SUBJECT : 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
- The head, capacity and pump efficiency will be
- The mechanical seal will have trouble
compensating for shaft movement and stuffing box
- 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
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.
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:
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.
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.
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.
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.
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
- 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
- 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
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.
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