would
fill the tank from the top because that is the only way I could
keep a constant head on the system and keep the pump close to its
best efficiency point. If I were using a rotary pump, I would fill
the tank from the bottom because the pump would be using less
power during the filling process (power is foot pounds or head
Lets talk about the fluids you'll be
- Centrifugal pumps work best with low viscosity fluids (like
water) that do not contain entrained air. A centrifugal pump has
to be primed before it can pump liquid.
- Rotary pumps work best with viscous (thick) fluids because the
viscous fluid fills the clearance areas as well as the pumping
cavities, and the less clearance you have in a rotary pump the
better it works.
- This means that rotary are more efficient than centrifugal
pumps when the fluid is viscous, but less efficient with low
viscosity fluids because of "slip" (the cross over point varies
from 700 to 1000 SSU).
- They also have the advantage of being self priming because
they can pump gases as well as liquid.
- Tight tolerances means more wear when you are pumping a slurry
or abrasive fluids. If you are pumping either of these you should
run at pump speeds well below those used for clean lubricating
liquids. In slurry applications the wear rate is proportional to
the speed. Caution : Be sure to keep the speed high enough to keep
all velocities within the pump and system above the critical
carrying velocity of the slurry.
- Specify pumping elements which combine soft and hard materials
to reduce abrasion and provide resistance to the solids imbedding
into the pump components.
- Since rotary pumps are positive displacement pumps and
slurries have an inherent tendency to settle and clog piping, over
pressure protection should be part of the system. Slurry service
precludes the use of many conventional relief valves, but rupture
discs and other options are available.
- The corrosion rate of the slurry should be a prime
consideration in selecting the pump materials. Most corrosion
resistant metals form a protective oxide layer (we use the term
"passivated" to describe this), that will be removed by the
slurry, increasing the corrosion rate of the metal
Effect Of Viscosity On The Pump And System Performance
- The net positive inlet pressure required (NPIPR) increases
with increasing viscosity.
- The required input power increase with increasing
- The maximum allowable pump speed decreases with increasing
- The pump slip decreases with increasing viscosity. This has
the affect of a slight increase in the gpm output.
- The outlet pressure does not increase with an increase in
- The centrifugal pump has a maximum, or shut off head
determined by the impeller diameter and shaft speed. The
centrifugal pump head changes as the capacity changes. As you
throttle or slow down the capacity, the head will increase at the
rate shown on the pump curve. If you double the speed of a
centrifugal pump it is capable of putting out four times the head
at the slower speed.
- Changing the speed of a rotary pump to vary its capacity has
little to no affect on its pressure output. The pressure is
determined by the resistance at the pump's discharge.
- The rotary pump will work against any back pressure, provided
you have the horse power or kilowatts to drive the pump. Unlike
the centrifugal design it does not have a maximum head or
pressure. Operating against a closed discharge valve will cause
the rotary pump to continue to build pressure until it either
overloads the motor, or damages a component. All of this means
that you will need a pressure relief valve in the discharge system
or built into the pump casing.
- If you double the speed of a centrifugal pump it will require
eight times the horsepower to drive it because the capacity will
double, but the head will increase four times.
- If you double the speed of a rotary pump it will require twice
the horse power because only the capacity will double.
- If you can get the fluid to a rotary pump it will pump it. The
trick is to get it there. Instead of the term NPSH (net positive
suction head) rotary pump people use the term Net Positive Inlet
Pressure (NPIP), but some people are hard to change, so the term
NPSH is still often used with rotary pumps.
- Centrifugal pump NPSH is determined by holding the speed and
suction pressure constant and then throttling the suction until
you get a 3% drop in discharge head. The test is a lot more
reliable if you use room temperature deaerated water to remove any
small amount of bubbles.
- Rotary pumps are often selected to move liquids with a low
vapor pressure point, or fluids with a lot of entrained bubbles.
This means that NPIP required (NPSH) is difficult to test. The
Hydraulic Institute establishes the point at the first indication
of any of the following.
- Cavitation noise is heard.
- A 5% reduction in capacity at constant differential
pressure and speed
- A 5% reduction in power consumption at constant
differential pressure and speed.
pumps present a few advantages over their centrifugal cousins. These
- Flow is independent of pressure. You can change the flow
without upsetting the pump's efficiency.
- The pump can handle high viscosity fluids efficiently.
- The pump is self priming
- You get a smooth pulse free flow of the liquid into the
- You can get the desirable high head low flow combination that
is need in many high pressure applications.
- They give you a non-shearing action that will not degrade
sensitive petrochemicals and polymers
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
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