Page 41 - HVAC Pump Handbook

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Piping System Friction
38 The Basic Tools

3.1.2 Pipe velocity is the designer’s
responsibility
It is also very clear from Table 3.2 that sizing all pipe, particularly
large pipe in the range from 20 to 42 in in diameter, requires a detailed
analysis of the entire piping system to achieve the economical size for
a particular installation. It cannot be based on a rule that limits pipe
velocity. Reiterating, it is the designer’s responsibility to determine
pipe size and maximum velocity. There are so many judgment calls in
the final selection of pipe diameter that it is not a simple process. For
a hypothetical example, if you have 12,000 gal/min flowing in a chiller
header in a central energy plant, you could use 20-in-diameter steel
pipe if the header is only 100 ft long. This would reduce the cost of the
piping and tees where the chillers are connected. On the other hand,
if a chilled water supply main runs for 1000 ft to a group of buildings,
you might use 24-in-diameter pipe to reduce the overall friction loss.
The cost of piping accessories and the length of pipe involved affect the
decision on the final pipe size. These are the evaluations that a good
pipe designer must make.
The physical pressure that the pipe must operate under and the
possibilities of corrosion as well as availability determine the schedule
or wall thickness of steel pipe. The designer should make the velocity
calculation and, therefore, the friction calculations based on the actual
inside diameter of the pipe to be used in the water system. The designer
should check the actual project conditions to ensure that the pipe
inside diameter to be used for each pipe size is available at the job site
at the time of construction.

3.2 Pipe and Fitting Specifications
Elements of an HVAC water system are connected together by means
of piping. In most cases, this piping is steel, although various types of
plastic piping are now appearing in this industry.
Most steel piping used in the HVAC industry for low-temperature
applications conforms to American Society for Testing and Materials
(ASTM) Specifications A53 or A120. Higher-temperature applications
such as high-pressure steam and high-temperature water may require
seamless piping per ASTM A106; local and ASME codes should be
checked for detailed pipe, flange, bolting, and fitting specifications for
particular applications such as high-temperature water and high-
pressure steam. Steel fittings follow American National Standards
Institute (ANSI) Specification B16.5, whereas threaded cast iron fittings
comply with ANSI Specification B16.4 and flanged cast iron fittings
with ANSI B16.1.

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