Page 65 - HVAC Pump Handbook

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Piping System Friction
62 The Basic Tools

friction losses through tees includes any data for this type of connec-
tion, which is known to have a sizable friction loss.
Item 6, multiple-duty valves, refers to a combination check, balancing,
and shutoff valve. These valves can be wasteful of energy, since they can
be set at an intermediate position and left there at a high friction loss.
These valves should not be used without downstream shutoff valves,
since the check-valve portion cannot be repaired without draining the
system. In most cases, the cost of this multiple-duty valve is so much
more than that for an ordinary check valve and shutoff valve that they
are not economical. Also, their friction loss is usually greater than that
for an individual check valve and butterfly valve (see Fig. 5.8).
Certain crossover bridges can be piping practices that waste energy
by using return pressure-reducing valves. The additional piping losses
also can increase the energy consumption of the total piping system.

3.4.1 Effect of fabrication on steel fitting loss
The great variation in steel pipe fitting friction loss is due to some
extent to the method of fabricating steel pipe assemblies. The fitting
losses listed previously are for factory-formed fittings such as tees
and elbows. Figure 3.4a describes the three ways that a steel tee can
be fabricated: (1) a factory-fabricated tee with dimensions conforming
to ANSI Specification B16.5, (2) a “fishmouth” fabrication in which
the end of the branch pipe is cut to a curve equal to the outside diam-
eter of the main pipe, and (3) an unacceptable fabrication that should
never be used. Unfortunately, if the inspection is of questionable qual-
ity, this type of fabrication may result. A fourth method that is avail-
able is an acceptable practice and is called a saddle tee.
It is obvious that the friction loss for the factory-fabricated tee will
be less than that for the field-fabricated tees, particularly for the type
3 tee, where the branch pipe projects into the main pipe. The loss for
the latter may be two to three times that for the factory-fabricated
tee. The smooth radii of the factory-fabricated tee reduce appreciably
the friction of the sharp corners of the type 2 tee and the reentrant
loss of the type 3 tee.
Another case of fabricated fittings is the 45- and 90-degree elbows.
Figure 3.4b describes (1) a standard production elbow and (2) a typical
field-fabricated mitered elbow. If a sizable radius is maintained on the
mitered elbow, its friction loss will be similar to that of the factory-
fabricated elbow. Often designers reduce their fitting losses through
elbows by specifying long-sweep elbows with a greater radius.
Field fabrication and selection of fittings have a great impact on the
overall friction loss in the piping of an HVAC water system. Continued

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