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Factors Affecting Flow Rates
In Pipes |
The major
factors affecting the flow of fluids through pipes are:
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The
velocity of the fluid.
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The
friction of the fluid in contact with the pipe.
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The
viscosity of the fluid.
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The
density of the fluid.
Fluid velocity
depends on the head pressure which is forcing the fluid through the
pipe. The greater the head pressure, the faster the fluid flow rate (all
other factors remaining constant), and consequently, the greater the
volume of flow. Pipe size also affects the flow rate. For example,
doubling the diameter of a pipe increases the potential flow rate by a
factor of four times. Pipe friction reduces the flow rate of fluids
through pipes and is, therefore, considered a negative factor.
Because of the friction of a fluid in contact with a pipe, the flow rate
of the fluid is slower near the
walls of the pipe than at the center. The smoother, cleaner, and larger
a pipe is, the less effect pipe
friction has on the overall fluid flow rate.
Viscosity (m),
or the molecular friction within a fluid, negatively affects the flow
rate of fluids. Viscosity and pipe friction decrease the flow rate of a
fluid near the walls of a pipe. Viscosity increases or decreases with
changing temperature, but not always as might be expected. In liquids,
viscosity
typically decreases with increasing temperature. However, in some fluids
viscosity can begin to
increase above certain temperatures. Generally, the higher a fluid’s
viscosity, the lower the fluid flow
rate (other factors remaining constant). Viscosity is measured in units
of centipoises. Another type of
viscosity, called cinematic viscosity, is measured in units of
centistokes. It is obtained by dividing
centipoise by the fluid’s specific gravity.
Density (r) of
a fluid affects flow rates in that a more dense fluid requires more head
pressure to
maintain a desired flow rate. Also, the fact that gases are
compressible, whereas liquids essentially
are not, often requires that different methods be used for measuring the
flow rates of liquids, gases,
or liquids with gases in them. It has been found that the most important
flow factors can be correlated together into a dimensionless parameter
called the Reynolds number, which describes the flow for all velocities,
viscosities, and pipeline sizes. In general, it defines the ratio of
velocity forces driving the fluid to the viscous forces restraining the
fluid, or:
At very low velocities of high viscosities, RD is low and the fluid
flows in smooth layers with the highest velocity at the center of the
pipe and low velocities at the pipe wall where the viscous forces
restrain it. This type of flow is called laminar flow and is represented
by Reynolds numbers below
2,000. One significant characteristic of laminar flow is the parabolic
shape of its velocity profile
At higher velocities or low viscosities
the flow
breaks up into turbulent eddies where the majority of flow through the
pipe has the same average
velocity. In the “turbulent” flow the fluid viscosity is less
significant and the velocity profile takes on a
much more uniform shape. Turbulent flow is represented by Reynolds
numbers above 4,000. Between Reynolds number values of 2,000 and 4,000,
the flow is said to be in transition.
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