Fine Abrasive Fluids in Positive Displacement Pumps
June 15, 2021
What is a Fine Abrasive Fluid?
An abrasive fluid is one that causes increased wear to the fluidic system by eroding surfaces through physical interaction. Erosion can occur from simple flow across a surface or by the abrasive particles interfering with internal component interactions within a pump.
The types of particles can be large contaminants such as those experienced in wastewater applications, or they can be finely integrated pigments in the liquid which are intentional. These include paint pigments, ink pigments, suspended carbon, metal pigments and chocolate (when suspended in milk, syrup, etc.). This article will deal specifically with suspended abrasive particles less than 1mm in size.
Common Particles Causing Abrasion
Naturally, the harder the solid particle the greater concern it is for pump wear. Hard pigments such as titanium dioxide (TiO2) (hardness of 6 on the Mohs scale) can quickly wear unhardened interacting metal components. However, even soft pigments can wear non-metal components such as plastic gears or rubber check-valves. If possible, all moving components should be harder than the pumped particulates. However, selecting the hardest materials comes with a cost which may not be necessary. Therefore, it is important to understand the fluid and communicate that clearly to the pump engineers.
The size of the particles will also affect the best pump choice. Pumps designed using ultra-hard materials such as Alumina (Al2O3) may bind if the pigment size is greater than the clearance in the pump. Pumps using soft components such as plastic or rubber may be more tolerant of large particles but will still be sensitive to abrasive wear.
Effect of Viscosity
The liquid’s viscosity affects the abrasiveness of the liquid/pigment combination. Higher viscosity fluids develop a more robust fluid film which cushions the pigments. This is especially true in configurations such as hydrodynamic (journal) bearings. Viscosity plays a lesser effect where intentional contact between components is required such as gear tooth interaction.
Studies have shown that abrasive wear is related to the square of the speed. For this reason, many pump suppliers will suggest oversizing the pump and running it at a slower speed. This is not ideal for hydrodynamic bearings and internal leakage. However, abrasive wear is typically the primary concern and the other effects can be overcome.
The type of pump selected is immensely important for abrasive fluids. Rotary positive displacement pumps typically have component-component contact. Using these pumps for abrasive liquids requires hard materials such as:
- Ceramics (Alumina or Zirconia)
- Carbides (Tungsten Carbide)
- Hardened steels
- Hard coatings
Each material has tradeoffs such as manufacturability, cost, corrosiveness, etc. The style of pump also affects the available materials. External gear pumps, for example, cannot use ceramic gears because the sharp corners will experience brittle fracture.
Pumps utilizing elastomers in the flow path, such as diaphragm pumps, often do not have component-component contact. However, the soft elastomer is subject to erosion simply by the abrasive material flowing around the edges at high velocity. Furthermore, the pulsed nature of some pumps is incompatible with certain applications such as continuous ink jet without including expensive pulse dampeners.
Fine abrasives are catastrophic to sliding seals. Sliding seals, by their nature, have soft materials sliding against hard materials. Small abrasive particles easily get between the sliding surfaces and quickly wear the soft seal material.
One solution is to use magnetic coupling such as that found in magnetically coupled gear pumps. These are an excellent solution for abrasive liquids provided the particle size is small and the particulate is non-magnetic.