Internal Leakage in Positive Displacement Pumps November 23, 2023 What is Internal Leakage? As mentioned in discussions on positive displacement pumps, achieving truly fixed displacement regardless of outlet pressure is only theoretical. In practice, various factors such as material flex, internal leakage (often referred to as “blow-by”), wear, and other variables introduce pressure dependence. Here, we delve into the specific details of internal leakage. Internal Leakage at Tips in a Gear Pump Causes and Nature of Internal Leakage Internal leakage occurs due to the imperfect fit between components within a pump assembly. No matter how precisely two components are manufactured, microscopic gaps will always exist, allowing fluids to seep through. Internal leakage is typically linearly related to the dynamic viscosity of the fluid, meaning it becomes more pronounced for low-viscosity fluids. While often regarded as a problem, internal leakage can sometimes serve a beneficial purpose. For example, in gear pumps, a certain amount of internal leakage is necessary to lubricate bearings, which rely on the flow from high-pressure areas to low-pressure areas to establish proper hydrodynamic bearings. In other pump designs, internal leakage is used as a safety feature to limit maximum pressure, preventing overpressure conditions. Nevertheless, internal leakage can compromise pump efficiency, so pump engineers must carefully manage its effects. Internal Leakage in Rotary Positive Displacement Pumps In rotary positive displacement pumps, internal leakage primarily arises from tip clearance and face clearance. Both contribute to reduced flow rates, diminished maximum pressure capacity, and compromised priming ability. Tip Clearance Tip clearance refers to the gap between the tips of gears, rotors, or lobes and the pump housing. Without outlet pressure, fluid at the tip surface moves at the tip’s velocity, while the fluid at the cavity wall is stationary, creating a linear velocity distribution. However, under high outlet pressure, the velocity curve can invert, causing some fluid to flow backward. At a “blocked flow pressure,” where the flow rate equals zero, the volume of fluid moving forward equals the volume moving backward (assuming other sources of leakage are negligible). Gear Tip Fluid Flow Modeling Tip Clearance Leakage Modeling leakage due to tip clearance is complex. Empirical data and physical models indicate that this leakage behaves like a hybrid between laminar flow (viscosity-dependent) and fluid inertia (density-dependent). Studies suggest that leakage scales with radial clearance (h) between h² and h³ and is inversely proportional to the tip length. Design Approaches to Minimize Tip Clearance Leakage Pump designers employ three strategies to mitigate tip clearance leakage:1. Reducing Tip Clearance: This approach requires high manufacturing precision and materials that resist deformation caused by temperature, residual stresses, and fluid absorption. Achieving consistency in tip clearance involves tight control over gears, housings, bearings, and shafts.2. Increasing Tip Length: Increasing the length of the tip reduces leakage but results in a decrease in the volume per revolution, impacting the pump’s overall capacity. Nevertheless, this trade-off can yield up to a 75% reduction in leakage.3. Increasing the Number of Teeth on the Gears: By increasing the number of gear teeth, the volume per revolution decreases, and more pressure seals are formed. This technique improves leakage performance, reduces noise, and generates Tip Optimized for Displacement Computational Fluid Dynamics Simulation of Gear Tip Leakage Face Clearance Face clearance is the gap between the rotating components and the stationary surfaces within a pump. It is often the largest contributor to internal leakage. Since the surface area is larger than in tip clearance, and flow through the gap scales with the cube of the clearance (h³), even small increases in face clearance can significantly increase leakage. Controlling face clearance is somewhat easier than controlling tip clearance, but it requires precision machining of the components to reduce gaps. Some pumps use PTFE gaskets between the housing components to seal against external leakage, but changes in the thickness of these gaskets due to wear or temperature can directly impact face clearance and, subsequently, pump performance. Internal Leakage Across Gear Faces Internal Leakage in Reciprocating Positive Displacement Pumps Reciprocating positive displacement pumps, designed for precision metering or dispensing, generally exhibit less internal leakage than rotary pumps. However, the effects of internal leakage are still relevant in these designs, especially when high precision is required. Check Valves A common source of internal leakage in most reciprocating pumps is found in the check valves that control the flow at the inlet and outlet. These valves are typically diaphragm check valves or