Understanding Pump Performance

Pumps are used to move fluids from one place to another and to add energy needed to overcome the resistance to flow posed by pipes, valves, and nozzles. Pumps are broadly divided into centrifugal and positive displacement types; both are used widely in the food industry. Detailed selection of specific pumps and design of fluid handling systems are typical engineering tasks, but anyone involved in food processing should have some basic understanding of pumps and fluid handling principles.

Pump manufacturers are rich sources of information. Waukesha Cherry-Burrell, now a division of SPX Process Equipment, Delavan, Wis. (www.spxpe.com), and Grundfos Pumps Corp., Olathe, Kan. (www.grundfos.com), are two examples of companies upon which I have relied. Waukesha’s Web site includes an engineering manual that can be downloaded. Grundfos publishes a hardbound handbook.

Centrifugal Pumps
Centrifugal pumps are relatively inexpensive and versatile. They operate by rotating an impeller in a circular case. The impeller has vanes that impart a velocity to fluid entering at the center of the case. The fluid is flung to the perimeter of the case and discharged through an exit. Centrifugal pumps can be designed with entrance and exit on opposite sides of the case or in other configurations.

The energy imparted to a fluid by a pump is expressed as a change in pressure across the pump, usually called the head, using units of pressure, such as pounds per square inch (psi) or equivalent height of the liquid, such as feet or meters. The latter expression derives from the common need to raise a fluid to a new elevation. For consistency, the friction losses through pipes, valves, and other equipment are often given in equivalent head units.

It is characteristic of centrifugal pumps that as their volumetric throughput increases, their output head decreases. This is shown in head curves, unique to each pump size and design. For any system, there is also a characteristic curve of head required against volumetric throughput. Typically, head required increases with flow rate. Where the two curves intersect is the operating point for that pump in that system.

As system requirements change, it can be necessary to adjust the performance of a pump. Centrifugal pumps can be controlled by a throttle valve on the outlet and can operate against a closed valve, if necessary. This is wasteful of energy and can cause a temperature increase in the fluid because the energy provided by the motor is converted to heat if it cannot be used to move the fluid.

A bypass loop around the pump provides an alternate form of control. This is also somewhat wasteful of energy, because more fluid is pumped than is required, but it is simple to implement. As will be noted later, positive displacement pumps cannot operate safely against a shut valve, but often do have bypass loops.

A less convenient way to adjust the performance of a centrifugal pump is to use a smaller impeller in the same circular housing. This reduces the flow and output head of the pump, but obviously must be done when the pump is out of service. It is not unusual to specify a larger pump case and drive than is initially required, and to install a smaller impeller with the intention of increasing the impeller when capacity must be increased.

Finally, pump performance can be controlled by adjusting the speed of the drive motor. This is commonly done by a variable frequency drive (VFD). A VFD consists of a rectifier, an energy storage device (capacitor), an inverter, and associated control circuitry. There may be a filter to protect other devices from noise. Alternating current (AC) at 50 or 60 cycles per second is converted to direct current in the rectifier, stored, and then converted to square wave pulses of varying width in the inverter. The effect is to modulate the motor speed, and hence the pump flow and head, in response to some control signal, such as pressure or temperature.

VFD control is more energy efficient than throttle valves or bypass loops and so can compensate over the life of apump for the higher initial costs.

To achieve higher output pressures than one pump alone can provide, centrifugal pumps can be connected in a series. When this is achieved within one case with one drive, it is a multi-stage centrifugal pump. In food processing, centrifugal pumps are often used in clean in place (CIP) circuits, where high velocities are desired. CIP solutions are water-based and so have low viscosities compared with many food fluids. Centrifugal pumps work well with lower-viscosity liquids.

The inlet to a centrifugal pump needs to be at a pressure higher than the vapor pressure of the liquid at pumping temperature in order to avoid vaporization of the liquid. If vaporization occurs, the pump may not operate, or cavitation may occur when vapor bubbles collapse as the pressure increases in the pump. This can damage the pump. The solution is to provide net positive suction head (NPSH), typically by having the feed supply to the pump several feet above the pump.

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Positive Displacement Pumps 
A positive displacement pump has little or no slip of the fluid and so can provide whatever output head is required to transport a given volume of fluid. The upper limit on output head or pressure is usually set by some mechanical feature, such as the shaft seal or case gasket.

There are several basic types of positive displacement pumps, including piston, rotary lobe, screw (progressing cavity), and gear pumps. Piston pumps provide a varying flow and so are often designed with multiple pistons to smooth out the flow—two or three are common. Homogenizers and high pressure feed pumps for spray dryers are triplex piston pumps. Most positive displacement pumps have pressure-relief valves and internal bypass loops to protect the pump and downstream equipment from excessive pressures.

Rotary lobe pumps such as those made by Waukesha and Fristam, Middleton, Wis. (www.fristam.com/usa), are found in many food plants because they can handle viscous fluids and are made in sanitary designs. Sanitary design of pumps implies that they are constructed of polished stainless steel, have curved rather than sharp intersections of planes, and can be disassembled and inspected easily with minimal tools. Shaft seals are designed to be easily cleaned. Often the face of the case can be removed by loosening wing nuts or wing bolts.

Rotary lobe pumps require a net positive inlet pressure (NPIPR) to ensure that fluid fills the pump cavity. This can be compared to the net positive inlet pressure available (NPIPA). Both are typically expressed in pressure units (psi). The requirement is a function of pump design, inlet diameter, flow rate, and viscosity. The available inlet pressure may be reduced by increasing flow because of increasing fluid friction and by increasing temperature because of increasing vapor pressure.

If a rotary lobe pump experiences difficulty because the available inlet pressure is less than required, there are several remedies. Slowing down the pump decreases the flow rate and reduces the required pressure. Increasing the inlet diameter reduces the friction loss. Shortening the inlet line length, reducing the number of fittings, and reducing the number of size and direction changes all help reduce friction losses.

Increasing the pump size reduces the inlet required pressure. Finally, elevating the feed tank, lowering the pump, or pressurizing the feed source all increase the available inlet pressure.

In one case, where a rotary lobe pump was operating near its pressure limit, a solution was to decrease the flow resistance of the system by putting some heat exchangers that had been linked in series into two parallel lines, thus cutting both the flow rates and length in half and reducing the pressure drop by about a factor of four.

Progressing cavity or screw pumps, such as those made by Moyno, a unit of Robbins & Myers Inc. Springfield, Ohio (www.moyno.com), typically have a rotating, screw-shaped device in a stator, which is often made of rubber or another polymer. Such pumps can tolerate abrasive fluids and, when properly selected, can transport fluid with suspended particles. Reversing rotational direction can reverse the flow, giving some flexibility in installation. Because the stator is intended to be soft and resilient, it can wear if abused. A situation in which stators disintegrated in use was traced to manufacturing defects, but the pumps were replaced to prevent recurrences.

Gear pumps typically have very close tolerances and are often used as metering devices because their delivery rates can be precisely controlled by motor speed control. In contrast to some other pump designs, they are usually disassembled for cleaning by hand and may need to be cooled before the parts fit back together. Rotary lobe pumps are usually bypassed for cleaning, but may have CIP solutions run through the case with lobes removed. As previously mentioned, flow rates for CIP are usually several times higher than flow rates for food products, so higher-speed pumps are used for cleaning, and the food pumps are treated as part of the system to be cleaned.

by J. Peter Clark,
Contributing Editor,
Consultant to the Process Industries, Oak Park, Ill. 
[email protected]