By Richard A. Rauth, KNF Neuberger, Inc., Trenton, NJ
For designers of small fluid-handling systems, failing to consider the many variable conditions affecting the operating environment for a pump can lead to problems.
Late in the design phase of a new product, a sterilizer manufacturer ordered a pump from a supplier without properly defining minimum and maximum pressure requirements for the system. The designer selected a pump with maximum pressure of 43 psi, which also was the minimum pressure required for the sterilizer to function, and didn’t consider other external conditions, such as altitude. When the sterilizers went to work in high-altitude locations, where absolute pressure is naturally lower, pump pressure dropped below 43 psi and, ultimately, the sterilizers malfunctioned. The costly fix was to revisit and redefine the operating pressure range (consistent with ambient conditions), and then modify the pump to conform.
Diaphragm pumps come in a range of sizes and configurations.
Such mishaps are all too common and stem from failing to take into consideration all the design parameters involved in selecting a pump. To avoid this scenario, a little knowledge of the basics of pumps and the right questions to ask when selecting a pump for an application goes a long way.
Pumps serve as dynamic subsystems and the days are long gone when designers were forced to compromise with standard products that could not be modified for a particular application. Today, customization is king. Tailoring a pump to perform perfectly within a system is the norm rather than the other way around, especially with evolving technology advances in manufacturing methods, materials, controls, and construction, providing you with an array of options, features, and application advantages.
Among the types of pumping systems delivering modest flow rates, diaphragm, peristaltic, and linear pumps are among the most commonly specified.
Diaphragm pumps use inlet and outlet valves to create pressure or vacuum and to transfer air, gases, or liquids within a system. 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, and check valves control flow into and out of the pump chamber. Designed without rotating or sliding seals, the pumps are particularly suited for applications requiring contamination-free pumping, corrosion resistance, and long service life.
Depending on the type of drive used, diaphragm liquid pumps can be categorized as high-speed and low-speed pumps. Low-speed diaphragm liquid pumps operate at speeds up to about 300 strokes/min, making these pumps especially suitable for metering. The flow rate can be varied by changing the stroke of the diaphragm or motor speed in response to a change in an electrical signal.
Many requirements for fluid transfer can be met with simpler means using high-speed diaphragm pumps. They operate preferably in the range from 2,500 to 3,000 strokes/min, 10 times as fast as their slow-running counterparts. Their size, on the other hand, is in inverse proportion to their speed, so that very compact dimensions are one of the particular advantages of high-speed diaphragm pumps.
Peristaltic pumps use a flexible tube progressively compressed by a series of rollers to induce liquid flow within a system. As the rollers travel along the tube, fluid is forced through the tubing and contained fluid will leak if the tube ruptures. Because the tubing can be sterilized, these types are used largely to transfer sterile fluids (such as blood-transfusion equipment or automatic liquid-feeding devices for hospital patients). Other applications include multi-channel pumping, where one pump transfers many fluids simultaneously through a number of tubes.
Linear pumps apply mechanical, magnetic, or pneumatic displacement to move the central portion of a diaphragm in linear fashion (instead of flexing the diaphragm with rotating elements found in other types). Energizing an electromagnetic coil, or solenoid, pulls the piston against a spring, causing fluid to enter the pump chamber. De-energizing the coil allows the piston to return, which forces fluid past the pump’s outlet check valve.
The Five Questions
Regardless of pump type, how can designers best arrive at the right pump for the right application? Here are five of the most important questions to ask to move the process forward:
1. Have the voltage and performance requirements been defined? The performance of a pump can vary significantly, depending on the equipment and the environment in which the pumping system will operate. In addition to considering rated design conditions, planning for variations in power supply, media temperature, pressure, and loading can help guide decision-making. A best practice is to define a pumping system’s performance requirements over a range of externally variable conditions, such as altitude and ambient temperature, rather than at a single operating point.
As a first step, general requirements should be evaluated, including space limitations for the pump, wetted materials, power available to drive the pump, and the target cost range. The focus can then turn to the pumping system’s tolerance to various system specs, in addition to the conditions for which the system is primarily designed.
2. What are the system’s electrical considerations? Defining a nominal voltage with an allowable tolerance range will be critical. What power will be available to start and operate the pump? If portability, speed control, or compatibility in worldwide locations is important, a dc-operated pump driven by an universal power supply provides more flexibility than an ac-driven pump. Available motor startup current is critical, especially when the pump must start against system vacuum or pressure. The pump may not start at all, or shut down on over-temperature, with overall system failure to follow.
3. Are vacuum and flow rates within acceptable ranges? If a pump can create a vacuum greater than the vacuum required by a device (as an example, the device contains soft tubing), excessive vacuum could cause the tubing to collapse, resulting in system shutdown or equipment damage. Alternatively, pumping systems that create pressure beyond a system’s range could damage connectors, sensors and other expensive parts downstream, causing external leakage, or even jeopardize operator safety. Properly applied pressure regulating devices are available to avoid such a scenario. Instead of defining a single point of vacuum rate and one flow rate, designers would do well to specify a tolerance range of flow, vacuum, and pressure.
4. Will the temperature fluctuate? While a pumping system may be rated for a specific media temperature, real-world ambient conditions can intrude. For example, the pump may be mounted inside of a machine or instrument where the localized temperature is significantly higher than rated temperatures due to lack of proper ventilation. This could ultimately lead to immediate pumping system shutdown and equipment failure. Understanding that high ambient temperature and improper ventilation can shorten the life span of any pumping system can help fine-tune the specification and avoid costly problems.
The internal workings of a diaphragm pump.
5. How will the duty cycle play out? Anticipating duty cycles can contribute toward savings on energy, overall design costs, and increase lifetime. While some pumps operate continuously at a fixed speed, these usually are the exceptions in today’s energy-conscious environment.
For example, medical analyzers perform many detection functions, such as optically reading the contents of a vial. While the analyzer’s control logic performs this function, fluid movement changes within the system. Since full pump performance isn’t needed during this inactive period, it will frequently be reduced in speed or turned off, ideally by the analyzer’s logical controls. Systems may require that the pump’s performance vary during fluctuating demand. In these cases, a brushless dc motor equipped with logical speed controls can provide this feature.
What if the pump is required to restart against a load? Most pump motors are sized to only start at no-load conditions on inlet and outlet. Similarly, a pump may be required to restart against vacuum or pressure, which most standard pumps cannot. Modifications, such as using a larger motor, can eliminate startup problems.
Other conditions to consider for a successful design include minimizing any liquid cavitation effects, the need for self-priming capability, chemical compatibility between pump and the fluid as well as handling any particulates in the media, reverse flow through the pump, and any system leakage.
Identifying and evaluating the above requirements, and then selecting a pump consistent with the application and operating conditions, goes a long way towards getting the design right the first time. Also, partnering with an experienced pump manufacturer early in the design stage can create an alliance that can help to lower costs and optimize pumping system performance.
KNF Neuberger, Inc.
Filed Under: Fluid power, Pneumatic equipment + components, Pumps