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.
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.