Today, many chemical companies are addressing numerous environmental and safety issues with a simple, yet elegant approach: double-walled pipe and tubing. Such systems, which feature a pipe within a pipe or encased in an outer covering, with an annulus between the two diameters, have been around for years. However, enhancements in materials, tools and services make double-walled piping faster to install, more reliable long-term and applicable to a broader variety of needs.
The great majority of those needs are in two applications, says Gary Sample, product manager for George Fischer, Tustin, Calif.: environmental — especially those involving wastewater and groundwater — and process safety in the plant.
U.S. Environmental Protection Agency (EPA) standards, published in 40 CFR Parts 280 and 281, mandate double-walled piping for many below-ground transport systems in wastewater treatment plants and at sanitary or hazardous-waste landfills or remediation sites. (These and other regulations affect fuel storage systems, tank farms, drainage or runoff from process plants, and some food-processing and related applications.) “EPA maintains a long list of hazardous materials for which double-walled piping transport is required,” Sample says. “If you have below-grade transport of any of these materials, you need double-walled piping.”
No equivalent national standard for process safety applications drives the use of double-walled piping. However, Raul Kottler, vice president for engineering at Valex Co., Ventura, Calif., a global supplier of electropolished steel tubing for the microelectronics industry, cites an important mandate in the heart of Silicon Valley: The Santa Clara Fire Chief's Association and Santa Clara County Department of Health developed local standards for the many semiconductor fabrication plants in that county. These standards specify double-walled containment systems for certain classes of toxic or corrosive chemicals — mostly gases — used in semiconductor fabrication. Some designers of chemical plants also opt for the approach for such materials.
Material choices
Perhaps the most critical discretionary choice for pipe system fabricators is the material of the double-walled system. In most cases, the applications involve little or no gauge pressure but can vary widely by temperature. For large-scale environmental applications, such as at landfills, economics dictates the use of high-density polyethylene (HDPE). It is a relatively low-cost polymer and can be assembled by butt fusion (welding). The faces of pipe lengths or fittings are heated, usually with an electrically powered resistance heater, then butted together. Pressure testing afterward ensures that the welds are defect-free.
“Our installations succeed because we're very careful about how pipe joining is done,” says Bill Lee, environmental sales manager at Lee Supply Co., Charleroi, Pa. He says the company uses HDPE pipe from Chevron Phillips Co., The Woodlands, Texas, and fusion machines from McElroy Mfg. Co., Tulsa, Okla. “We use field technicians who have been trained with the McElroy equipment, and each technician puts his own identification seal on the joint he forms,” Lee says. “It's a point of pride with us.” Lee Supply started out in the business of supplying water-transport equipment and services for the coal mining industry but has since expanded into environmental and wastewater-treatment applications, he adds.
Plastic Fusion Fabricators, Huntsville, Ala., similarly favors HDPE. Van Dobbs, sales and marketing manager, says that the company uses pipe from Rinker Materials Corp., West Palm Beach, Fla., and McElroy fusion machines. The company also has some experience with dual-walled pipe from Asahi-America, Malden, Mass. Dobbs notes that design specifications can be very tight for the chemical industry but looser for landfills or similar environmental projects. In particular, these installations sometimes opt for simply having the inner (carrier) pipe lying in the outer (containment) pipe, rather than being held in place concentrically by spacers or similar fittings (Figure 1). Field installation of such systems can be somewhat easier, because the outer pipe simply slides over the inner one.
HDPE finds use in many types of water transport applications. However, for chemical solutions, other materials generally are preferred. Asahi-America has just introduced a line of polyvinyl chloride (PVC) and chlorinated polyvinyl chloride (CPVC) dual-wall pipe, called Pro-Lok. The products are available in diameters of 1 in. to 10 in. Its older product lines include polypropylene (PP), polyvinylidene fluoride (PVDF) and ethylene-chlorotrifluoroethylene copolymer (ECTFE, also known as Solvay-Solexis' Halar thermoplastic). The Duo-Flo product line is available in PP, PVDF and ECTFE materials; the Duo-Pro line is available in PVDF and PP; and the Fluid-Lok line in HDPE.
At George Fischer, the workhorse product, Fuseal II, is made of PP. The company also makes a PVC outer pipe system, brandnamed Contain-IT. George Fischer has emphasized ease of assembly in how its system is designed, says Gary Sample. The Fuseal II line includes fittings that have wire coils embedded in them. To join pipe and fittings, the company's “electrofusion” machine is hooked up to the coil, and the joint is heated for a predetermined time period (Figure 2). Then the machine's cables are disconnected and the seam is allowed to cool, completing the joining. The company also offers an infrared heating device that allows joints to be melted together without any contact with the heating element, thereby providing a greater degree of purity in the vicinity of the joint.
The Contain-IT line of PVC comes with pipe and fittings sliced into halves, which are glued together with a two-part epoxy cement around the carrier pipe. The PVC is clear, allowing for a quick visual inspection of pipe integrity.
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.
Leak-detection options
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.
