The technology involved in coal mining around the world has advanced a great deal over the years, especially in terms of improving the health and safety of the miners. At the same time mining efficiency and production rates have also improved
Pumps, in their many different designs, perform a large number of vital tasks within the mining industry and as such they can be responsible for certain aspects of mine safety. However, with so many pumps in operation they can also make a considerable contribution towards improving the efficiency of the mine, especially in terms of energy consumption.
Mine safety, whether above or below ground, is the highest priority for all operators and maintaining a high standard requires considerable effort from all parties involved. This includes the manufacturers of equipment that forms part of the safety apparatus, requiring them to meet stringent performance criteria and successfully complete extended periods of testing.
With ever-tightening margins, the efficiency of individual pieces of equipment as well as that of operational and maintenance processes becomes more important. Making the right decision in terms of pump design can produce significant benefits for the safe and efficient operation of the mine.
Slurry pumps and pipelines form a crucial link between the mine itself and the processing and shipping operations. As such, the pumps must perform reliably in order for the mine to maintain operations. However, due to their size, energy consumption can be considerable and so improvements to their efficiency can have a substantial impact on operating costs.
The minerals are transported in suspension, in many cases over very long distances and the pumps must be designed to cope with large solids as well as corrosive fluids. These very harsh working conditions demand regular maintenance procedures to be conducted in order to keep the pump operating at its most efficient.
More recent designs are improving maintenance accessibility as well as using more durable materials to improve wear resistance and efficiency. With the aid of more sophisticated computer design systems, the development of slurry pumps is set to achieve efficiency levels close to those of clean water pumps.
Maintenance efficiency is also being improved with the introduction of condition monitoring technology which provides an early warning to the maintenance engineers about a potential issue. By addressing these before they develop into a serious breakdown, the downtime is greatly reduced, further improving the operating efficiency of the pumps.
Keeping any mine dry is a crucial aim for the operators and should form one of the key health and safety targets within the organisation. As such, the dewatering aspect of the mining operation requires very high levels of reliability which depends on selecting the most appropriate design and construction for the application in question.
For most requirements, the centrifugal pump remains the tried and tested solution. However, the need for improved efficiency and lower operating costs has led to some operators moving away from traditional centrifugal pumps and opting for alternative technologies such as oscillating discharge pumps.
As underground mine depths increase so the dewatering pumps have to be specified to cope with the new challenge. This can often come in the form of multi-stage, high pressure units, operating at around 100bar, which are capable of maintaining the desired water levels.
All pumps and their associated pipework are affected by the fluids that they are required to transport, with solids content and pH posing a constant threat to reliable performance.
Advances in materials technology have seen great improvements, with the use of chrome steels, duplex steels and high quality elastomers being used for more acidic applications.
In some applications, deposits can build up in the pipework which, if not resolved, can cause increased friction losses and therefore additional energy costs and a reduction in energy efficiency. One solution is to modify the pump arrangement to increase the fluid velocity, causing a scouring action to be created and, if properly managed, the rate of deposition can be equalised with the scouring rate.
Although this may increase the energy costs in order to attain the increased fluid velocity, this should be balanced against the maintenance costs of replacing or cleaning of all the dewatering pipework on a regular basis.
These costs should also include the figures for lost production if the mine cannot be safely operated without the system under maintenance in operation.
More recent developments have seen the introduction of on-board electronics in dewatering systems designed to help with energy saving and reducing pump wear. Pump manufacturers are pursuing design improvements that can reduce the Total Cost of Ownership (TCO) and improve reliability.
Roof support systems
While the dewatering pumps manage with water levels, so it is equally important to ensure that the roof of an underground mine remains in position while the minerals are being mined. This responsibility falls to the roof support systems that are powered hydraulically by a number of high pressure pumps.
The principle of longwall mining is the use of a large cutting head or shearer which traverses the coal seam breaking it up and depositing the coal onto an armoured conveyor to transport the coal away from the face of the mine. This operation is protected by a series of hydraulic roof supports which prevent the mine roof from collapsing by applying huge pressures to support the rock above.
