Guidelines for Selecting Small Pumps

by Eric Pepe, KNF Neuberger, Inc.

Failing to consider variable conditions can lead to big headaches in small fluid-handling systems.

A number of technological breakthroughs in fluid-handling pumping systems have occurred over the past ten years, including better materials, advanced controls, and more efficient constructions, giving engineers more design flexibility than ever before. Unfortunately, the advances often go to naught.

While a pump is generally the heart of a system, clear guidelines highlighting pump selection criteria are often lacking. Too often, standard products are purchased late in the design process to meet a system’s dynamic needs—in other words, fitting the system to the pump, rather than the other way around. Another common mistake is to design for a specific set of operating conditions while gailing to account for the unexpected.

Successful design

Identifying key pumping system requirements and communicating those needs to a supplier early in the design process leads to more efficient, longer lasting, more cost effective systems. Perhaps the single, most-important guideline to selecting the right pump for an application is to understand that pumps are subsystems—dynamic, interactive elements of the equipment in which they function.

Most equipment designers, on the other hand, tend to think of pumps as commodities, rather than as dynamic subsystems, because most pump suppliers provide standard products that cannot be modified to meet specific operating requirements. In the past, designers were generally forced to compromise between two less-than-optimum products, with no alternative in between. Today, however, more and more suppliers modify pumping systems to meet specific needs. Thus, a wide array of options is available. Tailoring the pump to the system, although often more expensive, is usually more economical in the long run.

Performance requirements

Among the types of pumping systems appropriate for modest fluid flow rates, diaphragm, peristaltic, and linear pumps are used most often. However, performance can vary dramatically depending on the equipment—and the environment—in which the pumping systems operate.

Small pumping systems available today are better than ever. However, getting the most out of them means not only considering rated design conditions, but also planning for variations in power supply, temperature, pressure and loading.

Pumping system performance varies as conditions within the equipment change. Failing to consider, for example, temperature or electrical power variations beyond defined tolerance limits could cause the pump to malfunction and the entire system to shut down. Therefore, defining a pumping system’s performance requirements over a range of conditions, rather than at a single operating point, can be vital to selecting an appropriate pumping system.

First, consider general requirements, including the size of the pump, construction materials, power available to drive the pump, and the target price range. This is a good way to "ballpark" the type and size of pump required. However, to zero in on the best pump for an application, it is critical to consider the pumping system’s tolerance to various system specifications, in addition to conditions for which the system is

Too often, pumps are considered mere commodity items. Actually, pumps are dynamic, interactive subsystems in the equipment in which they function. Choosing a standard product that does not exactly match operating requirements, rather than tailoring the pump to the system, leads to less-efficient operation and more headaches in the long run.

Primarily designed. One obvious requirement, for instance, is material compatibility of all wetted components with the pumped solution. However, if the pump will be cleaned with a different fluid, compatibility with this second material cannot be overlooked.

Electrical considerations

A less obvious factor is the power requirement. The pumping system may operate on 115 Vac, 60 cycles exclusively, or may require compatibility with both 50 and 60-cycle power. Voltage conditioning may be another factor, where a voltage transformer allows the pump to operate with 115 Vac in one environment and 220 Vac in another. Specifying a pump’s power requirement at 115 Vac is not enough, however. An allowable tolerance range is critical if the pump operates in a system where the voltage caries. Knowing the tolerance range is also important. If a pump designed to operate at 115± 10 Vac is located where the voltage may vary as high as 130V, the pump may shut down, resulting in overall system failure.

One case in point is a high-pressure liquid chromatography (HPLC) analyzer for which the designer specified a pump with a 115 Vac, 50 and 60-cycle motor power requirement. However, no voltage power tolerance was specified. Some analyzers were shipped to England where, after a transformer step-down, the voltage output was 10% higher than that in the United States. The pump’s motor coil received approximately 127 Vac at 50 cycles, which was beyond the motor’s capability and caused thermal overload and pump shut-off. Bottom line, this rendered a $100,000 HPLC analyzer useless and idled several lab technicians. Incorporated a larger motor with a specially wound coil resulted in a pump with an acceptable voltage.

Pressure and temperature

The same approach applies to other specifications. Rather than defining one vacuum rate and one flow rate, designers must specify a tolerance of flow rate for a range of vacuum pressure.

