Katz said the approximately 3,100 feet of Aldyl-A pipe is located in the Barron Park neighborhood. The pipe comprises about a mile or 0.5 percent of the city's distribution lines and was installed by PG&E about 30 to 40 years ago when Barron Park was unincorporated. The area was annexed to the city in 1975, and the city inherited the pipelines. But the utilities department has never used Aldyl-A in any of its other lines, Katz said.
Defective Aldyl-A pipe was made by DuPont Company between 1970 and 1972 and has a shortened lifespan, according to Gene Palermo, an industry expert who worked for DuPont and is now a consultant and president of Palermo Plastics Pipe Consulting in Tennessee. It is susceptible to brittle cracking and crazing when impinged by tree roots and rocky soils and has been implicated in numerous explosions and fires across the country. On Sept. 27, a leaky Aldyl-A pipe in Roseville, Calif., caused a fire that lasted eight hours. Another gas leak was found in the same pipe 2 miles away in early October, according to news reports.
PG&E spokesman Brian Swanson said there are about 1,200 miles of Aldyl-A pipe in its gas line network. DuPont manufactured Aldyl-A from 1965 to 1984, and only a small percentage of that pipe is defective, Palermo said. However, about 33 percent of Aldyl-A manufactured between 1970 and 1972 is considered problematic, he said.
In 1998 the National Transportation Safety Board issued warnings regarding the defective pipe and the federal Pipeline and Hazardous Materials Safety Administration issued an advisory bulletin in 2007. Palermo said the so-called "pre-1973" pipe has an oxidized inner wall that can crack if the pipe is impinged or squeezed by rocks, roots or flow-control devices.
Defective Aldyl-A pipe has a life expectancy of about 10 to 20 years, said Palermo, who has done pipe life-forecasting studies.
By comparison, other polyethylene pipe can last 160 years at ground temperatures of 70 degrees Fahrenheit — the approximate temperature of southern California soils, he said.
Katz said so far the city has not found any problems with the pipes. The city's operating pressure of 25 pounds per square inch (psi) throughout the distribution system is well below the 60 psi pressure at which PG&E operates its distribution system, Katz added.
But the city has aggressively tested the lines since learning they could be problematic. Palo Alto routinely tests one half of its entire network each year and the other half the next — well above the once in five years required by the Department of Transportation. But where Aldyl-A is known to exist, testing is more frequent, she said.
The city conducted a survey of the Aldyl-A pipes at the end of October on top of its routine leak survey, she said. The city will be performing leak surveys in Barron Park on an annual basis until the Aldyl-A pipelines have been removed from the system, which is expected by 2013, she said.
Although there is no reason to believe any Aldyl-A exists outside of Barron Park, "We do want to be sure we are being thorough about identifying all the Aldyl-A that might have been installed by PG&E in the annexed area, so we are continuing to research our records of drawings and information given to us by PG&E at the time of the transfer," she said. More specific locations will be available after the engineering phase, she added.
Gas is distributed from large transmission lines to distribution feeder mains, to distribution mains and then to the service lines, which link to the customer's meter, Swanson said.
Katz said the city does not have records that identity whether Aldyl-A was used in the city's service lines.
"When the mains are replaced in the next approximately 18 months, service lateral pipeline material will be checked and replaced if they indeed are made of Aldyl-A," she said.
Palermo said residents shouldn't be overly concerned. Plastic pipe — even the worst plastic pipe ever sold — has a lower failure rate than steel pipe, according to the American Gas Association, he said.
"In the order of magnitude, it is still better than steel pipe," he said.
Steel corrodes, unlike plastic, and steel mains were involved in such tragedies as the 1994 liquid natural-gas pipeline explosion in Edison, N. J., and an August 2000 explosion and fire that killed 12 people in Carlsbad, N. M., he said.