Pump Internal Leakage November 23, 2023 What is Internal Leakage? As discussed in the overview of positive displacement pumps, fixed displacement, regardless of outlet pressure, is only theoretical. Material flex, internal leakage (“blow-by”), wear, and other variables result in varying amounts of pressure dependence. Here we will look at the details of internal leakage. Internal Leakage at Tips in a Gear Pump Internal leakage results from the imperfect fit between components in a pump assembly. Regardless of how well the two components conform to one another, microscopic gaps will exist, and fluids will move through them. Internal leakage is typically linearly related to a fluid’s dynamic viscosity and, therefore, becomes more pronounced for low viscosity fluids. Internal leakage is not always undesired. Lubrication of gear pump bearings requires flow from areas of high pressure to low pressure to establish a correct hydrodynamic bearing. In some pumps internal leakage is used to limit the maximum pressure to prevent system over-pressure. Some pump designs eliminate internal leakage through the use of compliant materials with interference fits (i.e. seals). These strategies can eliminate nearly all internal leakage. However, they introduce sliding wear, which creates other problems. This article will not focus on sliding seals. Rotary Positive Displacement Pumps The two common sources of internal leakage in rotary pumps are tip clearance and face clearance. Not all pumps have both types, for example, vane pumps do not have tip clearance because the vanes actively slide against the wall. Internal leakage in rotary positive displacement pumps not only reduces flow rate but also reduces maximum pressure and ability to prime. Tip Clearance Clearance at gear, rotor, or lobe tips is an important source of internal leakage. Without pressure, the fluid reaches the tip velocity at the surface of the tip and zero velocity at the cavity wall, with a linear distribution in between. However, when the outlet has a high enough pressure, the middle of the curve can invert, resulting in some fluid flowing backward. At blocked flow pressure (flow rate equals zero) the volume of fluid moving forward is equal to the fluid moving backward (ignoring other sources of leakage). Gear Tip Fluid Flow Gear Tip Fluid Flow Unlike other forms of internal leakage, tip clearance is extremely complicated to model. Considerations of both experimental data and physics models suggest the leakage mechanism is a hybrid of laminar flow (dependent on viscosity) and fluid inertia (dependent on density). The data suggests a dependence of internal leakage between h² and h³, where h is the radial clearance, and linear to the inverse of tip length. Three methods are available to designers of internal and external gear pumps, gerotor pumps and lobe pumps to reduce the effect of leakage at the tips: Decrease the tip clearance. Doing so requires high, repeatable precision, excellent quality control and use of materials with minimal distortion due to fluid absorption, temperature differences, residual stresses and creep. These considerations must be applied to the gears, housing, bearings and shafts.Increase the length of the tip clearance. Doing so is a design choice because the tradeoff is a decrease in the volume per revolution. However, internal leakage is proportional to and can by 75% in many instances. Tip Optimized for Low internal Leakage