11 Tips for Electric Diaphragm Pump Selection

The fundamental design criteria for selecting an electrically operated diaphragm pump to handle air or gases has been well-covered around essential issues such as flow rates, inlet and outlet loads, chemical compatibility, voltage, and ambient and media temperature. But other equally important parameters are often overlooked. Here's a rundown of the most important:

1. External Leakage. This involves media escaping from inside the pump or outside air leaking in and diluting the sample. Depending on the application, a simple bubble-tight construction may be acceptable with various improvements, including use of a secondary safety diaphragm to provide leak tightness of <0.000006 mBar l/sec. The first step is to define the level of leakage that the system can tolerate and then accommodate from there.

2. Reverse Flow Leakage. The valves inside a pump typically are not designed to be absolutely tight when the pump is off. If absolute tightness becomes necessary, a pump modification, check valve or other option should be discussed with the pump designer's technical contact.

3. Through Leakage. The internal geometry of a pump is such that flow from inlet to outlet occurs with minimum loss. Ideally, the loss is only the force necessary to open the valves. In the "off" position this leads to the possibility of a siphoning effect through the pump. A simple pressure control valve, check valve with suitable cracking pressure, or other modifications to the pump and/or system should be considered as counter-measures.

4. Pulse Dampening. The reciprocating motion that produces flow performs in a pulsating manner. Methods of reducing the pulse can include a pulsation damper, two-headed pump with offset heads, an accumulator, reconfiguring placement of the pump in the system, and/or changing tubing type and length.

5. Audible Noise. The first step in mitigating noise is to determine its origin. Fluid noise (air or liquid) can be minimized through proper selection of mufflers and filters, while transmission of mechanical noise generated by the pump may be reduced with proper vibration pump mounts, proper hose durometer
selection, and/or sound-absorbing material around the pump.

6. Electrical Noise. Electronics within the pump, the entire system, or even neighboring devices may be sensitive to electrical emissions from ac or dc motors. Standards such as EN61000 have been established to address electrical emissions from pumps, as well as immunity to electrical noise damage from other devices.

7. Speed Control. Matching the pump's speed to actual requirements ultimately will cause less stress on the pump and other components in the system, consume less energy, create less heat, and generate less noise and vibration.

8. Start Against a Load. Most pumps are not designed to start (or restart) against a load. Modifications to the pump can allow for starting or restarting against a load throughout its life without long-term damage, while avoiding complex and costly system changes external to the pump.

9. Effects Of Humidity. Compressing a humid gas during the mechanical pumping process will result in condensate formation within the pump, potentially causing the motor to fail prematurely. A multi-port valve can offer more support for the valve's surface area and result in less strain and deformation. A multi-port valve also can effectively pump out accumulated water.

10. Cavitation. This condition occurs when the pressure of a liquid drops below its vapor pressure. The situation can be avoided (or at least minimized) by appropriately coordinating inlet tubing diameter and durometer, stroke length and suction speed.

11. Elevation. A pump's flow rate will vary, depending on its elevation. Most pump performance specifications are based on operation at sea level and will increase or decrease as elevation changes.

David H. Vanderbeck, is business development manager for KNF Neuberger Inc.
For more information, go to www.knfoem.com.