Page 258 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

Design index 111235
relatively brittle and is subject to graphitization, a form of cor- reformed after it has been set. The set is the curing process of
rosion. Its brittle nature allows for more dramatic failure modes the plastic and usually occurs under application of heat or in the
such as rapid crack propagation and circumferential breaks. presence of certain chemical agents. A thermoplastic, on the
Such failures are potentially much more severe than more duc- other hand can be repeatedly softened and rehardened by
tile failure modes commonly seen in today's pipe materials. increases and decreases of temperature, respectively.
Smaller diameter cast iron pipes have reportedly been more The most common thermoplastic piping material is
prone to failure. There is also statistical evidence that cast iron polyvinyl chloride (PVC). In the United States, PVC accounts
installed after 1949 (1 8-ft segments) experiences a higher fre- for the vast majority of all plastic pressurized water pipe and
quency of breaks than does pre- 1949 ( 12-fi segments) cast iron sewer pipe. It came into widespread use in the 1960s. but was
[51]. Alternate pipe materials have more satisfactory proper- first used in Germany in the 1930s [60]. PVC is very durable,
ties. In many locations, active efforts are being made to replace inert to water, corrosion resistant, and resistant to biological
all cast iron piping in gas service. A prioritization program to degradation. But it has less stiffness and impact resistance than
drive such replacements will often rate pipe sections based on some other pipe materials and can fail in a brittle fashion.
their proximity to occupied buildings, susceptibility to earth Polyethylene pipe is another popular plastic pipe. In the
movements, leak history, size, and operating pressure. In other United States, a majority of new and replacement distribution
areas, cast iron has been shown to provide centuries of good pipelines in recent years have been made from PE [21]. PE is
performance with no replacement programs planned. available in several formulations, some of which may be more
Today, rigid pipes are most commonly installed for low-pres- susceptible to environmental stress cracking. Stress corrosion
sure or gravity-flow water and wastewater applications. cracking is aphenomenon seen in higher stress conditions ifthe
pipe material is simultaneously weakened by its interaction
Flexible pipe with certain chemicals. PE is popular in gas distribution sys-
tems. Its flexibility offers a measure ofprotection against exter-
Steel is a flexible material and is the most commonly used nal forces caused by earth movements. It also allows the pipe to
material for high-pressure hydrocarbon transmission pipelines be crimped as a means to shut off flow. This weakens the pipe at
and high-pressure applications in general. Steel is also a com- the crimping location and generally requires a reinforcing
mon material for lower pressure municipal applications. The sleeve when the line is placed back in service, but is nonethe-
higher strength steels (>35,OOO-psi yield stress) are less com- less a valuable feature. A high-density PE formulation is avail-
mon in the lower pressure service seen in most distribution sys- able for higher pressure applications; a medium-density PE is
tems. When used as a gravity-flow conduit, steel pipe cross normally used in low-pressure applications. A substantial
sections are frequently noncircular and have a corrugated wall material cost savings is often associated with lower density PE
for a better strength-to-wall thickness relationship. Because versus high density, but this ofcourse has accompanying trade-
carbon steel is susceptible to corrosion, coatings and linings of offs in desirable properties.
bitumen-type materials, Portland cement, and polymers are Acrylonitrile-butadiene-styrene (ABS) is a material seen
common. The use of galvanized or aluminized steel is also an primarily in nonpressure applications (vents, drains, small-
anticorrosion option. diameter sewers). Polybutylene, cellulose acetate butyrate, and
Copper is sometimes used in lower pressure piping applica- styrene rubber are other less common thermoplastic materials
tions. Copper is susceptible to galvanic corrosion and is avery used in pipe manufacture.
ductile material. It is normally used in small-diameterpipes. Among thermosets, fiberglass reinforced plastic (FRP1 pipe
Ductile iron pipe is the more flexible iron pipe that has employs a thermoset resin and fiberglass for reinforcing. It is
replaced cast iron. The addition of magnesium in the formation used in both pressure and nonpressure applications, but is not
of the pipe has improved the material toughness. Ductile iron as common as the thermoplastics. Unraveling is a common fail-
pipe, as its name implies, is more fracture resistant than cast ure mode.
iron pipe. Because both external and internal corrosion are
potential problems, lining materials such as cement mortar and Joining
external wrappings such as polyethylene are used when soil
conditions warrant. Occasionally, cathodic protection has been In any pipeline design, provisions must be made to join pieces
employed in preventing corrosion in buried ductile iron. of pipe. There is a myriad ofjoining methods available for the
Although ductile iron is found in gas distribution systems, various pipe materials found in distribution systems. Welding,
today, it is mainly placed in water and wastewater service. bell and spigot connections, couplings, fusions, flanges. and
Plastics are now a common material for pipe construction. screwed connections can all be found in distribution piping. In
Advantages cited include low cost, light weight, ease of instal- many cases, the joint is structurally the weakest part of the
lation, and low corrosion susceptibility. Drawbacks include dif- pipeline. Joint type has been identified as a critical factor in
ficulties in line location after installation, susceptibility to pipeline susceptibility to seismically induced failures (see
damage (plastics generally are less strong than steels), some pages 112-1 13). Ensuring a continuous anticorrosion coating
degree of gas permeability, and certain difficulties in the join- or lining across a joint is also a challenge. The number ofjoints
ing process. Also, the buildup of static electricity charges in in a pipeline design depends on the length of pieces ofpipe that
plastic lines is a well-known phenomenon that requires special are to be joined. Although there are practical considerations
precautions to prevent possible sparking. such as the length of pipe that can be economically produced,
Two categories of plastics are available: thermosets (or ther- transported and handled during installation, the number of
mosetting plastics, FRF') and thermoplastics (PVC, PE, ABS). joints is normally minimized in a good pipeline design. The
The thermoset is characterized by its inability to be melted or evaluator should take note of the joining technique and its