Viscosity is defined as resistance to pouring, with higher
viscosity liquids affecting centrifugal pump performance in several
- 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 4°C
(39°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 almost always
select the wrong size pump.
Temperature is not the only variable when we look at viscosity.
There are three 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
- 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.
- 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 (20.2°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 a 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
- 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
- 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
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 tests 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. The following chart is typical:
POWER INCREASE NEEDED
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