Diaphragm Pump Principles of Operation
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Cavitation is a hydraulic condition which can exist in any type of pump. It is primarily a situation in which the pump is discharging less liquid than its rated capacity due to a reduction or lack of liquid supply to the pump intake. "Excessive suction lift, insufficient NPSH, or operation at too high a speed are common causes of cavitation. Pitting, vibration, and noise are common troubles stemming from cavitation. While severe cavitation is usually accompanied by excessive noise and damage tot he pump, mild cavitation may produce nothing more than a small reduction in pump efficiency and moderate wear of pump parts."1
Diaphragm pumps, like other pumps, "do not suck in liquids; they reduce pressure in the suction chamber, and external pressure, usually atmospheric, pushes the liquid into the pump. For any pump with a given size suction line, capacity or maximum speed is fixed by the existing net positive suction."2 A diaphragm pump can be particularly vulnerable to a "starved suction" condition because it is generally pumping a viscous, solid-laden slurry. In fact, most cases of low flow rates can be traced to starved suction conditions due to either too high a static lift, too long a suction line, or a combination of both.
Diaphragm Pump Principles of Operation
Engineering
Case Studies & Tutorials
Wilden On-Line
Catalog
It is likewise possible to experience cavitation even though the pump may have a "flooded suction." In this case it is due to trying to discharge more than can be pulled in through a suction line that is to long and/or too small in diameter. If a slurry is quite "thick" it will only be possible to pump a much smaller amount than the pump's full capacity.
Cavitation is harmful to diaphragm life because on the suction stroke the diaphragm is being pulled mechanically by the shaft connected to the pressurized diaphragm. There is an "unbalanced" pressure on the suction stroke higher. The lower the suction lift condition imposed on the diaphragm, the less the unbalanced mechanical load, and the longer the diaphragm life. Every diaphragm has a given number of flexes before failure. If the pump is cavitating, less liquid is being pumped per flex; therefore, diaphragm cost per gallon is increased.
Cavitation can be eliminated in air-operated double diaphragm pumps easier than with any other type of pump. Start the pump slowly by controlling the volume of compressed air to the pump by the use of a gate or globe valve. After the unit starts pumping, the throttling valve can be opened to increase capacity. The point at which further opening of the valve increases cycling rate without an increase in discharge rate will be the cavitation point, and the valve should be closed slightly. Further attempts to increase capacity should be oriented around a larger suction intake line, larger pump, reduced lift condition and/or a combination of these conditions.
Under conditions of limited flow due to cavitation, damage to the double diaphragm pump will most likely be limited only to the diaphragms, in comparison to numerous expensive components in other types of pumps.
1Tyler G. Hicks and Theodore W. Edwards, Pump Application Engineering, page 87.
2Tyler G. Hicks and Theodore W. Edwards, Pump Application Engineering, page 51.
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