A primer on viscosity
Viscosity is defined as resistance to pouring, with higher viscosity liquids affecting centrifugal pump performance in several ways:
- An increase in horsepower (kw) is needed.
- The head, capacity and pump efficiency will be reduced.
- The mechanical seal will have trouble compensating for shaft movement and stuffing box misalignment.
- The bearings will be subjected to higher radial loading as the pump shaft is displaced
- The sealed liquid may not lubricate the lapped faces if the fluid film thickness is less than 0.000040″ (one micron) at the seal’s operating temperature and face load.
Viscosity is a measure of the “thickness” of the liquid. Molasses and motor oil are thick or high viscous liquids. Gasoline and water are thin, low viscosity liquids. Do not confuse this viscosity with the specific gravity of the same fluid. Specific gravity is a measure of the weight of the liquid compared to an equal volume of 20° C (68° F) fresh water.
Motor oil has a low specific gravity (it floats on water), but a high viscosity of more than 500 centistokes. Mercury has a high specific gravity (13.7) but a low viscosity of only 0.118 Centistokes. It is important to note again that these two properties of a liquid are entirely independent of each other.
The viscosity of a liquid can change appreciably with a change in the temperature of the liquid, but seldom changes when the pressure is altered We all know that hot oil is “thinner” than cold oil, so we must always know the temperature of the fluid when the viscosity is to be measured. Without this information you will frequently select the wrong size pump.
Temperature is not the only variable when we look at viscosity. There are four classes of fluids that change their viscosity with agitation, and one that does not:
- Newtonian fluids are unaffected by the magnitude and kind of motion to which they are subjected. Mineral oil and water are typical of this type of liquid.
- Dilatant fluids increase their viscosity with agitation. some of these liquids can become almost solid within a pump or pipe line. We all know that with agitation, cream becomes butter. Candy compounds, clay slurries and similar heavily filled liquids do the same thing.
- Plastic fluids have a yield value which must be exceeded before flow will start. From that point on the viscosity will decrease with an increase in agitation. Tomato catsup is the best example of such a product.
- Pseudo-plastic fluids show a decrease in viscosity with an increase in agitation, but they do not have a yield value. Many emulsions fall into this category.
- Thixotrophic fluids exhibit a decreasing viscosity with an increase in agitation, although the viscosity at any particular rate of motion may depend upon the previous agitation of the liquid. Examples are: glues, non-drip paint, greases, cellulose compounds, soaps, starches, and tar.
Viscosity is expressed in “absolute” or “kinematic” terms. Let’s look at absolute first:
- The basic unit of absolute viscosity is the “poise”.
- The common unit for expressing absolute viscosity is the “centipoise” (1/100 of a poise)
- Water at 68.4°F (20,2°C) has an absolute viscosity of one centipoise
- Kinematic viscosity is different:
- The basic unit of kinematic viscosity is the “stoke”.
- The common units for expressing kinematic viscosity is the “Centistoke” (1/100 of a stoke ).
The two are related as follows:
KINEMATIC VISCOSITY = ABSOLUTE VISCOSITY/ SPECIFIC GRAVITY
Since the specific gravity of water at 68.4°F (20.°C) is almost one, it follows that the kinematic viscosity of water at 68.4°F is for all practical purposes 1.0 centistokes.
We measure viscosity with a viscosimeter and there are number of them available to chose from:
- The Saybolt universal version is the most popular in the United States, and is used to measure liquids of low to medium viscosities. The Saybolt Furol version is for high viscosity liquids. A measured volume of liquid is allowed to flow through an orifice of specified dimensions and the time that it took to get through is measured in seconds. This is called the SSU number (Seconds Saybolt Universal) or SSF number (Saybolt Seconds Furol). These numbers are widely published in various charts and are often used in addition to, or in place of the actual viscosity measured in centistokes.
- The Irany, Zahn and Redwood viscosimeters operate on the same principal. You can compare viscosity readings to each other by means of conversion factors or comparison charts that are widely available.
- The Brookfield Viscosimeter is the rotating type where a disc is rotated in the liquid to be tested. The drag is noted and read directly in centipoise. The rotating disc has approximately the same friction factor operating on it as the pump impeller, so it is the best instrument for reading the friction forces we find in a typical centrifugal pump.
- You should use these instrument to read non-Newtonian fluids and solid liquid mixtures. The solids tend to clog the small orifice in the other type instruments, giving high, false readings even though the liquid portion of the mixture is at a much lower viscosity.
There are tables available that list the viscosities of many common liquids at various temperatures. It is very obvious that even small changes in temperature can affect viscosity greatly, which will change the friction losses in the pipe fittings and valves.
Unfortunately there is no acceptable analytical method of predicting pump performance when the liquid has a viscosity different than water. Many test have been conducted, and the data formulated into charts and nomographs with the result being that your pump performance can be reasonably estimated for liquids of just about any viscosity.
- On February 18, 2018