There are two largely accepted facts about mining: 1) A mine's landscape is dynamic, and 2) Within that landscape, water is a mine's worst enemy. Navigating a mine's landscape with pumps designed to move and/or remove water has long been the challenge of mine operators.
Mine plans are developed in an effort to extract the highest quantity and quality of materials at the lowest cost. Using a combination of maps, geological surveys, drilling samples and further research, a mine plan maps-to the greatest extent possible-the entire environment below the subsurface of a mine site. A mine plan can change quickly, depending on factors like the vulnerability of the landscape, and the market value and concentration of the products being mined.
It is not uncommon for a mine plan to change from mining for copper to mining for molybdenum, based on the increased market value of molybdenum. In other words, mining may move from one side of a pit because research indicates a higher molybdenum concentration exists in another ore body within the mine. As the landscape of a mine changes-whether open pit or underground-so do the requirements and space availability for mine dewatering equipment.
Moving Water
Water is a constant threat to mining, whether it enters a mine through infiltration or comes from a storm. Mine operators must be able to manage the water where it is known to exist and quickly alleviate the problems associated with it when it rises unexpectedly. Mine dewatering pumps generally need to pump water horizontally-from a smaller sump to a larger sump-or vertically from the bottom of the pit up to the elevation of the mine, and then horizontally to a discharge point. The pumps most commonly found in mine operations include vertical turbine pumps, electric and hydraulic submersible pumps, horizontal multistage centrifugal pumps and horizontal single-stage centrifugal pumps.
Vertical Turbine Pump
Vertical turbine pumps are the best candidates for pumping clean, cold water. Custom-made, vertical turbine pumps can have multistage, enclosed impellers in bowl assemblies supported by a frictionless, round driveshaft. Requiring a sump with significant depth for submergence and construction of a platform that suspends the pump, the vertical turbine pump's worst enemies are abrasive matter such as grit and sand-which destroy the wear rings and bushings-and air-which, if drawn into the pump, will immediately compromise performance.
Vertical turbine pumps also require special considerations for critical speed; sump dimension and materials of construction; multi-staging (which is limited by bowl shaft horsepower capacity and shaft stretching); pre-lubrication (the line shaft bearings need lubrication before operation via a solenoid); protection from blasted "fly-rock" and maintenance. Though they require minimal operational cost, the associated construction and maintenance costs and downtime can be detrimental to mining operations.
Electric and Hydraulic Submersible Pumps
Electric and hydraulic submersible pump ends generally require varying levels of submergence while the power packs require level and ample space on land. Electric submersible pumps can handle generous volumes (up to 6,000-gpm) or significant heads (around 375-ft), but are limited to less than 1-in solids handling at higher heads. Electric submersibles should be installed semi-permanently, moved infrequently and maintained by a certified electrician or technician.
In most cases, electric submersibles (like vertical turbines) are run at very high speeds. While the necessary heads may be achieved at speeds of roughly 3,600-rpm, the potential for wear from pumping silt-laden water at high speeds increases exponentially, decreasing pump life. Although rubber lining of wear parts is an option, it is not a long-term solution compared to the hardened wearplates found in centrifugal pumps. Electric submersible pumps can be prone to seal, motor and/or electric power failure when water seeps along the power cord and into the motor. These types of failures can result in repair costs measuring roughly half the value of the pump. It is not uncommon for one single-stage centrifugal pump to outlast three comparable electric submersible pumps in a mine.
Because of the robust construction of the pumpend, hydraulic submersible pumps are ideal for pumping lightly abrasive materials. Like electric submersibles, hydraulic submersibles are capable of either sizable flows (to 6,000-gpm) or modest heads (to 280-ft). Hydraulic submersibles are the most easily installed submersible pump because they do not require the power or switch gear that an electric submersible does. Instead, hydraulic submersibles require refueling and the preventive maintenance associated with the diesel engine on the power pack.
Horizontal Multistage Centrifugal Pumps
Horizontal centrifugal pumps can be multistage, i.e., multiple impellers on a common shaft enclosed in the same pump casing. These pumps generate increased pressure as the discharge of one enclosed impeller is routed to the suction of another enclosed impeller within the casing. Enclosed impellers limit the solids handling sizes of these pumps. In addition, and much like their vertical turbine counterparts, the efficiency and effectiveness of horizontal multistage pumps is dictated by the life of the wear rings and thin, multi-vane impellers.
Suction lift and distance to the water on these pumps is limited, and a net positive suction head (NPSH) is a possible concern. If NPSH is compromised, cavitation will occur, thereby causing premature damage and shortening the pump life. While the multistage pump is technically a single pump, the multiple wearparts in the pump make it more susceptible to maintenance issues and costs associated with more moving parts. Horizontal multistage pumps are typically not easily repaired in the field, particularly when repairs to wear rings or impellers are required.
