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Cavitation and How to Avoid It
What is Cavitation?
All pumps operate by creating a low pressure at the inlet and allowing atmospheric (or system) pressure to push fluid into the pump. The process makes all pumps susceptible to a phenomenon called cavitation. Cavitation is the formation of vapor cavities (bubbles) inside of a liquid when the local pressure is decreased rapidly below the vapor pressure of the liquid. This forms a vapor bubble inside the liquid which typically lasts for a short time before collapsing back into a liquid. The collapse is violent, producing a loud popping noise and often damaging nearby surfaces. Even strong metals will be pitted when subjected to the strong, localized jet resulting from the bubble implosion.
Lifecycle of a Cavitation Bubble
Internal to pumps, cavitation often originates behind a moving part where localized, low pressure regions exist. This may not be noticed by the user, but it will begin to damage components within the pump and must be avoided. As the system inlet pressure is reduced, cavitation will become more prominent, producing pump fluctuations, emitting loud noise, and sometimes resulting in cloudy fluid at the pump outlet.
The point at which cavitation begins is very complicated. It is a combination of fluid viscosity, vapor pressure, density, temperature, hydraulic lift, atmospheric pressure, pump type, and pump speed. A precursor to cavitation is often growth of pre-existing gas trapped in the liquid. While these bubbles are not a risk to damaging the pump, they can reduce the accuracy of fluid delivery.
By far the most common occurrence of cavitation when using positive displacement pumps results from using long, small diameter tubing on the inlet of the pump. A general equation for the pressure drop through a tube is , where:
Q = flow rate
µ = dynamic viscosity
L = length of tubing
D = tubing inner diameter
Note that the pressure drop is dependent on D4 therefore, doubling the inner diameter of the tube will decrease the pressure drop by a factor of 16! The above equation is for laminar flow only (Reynolds numbers < 2320). For turbulent flow the equation is more complicated and dependent on density instead of viscosity.
Laminar vs. Turbulent Flow in Tubes
Tubing is not the only source of pressure drop often overlooked by hydraulic system designers. Inlet filters, check valves, and orifices are examples of components that increase the vacuum at the inlet. Check valves, in particular, must be carefully chosen as to not create too high of a vacuum.
Cavitation in Reciprocating Positive Displacement Pumps
Reciprocating positive displacement pumps do not typically suffer from internal, highly localized cavitation like high speed rotating pumps. However, they do have highly pulsed flow rate, which results in peak flow rates up to three times the average flow rate. More importantly, the frequent stop/start of the fluid generates an inertia-based vacuum on the inlet. As the pump begins to pull fluid in through the inlet, the fluid behind it must accelerate. As with viscous drag, long thin tubing is the worst for vacuum related to fluid acceleration (proportional to D2). These aspects of reciprocating pumps can surprise a system engineer who is designing for the average flow rate.
Pulsed Flow in a Reciprocating Pump
Cavitation in Rotary Positive Displacement Pumps
In addition to vacuum-producing drag previously mentioned, high-speed moving elements create a low-pressure region immediately behind the moving element. This is a risk in higher diameter components such as propellers or the impeller of a centrifugal pump and does not have as strong of an influence on smaller gear pumps. However, internal cavitation can occur in a gear pump at the gear mesh as the void opens between the two gears, and the newly created volume is quickly filled with liquid. This effect can be minimized with precision machined helical gears, generating a smooth opening of the gear mesh. Nevertheless, internal mechanisms within the pump can produce localized pressure drops as high as 0.1 bar in water at speeds above 3000 rpm.
Common Cavitation Locations in an External Gear Pump
Peristaltic and lobe pumps have a relatively strong pulsation in their flow profiles. This pulsation produces transient vacuums similar to those in reciprocating pumps. Therefore, care must be taken when implementing these pump types.
Net Positive Suction Head (NPSH)
NPSH is a common metric used by civil engineers. Centrifugal pump and turbine manufacturers in this industry often give their pumps an NPSH rating signifying the minimum pressure at the suction port needed to keep the pump from cavitating. NPSH is typically given in units of feet since this is the common unit of pressure (head) dealt with by civil engineers in the U.S. Positive displacement pumps in medical, food and beverage, and light industrial applications do not typically attach NPSH values to pumps because of the large variations in fluids, temperatures, speeds, and other operating conditions.
The best way to avoid cavitation is to engage the pump supplier early in the design of a hydraulic system. Pump engineers understand the sensitivity of their pumps, know what levels of vacuum are tolerable, and have significant experience tailoring hydraulic systems to avoid cavitation. Close collaboration between system designers and pump engineers will streamline the design process and avoid iterations late in the design cycle.