ASHRAE Journal - July 2019 - 47

COLUMN ENGINEER'S NOTEBOOK

FIGURE 1 One-pipe and two-pipe pre-insulated PEX.

TABLE 3 Hydrostatic design stresses and pressure ratings for SDR 9 PEX tubing

for water at different temperatures.
RATED TEMPERATURE (°F)

HYDROSTATIC
DESIGN STRESS (PSI)

PRESSURE RATING
FOR WATER (PSI)

200

315

180

400

100

630

160

Corrugated PE Jacket
Insulation

expansion from 220°F (100°C) down to 73°F (23°C) for a
particular PEX pipe.
One-Pipe

PEXa
Carrier
Pipe

Two-Pipe

Hydrostatic Pressure Ratings
The pressure rating of PEX piping is obtained from the
hydrostatic design basis (HDB) method as deﬁned in
ASTM D2837.4 The HDB ratings have been shown to be
useful indicators of relative long-term strength of thermoplastic materials. The Plastics Piping Institute (PPI)
develops policies and procedures for the recommendation of the estimated long-term strength for commercial
thermoplastic piping materials and includes this in its
"PEX Pipe Design Manual (HDB-Based) for Water, Oil,
Gas & Industrial Applications."5 PPI also develops policies and procedures for the recommendation of the
estimated long-term strength for commercial thermoplastic piping materials and includes this in Technical
Report-3, TR3.6 PPI Technical Note, TN-52,7 provides
guidance for determining appropriate design life calculations of PEX pipe and tubing in high-temperature
applications, deﬁned as operating temperatures above
180°F (82°C).
Table 3 shows the ASTM F8768 minimum long-term
hydrostatic design stress ratings and pressure ratings for
SDR 9 PEX tubing for various temperatures, when determined in accordance with procedures no less restrictive than those of PPI TR-3. The pressure rating is the
estimated maximum water pressure the tube is capable
of withstanding continuously with a high degree of certainty that failure of the tube will not occur.

Pre-Insulated Piping and Joining Methods
Pre-insulated PEX piping systems are available from
several manufacturers. In North America, pre-insulated
PEX piping comes in two primary conﬁgurations: single
pre-insulated pipe is available up to 4 in. (100 mm) nominal pipe diameter while two-pipe systems in a single
insulated jacket are available up to 2.5 in. (65 mm) nominal pipe diameter. Figure 1 shows pre-insulated single pipe

and two-pipe conﬁgurations. Two-pipe systems typically
work best when the temperature differential between
the two pipes is less than or equal to about 20°F (11°C)
T. Corrugated polyethylene jackets are used for the preinsulated piping system to allow ﬂexibility in bending and
resistance to thermal expansion when buried.
Because the maximum diameter of available preinsulated PEX pipe is relatively small, its use is limited
to applications with lower ﬂow requirements. In the
author's experience, this has been in applications routing from local heating plants to nearby buildings and
used in building connection branches from campus distribution vaults.
Different insulation materials and techniques are used
by various manufacturers. Fully bonded polyurethane
insulation is available by several manufacturers while
another uses unbonded layered PEX insulation. The fully
bonded polyurethane insulation is bonded to both the
PEX pipe and jacket and is more rigid, and has a lower
thermal conductivity requiring less insulation. The
unbonded layered PEX insulation has different layers of
PEX insulation that move independently for increased
ﬂexibility and tighter bending radiuses but requires a
larger outside jacket diameter due to a higher thermal
conductivity. For example, a 2 in. (50 mm) nominal pipe
with the fully bonded polyurethane insulation system
requires a 36 in. (914 mm) bending radius while the
unbonded layered PEX insulation requires only 18 in.
(457 mm).
An integral part of any pipe system is the method
used to join the system components. PEX is typically
joined with compression sleeve type ﬁttings. The
majority of systems use ﬁttings made from either DZR
(Dezinciﬁcation Resistant) brass or offer poly PEX
J U LY 2 0 19

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ASHRAE Journal - July 2019

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