For example, if a pump can create a vacuum greater than that required by a device—and the device contains soft tubing—excessive vacuum could cause the tubing to collapse, resulting in system shutdown or equipment damage. Likewise, pumping systems that create pressure beyond an instrument’s range can break connectors and other parts, causing damage and even personal injury.

Rather than defining a single vacuum or pressure rating, designers should specify a tolerance range. If a pump produces pressure or vacuum greater than that required for an application, broken connectors or collapsed tubing could result in system shutdown or equipment damage.

Case History

In one case, a sterilizer manufacturer came to a pump supplier late in the design stages of a new product without properly defining minimum and maximum pressure requirements. As a result, the manufacturer selected a pump with a maximum pressure of 43 psi, which was also the minimum pressure needed for the sterilizer to function. Some sterilizers were subsequently operated in Denver, where absolute pressure is lower due to the city’s altitude. Pump pressure dropped below 43 psi and the sterilizers malfunctioned. Redefining the operating pressure range with regard to ambient conditions and modifying the pump solved the problem.

In addition to illustrating the importance of defining key system parameters, this is a good example of why the operating environment—both inside and outside the system—must be considered.

The immediate temperature of the operating environment can also be a critical factor. While the pumping system may be rated for a specific temperature, the pump may be mounted inside a machine or instrument where—due to lack of ventilation, for example—the localized temperature is significantly higher than ambient.

The result, again, could be immediate pumping system shut-down and equipment failure. At the very least, factors such as high ambient temperature and improper ventilation can shorten the life span of a pumping system.

Duty cycles

Anticipating duty cycles (periods of operation and inactivity) is another way to save on overall design costs and reduce the possibility of system failure. While some pumps are simply turned on and then operate continuously at rated speed, this is often not the case.

Foe example, some medical analyzers perform a detection function, such as optically reading the contents of a vial, as part of their overall operation. While the analyzer performs this function, which can take from several seconds to several minutes, fluid movement within the system stops. Because the pump is not needed during this inactive time span, it is frequently turned off.

The period of inactivity, the demands made on the pump when the analyzer performs the next function, and many other factors can be critical to selecting the right pump for the job. For instance, the pump may be required to restart against a load. Most pumping systems must be at no-load conditions on inlet and outlet to restart properly. However, some pumps can be modified to start under these conditions.

In the same vein, a pumping system may need to restart against vacuum. Standard pumps, however, are not capable of restarting against vacuum. Specifying such a pump would subsequently create the need for a complex venting mechanism for the pump to restart properly, or the pump would need to be replaced.

Considering such system requirements early in the design process eliminates the need for retrofitting and the resulting greater costs and development time. Problems in the field are usually not due to a faulty pumping system, but to incomplete requirements being communicated to the supplier.

Key Pumping Systems

Three basic types of pumping systems currently meet most equipment designers’ needs:

Diaphragm pumps use inlet and outlet valves to create pressure or vacuum, as well as to transfer liquids under vacuum. Due to the absence of rotating or sliding seals, the pumps are particularly suited to applications requiring contamination-free pumping, as well as to those requiring corrosion resistance.
Peristaltic pumps use a flexible tube that is progressively compressed by a series of rollers to induce liquid flow within a system. Because the tubing can be sterilized, peristaltic pumps are used most often to transfer sterile fluids, for example, in blood transfusion equipment or automatic liquid feeding devices for hospital patients. Another application for peristaltic pumps is multi-channel pumping, where one pump transfers many fluids simultaneously through a number of tubes.
Linear pumps typically use mechanical, magnetic, or pneumatic displacement to move a diaphragm in linear fashion—rather than flexing the diaphragm with rotating elements, as with other types of pumps. An important advantage of linear pumps is that they can be designed to operate very quietly.

Diaphragm Pump

An elastomeric diaphragm clamped between the diaphragm head and compressor housing forms a leak-tight seal between pump chamber and crankcase. The rotating eccentric causes reciprocating motion at the diaphragm. Check valves control flow into and out of the pump chamber.

Peristaltic Pump

A series of rollers progressively compresses a flexible tube in peristaltic pumps. As rollers travel along the tube, fluid is forced through the tubing. Contained fluid can only leak if the tube ruptures.

Linear Pump

Energizing an electromagnet in this linear pump pulls the piston against a spring, causing fluid to enter the pump chamber. De-energizing the coil allows the piston to return, forcing fluid past the outlet check valve.