The Science Behind Pump Cavitation: Causes, Effects, and Solutions October 29, 2024 Fluid pumps play a critical role in many industrial processes, from transferring fluids to controlling pressure and maintaining flow. However, one of the most prevalent issues with fluid pumps is cavitation—a problem that can cause significant damage, increase maintenance costs, and reduce efficiency. As a leading fluid pump manufacturer, at Diener Precision Pumps we understand the importance of smart pump designs to minimise the risk of cavitation and optimise the pump’s performance and lifespan. In this blog, we’ll explore the nature of pump cavitation, how to identify it, and effective solutions to prevent it. What is Pump Cavitation? Fluid pumps operate by creating low pressure at the inlet, allowing atmospheric or system pressure to push fluid into the pump. This makes them susceptible to cavitation—a phenomenon where vapor cavities or bubbles form inside the liquid when the local pressure drops rapidly below the vapor pressure of the liquid. These vapor bubbles are short-lived but collapse violently, producing loud popping sounds and damaging nearby surfaces. Even strong metals can suffer pitting damage from the high-pressure jets created by the bubble implosions, which, if left unchecked, can eventually destroy the pump. Lifecycle of a Cavitation Bubble in Fluid Pumps Cavitation typically begins behind a moving part within the pump where low-pressure regions exist. Although users might not immediately notice it, cavitation gradually damages internal pump components. As inlet pressure decreases, cavitation becomes more pronounced, causing pump fluctuations, loud noises, and sometimes cloudy fluid at the outlet due to dissolved air. Understanding the onset of cavitation is complex and depends on several factors: fluid viscosity, vapor pressure, density, temperature, hydraulic lift, atmospheric pressure, pump type, and pump speed. Often, cavitation is preceded by the growth of pre-existing gas bubbles in the liquid, which, while not immediately harmful, can reduce the accuracy of fluid delivery. In industries like food processing and inkjet printing, the presence of gas bubbles leading to cavitation can significantly impact the accuracy of fluid delivery, often with visible or costly results. For instance, in the food industry, cavitation can cause inconsistent dosing when pumping sauces, syrups, or creams, leading to uneven filling or incorrect ingredient proportions in packaged foods. This not only affects product quality but may also result in waste or non-compliance with strict food standards. In the inkjet printing industry, cavitation and gas bubbles can disrupt the smooth flow of ink through the nozzles, resulting in streaks, incomplete prints, or blurred colors on paper or packaging. Even small inconsistencies in ink delivery can lead to costly reprints or customer dissatisfaction, especially in high-quality printing where precise color and detail are crucial. Inlet Restrictions The most common cause of cavitation in positive displacement pumps is using long, small-diameter tubing at the inlet. The pressure drop through a tube can be calculated using the following equation: ∆P = Q∙μ∙(128∙L)/(π∙D⁴) Where: Q = flow rate μ = dynamic viscosity L = tube length D = tube inner diameter The pressure drop is heavily influenced by the inner diameter (D⁴), meaning doubling the diameter decreases the pressure drop by a factor of 16! For laminar flow (Reynolds number < 2320), this equation is straightforward. However, in cases of turbulent flow, the calculation is more complex and dependent on fluid density rather than viscosity. Laminar Flow 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. As a precision pump manufacturer, Diener takes special care to design the internal flow paths to minimize restrictions that would lead to high fluid velocities and low-pressure cavitation zones. Allowing the fluid to move easily decreases the likelihood of cavitation and its destructive effects. Cavitation in Reciprocating Positive Displacement Pumps Reciprocating positive displacement pumps, mostly used for metering applications, generally do not suffer from the intense internal cavitation seen in high-speed rotary pumps. However, their highly pulsed flow can generate peak flow rates up to three times the average, creating inertia-based vacuums at the inlet during fluid acceleration. Long, thin tubing is particularly prone to vacuum-related issues during fluid acceleration. Pulsed Flow in a Reciprocating Pump Cavitation in Rotary Positive Displacement Pumps In rotary positive displacement pumps, cavitation can occur in specific zones, especially behind high-speed moving elements where rapid pressure changes create ideal conditions for vapor bubble formation. Although smaller gear pumps tend to experience less severe cavitation, they are still susceptible to internal cavitation at the gear mesh, particularly as voids open between gears and fill with liquid. This can lead to inefficiencies, vibration, and eventual damage if not managed properly. Precision-machined helical gears offer a practical solution by smoothing out the gear mesh opening and helping to minimize cavitation risk. However, despite these improvements, internal pump mechanisms can still cause localized pressure drops—especially at operating speeds exceeding 3000 RPM—leading to increased wear and tear on pump components. Understanding and addressing cavitation in rotary positive displacement pumps is essential for industries requiring reliable pump performance, such as chemical processing, oil and gas, and manufacturing. By choosing pumps with optimized designs, like helical gear configurations and suitable speed limits, operators can better manage cavitation effects, prolonging equipment life and ensuring consistent fluid flow under demanding conditions. Common Cavitation Locations in External Gear Pump Peristaltic and lobe pumps exhibit significant pulsation in their flow profiles, creating intermittent vacuums similar to those seen in reciprocating pumps. This pulsating action can lead to pressure fluctuations within the pump and connected systems, potentially impacting process consistency or causing unintended flow interruptions. As a result, careful consideration is required when implementing these pump types, particularly in applications sensitive to pressure variations. Proper setup, along with dampening accessories or flow-smoothing techniques, can help minimize these effects and ensure reliable, steady fluid delivery. Net Positive Suction Head (NPSH) Net Positive Suction Head (NPSH) is a critical metric for preventing cavitation in centrifugal pumps, widely used across engineering applications to ensure efficient pump operation. NPSH indicates the minimum pressure required at a pump’s suction port to avoid the formation of vapor bubbles that can lead to cavitation damage. By ensuring sufficient NPSH levels, engineers help prevent premature wear, reduce noise, and improve overall pump performance. In centrifugal pumps, achieving adequate NPSH is essential, especially in industries where precise flow and pump reliability are key, like civil engineering and large-scale manufacturing. However, in positive displacement pumps—commonly used in medical, food and beverage, and light industrial settings—NPSH ratings are typically not provided due to varied fluid properties and operating conditions. Instead, managing inlet pressure and system design takes precedence to avoid vacuum conditions and mitigate cavitation risks. For reciprocating pump systems, which are more susceptible to cavitation, maintaining proper inlet pressure, optimizing NPSH levels, and minimizing vacuum conditions are vital. Following these practices not only helps extend the life of your pump but also improves efficiency, reliability, and overall performance in demanding applications. Collaborating with Pump Engineers The best way to avoid pump cavitation is by involving pump engineers early in the hydraulic system design. Our expert pump engineers at Diener Preicison Pumps have in-depth knowledge of the different pump designs’ tolerance to vacuum conditions and can tailor solutions to minimize the risk of cavitation in line with the intended pump application and handling fluids. Close collaboration between our clients’ system designers and our pump engineers helps streamline the design process and avoid costly iterations late in development. By addressing cavitation proactively, you can protect your pumps, reduce maintenance costs, and ensure that your systems operate at peak efficiency for the long term.