Each roof support is capable of independent movement so once the shearer has passed by, the support can lower itself away from the roof and advance forward to fill the space left by the shearer. In this way the roof support system maintains a safe haven for the miners and their equipment to operate in, all the time moving forward as the coal seam is mined and allowing a controlled collapse of the roof.
The efficient operation of the roof support system directly affects the safety and productivity of the mine; any down-time on the system in such a difficult operating environment would be very counter-productive. The reliable performance of these systems is essential if the client is to maximise output from the mine and see a return on investment.
High pressure pump design
Due to the high pressures required to operate the roof support structures, the pumps use a reciprocating piston principle, usually powered by a mid-sized electric motor. A reciprocating pump discharges liquid by changing the internal volume of the pump and uses non-return valves (NRVs) at both the inlet and outlet ports of the pump. Typically these pumps can produce 1,000 bar in pressure, in this application they will typically be operating at around 350bar.
The major benefit of the reciprocating pump is that it produces a fixed volume of fluid displacement at a given speed and provides a constant flow, regardless of pressure. This means that variable capacity can be achieved by changing the pump speed. In contrast, the centrifugal pump would be forced up and down the performance curve, varying the flow which may cause problems for the application.
Due to the fundamental design of the reciprocating pump, it is beneficial to control the pumps by a variable speed controller which allows accurate control of the output pressure. For applications requiring increased flow rates, one or more pumps can be added in parallel to the original to produce the desired flow rate at the same pressure rating.
It is important that the pump design incorporates the necessary safety features, including a safety relief valve to protect against a dead head situation, when the system demand for flow has stopped, causing the pressure to rise instantly. The safety valve should be designed to allow the entire pump capacity to flow, while the opening point should be set at 10% above the operating pressure. In addition, the pipework associated with the safety relief valve should be properly sized to ensure adequate flow conditions.
A pump's sealing system is equally important when considering its safety as well as overall performance and reliability. For high pressure reciprocating pumps the task is typically performed by the stuffing box, which contains packing and lubricant in various configurations depending on the application. This packing is a service item and should be able to be replaced without too much interference with the main pump assembly.
Efficiency and reliability
Energy consumption ranks as one of the highest production costs and so adopting more energy efficient motors to power various pumping systems can have a significant effect on the overall profitability of the mine.
However, an incorrectly specified pump, even with a high efficiency, can be worn significantly in a short period of time, negating the improved efficiency and ultimately incurring higher costs. Most pump problems arise because the pump’s performance characteristics do not match the application requirements, either from the initial specification or as a result of a change in circumstances.
This lack of application awareness results in higher power consumption and shorter bearing and wear life and ultimately higher operating costs. The system parameters and pump performance must be matched carefully to ensure efficient, trouble-free operation.
For high pressure applications, reciprocating pumps offer additional benefits as the design offers greater mechanical efficiency, which means that a smaller prime mover can be used and less energy is consumed. By carefully selecting the correct motor and matching it to the pumping system, a more efficient combination can be created, resulting in lower TCO for the client.
Achieving an efficient design is one thing, but making it suitably reliable for a mining application as well requires a higher level of development. This requires a certain degree of 'over-engineering'; using materials and design concepts that may exceed the original specification, but which will afford a much greater level of reliability.
In today’s climate of matching plant equipment performance exactly to demand, there is less scope for engineering a machine to exceed targets. In heavy industries, such as mining, building something to last can make the difference between catastrophically expensive downtime and ultimate profitability.
Total cost of ownership
As with most industries, total cost of ownership (TCO) is a crucial benchmark that includes, not only the initial purchase cost, but also the costs for maintenance, lost production and energy. Trying to establish accurate figures for the TCO is very important in order to determine the long term costs associated with vital pieces of equipment.
TCO covers a wide range of costs and can be influenced by the design of the equipment. For example long life bearings, which do not require regular lubrication, can reduce maintenance costs, while components designed to be easily exchanged cut the time to effect a repair. Using high specification components within a robust design adds to the confidence of a reliable product, which can be borne out by extensive testing both during the design phase and the production cycle.
For more information, visit RMI Pressure Systems.