spring-loaded ball check valves. Leakage at the inlet can unintentionally create positive pressure during intake, while leakage at the outlet can cause liquid to be drawn slightly backward from the discharge port. Both scenarios result in a reduction of the effective dispense volume. Diaphragm Check Valves Diaphragm check valves consist of flexible rubber components placed over an opening, which close when in a steady state. The sealing effectiveness relies on the diaphragm’s natural, unstressed shape, along with back pressure, to prevent leakage. Different types of diaphragm check valves include free-floating discs, flexing elastomers, duckbill shapes, and umbrella designs. Over time, back leakage may occur as the diaphragm flexes, debris disrupts the sealing surface, or abrasive particles cause wear on the sealing or seat surfaces. Spring-Loaded Ball Valves Spring-loaded ball valves create a seal by pressing a ball tightly against a conical seat. The conical shape helps guide the ball into position, forming a strong seal. These valves are typically constructed from hard materials to extend their lifespan. However, the rigid nature of these materials means they lack the flexibility to fully conform to one another, which can create tiny paths for fluid to leak through.Despite advancements in check valve design and manufacturing, the fundamental characteristics that lead to leakage cannot be fully eliminated. For applications seeking to avoid these leakage issues, valveless piston pumps offer an alternative, though they come with their own unique challenges related to internal leakage. Piston Clearances Piston pumps and valveless piston pumps operate by having a piston slide within a cylinder. However, deviations in the piston’s straightness, size, circularity, or cylindricity can result in gaps where fluid can leak. The amount of this internal leakage depends directly on the outlet pressure, effectively reducing the dispensed volume. Internal Leakage in a Piston Pump The relationship between leakage and pressure in a piston pump can be described by the following equation: • P = outlet pressure• µ = dynamic viscosity• D = piston diameter• h = radial clearance• L = length of the piston Since flow is proportional to the cube of the radial clearance (h³), achieving high performance in a piston pump requires extremely tight clearances. For example, in precision applications with water, the clearance must be managed to be below 20µm, with leakage kept under 1% of the desired displacement for optimal performance. Piston Pump Internal Leakage as a Function of Clearance Piston Pump Internal Leakage as a Function of Clearance Obtaining clearances in the range of single-digit micrometers is challenging and requires careful consideration of variables such as piston shape, size, surface finish, thermal expansion, and machining techniques. Ceramic materials are particularly well-suited for this purpose due to their: • Low thermal expansion• Ability to be precision ground• Fine grain structure• Stability across a wide range of fluids However, choosing the right material is just the beginning. To achieve such tight tolerances, highly controlled precision machining and rigorous quality control processes are essential. This level of precision goes beyond standard ISO 9001 practices and demands expert knowledge and experience in delivering high-quality performance on a microscopic scale. Summary Internal leakage is a reality for positive displacement pumps. While it can sometimes be leveraged for lubrication or pressure control, excessive internal leakage reduces pump efficiency and can cause performance issues, such as reduced flow rates or the inability to maintain pressure. Managing internal leakage requires expert engineering, careful material selection, and precision manufacturing. Working with experienced pump engineers, like those at Diener Precision Pumps, ensures that internal leakage is minimized while maintaining optimal pump performance. By using advanced materials, precision machining, and rigorous quality control, Diener Precision Pumps designs pumps that achieve the tight tolerances necessary to minimize internal leakage, delivering reliable, efficient, and precise performance across a wide range of applications. Tip Optimized for Displacement Tip Optimized for Low internal Leakage Computational Fluid Dynamics Simulation of Gear Tip Leakage Face Clearance Face clearance is the gap between the rotating components and the stationary surfaces within a pump. It is often the largest contributor to internal leakage. Since the surface area is larger than in tip clearance, and flow through the gap scales with the cube of the clearance (h³), even small increases in face clearance can significantly increase leakage. Controlling face clearance is somewhat easier than controlling tip clearance, but it requires precision machining of the components to reduce gaps. Some pumps use PTFE gaskets between the housing components to seal against external