April 22nd,

A Complete Guide to Pumping Slurries

Slurries are specialized compounds found in many processing industries, including sanitary industries such as food, dairy, beverage processing, and biopharmaceutical manufacturing. Slurries combine properties of both liquids and solids, and so specialized consideration must be given when it comes to determining the type and size of slurry pump to use with them.

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This article will focus on:

• Defining what slurries are
• The types of pumps that may be used to pump slurries
• Factors to consider when selecting a slurry pump for a particular application
• A brief overview of a few of the sanitary processing pumps available from CSI suitable for pumping these unique compounds

What are slurries?

Slurries are mixtures of solids and liquids, with the liquid serving as the transport mechanism used to move the solid. The size of the particles (or solids) in slurries ranges from one micron in diameter up to hundreds of millimeters in diameter. The particle size significantly impacts a pump's ability to move a slurry through a process line.

All slurries share five essential characteristics:

1. More abrasive than pure liquids.
2. Thicker in consistency than pure liquids.
3. May contain a high number of solids (measured as a percentage of the total volume).
4. The solid particles usually settle out of the slurry's precipitate relatively quickly when not in motion (depending on the particle size).
5. Slurries require more energy to move than do pure liquids.

Slurries are further classified by industry into four classes based on how aggressive they are &#; Class 1 being the least aggressive and Class 4 the most aggressive. The pumping of slurries can have the following wear impacts on both pumps and pipeline components:

• Abrasions: including gouging, high-stress grinding, and low-stress grinding (applicable only with settling-type slurries).
• Erosion: the loss of surface materials caused by the action of the particles in the slurry being pumped. Erosion is primarily found with the pumping of settling-type slurries.
• Corrosion: caused by the electrical galvanic action in the fluid being pumped. Certain types of slurries (e.g., highly acidic or alkaline compounds) will have more impact on component corrosion than will more benign slurries.

Types of slurry pumps

As described below, there are several types of pumps that are suitable for pumping slurries. However, we must address a couple of critical considerations before considering which technology to use.

1. The size and nature of the solids in the liquid: the size and nature will affect the amount of physical wear on the pump and its components and if the solids will pass through the pump without being damaged.
   
   A concern for centrifugal pumps is the speed and shear inside the pump may damage the slurry/solids. In general, twin screw pumps allow for the least damage to solids in a slurry.
   
   
2. The corrosiveness of the liquid or slurry mixture: more corrosive slurries will wear pump components more quickly and may dictate the selection of the material from which the pump is constructed.

Pumps designed for pumping slurries will be heavier duty than those designed for less viscous liquids since slurries are heavy and difficult to pump.

Slurry pumps are typically larger in size than standard pumps, with more horsepower, and built with more rugged bearings and shafts. The most common type of slurry pump is the centrifugal pump. These pumps use a rotating impeller to move the slurry, similar to how a water-like liquid would move through a standard centrifugal pump.

Centrifugal pumps optimized for slurry pumping will generally feature the following in comparison to standard centrifugal pumps:

• Larger impellers made with more material. This is to compensate for wear caused by abrasive slurries.
• Fewer, thicker vanes on the impeller. This allows the passage of solids more readily &#; typically 2-5 vanes, compared to 5-9 vanes on a standard centrifugal pump.

For pumping abrasive slurries, these types of pumps may also be made from specialized high wear alloys such as AL-6XN® or Hastelloy® C-22®. Hardening stainless steel is also a common option for abrasive slurries, with Expanite and Armoly being two hardening processes.

For certain types of slurry pumping conditions, positive displacement pumps may be a more appropriate choice than a centrifugal-style pump.

These conditions include:

• A low slurry flow rate
• A high head (i.e., the height to which the pump can move liquid)
• A desire for greater efficiency than that afforded by centrifugal pumps
• Improved flow control

Common types of positive displacement pumps used in slurry pumping applications include:

Rotary Lobe pumps

These pumps use two meshing lobes rotating within a pump's housing to move fluids from the pump's inlet to its outlet.

Twin-screw pumps

These pumps employ rotating screws to move liquids and solids from one end of the pump to another. The screws' turning action creates a spinning motion that pumps material.

Diaphragm pumps

These pumps use a flexible membrane that expands the volume of the pumping chamber, bringing in fluid from an inlet valve and then discharging it through an outlet valve.

Selecting and operating a slurry pump

Choosing the right pump for your slurry application can be a complex task due to the balance of many factors including flow, pressure, viscosity, abrasiveness, particle size, and particle type. An applications engineer, who knows how to take all of these factors into account, can be a great help in navigating the many pump options available.

~ Matthew Sato, Applied Products Sales Manager, Ampco Pumps

In determining which type of slurry pump is best suited for your particular application, follow these four simple steps.

