ROTARY OR POSITIVE DISPLACEMENT PUMPS R019
Rotary pumps make up about 10% of the pumps we use in industry. They are frequently used as priming pumps.
Unlike the more common centrifugal design they are PD (positive displacement) pumps.
- These pumps will put out a constant volume of liquid regardless of the pressure they encounter. The put out the constant volume with each rotation of the shaft.
- They do not impart velocity to the liquid they are pumping.
- 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 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 components.
- The power available from the pump driver (normally an electric motor).
In my lectures I seldom talk about positive displacement pumps because, unlike centrifugal pumps there is very little you can do to modify them and 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 is still a parts replacing business, and you know how to do that without having to go through any special training. However, if you are going to be called upon to solve a specific pumping problem, you are going to need a basic knowledge of these pumps because they represent about 10% 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 section we will leave out the reciprocating types 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
- Progressive cavity
- Three screw
- Two screw
- One screw
- Flexible tube
- Sliding vane
- Flexible vane
The following table will give you a feel for the capabilities of some of the above designs:
In the following paragraphs we will investigate the main differences between these designs and the centrifugal pump that dominates about 90% of the chemical process market.
The Pump Curve
The specific speed or shape of the impeller determines the centrifugal pump curve shape. Although there are a number of head/capacity combinations possible, there is only one best efficiency point (BEP). If you want to match the best efficiency point (BEP) of a given size pump to your application, you are 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 whatever head is needed to move the fluid, but no more.
Rotary pumps do not have a best efficiency point (B.E.P.). 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 he only way I could keep a constant head on the system and keep the pump close to its best efficiency point (B.E.P.). 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 (remember that power is foot pounds (Kg meters) or head x capacity)
Lets talk about the fluids you will be pumping.
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 any liquid.
Rotary pumps work best with viscous 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 pumps are more efficient than centrifugal pumps when the fluid is viscous, but less efficient with low viscosity fluids because of “slip”.
- They also have the advantage of being self-priming because they can pump gases as well as liquid.
- Tight tolerances mean more wear if you are pumping a slurry or abrasive fluid. 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 that 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 dramatically.
The 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 viscosity
- The maximum allowable pump speed decreases with increasing viscosity.
- 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 viscosity.
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 resistance at the pump’s discharge determines the output pressure.
The rotary pump will work against any back pressure, provided you have the horsepower 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 horsepower 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 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.
Rotary pumps present a few advantages over their centrifugal cousins. These advantages include:
- 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 system.
- You can get the desirable high head low flow combination that is need in many high-pressure applications.
- Rotary pumps can run backwards,, but you will lose the advantage of their built-in over pressurization feature.
There is a downside also
- They do not work well with low viscosity fluids such as water; too much internal leakage and excessive wear
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.