5 key tips for specifying a pump
The days when pump system designers were forced to compromise with standard products that couldn’t be modified to their specific applications are long gone.
Now customization is king. And tailoring a pump to perform perfectly in a system is the norm. The key to finding the right pump for the job involves first understanding the pumps and then asking the right questions.
Let’s start with the pumps. Diaphragm, peristaltic, and linear pumps are among the most commonly specified among the types of pumping systems delivering modest flow rates.
Diaphragm pumps use inlet and outlet valves to create pressure or vacuum and to transfer air, gases, or liquids in a system. An elastomeric diaphragm clamped between the diaphragm head and compressor housing forms a leak-proof 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. With no rotating or sliding seals, diaphragm pumps are best suited for applications requiring contamination-free pumping, corrosion resistance, and long service life.
They can be used with many motor types. And the pumps can be categorized as high-speed or low-speed pumps, depending on the drive used. Low-speed diaphragm liquid pumps operate at speeds of up to 300 strokes per minute, making these pumps great for metering. The flow rate can be varied by changing the stroke of the diaphragm or motor speed.
High speed diaphragm pumps operate in the range from 2,500 to 3,000 strokes per minute – nearly 10 times faster than their slow-running counterparts. But their size is in inverse proportion to their speed, making their compact dimensions a particular advantage of high-speed diaphragm pumps.
Peristaltic pumps use a flexible tube compressed by a series of rollers to push the liquid through a system. As the rollers travel along the tube, fluid is forced through the tubing – and will leak if the tube ruptures. and contained fluid will leak if the tube ruptures. These pumps are often used to move sterile fluid (such as in blood transfusion equipment or automatic liquid feeding devices in hospitals) because the tubes can be sterilized. 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). 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.
Now that you’ve got the pump basics, the trick is finding the right pump for your application. Here are the five most important questions to ask in the process.
1. Have the voltage and performance requirements been defined?
The performance of a pump can vary dramatically, depending on the equipment and the environment in which the pumping system will operate. Planning for variations in power supply, pressure, media temperature and loading can help inform your decision. One 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, evaluate the general requirements 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?
It’s important to define a nominal voltage with an allowable tolerance range. Ask what power will be available to start and operate the pump? A dc-operated pump driven by a universal power supply can provide more flexibility than an ac-driven pump in locations where portability, speed control, or compatibility is important.
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 (for example, the device contains soft tubing), excessive vacuum could cause the tubing to collapse. The can lead to a system shutdown or equipment damage.
Also pumping systems that create pressure beyond a system’s range can damage connectors, sensors and other expensive parts downstream, causing external leakage, and possibly even jeopardize operator safety. Properly applied pressure regulating devices can help avoid such a scenario. So in lieu of defining a singly point of vacuum rate and one flow rate, look 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 interfere. The pump may be mounted inside of a machine or instrument where the localized temperature is significantly higher than rated temperatures because of lack of proper ventilation. Again, this can cause the pumping system to shut down and equipment to fail.
5. How will the duty cycle play out?
Anticipating duty cycles can save money on energy, overall design cost and increase the life of the pump. Though a few pumps do operate continuously at a fixed speed, they have become the exception in a new energy-conscious environment. Systems may require that the pump’s performance vary during fluctuating demand. In these cases, a brushless dc motor equipped with logical speed controls offers such a feature.
Modifications, such a using a larger motor, can also help in the case where a pump may have to restart against a load.
In addition to these questions, a successful design will also account for:
- minimizing any liquid cavitation effects
- the need for self-priming capability
- chemical compatibility between pump and the fluid as well as handling particulates in the media
- reverse flow through the pump
- system leakage
Asking the right questions and rooting out the answers will ensure that you get the right pump on your first try.