Pumps play a crucial role in the coal preparation process. Tony Lawson, engineering director at Weir Minerals Europe, explains how evolving pump design and condition monitoring are helping drive efficient production at coal mines around the world.
The complexity of a coal preparation plant varies according to the nature of the raw material, the requirements for the end product and the local regulations around water use and waste production. Some plants can require up to 15 different stages, whilst others have significantly fewer.
One thing that many plants have in common is that the coal is moved around the plant by slurry pumps. The pumps are vital components in the process, as the other pieces of equipment in the circuit – screens, hydro cyclones and separators, for example – are all gravity driven. It is slurry pumps that provide the energy to lift the material to the top of the plant, to transport it from one stage to the next, and to carry waste slurry to the disposal facilities.
This crucial role means that the performance of pumps has a direct impact on the efficiency of an operation. A unit that does not perform efficiently can become a major bottleneck in the process.
The curved vane centrifugal slurry pump was developed in the middle of the 19th century, and the basic principle remains the same today. However, the increasing sophistication of computational fluid dynamics (CFD) and computer-aided design has seen engineers make significant advances in terms of understanding the complex flow of slurries and how it interacts with the impeller and casing inside a pump.
The research that has taken place in recent years has allowed engineers to continue to improve the efficiency, versatility and ease of repair of pump units and this has meant significant reductions in the total cost of ownership of pumps – the factor that determines the value delivered to the customer.
It was this challenge that led the research and development team at Weir Minerals to develop the latest model of Warman® slurry pump – the Warman WBH® pump.
Over the past 50 years, Weir Minerals’ best selling pump – not only for coal processing but also across many other applications in the mining industry – has been the industry standard Warman AH® pump.
In order to further enhance the performance of this benchmark pump, engineers at Weir Minerals took on the challenge of redesigning the whole pump. The aim was to design a brand new pump that would outperform the AH series in every way – wear life, efficiency, safety and ease of maintenance.
The primary challenge of the project was to redesign the hydraulics of the pump to deliver slurry flow through the unit that was less turbulent, and therefore more efficient, and less likely to cause rapid wear to internal parts.
CFD modelling was used to develop wear prediction and performance analysis models. This revealed the areas of highest turbulence and the impact of particles against the impeller and casing for a range of different slurry mixtures. Digitally manipulating the shape and re-running the models allowed the design to be refined to deliver an optimised performance for the pump’s intended duties.
This digital design process was supported by wear parts data from existing AH pumps, gathered over a period of several years, by examining worn impellers and casings that had been used on different kinds of slurry application.
One outcome of the design process was a new impeller which made a radical departure from the five vane design of the AH pump, replacing it with a new four vane arrangement that delivers improved slurry guidance and smoother hydraulic flow.
This new impeller, known as the Warman WRT® impeller, is also backwards compatible with the AH pump, bringing the benefits to those not looking to invest in a whole pump upgrade. Streamlining the volute liner and redesigning the cutwater to reduce turbulence delivered further improvements.
Since the new unit was launched in 2011, on-site performance has demonstrated the efficiency gains that have resulted from this ground-up redesign. The lifespan of wear parts has been increased, fuel consumption has been reduced and improved net positive suction head (NPSH) characteristics have been achieved.
Built for purpose
Another recent innovation in the Warman family of pumps is the Warman MDC® pump series, which has been designed specifically with the coal industry in mind. Featuring an unlined pump casing and a three-vaned impeller with maximised clearance, the pump is able to pass the very large particles common to coal processing applications.
The simple, low cost ‘wet end’ of the pump – the parts that come into contact with the slurry – can be mounted to existing Warman mechanical ends – the bearing assembly, drive shaft and mount – making on-site upgrades straightforward to implement.
In addition to re-designing and evolving existing units, Weir Minerals has also achieved considerable efficiency gains by finding innovative new uses for existing products.
A good example of this is the increasing popularity in the coal industry of pumps that were originally designed to work efficiently with froth slurry streams containing a high proportion of air, which causes a major challenge for traditional centrifugal pump designs.
One of the key objectives of modern coal processing circuits is to optimise the removal of water from the waste slurry stream, creating a very thick paste. This is beneficial, as it maximises the capacity of tailings facilities, a common limiting factor in coal production, while also reducing levels of water loss in the process.
A little over 15 years ago, Weir Minerals developed the Warman AHF® froth pump, a modification of the Warman AH pump that features an oversized inlet and a four-vane impeller with innovative inducer blades that scoop the slurry at the inlet and help feed it into the pump, avoiding air locking and blockages.
