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.Ĭentrifugal Pump Viscous Pumping Considerations 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. The most common type of rotodynamic pump is the centrifugal (radial) type. Rotodynamic pumps are kinetic machines in which energy is continuously imparted to the pumped fluid by means of a rotating impeller, propeller or rotor. PD pumps displace a certain volume with every revolution of the shaft, minus any volumetric leakage (slip).Ī centrifugal pump is within the rotodynamic pump family. Reciprocating and rotary pumps are within the positive displacement (PD) family. We will look at three types of pumps specifically: centrifugal (Image 3), reciprocating (Image 4) and rotary (Image 5). 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.ĭepending on the pump type, the impact of liquid viscosity is different. By comparison, light lubricating oils may have a viscosity of 100 or 200 SSU. For example, water has a viscosity of approximately 31 SSU at 60 degrees Fahrenheit (F). 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.
A common way to measure kinematic viscosity is the Saybolt Seconds Universal (SSU) (see Image 2). Kinematic viscosity is designated by the Greek letter ν (Nu). It is common to express viscosity relative to its density, which is known as kinematic viscosity. Example of SSU viscosity measurement)Ībsolute viscosity, μ (Mu), is the quotient of the shear stress (or force per unit area) divided by the shear rate.
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