Gunther, Sandstrum
# 2001 Munich
Washington Gas has been researching a number of innovative methods for the repair and replacement of older large diameter cast iron mains. We considered direct burial, rolldown, U Lining, PIM, and guided boring. The ideal replacement technique would allow for a fully rated (64 psi or 5 bar) system which would have a low installation cost, a minimum 50 year life, rapid and easy installation, and provide a safe, secure, all- fused system. Trenchless technology is very attractive due to the low cost and minimal disruption to our customers. We wanted to replace an older 24" cast iron pipe operating at 20 psi (1.4 bar) with an inserted 16" IPS plastic main rated for 64 psi. Washington Gas would require the following performance criteria : 1. The polyethylene (PE) pipe must be rated at 64 psi and yet have the maximum internal diameter. Thus high density Standard Dimension Ratio (SDR) 17 was chosen. 2. The PE pipe must have a very high resistance to slow crack growth because of the potential stresses and damage that would be inherent in insertion. 3. The pipe must have the highest resistance to rapid crack propagation since it would be unrestricted inside several sections of a 24" cast main. Furthermore, it would be possible to have dramatic swings in pipe temperature, and if the pipe were to be impacted with enough force to break through the casing rapid crack propagation (RCP) could, in theory, initiate. 4. Controlling the gas flow is also critical, therefore a 16" squeeze off tool was developed and tested by Mustang Manufacturing. Ideally, the PE pipe would have to be able to be squeezed to 100% shutoff with no damage or reduction in usable life. 5. Joining the PE pipe was also an important consideration. It was imperative to develop and test plastic-to-steel transition fittings as well as complying with the requirements of the Plastic Pipe Institute (PPI) universal procedure (PPI TR - 33/99). We selected a material that would meet all of these criteria and that is PE-100. It is impervious to RCP because it has a critical temperature of zero degrees Fahrenheit (-18 degrees Celsius), and has a slow crack growth resistance of over 2000 hours in the PENT test. PE-100 can withstand substantial backhoe hits and mechanically induced damage that would cause failures in other piping systems. The PE-100 material we chose was Phillips 8300 pipe made from Solvay TUB 121 resin. On August 21, 2000 Washington Gas installed the first 16" IPS PE-100 material in the United States. We will monitor its performance closely over the next 10 years.
