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The
pipe bursting tool is lowered into the launch pit. The
development of guide head (schnozz) technology has made
pipe bursting more effective in cast iron gas pipes.
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The development of pipe bursting
in North America has been the exact opposite of its development
in Europe. In the United States, and to a great extent North
America, pipe bursting has a strong hold on the sanitary and
storm sewer rehabilitation and replacement market. In Europe,
however, pipe bursting was originally developed to replace aging
small diameter gas distribution networks.
Now the technology is beginning
to become more prevalent in the gas market in North America.
Large gas utilities and providers are pushing the acceptance
of pipe bursting. PG&E is a prime example of a provider
that has discovered the benefits pipe bursting brings to
the gas industry. This is due in large part to the advances
manufacturers of pipe bursting equipment have made in making
pipe bursting more effective for small diameter cast iron
gas mains.
In North America, guide head (schnozz)
technology developed during the early 1990s. As pipe bursting
gained exposure to various types and sizes of pipe, it also
encountered new challenges. Cast iron lines in particular presented
problems.
Cast iron pipe tends to break several
feet in front of a standard pipe bursting tool during the bursting
process. This causes the potential for two major problems. First,
because the shards of cast iron pipe are so sharp they can actually
cut winch lines. Second, pipe fragments can build up, making
the host pipe impassable. This can cause the tool to change
direction and veer off course.
The guide head adds needed tool length
and serves as a guide through the pipe. It also allows the tool
to get into the pipe and break it at its weakest point, the
inside. Moreover, it means that the diameter of the bursting
tool can be bigger than the diameter of the host pipe. Therefore,
under appropriate circumstances, more powerful tools can he
used and longer runs accomplished.
The guide head also helps protect
the winch line. In many configurations, the winch line is connected
to the front of the guide head instead of the front of the tool.
This keeps the line ahead of the actual bursting and out of
the way of shards of pipe.
In addition, the guide head protects
the lead end of the tool from wear. Several different configurations
are available. Sometimes guide heads are fabricated on site
to meet the specific needs of a particular job.
Cutting blades can also be used in
conjunction with a guide head. The blades can be welded directly
to the guide head and, again, different configurations are used
in different situations. The blades focus the percussive action
of the bursting tool into pressure points and can greatly enhance
overall bursting power.
Tool and expander configurations
also play a part in successful gas bursting. Using a small
rear expander is common because many times gas lines being
replaced through pipe bursting require a larger size casing
pipe or sleeve to be installed first. These advances in
pipe bursting technology have prompted PG&E to give pipe
bursting a closer look for portions of their pipeline replacement
program.
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Cutting
blades placed on the head of the bursting tool help focus
the percussive action of the tool, enabling it to break
through cast iron mechanical joints.
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PG&E began its gas pipeline replacement
program in 1985. The program covers the replacement and rehabilitation
of over 2,500 miles of pipe, much of which is cast iron. Even
halfway through the plan, PG&E is still keeping an eye on efficiency.
At an estimated total project cost of $2.2 billion, it is necessary.
That is why PG&E is willing to take the time to investigate
new pipe rehabilitation and replacement techniques that could
save them time and money. Pneumatic pipe bursting is the latest
tool to find its way into the company's tool box.
For their first bursting project
in over 10 years, PG&E chose a 700-foot section of 4-inch
cast iron in San Jose, CA. They enlisted the technical support
of TT Technologies, a manufacturer of pipe bursting tools
and equipment based in Aurora, IL. The project called for
the 4-inch cast iron main to be replaced with 4-inch Medium
Density Polyethylene (MDPE). A 6-inch HDPE sleeve would be
installed during the burst, then the 4-inch MDPE gas main
would be inserted into the 6-inch casing. PG&E Senior
Construction Specialist Lenny Caldwell said, "We do a tremendous
amount of directional drilling, a lot of dead insertion, live
insertion, pipe splitting and pipelining.
"We view pipe bursting as another
tool that works well for us in certain situations, specifically,
size for size replacement of low pressure systems."
Pipe bursting was well-suited for
this particular situation for several reasons. First, the existing
paving in the area was very thick: 15 inches. A trenchless solution
was very desirable because it would avoid the high repaving
costs associated with open cut. Second, the line would remain
a low pressure distribution line requiring a 4-inch replacement.
The new 4-inch line would be inserted into a casing. Pipe bursting
provided an easy method for upsizing from 4-inch cast iron gas
main to 6-inch HDPE casing.
TT Technologies Pipe Bursting Specialist
Collins Orton said, "The 700-foot run was divided into three
sections based on the number of services involved. PG&E wanted
to restore full service to the customers by the end of each
day. To accomplish this, we needed to maximize our time and
work with smaller sections of pipe."
PG&E maintained temporary service
to residents during each burst. Caldwell said, "In certain areas
we will tank the individual homes with gas cylinders or manifold
a tank together and feed several homes at once. Ultimately,
temporary service is determined on a project by project basis."
For the actual bursting, a 5.75-inch
diameter PCZ 145 Grundocrack pneumatic pipe bursting tool from
TT Technologies was chosen. The tool was equipped with a 6-inch
rear expander with a 7-inch outer diameter and fitted with a
guide head specifically designed for bursting cast iron. A 10-ton
constant tension Grundowinch was used to guide the tool through
the host pipe.
Orton said, "A guide head was essential
to the success of this project. We encountered bell joint leak
clamps on all bell and spigot joints, as well as double strap
iron service saddles that could have hindered the bursting process
if a guide head wasn't use."
After tweaking the tool configuration,
the actual bursting went smoothly. The bursts varied from 25
to 50 minutes each. Even before the 6-inch casing was in place,
the PG&E crew exposed each service connection. After the casing
was installed, crews cut away sections of it at each service
connection. The 4-inch carrier line was then inserted into the
6-inch casing by hand and connections were made.
Orton said, "Service tees were fusion
welded directly to the 4-inch carrier line. The tee makes the
actual connection to the line. It comes equipped with a built-in
single-use cutting system. After the cut is made, the mechanism
retains the coupon to keep the line free from obstruction."
This service connection process took
place after each burst and allowed PG&E to restore full service
to each customer at the end of the day.
Since this bursting project, PG&E
has done several others. According to Caldwell, plans call for
applying the technology in situations where it is most beneficial.
With over 1,000 miles of pipe remaining to be replaced, pipe
bursting tools have found a home in the PG&E toolbox.
Pipeline & Gas Journal,
June 1999, Pages 38-39
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