Metso’s Europe, Middle East and Africa director for mining flow control, Steve Sedgwick talks about Metso’s approach to slurry pump design and the key features that enable the company to offer the lowest possible total costs of ownership.
Slurry pumps include various types of heavy-duty centrifugal pumps used for the hydraulic transportation of solids. “Slurry pumping involves a varied portfolio, depending on the media being pumped, but slurry pumps are almost always designed and chosen based on their wear performance,” begins Sedgwick.
The Metso slurry pump range covers the pumping of any type of materials, primarily ground rock, including large particles using dredge pumps that can handle particles of over 150 mm. “The typical discharge from the mill in a minerals processing application is in the 100 µm to 250 µm average particle size range, combined with large rocks and a steel content caused by mining activity and broken mill balls (scats), which are all pumped as very dense slurries that cause high wear rates on the pump’s internal components,” he explains.
“Metso can offer the full range of duties and wear lining options, from rubber-lined to high chrome white cast iron, but to get the best value, the pump has to be well suited to the slurry it is pumping,” he adds.
Showing a summary diagram of the slurry pump range, Sedgwick says that Metso’s approach to handling the diversity of different slurry types starts with the American Hydraulic Institute’s wear category classification. “For a Category 4 slurry, for example, we recommend an impeller aspect ratio of 3. This is the ratio between the pump’s outside diameter and the internal impeller diameter at the eye of the impeller – OD/ID. Our flagship MD pump is designed to meet these extremely abrasive Category 4 needs,” he tells MechTech.
The aspect ratio is a simple number that sets the basic design limitation for any centrifugal pump subjected to wear. If the aspect ratio is large, then the size of the pump has to be made larger to accommodate the larger impeller. This allows the impeller to rotate at a slower speed for the duty required, and it increases the impeller vane length and thickness, which slows its deterioration rate.
But the larger size is associated with higher costs. “A Category 4 MD pump might have an impeller with an OD of 600 mm and an ID of 200 mm. A Category 3 with an aspect ratio of 2.5 that can produce the same duty only needs an impeller with a 500 mm OD for the same inlet size. So a pump capable of producing the same flow and head has an impeller, a volute and a frame size that are all physically smaller by some 25%, making the pump significantly cheaper, easier to handle and to maintain,” Sedgwick explains.
“Hence the importance of selecting the appropriate aspect ratio for the slurry being pumped. It is the starting point in selecting the most cost effective pump,” he advises.
Metso’s slurry pump selection chart and its pump selector software, Pumpdim, use the aspect ratio as the primary variable in organising its range for different slurry requirements. “The pink colour (HM and HR range), for example, represents an aspect ratio of around 2.5. Our slurry pumps are organised on the selection chart in order of reducing aspect ratio, that is, from most abrasive (3) to least abrasive (2), with Metso’s MD pumps being the most suited to cope with the highest abrasion levels,” Sedgwick notes.
Another differentiator for Metso is its focus on limiting the inlet fluid velocity of its pumps. “For heavy slurries we keep our inlet velocity down to below 5.5 m/s to minimise impact damage to the impeller from sharp, coarse and heavy solids. From a hydraulic design perspective, we also try to make sure that the pump always operates just to the left of the best efficiency point (BEP), whilst maintaining a relatively low inlet velocity. This results in the highest possible efficiency, minimum internal turbulence and the longest wear life. Wasted energy has to go somewhere, into vibration or turbulence, for example, which may cause bearing failure and accelerated wear,” Sedgwick points out.
Addressing the design innovations that improve the hydraulics for abrasive, applications, Sedgwick says: “A pump is always going to wear when pumping slurry. On some applications, the best products might only last for 400 to 600 hours, which can be less than a month. This makes it important for operators to choose pump designs that extend wear life and make it easier to maintain hydraulic efficiency.”
As an example, he says that a slurry pump does not operate well with a big gap between the impeller and the front casing liner. “Pumps accelerate fluid under centrifugal action and convert this kinetic energy into pressure energy. If there is a gap between the suction wear plate liner and the impeller, fluid under pressure at the outlet is forced back to the inlet through this gap. This process is known as recirculation.
“The more high-energy material that is flowing around the front of the impeller, the higher the wear rate on the liner – and the larger the gap the higher the recirculating flow. So this gap must be kept to a minimum at all times,” says Sedgwick.
