At Asahi-America, the new PVC/CPVC Pro-Lok line is also sealed with conventional PVC cement. The other materials make use of a heat-fusion device with heating clamps that go around the joint. Asahi has a patented device, informally known as the “Dogbone” that allows pipe lengths or fittings to be simultaneously clamped together and that provides the concentric support to keep the inner carrier pipe in place. “Depending on how the system is engineered, the Dogbone can be used to lock down thermal expansion of the pipe when it is heated, or to direct that expansion to a particular place in the system,” says Asahi's James Leary.
Both the Asahi Duo-Pro line and the George Fischer Fuseal II line are available with the inner and outer pipes already assembled, and with fittings in place. Their joining machines are designed to handle both seams (inner and outer) at the same time, or in staggered fashion.
ECTFE has the highest temperature rating — as much as 300F — of these materials, according to Asahi literature. PVDF is rated to 250F, and PP, which is available in a range of compositions, to 200F.
Asahi's Leary says that in recent years, an unusual situation has arisen in the wastewater treatment field: stress cracking of PP and PVC piping caused by higher concentrations of sulfuric acid, which is used for neutralization and preventing biofouling in certain treatment systems. “Systems were failing prematurely, especially in the southeastern United States,” he says. “Our investigations found that the fertilizer industry, which is a key supplier of industrial-grade H2SO4 to wastewater treatment plants, was now selling purer acid, yet at a lower cost than previously. So, as plant operators switched to the higher-grade acid, they were encountering problems caused by sulfur trioxides attacking the pipe.” The choice for plant operators, says Learly, is to switch to more impervious materials for the pipe or to dilute the concentration of H2SO4 that they were handling in their systems.
Valex's dual-wall steel tubing most commonly is used in semiconductor plants, where highly toxic gases are piped from cylinders or gas cabinets to processing equipment. Pipe lengths or fittings are welded together, usually with an orbital welder, Valex's Kottler says. Extra care is taken inside the gas cabinet where the outer containment tube ends; the inner carrier tube must have a secure connection to the flow-control valve or manifold, he adds. For high-purity microelectronic applications, Valex provides electropolishing of the inner walls of the tubing — even with fine instrument tubes that are 1/8 in. in diameter.
EPA rules on dual-containment systems specify that a continuous monitoring system needs to be in place for detecting leaks from the carrier pipe to the containment pipe. When these systems were first installed, many used a conductivity cable, which would short out if it encountered a puddle of leakage, along with the accompanying monitoring station and warning system. But Plastic Fusion's Van Dobbs says that many system designers have moved away from that technology because of problems with false positives and the cost of installing the system. “These cable systems can run $7 to $8 per each foot of pipe, which approaches the cost of the pipe itself,” he says.
One alternative, at least for landfills or tank farm sites with below-ground piping, is to design the system with sump areas or low points, and then to run a sample-point pipe vertically from that area. The plant operator could choose to install electronic probes at that point or simply set up a schedule according to which a technician would visually inspect the sump on a routine basis.
Condensation left from the original installation can cause cable systems to fire off essentially as soon as the project is complete, says George Fischer's Sample. “If the pipe is being installed during summer months when the temperature is 85 and the humidity is high, and the system is going below grade and will reach 65, you'll have condensation,” he says. One way around this, while using the sump approach, is to calculate how much condensation will collect and to set a sampling probe at a certain height about that calculated volume of condensation.
Leak-detection suppliers have responded with products that go well beyond mere detection. Ronan Corp., Woodland Hills, Calif., for example, has a Series X76LVC system that uses probes to monitor not just liquid level, but also vapor density and pressure decay (a way to sense a failure in the outer containment wall). Asahi-America distributes sensing systems from PermAlert, a business unit of Perma-Pipe, Niles, Ill.: PAL-AT, a cable-based system, as well as LiquidWatch, a linked sensor system that monitors low points in a pipe network.