ASHRAE Journal - September 2014 - 55

COLUMN ENGINEER'S NOTEBOOK

cause of underground piping system failures.
"Underground Corrosion" Circular C-579"1 shows that
ferrous metals including carbon steel, wrought iron
and ductile iron corrode at essentially the same rate
underground. The apparent corrosion resistance of
ductile iron is attributed to the fact that graphitized
ductile iron retains its appearance as a pipe even
when much of the iron is gone. Underground corrosion of ferrous metals occurs through electrolytic and
galvanic corrosion. For more information on causes
refer to "Causes of Underground Corrosion" Technical
Paper 82."2 Most of the underground exterior-piping
corrosion failures witnessed by the author have originated at piping joints due to lack of corrosion protection or poor workmanship and inspection during
installation.
Ductile iron (DI) and polyvinyl chloride (PVC) pipe and
fittings may be joined to themselves and to other piping
materials using various mechanical joining methods
including gasketed push-on joints, flanges, mechanical
joint (MJ) adapters, and transition fittings. All mechanical joint products used with pressure piping systems
must provide restraint against pullout. Joining devices
and components with joints that seal but do not restrain
must be provided with additional external bolt-on
restraints or thrust blocks.
It is important to caution that gasketed mechanical
joints on PVC and DI pipe will experience leakage. This
is normally acceptable in domestic water distribution
systems but may not be desirable in large chilled and
heating hot water systems. Allowable leakage from these
joints can be calculated using AWWA C6003 and C605.4
Experience has shown that heating hot water systems
will leak at a higher rate than chilled water systems
when using gasketed push-on joints.
PVC and high-density polyethylene (HDPE) pipe
resist typical aging effects because they do not rust,
rot, corrode, tuberculate, or support biological
growth, and they resist the adherence of scale and
deposits. PVC and HDPE used in chilled water piping
applications are electrically nonconductive polymers
and not adversely affected by naturally occurring
soil conditions. As such, they are not subject to galvanic action and do not rust or corrode. The surface
characteristic of PVC and HDPE pipe are classified as
"smooth" pipe and as such, they offer lower resistance
to the flow of fluids compared to steel and iron pipe.

TABLE 1 Service pipe materials.
PIPE MATERIAL

SERVICE

RELATIVE COST

REFERENCE STANDARDS

Carbon Steel

CHW, HHW

$$

ASTM A53

Type K Copper

CHW, HHW

$$$$

ASTM B88

Ductile Iron

CHW, HHW

$$$

AWWA 151

PVC

CHW

$

AWWA C900 (4 to 12 in.),
AWWA C905 ( 14 in.)

HDPE

CHW

$

AWWA C906

PEXa

CHW, HHW

$$

ASTM F877 ( 5 in.)

HDPE provides an additional benefit because it can
be welded using heat fusion, not relying on a gasketed
mechanical joint. The principle of heat fusion is to heat
and melt the two joint surfaces and force the melted
surfaces together, which causes the materials to mix
and fuse into a monolithic joint. When fused according
to the pipe and/or fitting manufacturers' procedures,
the joint becomes as strong as, or stronger than, the
pipe itself in both tensile and pressure properties
(see "PE Pipe-Design and Installation" AWWA Manual
M555).
Crosslinked polyethylene (PEX) is a modified polyethylene material, typically high-density polyethylene
(HDPE) that has undergone a change in the molecular
structure using a chemical or a physical process whereby
the polymer chains are permanently linked to each
other. This crosslinking of the polymer chains results in
improved performance properties such as elevated temperature strength, chemical resistance, environmental
stress crack resistance, resistance to slow crack growth,
toughness, and abrasion resistance. This piping provides
a plastic alternative for heating hot water applications.
The service temperature of PEXa can be 180°F to 203°F
(82°C to 95°C) with short-term exposure to 210°F (99°C).
The typical pressure rating is 100 psig at 180°F (690 kPa
at 82°C).
Hydraulic shock is the term used to describe the
momentary pressure rise in a piping system that
results when the liquid is started or stopped quickly.
The momentum of the fluid causes this pressure rise;
therefore, the pressure rise increases with the velocity
of the liquid, the length of the system from the fluid
SEPTEM BER 2014

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ASHRAE Journal - September 2014

Table of Contents for the Digital Edition of ASHRAE Journal - September 2014

Contents
ASHRAE Journal - September 2014 - Cover1
ASHRAE Journal - September 2014 - Cover2
ASHRAE Journal - September 2014 - 1
ASHRAE Journal - September 2014 - 2
ASHRAE Journal - September 2014 - Contents
ASHRAE Journal - September 2014 - 4
ASHRAE Journal - September 2014 - 5
ASHRAE Journal - September 2014 - 6
ASHRAE Journal - September 2014 - 7
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ASHRAE Journal - September 2014 - SCover1
ASHRAE Journal - September 2014 - SCover2
ASHRAE Journal - September 2014 - S1
ASHRAE Journal - September 2014 - S2
ASHRAE Journal - September 2014 - S3
ASHRAE Journal - September 2014 - S4
ASHRAE Journal - September 2014 - S5
ASHRAE Journal - September 2014 - S6
ASHRAE Journal - September 2014 - S7
ASHRAE Journal - September 2014 - S8
ASHRAE Journal - September 2014 - S9
ASHRAE Journal - September 2014 - S10
ASHRAE Journal - September 2014 - S11
ASHRAE Journal - September 2014 - S12
ASHRAE Journal - September 2014 - S13
ASHRAE Journal - September 2014 - S14
ASHRAE Journal - September 2014 - S15
ASHRAE Journal - September 2014 - S16
ASHRAE Journal - September 2014 - S17
ASHRAE Journal - September 2014 - S18
ASHRAE Journal - September 2014 - S19
ASHRAE Journal - September 2014 - S20
ASHRAE Journal - September 2014 - S21
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ASHRAE Journal - September 2014 - Cover4
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