A Brief Introduction to Centrifugal Pumps Part 5 - Specific Gravity and Viscosity

Joe Evans, Ph.D

http://www.pumped101.com

SPECIFIC GRAVITY AND VISCOSITY

Specific Gravity (or why head is usually expressed in feet)

The Specific Gravity of a substance is the ratio of the weight of a given volume of the substance to that of an equal volume of water at standard temperature and pressure (STP). Assuming the viscosity of a liquid is similar to that of water the following statements will always be true regardless of the specific gravity:

  1. A Centrifugal pump will always develop the same head in feet regardless of a liquid’s specific gravity.
  2. Pressure will increase or decrease in direct proportion to a liquid’s specific gravity.
  3. Brake HP required will vary directly with a liquid’s specific gravity.

The Figure below illustrates the relationship between pressure (in psi) and head (in ft) for three liquids of differing specific gravity.


Figure 7

We can see that the level in each of the three tanks is 100 feet. The resulting pressure at the bottom of each varies substantially as a result of the varying specific gravity. If, on the other hand we keep pressure constant as measured at the bottom of each tank, the fluid levels will vary similarly.

A centrifugal pump can also develop 100' of head when pumping water, brine, and kerosene. The resulting pressures, however, will vary just as those seen in Figure 7. If that same pump requires 10 HP when pumping water, it will require 12 HP when pumping brine and only 8 HP when pumping kerosene.

The preceding discussion of Specific Gravity illustrates why centrifugal pump head (or pressure) is expressed in feet. Since pump specialists work with many liquids of varying specific gravity, head in feet is the most convenient system of designating head. When selecting a pump, always remember that factory tests and curves are based on water at STP. If you are working with other liquids always correct the HP required for the specific gravity of the liquid being pumped.

The Effect of Viscosity

Viscosity is a fluid property that is independent of specific gravity. Just as resistivity is the inherent resistance of a particular conductor, viscosity is the internal friction of a fluid. The coefficient of viscosity of a fluid is the measure of its resistance to flow. Fluids having a high viscosity are sluggish in flow. Examples include molasses and heavy oil. Viscosity usually varies greatly with temperature with viscosity decreasing as temperature rises.

The instrument used to measure viscosity is the viscometer. Although there are many, the Saybolt Universal is the most common. It measures the time in seconds required for a given quantity of fluid to pass through a standard orifice under STP. The unit of measurement is the SSU or Seconds Saybolt Universal.

High viscosity can gum up (pun intended) the internals of a centrifugal pump. Viscous liquids tend to reduce capacity, head, and efficiency while increasing the brake HP. In effect this tends to steepen the head - capacity and HP curves while lowering the efficiency curve.

The performance of a small Centrifugal Pump, handling liquids of various viscosities, is shown graphically in the figure below.


Figure 8

Normally, small and medium sized centrifugal pumps can be used to handle liquids with viscosities up to 2000 SSU. Below 50 SSU the Characteristic Curves remain about the same as those of water; however, there is an immediate decrease in efficiency when viscosity increases over that of water. Viscosities over 2000 SSU are usually better suited for positive displacement pumps.

About the Author

Joe Evans lived in beautiful Rhododendron Oregon and retired from Pump Tech Inc on 12/31/15. Since entering graduate school, a continuing interest has been one of computer control of mechanical and electronic systems. It began with the introduction of the minicomputer, in the late sixties, and continued with the advent of the PC and PLC in the eighties and nineties. He accidentally entered the pump industry in 1986 and has been trapped there since. He is passionate about the sharing of knowledge and its ability to replace memorization with understanding.