Designers are usually familiar with the flow and pressure needs of their applications and what they plan on pumping, but that’s just the start. There are many factors specific to each application that need to be evaluated before the correct pump can be selected. Most of these factors can be grouped into environmental, system, and media issues. Consulting early in the design process with a pump company helps ensure that the pump selected will perform as expected while minimizing ongoing maintenance.
Most engineers design and test at or near atmospheric pressure and often do not consider effects that must be accounted for if their equipment will be used at higher elevations, such as in Denver or Mexico City. Elevation affects many aspects of pump performance, including mass flow and lowering the maximum vacuums and pressures that can be achieved. Additionally, as the airflow mass decreases, it affects cooling in the motor and bearings.
Humidity is another important factor, especially if ambient air is being pumped. If the humidity is too low, moisture may be pulled out of elastomer parts and can also shorten the life of bearings and other motor components; if it is too high, condensation can form, especially in the pump head. Once liquid forms, the pump’s internal components must be able to work with the noncompressible and potentially corrosive media. The situation is even more common with multi-stage pump heads. Other important environmental issues to consider are salt air, temperature extremes, and use in hazardous locations.
Just like in a tangled garden hose, the more resistance there is in a pumping application, the more force it takes to push a fluid through it. Every flow-path characteristic (tube ID, material, finish, length, orifices, valves, fittings, elbows, mufflers, etc.) has a pressure drop across it. Adding them all up determines the total change in pressure the pump must overcome to generate flow. Restrictions on the pump’s inlet are especially significant because 14.7 psig is the largest possible suction difference at sea level. Pumps should also be sized to produce the required flow rate even when the filters are dirty and need servicing.
Designers should know what the pump’s duty cycle will be. Often there is no clearcut answer as to whether it should run continuously or be cycled on and off. If so, a list of factors should be weighed to determine the best solution for the application. These factors include motor type (ac, brush, or brushless dc), noise limits, desired life of the pump, battery life per charge, susceptibility to current spikes and vibrations, heat generation, and load at start up and shut down. Controlling the motor’s speed is a popular and successful way to address these factors. Running the pump at lower speeds also saves power and extends pump service life as well as other components upstream and downstream of the pump.
Media can have a large impact on the system pump choice. Aside from the obvious chemical compatibility issues affecting pump component material selection, other aspects must be examined.
Nearly all fluidic components are affected by particles in the gas or liquid being pumped, and the particles’ shape and hardness can be as important as their size in determining how detrimental they will be. For instance, the gears in gear pumps can be abraded by particulates, potentially shortening their service life to unacceptable levels. Fibers can cause potential issues with certain pump technologies. Ideally, particulates should be removed from media by eliminating their entry and/or development or be filtered out on the inlet side of the pump. If elimination is impossible or particles are an intrinsic and necessary part of the media, then their characterization is important to proper pump selection. Application designers should tell all pump and component suppliers what type and how prevalent particles can be expected so they can modify designs or supply the appropriate filtration.
Particles tend to build up over time, increasing the pressure drop and even completely blocking flow in some cases. So turbulent flow can be helpful, or flushing may be necessary. Debris and particles not only affect the pump’s performance, they can create conditions whereby the pump operates beyond its design range.
It is particularly challenging for pumps to handle mixed liquid/gas media, which can occur when aspirating a liquid from a well plate or waste-liquid container. For such applications, diaphragm pumps are ideal because they withstand potentially aggressive liquids and are adept at handling the self-priming and increased flow requirements presented when transferring mixed media. Other media-related challenges that may influence pump selection decisions include liquids near their vapor pressure and gases other than air.
Other Factors to Consider
A trusted pump manufacturing partner will have the application experience necessary to provide guidance through many of the common pitfalls of system design. Some other possible factors to consider include pump orientation and mounting, system sensitivity to pulsation, self-priming needs for liquid pumps, flow rates for the pump when it’s just moving air and just moving liquids, safeguards against downstream blockages (especially if dangerous media are involved), internal and external leakage, regulatory and certification documentation requirements, etc. Limited current and variable supply voltage, as well as interactions with other components and the sensor feedback available, are also some of the more important factors that should be discussed.
Many conditions present themselves over the lifetime of a system. Designers must anticipate them all if the device is to be successful. The best solution is then determined by assessing the customer’s unique application needs and weighing the possible solutions with a trusted pump supplier. This consultative and thorough approach is the path that most often leads to success.
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