More recently, during testing, Weir Minerals engineers discovered that the flow inducing properties of the impeller design meant that the Warman AHF pump also delivered excellent performance in applications involving highly viscous slurries, moving them with higher efficiency than traditional centrifugal pump designs.
Weir Minerals has seen many coal mining customers adopting this solution and re-purposing their existing AH pump for thickened waste flow duties by applying the AHF pump modification, rather than investing in a new pump that is purpose built. This example clearly illustrates why, for those customers looking to minimise capital expenditure, making modifications to existing designs can be an excellent way of achieving efficiency gains that can quickly justify the expenditure.
Belt drive systems are one of the most popular means of delivering power to pumps, but they can also be a source of inefficiency, largely because of the need to replace drive belts periodically.
Pumps on-site often run with poorly aligned or tensioned belts, and this can lead to significant performance issues, not only because of increased downtime resulting from worn out belts, but also through lost energy as a result of belt slipping or bearing damage through vibrations.
A hydraulic belt tensioner such as Weir Minerals’ Gemex® system solves this problem by effectively providing a quick-release system for drive belt replacement. Rather than needing to re-align and retension the system every time the belt is changed, the hydraulics will return it to the optimum settings almost instantly.
Smart monitoring and maintenance
The high impact nature of a coal processing plant means that wear and tear is an unavoidable factor and, no matter how advanced the technology, maintenance will always be required throughout the lifespan of a pump for optimum performance to be sustained. It is therefore crucial that the right repair and maintenance strategies are in place for any given project so that potential problems are identified before expensive failures are allowed to take place. This also ensures reaction is fast if any unexpected issues arise.
The traditional approach to pump maintenance is reactive – to wait until major warning signs show themselves before taking action to make repairs. This is often caused by a commitment to keep the process running whenever possible, and only interrupting it when absolutely necessary.
In reality, approaching maintenance this way brings with it a number of demonstrable disadvantages. The first of these is that wholly reactive maintenance means accepting that sub-optimal performance and unexpected failures are inevitable. This should not be the case, as reacting to a problem in the process can prove much more costly in terms of downtime than would be the case with a well managed programme based on condition monitoring and scheduled servicing.
Poor performance or failure of pumps has a direct effect on the productivity of the entire process and an unforeseen issue can cause it to grind to a halt altogether. The duration of the resulting interruption can vary enormously depending on the proximity of qualified maintenance engineers, how long it takes to diagnose the cause of the problem and how readily available any necessary parts are. In the worst cases, processes can be halted for a matter of days – at potentially significant cost to the business.
Scheduling regular system checks will allow the condition of critical parts to be monitored over time. This means that replacements can be made before excessive wear leads to sub-optimal performance or failure.
Examining the condition of internal parts such as the lining and impeller will mean taking it out of action for a short time. This is often worthwhile in order to avoid unexpected repairs and long periods of poor efficiency.
The frequency of these tests should be determined based on a good knowledge of the average operating lifespan of individual parts at the relevant levels of duty. This way, checks will be performed frequently enough to maintain efficient performance, but not more often than necessary, avoiding excessive costs.
Non-interruptive monitoring should also be carried out regularly, as this provides a way for any issue arising between scheduled checks to be noted. Temperature and vibration measurement of any moving parts, energy consumption and flow-rate monitoring can all be carried out while a pump is operational – supplying useful information without costly downtime.
All mechanical seals should also be regularly checked and, if necessary, adjusted – especially those around the drive shafts of pumps.
Advanced monitoring systems are available that use handheld devices connected to a central database to guide on-site personnel through the process. This ensures that no part is missed and that all monitoring data is stored centrally so maintenance can be properly prioritised across a whole site. In 2013, Weir Minerals launched a proprietary condition monitoring system, which is used by its maintenance engineers and is available to its customers. Many cases have been seen where automating the maintenance process at coal mining sites has delivered a significant improvement in equipment performance.
Systems, conditions and acceptable performance margins vary between different sites, so there cannot be a standard solution that meets the requirements of all. Consulting with an equipment supplier or maintenance expert is advisable to ensure that the right level of monitoring is in place.
It is Weir Minerals’ mission as a pump manufacturer to deliver continuous improvement for its customers, whether in terms of increases in energy efficiency, reduction in the frequency at which parts need replacing or the ease with which this can be done.
It is important for those operating coal processing plants to keep abreast of the developments both in pump technology and in the support strategies that many suppliers are now beginning to adopt in order to ensure that the pumps driving their process are performing as well as possible.
Ultimately, using inappropriate or ill maintained systems can have a negative impact on the bottom line, but fit-for-purpose pumps running at optimum efficiency will bring significant production benefits.