In physics, viscosity is a measure of how thick or thin a fluid is. In the world of pumping, we think of it as a measure of a fluid’s resistance to flow. For our purposes, we consider a liquid with a viscosity of more than 750 SSU (Saybolt Second Universal) to qualify as a viscous liquid. If a liquid falls within the “viscous” range (such as molasses, sugar syrup, or glycerin), pump selection must take the viscosity factor into consideration to ensure that product flow is trouble-free. We have a saying here at Viking Pump regarding viscous liquids: "If you can get the liquid into the pump, we can pump it out."
Characteristics of viscous fluids
Regardless of application, the goal is to maintain flow efficiency through the consistent and continuous movement of product. This is a challenge with viscous liquids, since efficiency tends to go down as viscosity increases. Furthermore, viscosity changes and patterns can occur while pumping through your system, thus, affecting flow and other measurements (like pressure and line loss). Here are some ways in which viscous liquids may behave:
- Newtonian Liquids – liquids that have a constant viscosity, independent of the rate of shear. Examples include lube oils and fuel oils.
- Non-Newtonian Liquids – liquids that change in viscosity when sheared.
- Thixotropic liquid — viscosity decreases as the rate of shear increases, making it difficult to start flowing, but once started, difficult to stop flowing. Examples are emulsions, industrial rubber cements, and ketchup.
- Dilatant liquid — viscosity increases as the rate of shear increases, also called “shear thickening.” Examples are starches and clay slurries.
- Laminar Flow — the rate of flow varies across the diameter of a pipe, with those along the walls moving very slowly, while those at the center are moving more rapidly. The result is a series of concentric rings of liquid moving through a pipe. This effect, which occurs with almost any viscous material, impacts pressure drop and line loss calculations.
Types of parameters
Overall quality of performance can be hindered by running high-viscosity liquids. To avoid common problems, such as low capacity, noisy operation, overloaded motor, etc., when handling viscous liquid, consider the following parameters:
- Pumping temperature — The viscosity of a liquid varies inversely with temperature. As temperature goes up, viscosity goes down, and vice versa. For some liquids, it is common to heat the liquid to reduce the viscosity. In these cases, it is important for the pump to be constructed of materials that can withstand high heat.
- Suction piping —Inadequately sized and/or overly long suction piping restricts the rate of flow and can even lead to pump starvation and cavitation. To avoid this, use large diameter pipes and keep the suction lines short by placing the pump as near as possible to the supply tank.
- Pump port size — An over-sized suction port is recommended for high-viscosity pumping conditions. Larger ports eliminate the need for a reducer fitting at the pump and a restriction at the port, while also providing a smoother flow after the liquid gets into the pump.
- Reduced pump speed and capacity — Depending on the liquid viscosity and suction piping, the pump speed may need to be reduced from the catalog recommended maximum. This is done to make sure the pump does not starve or cavitate and high volumetric efficiency is maintained.
- Discharge piping size — Different pipe sizes result in different discharge pressures, which can adversely affect production, operating costs, power consumption, maintenance, and equipment wear. Things to consider in pipe size selection include pipe elevation, pipe friction, need for supports and/or pressure relief valves, etc.
- Extra clearance — Providing extra clearance for highly viscous liquids helps ensure smooth pump operation. Adjustments to running clearances between all parts in relative motion have been added to Viking’s internal gear pumps based on extensive performance testing over the years.
Help with pump selection
As you know, not all pumps are created equal. Pump construction and material are especially important to the proper handling of viscous liquids, as adequate strength is required to withstand the extra pressure and stress from high power and torque. Viking Pump has designed options with this in mind; for example, our steel rotor is stronger than iron and, therefore, can handle all viscosities.
Contact a Viking representative to help make a good pump selection for your viscous liquid application.