User Guide

Plumbing Design Assistance Manual l Chapter 4 – Pipe sizing l 35
Chapter 4:
Pipe sizing
Standard
dimension ratio
Standard dimension ratio
(SDR) is a term used in
describing the size of PEX
piping — it is the conceptual
equivalent of a pipe schedule.
Dimension ratio (DR) is the
average outside diameter
(OD) of PEX piping divided by
its minimum wall thickness.
Temperature and
pressure ratings
Temperature and pressure
ratings for PEX piping are
determined by the Plastics
Pipe Institute (PPI) as required
by the ASTM F876 standard.
The minimum burst pressure
per F876 is 480 psi at 73°F
(22.7°C) for ½" PEX and 475
psi at 73°F (22.7°C) for ¾" and
larger PEX.
Note that Uponor PEX pipe
can withstand burst pressures
up to 800 psi at 73°F (22.7°C)
without failure, so designers
can feel comfortable designing
Uponor PEX pipe up to its
maximum temperature and
pressure limits.
To start the evaluation, pipes
of all sizes are empirically
tested to ASTM D2837 to
determine the hydrostatic
design basis (HDB); this
test method is used for all
polyethylene-based piping.
That data is then multiplied by
0.5 design factor to determine
the hydrostatic design stress
(HDS). The HDS is then run
through an ISO equation (ISO
R-161-1690) to determine the
temperature and pressure
limits of the pipe.
ISO Equation 2S/P = R-1
Where S = HDS, P = psi, R = SDR
Pipe sizing an
Uponor AquaPEX
plumbing system
Uponor AquaPEX pipe is
manufactured to have an
outside diameter (OD) equal
to copper tube size (CTS)
dimensions and a wall
thickness with a standard
dimension ratio (SDR) of 9
(i.e., wall thickness is one-
ninth the pipe OD).
Due to the thickness of PEX,
which provides superior
insulation and durability
characteristics, the inside
diameter (ID) of Uponor
AquaPEX pipe is slightly
smaller than that of copper
pipe. However, Uponor PEX
is three times smoother
than new copper pipe. This
smoothness means it can be
designed at higher velocities,
thus reducing the difference in
ow characteristics between
PEX-a and copper (resulting
from PEX-a’s smaller ID).
To calculate pressure loss for
an Uponor AquaPEX plumbing
system, refer to the Uponor
PEX friction loss tables
in Appendix B or online at
uponorpro.com in Technical
Support/Manuals/Plumbing.
ASTM F876 temperature and
pressure ratings for SDR9 PEX
Rated
temperature (°F)
Hydrostatic design
stress (psi)
Pressure rating for
water (psi)
73.4 630 160
180 400 100
200 315 80
Table 4-1: ASTM F876 temperature and pressure ratings for
SDR9 PEX
s
D
SDR = D/s
D = Outside