In both underground and above ground mines, quarries, and construction sites, water can accumulate and interfere with operations, necessitating its management and removal.
The process to achieve this is referred to as dewatering. Water is essential to mineral processing to recover valuable ores. Following their extraction, large sediment or tailings ponds are used to collect the tailings laden water from these processes.
After settling out of the water in these tailings ponds, water is recycled back to the process plant to be recycled using dewatering technology.
Dewatering usually employs a pump. Flexibility is important in dewatering applications due to variables including inclement weather and changes in mine planning. Consequently, applications are often diesel engine powered mobile plants as opposed to electrically powered fixed plants.
Timely management of water level
Due to the need to keep mines dry, the volume and ullage of dewatering catchments are often relatively small. This, coupled with the highly variable nature of inflows to the catchments and the action of dewatering pumps necessitates timely management of the water level. Because of thermodynamic constraints the pump must be located close to the water source in order to avoid the damage and loss of flow caused by cavitation. To prevent flooding of the pump and potential cavitation, frequent movement of the pump unit is required. Dewatering pumps, especially larger units, are typically skid mounted and require dozers or other equipment to move around site, which can cause costly interruptions to this equipment’s utilisation in mining operations.
Owing to the costs associated with frequent movement of the pump units, increasing suction lifts, defined by the vertical height from the water level to the impeller eye, are highly desirable. This lift and the necessary priming of the pump are usually achieved through the use of a vacuum primed pump system or submersible pumps. This suction lift is constrained, however, by the Net Positive Suction Head required (NPSHr) of the pump which can be thought of as the minimum pressure energy required for the pump to operate.
Speed Requirements and the Influence on Suction Lift Performance
Based on the mobility requirements mentioned previously, the pumps typically feature diesel powered engines. Direct coupling is usually employed as the initial cost and ongoing maintenance of gearboxes is excessive. When directly coupled to a diesel engine, the pumps are required to operate at relatively high speeds, usually between circa 1000RPM and 3000RPM. Centrifugal pump flow performance drops with increasing speed, resulting in competing requirements.
Efficiency, Speed and Lift Trade-Off
While increased speed reduces suction lift, it also has the effect of improving pump efficiency. There are a number of other operational considerations attributed to the solids handling requirements of the pumps and the importance of preventing efficiency loss caused by premature wear. Wear performance is considered against speed and vane thicknesses to measure Total Ownership Cost.
Pump duty considerations also come to bear on this trade-off. Mine dewatering operations typically have relatively high head requirements based on economic pit depths and underground applications, and wherever possible single stage applications are desirable based on the reduced rotating equipment maintenance requirements. Therefore, since centrifugal pump head development is largely a function of speed, impeller diameter and impeller vane exit angle, and increasing impeller diameter is correlated to increasing pump capital cost, higher pump speeds are also desirable from this perspective.
Diesel Engine Consideration
As higher diesel engine speeds typically result in increased power outputs relative to engine size, increasing pump speeds can reduce engine capital costs.
Matching of the pump impeller diameter and, indirectly, the pump torque curve to the engine is also an important consideration. Running the diesel engine at high loads and below its rated speed, results in a ‘lug’ condition. Lug conditions can cause high exhaust manifold temperatures in turbo charged engines, as they are not running at their optimum speed and have less air filling the cylinders. This can also lead to higher cylinder temperatures and associated mid to long term effects on engine valves and other components. Additionally, when operating under ‘lug’ conditions, the reduced speed of cooling fans reduces heat rejection and can be detrimental to the wear life of the engine.
When considering mobile dewatering applications, it is important to evaluate all of the above points. Weir Minerals’ dewatering portfolio covers a variety of dewatering solutions for its customers. Multiflo® dewatering pumps are uniquely designed to balance and optimise the conflicting requirements of pump moving, operating and fuel costs, pump efficiency and wear, pump unit capital cost in addition to diesel engine considerations and performance. Visit the company’s mine dewatering page for more information.
Author: Jeremy Hanhiniemi, Engineering Manager, Weir Group
Read the article online at: https://www.globalminingreview.com/product-news/11062018/pump-design-considerations-for-mobile-mine-dewatering-applications/