“Recirculation also reduces the specific energy of the pump, ie, the energy used per m3 of flow, since some of the flow is being recirculated,” he adds. “So this front liner gap has to be adjustable. The conventional way is to push the impeller forward on its shaft until it hits the front liner, then to shift it back a little for clearance.
“On our MD pumps, however, we have a front liner that can be adjusted independently of the impeller shaft. The front adjustment can be made by moving the suction wear plate liner forward, without disturbing any of the shaft components,” he explains, adding: “Big pumps have big motors, bearings and couplings. They also have a water-fed gland seal that is disturbed by moving the shaft. By moving the front liner, the adjustment becomes significantly easier and less time consuming.”
Sedgwick points out another advantage: “This also makes the pump double adjustable – the front and the back impeller gap can both be restored at same time. Some recirculation also occurs behind the impeller. For the longest possible impeller and liner life, it is best to occasionally move the shaft back to close the gap behind the impeller and then to adjust to suction wear plate forward to close the front gap,” he explains.
A critical component for slurry pumps is the shaft sealing system, which has to keep highly abrasive particles suspended in water away from the rotating elements on the shaft. “Our MD pumps are all fitted with water-fed Metso EnviroSetTM gland seals, which reduce water consumption by 50 to 60% compared to traditional gland-based solutions,” he reveals.
“Gland flush water is used to wash abrasive particles away, keeping them from migrating between the packing material and shaft sleeve, where they can cause considerable damage and premature failure of parts,” he explains. The EnviroSet solution has a built-in lantern ring and a helical grooving system that imparts a centrifugal flow to the flush water. This throws the particles in the fluid to the outer bore of the separation chamber, resulting in a more than 50% water saving over most conventional glands.
Says Sedgwick: “It all comes down to total costs of ownership (TCO), which far outweigh the slightly higher investment costs for a better pumping solution. If the life of pump can be extended; the number and costs of spares reduced; and the reliability improved, then fewer un-planned stoppages occur and the total cost over the pump’s life can be significantly reduced. Maintenance intervals, liner refit times, unplanned shutdowns and breakdowns can all be reduced, contributing to much better production uptime for the mine,” he argues.
To simplify pump maintenance, MD pumps have a back pull out facility. “Pumps are connected to pipes via flanges with many bolts and it can take a long time to disconnect the pipework to access and inspect the internals. Metso MD and most of our other horizontal pump ranges feature the back-pull out design, which is great for routine inspection or repair: the bearing frame and rotating element can be removed as a unit. This enables the impeller and gland seal to be replaced rapidly, without having to disconnect either the suction or the discharge pipework,” he explains.
Sedgwick adds: “We have a release mechanism incorporated into the design on larger pumps, to free the impeller from the shaft. The biggest cost to a mine is stopping production, so everything we can do to enable a repair or change-out to be completed more quickly saves the mine money.”
As an optional extra, Metso also offers its maintenance slide base to make it even easier for personnel to access and repair its pumps. “This once off expense can save considerable amounts of time over the lifetime of a pump, particularly in situations where a pump is being relined every 400 hours,” he suggests.
Describing a health and safety related innovation, he says that the mining charter insists that a safety guard with captive fasteners is fitted to pumps to prevent access to moving components. “For efficient water use by the gland follower, however, these need to be adjusted regularly, to reduce the water flow from a gush to a trickle. HSE says you have to stop the pump to adjust the gland, but the water stops flowing when the pump stops, making this difficult.
“So Metso has designed a telescopic guard to allow the gland to be safely adjusted without removing the guard or stopping the pump. This is part of the MD design concept, with its strong focus on simplifying maintenance tasks,” Sedgwick says.
“In spite of all of the cleverness that has gone into these designs, our pricing remains very competitive, Sedgwick assures. “Along with simplified maintenance, we aim to offer more competitive spares pricing, lower total operating costs and longer component life,” he assures.
“Modern product manufacturers are making their products more cost efficient by looking at every nut, bolt and washer in order to save weight, costs and improve efficiency. So mines should be doing the same, first by selecting the best pump for the particular slurry being discharged and, second, by looking at TCO and the whole spectrum of savings that can be generated by using products designed according to modern principles – products such as ours,” Sedgwick concludes.