Slurry Pumps: Picking the Best Type for the Task - Coal Age

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.

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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 estimated 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 prep plant or thickener-underflow discharge point, or serving a pipeline carrying abrasive slurry. 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.

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.

Pumps, unsurprisingly, can also fail to perform adequately if specified or installed incorrectly.

Look Beyond the Pump

The industry’s continuous drive to increase production from existing assets makes it important to view 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.”

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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.”

Expanding Future Options

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.

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,” said Will Pierce, manager of engineering, Schurco Slurry. 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.

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Coal-fired power stations remain a major source of energy, especially in developing countries. As of , the electricity generation facilities account for roughly 36% of global electricity production. In some countries, like China and India, coal's share in power generation exceeds 50%. Despite the rapid growth of renewable energy, fossil fuel continues to play a key role in the global energy mix due to its relative affordability and widespread availability.

However, thermal power stations are also one of the largest sources of air pollution. When black rock is burned, it releases significant amounts of sulfur dioxide into the atmosphere. Once airborne, SO₂ contributes to acid rain, which harms ecosystems and infrastructure. It also leads to respiratory issues in humans, particularly asthma and bronchitis. In addition to SO₂, combustion power plants emit nitrogen oxides, further contributing to air quality problems. In addition to gaseous pollutants, burning carbonaceous material releases particulate matter that can penetrate deep into the lungs, causing serious health problems. These emissions also contain heavy metals such as mercury, lead, and arsenic, which result in long-term contamination of soil and water resources.

Coal is primarily made up of sulfur and carbon, and when it burns, it releases carbon dioxide, which exacerbates the greenhouse effect and accelerates global climate change, as well as sulfur dioxide. To mitigate SO2 emissions, power stations typically install specialized equipment. What is a coal plant scrubber? It is a device used to clean sulfur dioxide from the flue gases of the electricity generation facilities. Its main purpose is to reduce emissions of the substance, which forms during black rock combustion and is a major contributor to air pollution at the facilities. By installing purification units, power plants can significantly lessen their environmental impact, making them essential for meeting emissions standards.

How does a coal scrubber work? The structure works by exposing polluted flue gases to a liquid filtering agent, typically an aqueous solution of alkaline materials like lime or limestone. As the sulfur dioxide-laden airflow passes through the air purifier, it is sprayed with a fine mist that absorbs the compound. This triggers a chemical reaction that produces sulfates or sulfites. The key byproduct of this process is gypsum, which can be extracted and repurposed for use in the construction industry.

Airstreams pass through nozzles that spray reagents, allowing for capture of SO2. After treatment, air is released through the stack with markedly reduced sulfur content. The reacted liquid is then directed to a regeneration or disposal system. Wet configurations are capable of removing up to 95-99% of SO₂ from the mixture, making them one of the most effective pollution control methods. Let’s explore a few types of a coal scrubber.

A hollow build uses spray towers to inject liquid medium into the airflow as it moves through an empty column. The gas rises through the column, coming into contact with the absorbent droplets, allowing for SO2 absorption. The absence of packing material reduces hydraulic resistance, lowering the energy costs associated with airstream pumping. This type of equipment is easier to operate and maintain, as there are no elements that can become clogged with sludge. However, optimal nozzle configuration is necessary to ensure effective liquid distribution. Its main advantage is high efficiency with relatively low pressure. The Venturi devices comprise a second section in addition to the main column. This section consists of converging and diverging sections that create a low-pressure zone, facilitating the formation of a strong vortex. This configuration further enhances phase mixing. A key advantage of the Venturi tube is its ability to significantly increase stream velocity, which aids in capturing not only gaseous pollutants but also solid particles—an important consideration for thermal power stations, where emissions often contain high levels of soot. Additionally, this design of the wet scrubber coal plant system allows for a reduction in unit size while still ensuring high performance. Generation facilities are subject to stringent pollutant release regulations. For example, according to EPA guidelines, the allowable concentration of sulfur dioxide in discharges varies by state, typically not exceeding 130-200 ppm on average. To monitor the levels of harmful substances such as sulfur dioxide, nitrogen oxides, carbon dioxide, and particulate matter, coal scrubber emission monitoring is implemented at these facilities.

The primary method employed is continuous monitoring systems, which provide real-time measurements of these contaminants' concentrations. Sensors are installed directly in the flue gas ducts and regularly transmit up-to-date output data to plant personnel. This process is crucial for preventing exceedances of permissible levels and optimizing the operation of pollution control technologies, including coal plant scrubber. The effectiveness of the equipment in removing SO₂ and other pollutants depends on several factors. The key parameter is the depth of contact between the flue gases moving from coal boiler to scrubber system for purification and the absorbent, which is typically an aqueous solution of limestone or lime. Modern wet setups utilizing these reagents can achieve removal rates of up to 95-99% for SO₂ in emissions from generation facilities. A critical factor affecting contact time is maintaining an optimal gas-to-liquid ratio.

Temperature also plays a crucial role; lower vapor temperatures increase the solubility of SO₂ in the medium, thereby enhancing absorption efficiency. However, excessively low temperatures may lead to condensation and corrosion within the structure, making it essential to strike a balance. Typically, the temperature of the flue gases entering a coal power plant scrubber ranges from 120 to 160°C.

The quality of the neutralizing agent is equally crucial. For example, insufficient concentrations of alkaline reagents in the solution can diminish the effectiveness of sulfur dioxide removal. Regular monitoring of the absorbent's pH is necessary; as excessively low acidity levels indicate that a significant portion of the alkali has already reacted, requiring the addition of fresh material. The first factor is the capacity of the thermal coal scrubber, which is defined by the volume of flue gases processed per unit of time. The higher the throughput, the greater the cost of the equipment; for example, a high-capacity device for a 500 MW plant can range from $5 million to $10 million.

The second factor is the type of absorbent used. Solutions of lime, limestone, sodium hydroxide, and other reagents are employed for sulfur dioxide removal. Their cost also impacts the overall coal plant scrubber cost and relates to the third factor—construction materials. Selecting corrosion-resistant materials, such as stainless steel, significantly increases expenses.

The fourth factor is the complexity and type of the chosen system, which may include additional components such as Venturi tubes. The main types of suitable devices have been discussed above.

The fifth factor is the installation and operational costs, which can represent up to 20% of the initial equipment expenses. This encompasses installation, setup, and future operational costs related to maintenance and part replacements.

These factors collectively make the coal scrubber cost multifaceted, varying based on the specific conditions and requirements of the project. Coal scrubber maintenance encompasses regular inspections, cleaning, and repairs to ensure the long-term effective operation of the entire configuration. The first step involves routinely checking mechanical components, including pumps, fans, and spray towers, for any signs of wear or damage.

It is also crucial to periodically evaluate the condition of the absorbent, as a decline in its activity over time might reduce cleaning efficiency. This is achieved by analyzing alkaline washing solutions for sulfates, sulfites, and pH levels. If necessary, the solution is replaced or the dosage of reagents is adjusted.

Additionally, cleaning the packing layer, filters, and collectors to remove accumulated coal scrubber sludge is a vital aspect of maintenance.

Emission monitoring technologies must be regularly calibrated to accurately measure pollutant concentrations. Replacing worn components, such as seals and gaskets, helps prevent leaks and ensures the system's integrity.

The frequency of maintenance depends on operating conditions and the type of coal gas scrubber; however, in most cases, preventive maintenance should be performed at least once a month. This does not apply to monitoring parameters necessary for continuous operation.