Step 1

Determine the nature of the materials being pumped

Consider the following:

• The particle size, shape, and hardness (impacts on the likelihood for abrasion and corrosion of the pump's components)
• The corrosiveness of the slurry solution
• If the exact in-pump viscosity of the product is unknown, CSI can help

Step 2

Consider the pump's components

If it's a centrifugal pump, is the design of and material used to construct the impeller appropriate for pumping slurries?

• What is the material used to construct the pump constructed? Higher alloys such as AL-6XN or Hastelloy C-22 are best for pumping highly abrasive slurries.
• Are the pump's discharge components appropriate for the slurry to be pumped?
• What is the best sealing arrangement for the application?
• Will the solid size pass through the pump?
• How much damage to the solid can the customer tolerate?

It's also important to consider the chemical compatibility of the slurry with any elastomers in the pump. Once one has addressed both the nature of the slurry and the components of different types of pumps, you may select the potential candidate slurry pump for the application.

Step 3

Determine what size the pump should be

The most important thing here is figuring out the pump horsepower needed to deliver a specific fluid flow rate at the desired or required differential pressure. Look at the following:

• The concentration of solids in the slurry &#; measured as a percentage of the total volume.
• The length of the pipeline. The longer the pipeline, the greater the slurry-induced friction that will need to be overcome by the pump.
• The slurry pipe diameter.
• The static head &#; i.e., the height to which the slurry must be lifted in the piping system.

Step 4

Determine the pump's operating parameters.

To reduce component wear, most centrifugal slurry pumps run at fairly low speeds &#; typically less than rpm. Find the sweet spot that allows the pump to run as slowly as possible but fast enough to prevent solids from settling out of the slurry precipitate and clogging the lines.

Then, lower the pump's discharge pressure to the lowest point possible to further reduce wear. And follow proper piping layout and design principles to ensure constant and uniform delivery of the slurry to the pump.

Alfa Laval

The SolidC Series of centrifugal pumps from Alfa Laval are designed to meet the hygienic requirements of the food, dairy, beverage, personal care, pharmaceutical, light chemical, and water industries.

Suitable for general applications, intermittent product pumping, and clean-in-place (CIP) duties, SolidC pumps combine efficiency with competitive pricing and are compliant with 3-A, CE, FDA, and EHEDG standards.

Ampco

The LC/LF/LD Series are high-efficiency sanitary duty pumps offering heavy-duty cast construction and a 304 stainless steel adaptor. These pumps handle food-grade products such as mashes and food slurries containing small suspended solids with ease.

The Z Series pump is an industrial-grade centrifugal pump designed for high flow rates in applications where corrosion resistance is of maximum importance. This pump is available in multiple alloys, including 316, 316L, and Duplex stainless steel, nickel aluminum bronze, and Hastelloy C-22.

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Positive displacement pumps

Alfa Laval

The OS Series twin-screw pump is a robust product that meets stringent hygienic standards. It is designed with low pulsation characteristics and can handle both solids and abrasives easily, so it's excellent for use in slurry pumping applications.

The company's basic rotary lobe pumps, the OptiLobe Series, are known for their simplicity of installation, operation, and maintenance.

The SRU Series rotary lobe pump is Alfa Laval's premier offering for the gentle handling of sensitive process fluids.

Ampco

The AL Series positive displacement lobe pumps offer ease of maintenance, CIP performance, uptime reliability, and economical pricing.

The ZP Series rotary piston positive displacement pump features a 304 stainless steel gear case and a front-loading seal for easy maintenance.

For food-grade applications, these ZP pumps can be used for slurries such as fruit fillings and concentrates, curds, relishes, and potato salad.

The front-loading design allows the seal to be easily maintained or changed, and the newly designed rotor case eliminates all dead zones

ITT Bornemann

Bornemann SLH twin screw pumps are self-priming, positive displacement pumps. The SLH Series is the best solution for universal or aseptic processes and applications with high viscosities, high pressures, or sensitive media.

The pump's design provides complete axial balancing of the rotating elements and eliminates all metal-to-metal contact within the pump. SLH can handle virtually any non-homogeneous fluid, regardless of viscosity, lubricity, or abrasiveness.

Contributing Author

Matthew Sato is the Applied Products Sales Manager at Ampco Pumps Co. With nearly a decade of pump sales and an engineering background, he is well versed in pumping principles and technology.

ABOUT CSI

Central States Industrial Equipment (CSI) is a leader in distribution of hygienic pipe, valves, fittings, pumps, heat exchangers, and MRO supplies for hygienic industrial processors, with four distribution facilities across the U.S. CSI also provides detail design and execution for hygienic process systems in the food, dairy, beverage, pharmaceutical, biotechnology, and personal care industries. Specializing in process piping, system start-ups, and cleaning systems, CSI leverages technology, intellectual property, and industry expertise to deliver solutions to processing problems. More information can be found at www.csidesigns.com.