Horizontal Single-Stage Centrifugal Pumps
Within the last five years, portable single-stage, close-coupled, automatic self-priming extreme high lift pumps have been introduced to the mining market. Capable of 600-ft heads using a single impeller with a diameter up to 20-in, one extreme high head pump can provide the same pumping capacity as several multistage models. Available with diesel engines or electric motors, extreme high head pumps provide power source flexibility and can be automatically controlled with a digital controller on the diesel engine, or a variable frequency drive (VFD) on the electric motor. These pumps have a suction lift of 28-ft (sea level), meaning they do not need to be submerged, but the suction elevation from the source to the impeller's eye can be no greater than 28-ft, largely eliminating NPSH concerns.
The automatic priming system on these pumps has no moving parts and requires little maintenance. Single-stage pumps have field-adjustable hardened front and rear wearplates and a hardened impeller to handle lightly abrasive applications over a long life cycle. The mechanical seal is cooled and lubricated by an oil bath and is not exposed to the abrasive-laden water. While the hardened wear materials work to combat the abrasives found in mine dewatering, the oil bath seal works to cool the rotating parts, providing dry-running.
Available on a skid mount or highway trailer, these portable pumps are built to be transported easily from site to site within a mine. Single-stage centrifugal pumps are also available with stainless steel and nickel chromium steel wet end components, making them more suitable for mine water with acid content. Like their other diesel-driven counterparts, diesel single-stage pumps require refueling and general repair and maintenance of the diesel engine.
Another commonly used method for pumping in mines is to stage single-stage centrifugal pumps in series on various levels of the mine, pushing up from lower to higher levels and out of the mine. The disadvantage of pumping in series is the complexity of pump control required for reliable service. With pumps in series, each pump is dependent upon the other and must react accordingly to changes in pumping performance. When one pump in the series fails, the entire pumping system shuts down until repairs are made.
Five Important Features of a Mine Dewatering Pump
No two mine dewatering operations are the same, but it is important to try to understand the frequency and contributing factors to water infiltration, and to attempt to alleviate those issues through adequate mine planning and dewatering equipment. When evaluating the effectiveness of an existing or developing mine dewatering plan, consider the following pump features:
1. Solids Handling Capability and Acid Content- Mine water can be laden with silt, sand and rock. Pumps with solids handling capabilities have a longer life cycle. In addition, choosing a pump with corrosive-resistant wearparts, such as stainless steel, helps to combat the destructive chemical breakdown associated with the high acid content in acidic mine drainage.
2. Portability- Pumps that require heavy civil construction (a platform) and a large footprint for setup are not the most ideal pumps for mining. In addition, anything requiring an external electrical power source loses its portability. As previously noted, the mining plan and landscape of the mine can change quickly and the onset of flooding can be dramatic. The more mobile the pump set, the better.
3. Automatic Operation- Flexibility is the key to success in mining applications. Diesel-driven centrifugal pumps, for instance, are available with programmable controllers that automate pump operation based on a pre-determined performance variable. The slow and steady operation of the pump minimizes the shock to the drive components and piping, increasing the pump's longevity. In addition, automatic controls improve reliability in rapid stormwater infiltration events, reduce labor costs associated with pump watches and decrease EPA-mandated emission levels.
Likewise, larger mines with a greater mine footprint have addressed rising fuel costs and overburdened local power grids by maintaining electrical power plants. These plants operate independently to supply power to anything from electric shovels to mine camps throughout the mining site. The availability of electric power throughout mines opens the option for electric-driven, single-stage centrifugal pumps with automatic controls. In this case, a level transducer transmits a signal to the VFD when the level in the sump pit rises to a pre-determined height. The VFD functions to its pre-programmed settings to slowly ramp up the pump to the necessary speed, pump the water level down consistently and efficiently and slowly turn down the speed until the pump turns off.
4. Easy Maintenance- Whether above or below ground, space is at a premium in a mine footprint. Ideally, mine dewatering pumps could be moved up and out of a mine, and/or would have enough space around them for maintenance technicians to perform repairs. Since that is not the case, platforms for vertical turbine pumps must be accessible so that maintenance technicians can assess the condition of the pump. Multistage pumps should also have enough space around them for repair. Single-stage portable pumps require minimum space for repair; their close-coupled configuration means no shaft alignment is necessary in the field when the pumps are moved from location to location.
5. Parts Availability- Mine flooding will not stop while pump parts are on order. Ideally, a pump would need limited parts and service, but mining conditions are not ideal. It is important to choose a pump that can live up to the conditions of the mine in an effort to minimize the downtime associated with ordering parts. If a pump has been custom-built for an application, reordering parts not stocked by the manufacturer can take 12 to 14 weeks, drastically affecting production.
Wherever water appears in a mine, it generally needs to disappear just as quickly. Understanding the landscape of the mine in question and capabilities of the pump dewatering options can go a long way in ensuring that mine operations continue uninterrupted, and that mine safety remains intact.
Pumps & Systems, September 2008