The pump is the heart of mining slurry operations. By paying close attention to the sealing solution used in the gland area of slurry pumps, mining operators can increase productivity – and profitability.
Slurry pumps are the workhorses of mining operations, helping to efficiently transport ore in the form of slurry throughout the site. Unfortunately, they are also often one of the more trouble-prone parts of most plants, with the abrasive nature of mining slurries taking a high toll on pump components.
A particular stress point is the gland area, where mechanical seals or packing are used to prevent leakage. Sealing solutions that are of poor quality or that are ill-suited to the application at hand can greatly increase maintenance demands, and lead to unplanned stoppages and excessive water use.
So, how can mining operators optimise the performance of their slurry pumps and improve the productivity and profitability of their plants?
The first step is to understand the challenges that gland sealing solutions face and the requirements needed for success in the operating conditions present. The next is to understand how to choose either an efficient mechanical seal or high-quality packing that suits the application.
Understanding the Challenges
One key consideration looking for a gland sealing solution is the hardness and abrasiveness of the slurry. Is the material being transported sharp and abrasive or round and soft? The Mohs scale of mineral hardness, which ranges from talc at one extreme to diamond on the other, can be used in the assessment. Naturally, the further down the processing order of operations a slurry is used, the rounder the particles are likely to be. While such particles are less abrasive, very fine particles can be problematic in that they may damage the small gaps between pump and seal components, such as between the seal faces. Not every pump in a process will be subject to the same levels of abrasion. Hence, pump OEMs have become very adept at maximising wear life through the use of different liner and impellor materials, ranging from extremely hard high-chrome irons to resilient rubbers and polyurethanes.
Another key factor is the nature of the slurry. The amount and weight of solids being carried in the slurry will have implications on wear. A mining slurry that is 35 percent solids will be far more abrasive than one that is five percent solids. Heavy mineral slurries also behave very differently to lighter fibrous slurries, which tend to absorb a lot of water and can potentially starve seal faces of the necessary lubrication. Care should be exercised to ensure that seal faces are adequately lubricated at all times, with particular attention paid to the temperature of the slurry and the rotational speed of the pump.
Often, the only available water to transport the slurry is hypersaline ground water. This, along with high temperatures and chemicals that may be present as a result of the chemical extraction process, present yet another level of complexity that needs to be considered during seal selection.
Armed with this information, operators can make a more informed choice about whether to employ packing or a mechanical seal. If a mechanical seal is chosen, fine tuning of its design to improve the abrasion resistance can be undertaken using items such as polyurethane covers, which enhance component life.
Choosing a Mechanical Seal
Mechanical seals are widely used in some mining settings, but still relatively uncommon in others. Among the applications with the greatest uptake are nickel, copper, alumina and mineral sand mining operations. Adoption is slower within gold, coal and iron ore. While usage is varied, mechanical seals have the potential to bring enormous efficiency benefits in terms of both equipment uptime and reduced water usage to all types of mining operations, provided a quality seal is chosen and properly installed.
So, what factors should be considered when choosing a mechanical seal? It’s important to remember that the mechanical seal faces will be lubricated by process water which can often be dirty/salty/hot/acidic or alkaline. For this reason, slurry seal faces are typically made from extremely hard and chemically resistant materials. Self-sintered silicon carbide and tungsten carbide are two materials commonly used.
There are also a number of key design features that good slurry seals should have. First, the mechanical seal should be a stationary-sprung design to help it cope with the higher levels of misalignment inherent in slurry pumps. It should have non-clogging springs, preferably outside the slurry process, to ensure that the springs continue to operate as designed throughout the life of the mechanical seal. And it should have micro-polished, dynamic O-ring surfaces to assist in the smooth operation of the seal faces as internal pump pressures change during operation. It should also have the flexibility to add erosion protection features, like polyurethane, in applications where the slurry is extremely abrasive.
A good mechanical seal should also be flexible enough to add support features to prolong operating life, such as quench/drain and flush. Quality mechanical seals have line-to-line seal faces, which ensure that slurries have a difficult time penetrating between seal faces and causing premature damage and uneven face wear. Lastly, slurry seals should be designed with generous cross sections and a robust drive mechanism that can mount on hardened pump sleeves, allowing for inventory rationalisation.
While mechanical seals provide optimised levels of performance, not every application is best sealed with this solution. Packing may in some cases be the only viable solution due to the extreme nature of particular applications, such as those where the packing acts as an additional bearing surface, stiffening the rotor. In such cases, packing can create a reliable seal, provided a quality product is used.
So, what makes for quality packing? First, the yarn needs to be sufficiently strong to prevent the slurry from penetrating into the fibre. It should be woven in a manner that creates a torturous leak path.
The packing needs to be low friction for low energy consumption and to reduce any damage to the rotating shaft, despite the presence of slurry. It should also be flexible enough to transfer the axial energy into radial load and maintain a positive seal for prolonged periods to reduce the amount of follower adjustments required during the life of the packing.
A. W. Chesterton Company offers quality solutions for both packing and sealing applications. Chesterton® specialists can help guide operators in the selection of a sealing arrangement to best suit their applications, without compromise or bias to one particular solution.
In conclusion, slurry pumps play a crucial role in mining operations. By examining operating conditions and requirements and choosing an appropriate sealing solution, pump uptime can be increased, water consumption reduced, and high productivity maintained.
or why not read Slurry Pumps: The Most Critical Asset in the Mining Industry
This article was featured in Pump Industry Magazine Winter Edition 2022.