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Piping System Friction
40 The Basic Tools

3.3 Steel Pipe Friction Analysis

As water flows through pipe, friction is generated that resists the
flow. Energy is required to overcome this friction, and this energy
must be derived from (1) pumps, (2) reduction in system pressure, or
(3) changes in static head. How this is done in actual practice requires
an evaluation of the basic equation for fluid systems: the Bernoulli
theorem. The total hydraulic head at any point in a piping system can
be computed by this theorem:

H Z hg hv (3.2)

where H total system head, ft
Z static head, ft
hg system pressure, ft H O
2
hv V /2g, velocity head, ft
2
For example, assume the following:

1. The pipe is 10 ft above the ground, which, in this case, is assumed
to be the datum for all energy measurements. (Often, this datum is
the elevation above sea level.)
2. The pressure in the pipe is 40 psig.
3. 200 gal/min of water at 50°F is flowing in a 4-in-diameter pipe. At
2
this flow, from Table 3.5, the velocity head V /2g equals 0.4 ft.
The total head H in the 4-in pipe is 10 40 2.31 0.4 102.8 ft.
This is the hydraulic gradient at this particular point in the
piping.

Equation 3.2 is for a frictionless system. For practical applications
of the Bernoulli equation, the friction of the system from one point to
another must be included in the equation. This is usually expressed
as an additional term Hf in feet.
Bernoulli’s theorem must be studied carefully to ensure that it is
understood fully. This theorem states simply that the total energy is a
constant in a system and that all energy must be accounted for, in any
analysis. A typical application of this theorem is in the use of a hot or
chilled water distribution system referred to as distributed pumping,
which will be demonstrated in several places in this book.
Distributed pumping is based on the Bernoulli theorem, which
states that energy for pipe friction can come from three sources: (1) ele-
vation, (2) system pressure, and (3) velocity head. Distributed pumping
derives its system distribution friction head from the second source,
namely, system pressure. Distributed pumping appears to be difficult

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