52
TECHNICAL
Considerations
for pipe sizing
Sizing a water distribution system for a commercial
space requires consideration of a number of factors,
from pressure and fixture count to pipe material.
By Tom Attenweiler and Don Townley
Calculating all contributing factors is essential to
meeting code requirements, as well as ensuring
proper operation of the system.
Russ Chaney
In the spirit of the sharing
of unique experiences that
shape the plumbing industries
in our respective nations, the
following article looks at how a
number of factors are essential
to meeting code requirements
and proper operation of the
water distribution system.
Written by Tom Attenweiler
and Don Townley of Lubrizol,
it is the next in a regular series
of similar articles that will run in
this magazine.
TABULATING LOSSES
The first step in pipe sizing is determining the water
pressure coming into the building from the city; for
commercial structures, there is pressure regulation at
the water meter, which is where engineers and designers
should start tabulating friction losses.
Understanding the friction losses contributing to a system
enables the designer to translate those losses into an
average pressure drop per 100 feet for the system. The
pressure drop of a system is a fundamental parameter
required for the calculation of the system velocity and,
ultimately, the flow rate (aka, ‘demands’).
Plumbing codes have established guidance for the number
of fixture units that can be serviced by a given flow rate. In
addition to its dependence on pressure drop, flow rate is
also dependent on the inner diameter of the pipe. Because
of this relationship, the size of the pipe can be adjusted to
achieve the necessary flow rate that is required to service
the desired number of fixture units. However, the maximum
CPVC pipes expand when heated and contract when
cooled. Engineers must factor this expansion into the
system design.
March 2019 Volume 25 I Number 1
number of fixture units capable of being serviced is
dependent on the available pressure in the system.
Pressure coming into the system is typically limited by
code to 80psi, though most systems average around
40–50psi. Factors such as altitude may come into play,
as well. Generally, the pressure needs to remain around
15psi once it reaches each fixture.
Determining if the 15psi of pressure at the fixture level will
be reached requires adding up the friction losses in the
system. Manually or using a sizing calculator, input incoming
pressure and then list known pressure losses. Every element
in the water system, from the meter to a bend, results in
pressure losses. Typical friction losses/gains include:
• Water meters
• Pressure reducing valves
• Submeters
• Elevation loss/gain.
Once you have tabulated the friction losses, you can
determine the total pressure drop, which is reported on
the building plans.
SELECTING THE PIPE
Taking the knowns — friction losses, pressure drops,
supply pressure, and the number of fixtures — code
tables will indicate the flow rate (demand) needed to feed
those fixture units. This flow rate enables the designer
to determine what size pipe is required to achieve the
proper flow rate to feed those fixtures.
The water pipe should be sized as outlined in section
6.10 and appendices A and C of the Uniform Plumbing
Code. Some of the items that need to be considered are:
1. Daily service pressure – Reference section A102.1
2. Friction loss through the water meter – Reference
section A102.2
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