leakage, but changes in the thickness of these gaskets due to wear or temperature can directly impact face clearance and, subsequently, pump performance. Internal Leakage Across Gear Faces Internal Leakage in Reciprocating Positive Displacement Pumps Reciprocating positive displacement pumps, designed for precision metering or dispensing, generally exhibit less internal leakage than rotary pumps. However, the effects of internal leakage are still relevant in these designs, especially when high precision is required. Check Valves A common source of internal leakage in most reciprocating pumps is found in the check valves that control the flow at the inlet and outlet. These valves are typically diaphragm check valves or spring-loaded ball check valves. Leakage at the inlet can unintentionally create positive pressure during intake, while leakage at the outlet can cause liquid to be drawn slightly backward from the discharge port. Both scenarios result in a reduction of the effective dispense volume. Diaphragm Check Valves Diaphragm check valves consist of flexible rubber components placed over an opening, which close when in a steady state. The sealing effectiveness relies on the diaphragm’s natural, unstressed shape, along with back pressure, to prevent leakage. Different types of diaphragm check valves include free-floating discs, flexing elastomers, duckbill shapes, and umbrella designs. Over time, back leakage may occur as the diaphragm flexes, debris disrupts the sealing surface, or abrasive particles cause wear on the sealing or seat surfaces. Spring-Loaded Ball Valves Spring-loaded ball valves create a seal by pressing a ball tightly against a conical seat. The conical shape helps guide the ball into position, forming a strong seal. These valves are typically constructed from hard materials to extend their lifespan. However, the rigid nature of these materials means they lack the flexibility to fully conform to one another, which can create tiny paths for fluid to leak through.Despite advancements in check valve design and manufacturing, the fundamental characteristics that lead to leakage cannot be fully eliminated. For applications seeking to avoid these leakage issues, valveless piston pumps offer an alternative, though they come with their own unique challenges related to internal leakage. Piston Clearances Piston pumps and valveless piston pumps operate by having a piston slide within a cylinder. However, deviations in the piston’s straightness, size, circularity, or cylindricity can result in gaps where fluid can leak. The amount of this internal leakage depends directly on the outlet pressure, effectively reducing the dispensed volume. Internal Leakage in a Piston Pump The relationship between leakage and pressure in a piston pump can be described by the following equation: • P = outlet pressure• µ = dynamic viscosity• D = piston diameter• h = radial clearance• L = length of the piston Since flow is proportional to the cube of the radial clearance (h³), achieving high performance in a piston pump requires extremely tight clearances. For example, in precision applications with water, the clearance must be managed to be below 20µm, with leakage kept under 1% of the desired displacement for optimal performance. Piston Pump Internal Leakage as a Function of Clearance Piston Pump Internal Leakage as a Function of Clearance Obtaining clearances in the range of single-digit micrometers is challenging and requires careful consideration of variables such as piston shape, size, surface finish, thermal expansion, and machining techniques. Ceramic materials are particularly well-suited for this purpose due to their: • Low thermal expansion• Ability to be precision ground• Fine grain structure• Stability across a wide range of fluids However, choosing the right material is just the beginning. To achieve such tight tolerances, highly controlled precision machining and rigorous quality control processes are essential. This level of precision goes beyond standard ISO 9001 practices and demands expert knowledge and experience in delivering high-quality performance on a microscopic scale. Summary Internal leakage is a reality for positive displacement pumps. While it can sometimes be leveraged for lubrication or pressure control, excessive internal leakage reduces pump efficiency and can cause performance issues, such as reduced flow rates or the inability to maintain pressure. Managing internal leakage requires expert engineering, careful material selection, and precision manufacturing. Working with experienced pump engineers, like those at Diener Precision Pumps, ensures that internal leakage is minimized while maintaining optimal pump performance. By using advanced materials, precision machining, and rigorous quality control, Diener Precision Pumps designs pumps that achieve the tight tolerances necessary to minimize internal leakage, delivering reliable, efficient, and precise performance across a wide range of applications. Share