To understand how the viscosity of a liquid affects a pumping system, it is important to understand what viscosity represents. By definition, viscosity is the property of a liquid that causes it to offer resistance to shear stress such as that caused by liquid flow, primarily in the area of the pipe wall.
Image 1 illustrates this by showing the velocity profile of a liquid relative to a static boundary surface. At the static boundary surface or pipe wall, the velocity of the liquid is zero. As the distance increases from the static surface, the velocity of the liquid increases. The force per unit area is a function of the velocity gradient v/d, which is the maximum velocity of the fluid, v, divided by the distance, d, from the static surface.
Absolute viscosity, μ (Mu), is the quotient of the shear stress (or force per unit area) divided by the shear rate. It is common to express viscosity relative to its density, which is known as kinematic viscosity. Kinematic viscosity is designated by the Greek letter ν (Nu). A common way to measure kinematic viscosity is the Saybolt Seconds Universal (SSU) (see Image 2). This refers to the length of time it takes for a measured quantity of liquid at a specific temperature to drain from a container with a measured orifice in the bottom. For example, water has a viscosity of approximately 31 SSU at 60 degrees Fahrenheit (F). By comparison, light lubricating oils may have a viscosity of 100 or 200 SSU. More viscous lubricating oils have viscosities in the thousands of SSU, and extremely viscous fluids—heavy tar, for example—have viscosities as high as 1 million SSU.
Depending on the pump type, the impact of liquid viscosity is different. We will look at three types of pumps specifically: centrifugal (Image 3), reciprocating (Image 4) and rotary (Image 5).
Reciprocating and rotary pumps are within the positive displacement (PD) family. PD pumps displace a certain volume with every revolution of the shaft, minus any volumetric leakage (slip).
A centrifugal pump is within the rotodynamic pump family. Rotodynamic pumps are kinetic machines in which energy is continuously imparted to the pumped fluid by means of a rotating impeller, propeller or rotor. The most common type of rotodynamic pump is the centrifugal (radial) type. In centrifugal pumps, the liquid enters the impeller axially at the impeller eye and progresses radially between the vanes until it exits at the outside diameter and is collected in a diffusor or a volute arrangement as shown in Image 3. It is important to consider how these types of pumps are different and the physics involved because these disparities result in significantly different operation with respect to viscous liquids.
Centrifugal Pump Viscous Pumping Considerations
It is industry standard to test the performance of centrifugal pumps with clear water per ANSI/HI 14.6 Rotodynamic pumps for Hydraulic Performance Tests. The performance of a centrifugal pump is affected when handling viscous liquids because of the increased friction when the impeller rotates and the resistance to flow compared to water test. A marked increase in input power due to reduced efficiency and a reduction in head and rate of flow occurs with viscous liquids compared to water.
The performance curve in Image 6 shows the water performance and the corrected viscous performance for the application liquid, which has a viscosity of 1,000 SSU and specific gravity of 0.9. The viscous data should be corrected from the water performance test per Hydraulic Institute standard ANSI/HI 9.6.7 Effects of Liquid Viscosity on Rotodynamic Pump Performance. ANSI/HI 9.6.7 was used to correct the performance as shown in Image 6. This standard prescribes an empirical method based on test data available from sources throughout the world.
The HI method enables pump users and designers to estimate performance of a particular rotodynamic pump on liquids of known viscosity, given the performance on water. The procedure is important so that the suitable pump and driver is selected for a required duty on viscous liquids. Not shown in Image 6 but also a concern is the increase of the required net positive suction head (NPSH) where a 3 percent head loss is noted (NPSH3) as well as increased required starting torque with viscous liquids. Consideration for these are outlined in ANSI/HI 9.6.7.