Engineering and Mining Journal - Whether the market is copper, gold, nickel, iron ore, lead/zinc, PGM, diamonds or other commodities, E&MJ takes the lead in projecting trends, following development and reporting on the most efficient operating pr
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JUNE 2014 • E&MJ; 137 www.e-mj.com P R O C E S S I N G S O LU T I O N S toring equipment. While generally a more reliable option than pressure sensing—as the temperature increases immediately when valves are closed—this option is again expensive as probes or thermocou- ples not only have to be hard-wired to the breaker but are also subject to the effects of slurry build-up, again impact- ing their performance and ability to pro- vide the rapid, reliable data needed to trigger a shutdown. Strain gauges are a further option for specifiers and system designers, but again are not always found to be reliable, while fusible plugs rely on the same tech- nology as temperature and pressure sens- ing and so do not overcome the issues associated with these technologies. Meanwhile, pressure relief valves again add to cost while their presence may compromise the leak-tightness of the whole system and they may not react quickly enough to relieve a rapid pressure build-up. And while most pumps are fit- ted as a matter of course with safety valves, these components require regular testing and maintenance to ensure they will come into action at the desired pres- sure. Indeed, the issue of maintenance is a key one given that many mines operate a continuous shift pattern, meaning that downtime for routine maintenance on pumps and associated components has to be minimized. Concerns over the effectiveness and cost of these technologies, as well as the need for a truly maintenance-free solu- tion, led to the development of the first bursting discs or rupture discs. A rupture disc is a sacrificial part containing a domed membrane, which fails instant- ly—within milliseconds—at a pre-deter- mined pressure and cannot reseal itself. This is ideal in scenarios where pressure may be subject to rapid build-up, and other forms of pressure relief may be unable to respond quickly enough. Many of the early discs were made from foil, in many ways an ideal material, but one that is relatively delicate and can be susceptible to damage such as bend- ing and scratching, which is most likely to occur when the disc is inserted into its holder. In these instances, the perform- ance of the disc may be compromised, with the most likely scenario being that it will burst at too low a pressure, resulting in unnecessary downtime and the cost of a replacement. This cost is by no means insignificant, while ancillary items such as the special disc holder raise purchase costs further, meaning that foil discs rep- resent an expensive option given their relative fragility. The issues with foil discs drove lead- ing materials companies to seek a more cost-effective and practical solution using a more robust material, which did not require a special holder, and was less prone to damage during the installation process, and so to premature failure resulting from accidental alterations to the disc's shape or surface profile. The solution came in the late 1980s in the form of the first graphite discs. These were (and still are) designed to fit between the bolts within the standard ANSI flanges found on most pumps, elim- inating the need for a separate holder and easing installation. Furthermore, with graphite being a harder and tougher mate- rial, the discs can withstand a certain amount of scratching with no compromise to their burst pressure, and can operate at a broad range of temperatures between -50°C to 250°C. The use of a PTFE mate- rial bonded to the disc optimizes resist- ance to any alkalis present and ensures their presence does not impact the disc's service life or performance. The flexibility and versatility of graphite enables the production of discs to very precise customer parameters—in fact, there are very few, if any, "standard" products on the market. Rather than rely- ing on calculations using pump casing pressures, burst pressure is typically cal- culated by taking the working pressure and adding 75%, meaning a pump with a working pressure of 1 bar will require a disc with a burst pressure of 1.75 bar. Physical destructive testing of sample discs from individual batches before they leave the facility guarantees that the discs will operate to the agreed customer parameters, with test certificates provided to ensure that local and international safety regulations are satisfied. The sophistication of modern manu- facturing techniques even allows the pro- duction of discs that can cope with nega- tive or vacuum pressure. For these appli- cations, for example where the system is subject to hydraulic back pressure, discs can be manufactured with a vacuum bar inside the orifice, meaning they will still operate effectively in the presence of any vacuum "pull." Discs can be manufac- tured in diameters of between 0.5 in. and 16 in. in a variety of thicknesses to suit virtually any application. If fitted correct- ly, graphite rupture discs have an almost unlimited service life, require no interim maintenance, and can typically be changed within a matter of minutes. While they are frequently used in con- junction with a safety valve, a rupture disc on its own is more than capable of ensur- ing that pre-determined pressure within a pump cannot be exceeded. The latest development has seen the introduction of "centerline" discs to com- pletely negate the issue of scratch dam- age. The versatility and cost-effectiveness of graphite discs has seen them tested against other methods specified by many of the leading international mining com- panies as the preferred system for protect- ing pumps from excess pressure build-up. The information contained in this article was provided by Morgan Advanced Materials, headquartered in Windsor, Cornwall, England. For further information, visit www.morganadvancedmaterials.com. If fitted correctly, graphite rupture discs designed to guard against pump explosion have an almost unlimited serv- ice life, require no interim maintenance, and can typically be changed within a matter of minutes. EMJ_pg136-137_EMJ_pg136-137 6/2/14 3:21 PM Page 137