From motor size and pump speed to wear life and operating costs, an imposing array of choices face a buyer intent on reaching and maintaining optimum pump performance. Here are some tips from experts.
By Russell A. Carter, Contributing Editor
Slurry pumps are essential for moving hard-to-handle, high solids-content fluids and sludge, and annual demand for these pumps reflects just one aspect of the significant space they occupy in several industry sectors. The global market for all types of slurry pumps is estimated at well more than a billion dollars each year, and although those sales represent only a single-digit portion of overall pump sales, slurry pumping costs take up a lot of space in mining’s collective energy budget. Process equipment supplier Metso estimates that slurry pumps account for only about 5% of centrifugal pumps — the most common type used for this purpose — installed throughout the mining industry, yet this small segment represents up to 80% of the industry’s total operational pumping costs.
The space they physically inhabit in a mining operation is typically harsh — at the bottom of a sump, a mill or thickener-underflow discharge point, or serving a pipeline carrying abrasive ore or tailings. Their duty cycles range from continuous to sporadic depending on the application, often with highly variable flow rates and particle sizes. Internal wear can be severe in some applications, with as much as 2 mm of material a day disappearing from crucial component surfaces. Due to the increased probability of high wear rates from the materials being transported, pump builders add thicker, heavier components and/or internal liners, making slurry models larger and heavier than their water-pump brethren.
The wide range of pump-performance requirements encountered at thousands of mine, mill and other industrial sites requires an equally wide variety of pump types, sizes and mounting configurations. Two recent product introductions illustrate the range of available choices.
Going Big, Going Mobile
Late last year GIW Industries announced that it had developed the TBC-92 slurry pump specifically for use in oil sands operations. Named for its 92-in.-diam (234 cm) impeller, GIW claims the TBC-92 is the largest and heaviest slurry pump available in the mining industry. GIW also noted that installation of the TBC-92 marks an important company milestone: it now has pumps in service at all operating Canadian oil sands hydrotransport applications.
Overall, GIW’s TBC line comprises a series of horizontal, high-pressure, end-suction centrifugal pumps with a conventional single-wall design in which the pressure load against the shell is transferred to non-wearing side plates, which are held together by large tie bolts to ensure safety. TBC pumps components include abrasion-resistant white iron, ductile iron and special alloys to match duty requirements. Alternative wear liner materials such as urethane and neoprene are also available.
The company said the TBC-92 retains the best features of models that preceded it, including many elements from the TBC-84 Super Pump, and has features from GIW’s MDX product line. “This pump incorporates lessons learned over the years from operating in the oil sands, and features our latest hydraulic and wear technologies,” GIW Business Development Manager Mollie Timmerman said. “Because this is the heaviest TBC pump we have ever designed, particular attention was given to maintainability, as well as material selection and construction of the pressure-containing components.”
The TBC-92 weighs about 209,000 lb (95,000 kg) — roughly equivalent, said GIW, to a fully-loaded Airbus A321 airliner — with a casing that alone weighs 34,000 lb (15,400 kg). For ease of maintenance, customers are provided with custom-lifting devices to facilitate safe removal and installation of wear components. The pump also features a long-lasting suction liner that can be adjusted without shutting the pump down, according to the company.
At the other end of the size and portability scale, Gorman-Rupp’s transportable SludgeKat self-priming, positive displacement hydraulic piston pump is designed for convenient pumping of sludges and slurries from clarifying pits, wastewater treatment, mud pumping, environmental cleanup and similar applications.
The SludgeKat has 4-in. (100-mm) suction and discharge ports and is capable of flows up to 226 gpm (14.3 lps) and heads up to 390 ft (118.9 m). Depending on the product being pumped, SludgeKat can pass up to 2.4-in.-diam solids without damaging or clogging the pump. Units are equipped with Kohler Tier IV diesel engines.
Each SludgeKat comes standard with a wheel kit. The pump end frame is mounted to a 52-gallon (197-l) fuel tank base and offers a full-load run time of 25.5 hours. The pump end frame can be detached from the unit and when connected to optional 150-ft (46-m) hoses, provides increased portability around the job site.
