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.
Great information, thanks Dave. I haven't used many diaphragm pumps, but I am starting on a chemical process project next month, so I need to bone up a little. For your readers benefit, I also ran across this interesting piece on diaphragm pumps.
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