Effects of Viscosity on Positive Displacement Pumps June 22, 2021 Does Viscosity Affect Positive Displacement Pumps? A common misconception is that since positive displacement pumps move fixed volumes of fluid mechanically, fluid viscosity has little to no effect on their output. However, this assumption can lead to severe problems in a hydraulic system if the effects of viscosity are not well understood. The key to preventing these issues lies in clear communication between system designers and expert pump engineers, such as those at Diener Precision Pumps, who can ensure that the pump design is optimized for the intended application and the specific fluids being handled. What is Viscosity? Viscosity refers to the internal friction of fluids and is a measure of how much resistance they offer to flow and deformation. For example, water has a low viscosity, while honey has a high viscosity. In fluid systems, higher viscosity increases drag on surrounding surfaces, meaning more force (in the form of pressure) is required to move the fluid. Viscosity is typically measured in two ways: kinematic (ν) and dynamic (μ). These are related by the equation ν=μ/ρ, where ρ represents the fluid’s density. Dynamic viscosity is more commonly used in laminar systems and is expressed in centipoise (cPs), where 1 cPs equals 10^-3 Pa-s. The majority of system challenges arise from handling high-viscosity fluids since most fluids encountered are thicker than water (1 cPs). Temperature also has a significant impact on viscosity. For instance, the viscosity of a 50% water/glycol solution can change nearly fivefold between 0°C and 50°C, while the viscosity of water changes by three times within the same temperature range. This is why careful consultation with pump engineers is essential to avoid performance issues due to temperature fluctuations. Effects of External Pressure Drops One of the often-overlooked factors in hydraulic systems is the pressure drop caused by tubes. In laminar flow, this pressure drop is directly proportional to the fluid’s viscosity. A 100 cPs fluid, for example, will experience 100 times the pressure drop in a tube compared to water. These pressure drops affect both the pump outlet (adding pressure) and the inlet (creating negative pressure). In reciprocating pumps, the pulsed flow magnifies these effects, potentially causing pump cavitation, dosing irregularities, and accelerated pump wear. Understanding the flow regime, particularly whether it is laminar or turbulent, is critical in avoiding pressure-related issues. System designers must work closely with pump engineers to ensure that tubing sizes and other system components are appropriately matched to the fluid’s viscosity. Effects of Internal Pressure Drops Pumps not designed for high-viscosity fluids can experience flow restrictions within internal channels, leading to the same pressure drop issues discussed in external systems. This is why it’s essential to involve experienced pump engineers early in the design process to tailor the pump for the fluid being handled and prevent unintended consequences. Risk of Internal Leakage Positive displacement pumps are subject to internal leakage through micro-gaps between sliding components or seals. While higher viscosity fluids are less prone to leakage, they can reduce the pump’s efficiency under certain operating conditions. Having pump engineers optimize the design to match the viscosity of the fluid ensures more reliable and efficient operation. Pump Capacity and Power Higher viscosity fluids require more force to move, meaning the power required to operate a pump increases as viscosity rises. For example, stirring honey takes significantly more effort than stirring water, and the same principle applies to pumps. When external gear pumps handle high-viscosity fluids, more power is needed just to keep the gears turning. This can reduce the pump’s capacity if the motor size is not appropriately selected. To avoid performance degradation, system designers must collaborate with pump engineers to ensure the motor and pump are correctly sized for the fluid’s viscosity. Risk of Cavitation As already discussed, high-viscosity fluids can increase the vacuum at the pump inlet, raising the risk of pump cavitation. Although high-viscosity fluids produce less damaging cavitation jets than lower-viscosity fluids, cavitation still needs to be avoided for reliable performance. Engaging expert pump engineers will help in identifying and mitigating potential cavitation risks through careful system design. Lubrication Higher viscosity fluids often provide better lubrication for hydrodynamic bearings, such as those found in magnetically coupled gear pumps. This can extend the life of the pump since the fluid supports the bearings and reduces solid-to-solid contact. However, this benefit may be offset by other negative effects, such as increased power consumption and pressure drops. An experienced pump engineering team can balance these factors to optimize performance. Hydrodynamic Bearing Conclusion Fluid viscosity plays a critical role in how positive displacement pumps perform within a system. Since most system designers do not control the fluid’s viscosity, it is essential to be aware of how it will impact both the system and the pump. Involving expert pump engineers like those at Diener Precision Pumps early in the design phase can prevent costly redesigns and delays. Their expertise ensures that the pump is optimized for the fluid’s characteristics, leading to better performance and longer-lasting systems.