In the space between these two very different pump solutions lies an array of conventional horizontal and vertical centrifugal models, submersibles and other types offering a wide range of performance characteristics that can be applied to specific slurry pumping requirements (see sidebar). It’s not uncommon to find that a pump intended primarily for one application can perform well in a different role.
For instance, an iron ore mine in the Northern Cape of South Africa reportedly benefitted from the versatility of a Grindex
Bravo 800 pump — a submersible model that, in this case, was dry-installed by pump distributor Integrated Pump Technology to improve slurry flow by acting as a booster, mounted on the wall of one of the mine’s settling-pond dams. An existing pump had failed due to extreme wear from the heavy sedimentation level in the pond. In the new setup, a submerged unit pumps slurry from the pond over a distance of about 30 m at a head of 8 m to 10 m to the Bravo 800 mounted on the dam wall. The Bravo 800 provided the wear resistance required by the application, and at a rated output of 45 kW and shaft speed of 1,475 rpm, can output almost 100 l/s in this application.
Pumps, unsurprisingly, can also fail to perform adequately if specified or installed incorrectly. Tsurumi America, which makes and markets a full line of light, medium and heavy-duty submersible slurry and wastewater pumps, reported earlier this year on a situation that involved both problems: One of the largest gold mining operations in Nevada was experiencing unplanned retention-pond pump breakdowns that cost the mine an average of $40,000 to $50,000 each in lost production, and repairs usually only lasted about three months or less. The mine attributed the frequent breakdowns to faulty repairs and brought in Tech-Flow, a nearby Tsurumi Pump distributor and equipment service company, to handle future repairs and also conduct a site study to determine if the overall performance of the mine’s submersible pumps could be improved.
Jace Church, territory sales manager at Tech-Flow, said, “We found the original pumps were not properly installed because they were mounted too high out of the water. This caused air bubbles to enter the pump, leading to cavitation, which led to the pumps breaking down. They were also using municipal pumps that are typically used in wastewater applications, so they were not entirely suited for this purpose.”
The mining company chose to replace the existing pump with one designed for the application, in this case Tsurumi’s GSZ-150 electric submersible. The duty cycle is intermittent — between 75%-90% — and operates at 110-120 ft of head, pumping 2,200-2,600 gpm. “They are pushing a year and a half now with no issues, which correlates a lot with properly submerging the pumps,” Church explained.
The combination of a large selection of pump models and the capability of many of those models to solve a variety of pumping problems might make it seem easy to choose an optimal unit for a given application. However, concerns about long-term maintenance costs, liner or wear-part interchangeability for maximum life, and options for minimizing energy consumption can muddy the waters. Further complicating the selection process are other factors that need to be considered — not just a pump’s performance characteristics, but also sump sizing, piping specs and the properties of other components associated with a mill’s process-flow setup.
Look Beyond the Pump
The industry’s continuous drive to increase production from existing assets makes it important to view a mill’s pump systems as one part of a much larger picture. In a recent blog post, Metso’s head of pump product management and marketing, Chris Wyper, outlined some important points to consider about pumps when aiming for plant-wide production increases. Among his recommendations:
Ensure motor power availability: “A well-designed plant has enough power allocated to mill pumps. Pumps typically operate on variable speed drives, meaning there are many process variables affecting speed and, finally, the power draw. It is a good idea to look at SCADA data on historical power drawn to better estimate the amount of power that would be available for tonnage increases. Rather than using engineering data sheets that are somewhat oversimplified, it is beneficial to use a point cloud type plot showing flow and pump pressure as a function of time. This information makes it possible to determine the optimal size of all the pumps and cyclones for the plant.”
Consider gearbox cooling at higher power: “As pump duty is increased, it usually also increases the power transmitted through the gearbox. This means that the amount of heat increases as well: a gearbox that is sized marginally for air to air cooling may overheat with higher continuous duty. Consideration must be given to the cooling capacity of the lubrication system, particularly at higher ambient temperatures and altitudes.”