Slurries can be found in a range of industries including power generation, mining, steelworks, foundries etc and offer a convenient means of handling solid particles. A slurry is typically made up of a mixture of a carrier fluid (usually water) and solid particles. Slurries can usually be described as settling or non-settling. 

Settling slurries consist of large or coarse particles which can exhibit an unstable mixture. Many slurry systems consist of coarse particles in transit and operating under these conditions requires careful consideration of wear as well as flow and power calculations when making a pump selection.  

Non-settling slurries on the other hand consists of fine particles/solids which tend to exhibit lower wear compared to settling slurries. Care and attention is however also required when making a pump selection as Non-settling slurries do not behave as a standard liquid or Newtonian fluid.  

The pumping and transportation of slurries in piping systems therefore represents one of the most challenging fluids to transport, particularly at design stage when we may need to predict the slurry characteristics.

There are a wide range of pumps used for pumping slurries though the most common pump used is a centrifugal pump. Centrifugal slurry pumps need to withstand wear imposed by the transportation of the abrasive, erosive and sometimes corrosive slurry material. Centrifugal slurry pumps typically have wider and heavier impellers which accommodate the large particles being transported. 

Centrifugal slurry pumps are often comparatively larger than their liquid pump counterpart to achieve lower operating speeds and a lower wear rate of the pump. They also usually require more power to operate because they are less efficient compared to their liquid counterpart. The slurry pump shafts and bearings also need to be much more robust and rigid. 

Centrifugal slurry pumps are also often provided with a metal (hard alloy) or rubber lining to protect the pumps casing from abrasion or erosion caused by higher pressure and circulation. Open impellers are often used on centrifugal slurry pumps as they are the least likely to clog. 

As slurry pumps tend to operate in a harsh environment, the pump should be selected to operate at a speed which is as low as possible to reduce wear however, conversely, the pump must operate at a high enough speed so as to avoid the slurry particles settling and potentially blocking the pipeline. It&#;s also worth considering reducing or optimising the pump discharge pressure to the lowest practical point to reduce wear. In any case, the piping system should be designed to optimise the performance of the pipeline and pump. This can be a difficult task as with slurry systems, the operating conditions such as particle size and concentration can vary considerably. 

The performance characteristics of a centrifugal pump pumping for example, water, is noticeably different to the same pump pumping a slurry. The difference depends on the slurry characteristics. Factors such as solid particle size, shape and density all effect the overall slurry properties and as such, pump performance. It&#;s no surprise that the pump, head, efficiency and power are all affected and in an unfavourable direction. Figure 1 gives an overview of the expected performance curve profile for a centrifugal pump pumping water vs slurry. 

Figure 1: Capacity Curve for Centrifugal Pump Transporting Water vs Slurry.

In order to size and select a centrifugal pump suited to a slurry application, the starting point is to source good quality physical property data for the slurry and the solids to be transported. This then should be used in conjunction with the design pump flow rate, head requirement and piping system characteristics. A suitable modelling software tool should ideally be utilised to help characterise the slurry, size the pump required and predict the performance of the vendor pump in the system. 

A suitably sized and selected pump should be capable of overcoming the pressure losses incurred by the system pipework and fittings and the flowing slurry velocity should not fall below the critical velocity of the transported slurry. The critical velocity is of course the velocity at which solid particles settle in the pipe so it&#;s important to keep the slurry velocity above this minimum value. However, pumping the slurry at too high a velocity increases the pressure losses which can result in wear of the piping system. Conversely, pumping the slurry at too low a velocity can of course lead to settling/sedimentation in the pipeline which also increases pressure losses in the system. It&#;s therefore vitally important to establish or predict this critical velocity value. There are many approaches to this task. 

A good software tool can help identify the slurry critical velocity as well as system trouble-spots and issues such as sanding/settling and the potential for pipe blockage. Software can therefore be very powerful in terms of optimising the performance of a pumped slurry piping system. 

Another item which needs consideration when selecting a centrifugal pump for a slurry system is the Net Positive Suction Head available, (NPSHa). The inlet pressure of the pump must exceed the vapor pressure of the liquid/slurry within the pump. The NPSHa is dependent on the vapor pressure of the fluid, ambient air pressure, density of the fluid and the level in the sump. In any case, it is important that the Net Positive Suction Head available (NPSHa) is not less than the NPSHr. If the NPSHa is less than the pump NPSHr, vapor bubbles occur in the impeller and when these bubbles reach an area of higher pressure, they can collapse causing damage to the pump impeller and volute. This condition can also exhibit noise, vibration and the efficiency of the pump is reduced. 

This blog outlines just some of the conditions for consideration when evaluating pumps for slurry piping systems. It in no way attempts to address all aspects of slurry flow which in itself, can be a complex problem. 

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