By Marc Carriere
As mines reach increasingly greater depths, piping water to and from the surface becomes ever more challenging. Mining contractor Cementation faced this obstacle when designing a chilled water line for the Resolution Project, located in the historic Pioneer Mining District just east of Superior, Arizona. With plans to sink exploratory shafts to 7,000 ft, Resolution Copper Mining needed a pipe-joining solution capable of handling the immense head and end loads, while offering flexibility to accommodate expansion and contraction. Victaulic provided a solution that met these specifications, and also reduced installation time and total installed costs.
The Resolution Project, 55% owned by Resolution Copper Co., a Rio Tinto plc subsidiary, and 45% BHP Copper, is currently in the pre-feasibility phase. Exploration conducted in 2001-2003 revealed a large copper ore body more than a mile below the surface—a deposit that could prove to be one of the largest copper ore bodies ever found in North America, and possibly the world.
In 2010, Rio Tinto reported an inferred resource of 1.624 billion metric tons (mt), containing 1.47% copper and 0.037% molybdenum—an increase over its 2008 announcement of 1.34 billion mt. If additional drilling verifies these numbers, a mine of this size would be capable of producing 25% of expected future U.S. copper demand over several decades. Production via panel caving, which is likely to begin in 2020 if evaluation and feasibility studies, and a land exchange are successfully completed, is predicted to span 40 years.
In addition to technical and environmental studies, the work scheduled for the next several years includes dewatering Shaft No. 9 of the former Magma mine, which was shut down in the late 1990s, and sinking a new exploratory shaft, No. 10, to 7,000 ft below the surface. As of April 2012, Shaft No. 10 had reached a depth of about 5,500 ft, with operations progressing at about 10 ft per day under 24-hour operations.
As mining operations progress deeper, the temperature of the rock increases. According to a paper by T. Payne and R. Mitra of the University of New South Wales, Sydney, Australia, rock within 50 m (~164 ft) of the earth’s surface maintains a temperature equal to that of the average air temperature. Between 50 m and 100 m (~164 ft to 328 ft), the gradient varies based on atmospheric changes and circulating groundwater. Below that zone, temperature almost always increases with depth. However, the rate of increase with depth (geothermal gradient) varies considerably with both tectonic setting and the thermal properties of the rock.
At the Resolution Project, temperatures can reach 130°F to 140°F at current depths. To keep workers comfortable, cooling in the form of a chilled water line was necessary. Of course, running water to a depth of several thousand ft—and an eventual depth of 7,000 ft—presents several challenges.
The first challenge was designing a piping system capable of withstanding the head and end-load of the vertical installation. The piping system had to withstand a maximum working pressure of 3,500 psi. In addition, piping hung vertically to such depths creates a tremendous amount of end-load, so the system would have to be capable of withstanding the load, ideally without extra pipe supports.
The second challenge was accommodating the temperature variations produced by the chilled water running into a hot shaft, and the resulting expansion and contraction of the piping system.
The initial plan for the chilled water line was to join the 8-in. schedule 80 pipe with 900-lb flanges. (As the depth of the shaft increases, Schedule 120 and Schedule 160 pipe will be used.) However, flanges would not provide the flexibility required to accommodate the temperature variation. As a result, Cementation sought alternatives.
Cementation evaluated two different joining systems; the first was a non-restrained flexible coupling, the second was the Victaulic Style 808 double-grooved coupling. Both pipe-joining methods had the ability to accommodate the high pressure, expansion and contraction, but the 808 was also a restrained joint. The other option would have required restraint systems on either side of the piping, increasing the installation time and cost.
Ultimately, Cementation chose the Victaulic 808 high-pressure coupling. It is a double-bolted coupling designed for use with Schedule 80 or heavier grooved steel pipe. Typical applications include paste backfill, high-pressure slurry and other solids piping systems. With the ability to withstand working pressures of up to 4,000 psi—the highest pressure rating available for any grooved mechanical pipe joining system—the 808 was well suited for the 3,500 psi chilled water system.
The system provides superior joint integrity at high pressures while maintaining a degree of flexibility to facilitate joining and accommodate up to about 1/4 in. of linear movement per coupling. This flexibility provided the necessary leeway for the thermal expansion and contraction of the pipe when subjected to the heat within the shaft.
The coupling keys of the 808 engage four grooves—two on each pipe-end. The double-groove engagement of mating coupling keys provides joint integrity at high pressures by distributing pipe end loads. Maximum end load from all internal and/or external forces to which the joint can be subjected under working conditions ranges from 103,410 lb on 6-in. schedule 80 pipe, to 319,190 lb on 12-in. schedule 160 pipe. This end-load performance gave Cementation the ability to assemble the system without extra pipe supports.
The heavy-duty ductile iron housings of the Style 808 ensure firm, secure engagement of the pipe. The housing design provides increased strength at points of high stress for high-pressure security. Double bolting on each side reinforces the double-groove positioning for positive high-pressure reliability.
The profile of the double groove is the same as that of a standard single cut groove, with the exception that there are two grooves on each pipe-end instead of one. The 808 does not require special weld-on nipples or collars, eliminating additional fabrication requirements.
In addition to accommodating thermal expansion and contraction, high pressure and end loads, the couplings also offered quick and simple installation. In shafts with limited light and space, pipe joining shouldn’t be more complicated than necessary, and comparatively speaking, the 808 offers one of the simplest installations for high-pressure systems. The couplings are installed by positioning the gasket over the abutted pipe ends, then assembling the housings over the gasket so that the coupling keys engage the grooves. The housings are bolted together until the bolt pads meet metal-to-metal and the required torque value is met. The speed of installation led to a reduction in labor, thus reducing total installed costs compared to the alternative joining methods considered.
So far, the contractor has installed more than 200 couplings—one about every 20 ft on the chilled water supply and return line. The line was pressurized and put into operation over a year ago, and the company reports that it’s very happy with the solution. In fact, Cementation plans to use the 808 for high-pressure dewatering and discharge lines as well. Victaulic couplings are also used to join compressed air lines and other various process water lines at the Resolution Project. In addition, Victaulic grooved butterfly valves are used in and around the pressure-reducing stations.
If all goes as planned at the Resolution Project, the construction process, from exploration through construction development, will last more than 10 years. (The formal construction development phase is planned for 2014-2018.) Every decision regarding system components is a commitment given those circumstances, but Cementation knew they could rely on Victaulic and its more than 85-year history in the piping industry, as well as its ongoing customer support and technical service.
As ore reserves close to the surface are depleted, mining operations will inevitably extend deeper into the earth. With depth comes pressure and other environmental factors. Fortunately, the pipe-joining technology is available to solve these challenges. Faced with the difficulty of designing a pipe-joining system capable of withstanding 7,000 ft of pressure, end load and temperature variation, Cementation found a reliable solution in the Victaulic 808 high-pressure couplings.
Marc Carriere is global mining manager for Victaulic, a leading producer of mechanical pipe joining systems. The company develops products for a full range of mining, industrial, commercial and institutional piping system applications. For more information, visit www.victaulic.com/mining.