Ensure gland seal water pressure at higher heads: “The pump gland seal water system should be sized so as to be able to deliver a constant flow of gland water under all operational conditions. This applies to the pump duty, including any increase in head due to tonnage increases. It should also be checked that the gland seal water system is adequate when other demands are placed on it, such as hose downs or flushing.”
Take a close look at pipe sizing: “If you double the speed, the rate of material loss increases 16-fold and the rate of abrasive wear on the surface is approximately proportional to the fourth power of velocity. If there is a significant increase in input, it is necessary to consider whether the pipe sizing is optimal. The right size allows friction losses and wear to be minimized. Of course, if there is a large variation in flow, then minimum velocity to prevent settling should be examined.”
Prepare for crash stops by calculating floor sumps: “In the case of a plant crash-stop, prepare for the maximum inflow based on calculations on the live volume of floor sumps. This may include the mill static overflow and any dump valves to empty pipes and sumps. If sump size is increased or the mill volume changed, then the sumps may be undersized. In this case, the existing sumps can be deepened or enlarged, to deal with the volume, or then additional sumps created. Typically, mill sumps should be separated from the other sumps in the plant due to the possibility of mill balls entering the sump.”
As industry-wide figures indicate, slurry pumping can serve as a prime example of purchased capital equipment where operating and maintenance (O&M) costs rapidly eclipse the initial procurement cost. A myopic view of TOC (Total Cost of Ownership) factors when selecting a pump can result in a variety of bad outcomes ranging from the need to prematurely replace an inadequate unit, to sky-high maintenance costs and production losses from unscheduled downtime. Conversely, pump OEMs and aftermarket suppliers are increasingly cognizant that their customers can’t always predict future events and consequently are expanding their product and services portfolios to provide affordable options when mining conditions, maintenance resources or technology changes occur over time.
Understanding the differences in industry-sector preferences and practices is important for a pump supplier in order to tailor its offerings to provide maximum value to the customer, according to pumpmaker Schurco Slurry’s vice president of engineering, Will Pierce, who told E&MJ that hard rock and coal operators, for example, typically might take different approaches to pump mainten-
ance — and for good reasons.
He explained that even though basic pump maintenance procedures — checking clearances, adjusting the impeller, inspecting the seal or packing for damage or leaks and ensuring proper lubrication — will always be similar in both hard rock and coal, the specific application dictates the frequency of preventive maintenance checkups.
“Heavy media separation is probably one of the highest wear applications in coal, and we would recommend checking a pump, depending on the tonnage being moved and hours of operation of course, every 1,000 hours. Yet there are gold mines in Nevada and Alaska that have a wet-end maintenance interval of 500 hours. They go in at 450 hours and tear everything out, because they know if they wait until 550 hours, they’ve got a shelled-out pump and it’s now an emergency situation.”
The two mining sectors also can differ in how they collect, analyze and act on pump operational data for maintenance planning. “Most coal prep plants have PLC integration with the equipment, but they often have fewer data inputs than large hard rock plants, which may additionally monitor pumps for vibration, pressure and flow. Those plants are set up to monitor trends and can proactively plan for maintenance based on trend data,” Pierce noted.
The differences also can apply to how pump maintenance is performed. “Generally, hard rock operations won’t send pumps out for repairs. They usually have well-staffed maintenance departments and their own shops,” said Pierce. This provides an opportunity for suppliers to offer maintenance kits, which Pierce explained are “a type of a vendor-managed maintenance program that the mine operator executes. We manage the kit, making sure it always has every part in it that the end user might need to perform any repair on the pump. When they perform a repair, if there are 30 parts in the repair kit, they might only consume seven on that repair. We will audit those kits and replenish the parts that are consumed so that any time a maintenance department would need to perform a repair, their kits are always fully stocked.”
