Alternate Pump Types: Positive Displacement (Rotary/Reciprocating)

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By Allan R. Budris

All of the writer's Pump Tips & Techniques columns to date have dealt with the centrifugal (rotodynamic) pump type, because that is the most popular and the one most often used in the Water and Wastewater markets. However, there are other pump types which have distinct efficiency and other advantages in certain (special) applications where centrifugal pumps are not well suited, such as for high viscosity liquids, very low specific speeds (very low flow rates at very high pressures), self priming, and vapor handling.

The most common alternative pump types fall into the positive displacement category, rotary and reciprocating. Instead of using the high fluid velocities in a centrifugal pump impeller to move liquid and generate pressure, positive displacement pumps rely on some mechanism to physically capture a liquid volume at the pump suction, and then move it along to the discharge where the capture volume is opened up and the liquid physically forced out.

Rotary Pumps

A rotary pump is a positive displacement pump consisting of a chamber containing gears, cams, screws, vanes, plungers or similar elements actuated by the relative rotation of the drive shaft to the casing, and which has no separate inlet or outlet valves. These pumps are characterized by their close-running clearances. There are seven common basic types of rotary pumps identified by the type of pumping element.

  • Vane (blade, bucket, roller or slipper)
  • Piston (axial or radial)
  • Flexible member (flexible tube, flexible vane, or flexible liner)
  • Lobe (single or multiple)
  • Gear (external or internal)
  • Circumferential piston
  • Screw (single or multiple)

The most popular of these pump types are gear pumps (they are good at handling high viscosity liquids - to 5,000,000 ssu), and screw pumps for medium to high pressures (to 2,500 psi), and very high viscosities (to 200,000,000 ssu),

Most rotary pumps are self-priming and therefore also possess certain abilities to handle fluids consisting of liquids with entrained vapor. The capacity (flow rate) of a rotary pump varies with speed, but may be affected by the pressure because of the effect of pressure on low viscosity slip (see figure 1). Other than this slip (internal circulation) with low viscosity liquids, the only way to change the flow rate of a rotary pump is by changing the speed. The discharge pressure is determined by the down-stream system resistance.

Most rotary pumps are not designed to be run against a closed outlet unless suitably protected against excess pressure (with a relief valve). Rotary pumps with minimal slip flow can create very high pressures if the system is incorrectly selected, or a valve in the discharge line is closed. Slip reduces with increased viscosity, and increases with increased internal clearances.

Generally, rotary pumps are designed to operate with close-running clearances and with wetted internal surfaces. Therefore, they are not always suited to being run dry or for handling fluids containing abrasive solids. However, some rotary pumps, such as single screw pumps with rubber housing liners, and flexible member pumps do have some capability to handle abrasives.

Rotary pumps have a constant torque (for constant viscosity), in contrast to centrifugal pumps whose torque is proportional to the speed squared. Therefore, typical power absorbed curves (see figure 2) vary linearly with speed and pressure, at constant viscosity. An increase in viscosity will increase power.

Figure 3 shows how the NPSHR / RNIP (Required Net Inlet Pressure) of a rotary gear pump changes with pump speed and viscosity. This determines the maximum speed for a rotary pump for a given viscosity. Slower speed gives more time for the liquid to fill the opening voids (between the teeth). The bigger the voids (teeth), the slower the pump. This figure came from a paper the author published in 1980, which is the only paper on rotary pump suction performance. Cavitation can cause airborne noise and sometimes physically damage components within a rotary pump. Dissolved and entrained gas also produce noise in much the same manner as cavitation, but without the damage,

One of the draw backs of a rotary pump, compared to a centrifugal pump, is the higher periodic pulsations generated by the opening of the fluid pockets in the pumping rotor(s), which causes greater noise and pressure pulsations. The larger the rotary pumping pockets the greater the noise, which can require system pressure dampening devices. Also, rotary pump life can be shorter then for a centrifugal pump due to the close running clearances which can open up over time.

Reciprocating Pumps

The basic reciprocating pump types are the Plunger & Piston. Reciprocating power pumps are constructed in horizontal and vertical; single-acting and Double-acting; Piston or Plunger; and Simplex, Duplex or Multiplex types. A reciprocating power pump is one driven by power from an outside source applied to the crankshaft of the pump, much like a car engine. It consists of a liquid end and power end.

Probably the biggest advantage to this pump type is the very high pressure capability, up to 40,000 psi. These pumps move a discrete volume of liquid on each stroke, so in order to change the flow rate the speed must be changed (see figure 4). The maximum flow/speed is limited by valve size, bearing life, inertial loads or NPSH. Flow is independent of pressure. The input power for plunger and piston pumps are proportional to speed (flow rate) and pressure, plus the mechanical energy losses, which can be relatively high at slow pump speeds. Efficiencies are relatively high at higher speeds. At constant pressure, energy savings is proportional to speed reduction, not to the 3rd power for a centrifugal pump, unless pumping into a friction system, where pressure changes with flow.

Draw backs of reciprocating pumps are the high price tag, high maintenance costs, very high pressure pulsations in the suction and discharge lines (which require a dampening device), and the requirement for a discharge relief valve. The insertion of a discharge relief valve of suitable size for the capacity of the pump, set to open at a pressure above the operating discharge pressure required of the pump, is mandatory because of the safety it affords.

Air operated diaphragm pumps are another type of reciprocating pump, but which are powered by shop air and limited in pressure (to the air pressure) and flow rate. Efficiencies are also quite low. They are normally used in hazardous environments where electricity to drive a motor could present a problem. They are also easily portable.

Conclusions

As you can see, when a pump application becomes difficult, or very inefficient for a centrifugal pump, there are other pump types that should be considered, which may provide higher efficiency, and/or longer life. It should be noted that this column does not cover all of the pump types available. Visit the Hydraulic Institute website (www.pumps.org) to see a more complete list, and additional details.

References:

1. Allan R. Budris, P.E., "Rotary Gear Pump Suction Performance", Chemical Engineering - 1980

2. Hydraulic Institute Pump Standards

About the Author: Allan R. Budris, P.E., is an independent consulting engineer who specializes in training, failure analysis, troubleshooting, reliability, efficiency audits and litigation support on pumps and pumping systems. With offices in Washington, NJ, he can be contacted via e-mail at budrisconsulting@comcast.net.

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