He continued: “Some coal plants that we work with have a different philosophy for managing pump maintenance. They keep a spare pump on hand. When they have a pump problem, their crew removes all ancillary equipment from the unit that’s being replaced, installs that equipment on the spare pump, and puts the spare online. With the worn pump out of service, they can perform the maintenance in a workshop on site or send it out to the manufacturer or a third-party shop.”
Manufacturers are also looking at ways to incorporate more performance flexibility into their pump models and ease some of the concerns associated with necessary pump modifications. “For example, we are developing a line of pumps designed with a solid casing with replaceable all-metal, liner-like elements. The metallurgy for these wear components is a novel enhancement to the proven 27%-28% chrome white iron that the industry has used for decades. We have hard rock customers that started with rubber liners 20 years ago, now they’re in a different ore deposit at the same mine and the material is sharper or has different abrasive characteristics and the rubber isn’t lasting. With the shell we’ve developed, they’re able to convert to a completely metal lined pump without major impact to the overall installation through using backward compatible adapter plates,” Pierce explained.
The new design also offers Shurco’s coal clients notable benefits: “Our coal customers almost always use metal-lined pumps, but the industry is very price-sensitive right now, so this new development doesn’t have the traditional massive ductile iron outer shell and metal liner — instead, it has replaceable metal wear components. There’s no quality compromise on the pump’s internal components, no change in wear or hydraulic performance. It’s just a lower-cost alternative.”
Designing for Durability
A rule of thumb when selecting a slurry pump is to look for the most robust pump, in terms of performance, wear resistance, power and maintainability, that falls within the service class rating for the type of material being pumped. Even that simple process can be complicated when special circumstances arise, such as unusually high mechanical wear experienced in a specific application, or intermittent operation rather than steady running. Pump manufacturers generally have vast knowledge of what works and what doesn’t under many conditions, and they incorporate the features that do work into their latest designs. For example:
FLSmidth Minerals expanded its line of Krebs millMAX slurry pumps with the introduction of the millMAX-e, which features a unique wear-ring design that the company claims solves grinding and recirculation problems within the pump by maintaining clearances between the impeller and the suction side. By maintaining the design performance without increasing the speed, the wear ring extends the life of all wet end parts and reduces power consumption.
The millMAX-e model is unlined and offers a compact, space-saving exterior design aimed at reducing capital and replacement costs as well as motor-power requirements. However, according to the company, millMAX-e’s power frame uses the same bearing and shaft components as the equivalent millMAX power frames and is capable of handling applications requiring high speed and power. The millMAX-e is equipped with the patented Krebs pump belt tensioning system that allows users to quickly change out v-belts without having to realign the sheaves.
Tsurumi Manufacturing’s entries in the mining-class slurry pump market include its GPN and GSD series, rated at motor outputs of 7.5-50 hp (5.5-75 kW) kW and 50-100 hp (37-75 kW), respectively. Both series comprise submersible three-phase, high head and high volume heavy-duty slurry pumps driven by a four-pole motor. They are equipped with high-chromium cast iron agitators that the company said assist in smooth handling of settled materials. Motors are enclosed by a water jacket that assures efficient cooling even when the motor is exposed to air. Pumps in this series incorporate seal pressure relief ports that prevent pumping pressure from affecting the shaft seal.
Finland-based Flowrox’s heavy-duty CF-S horizontal centrifugal pump is the first in a series of centrifugal pumps to be introduced by the company and capable of continuously pumping highly abrasive and dense slurries. The company said the new pump can provide flows from as low as 2.3 m3/h to more than 4,000 m3/h at heads exceeding 76 m. The pump’s split-case design is claimed to provide a good balance between efficiency and wear, and models are available with a range of liner material options. The pump is compatible with Flowrox’s Digital Services platform, a customized IIoT-based process data collection and analysis system.
MBH Pumps unveiled the Ni-Hard series submersible slurry pumps, designed and built to pump slurries containing abrasive solids up to 65% by weight. These heavy-duty pumps, according to the supplier, are equipped with an external agitator that breaks settled or compacted solids, while its adaptive spiral plate technology delivers higher pumping with less energy consumption.