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.
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:
- More abrasive than pure liquids.
- Thicker in consistency than pure liquids.
- May contain a high number of solids (measured as a percentage of the total volume).
- The solid particles usually settle out of the slurry's precipitate relatively quickly when not in motion (depending on the particle size).
- 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.
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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.
- 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 1200 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.
Slurry pumps offered by CSI
Not all pump types are capable of pumping slurries, but several hygienic-design pumps offered by CSI are up to the task.
Centrifugal Pumps
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 2205 stainless steel, nickel aluminum bronze, and Hastelloy C-22.
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.
A Guide to Choosing the Right Pump for Hygienic Applications
This guide is intended for engineers, production managers, or anyone concerned with proper pump selection for pharmaceutical, biotechnology, and other ultra-clean applications.
Next Steps
Given the relative complexity of all these variables, calculating the exact pump and motor size to use for a specific application can get fairly technical. Consult with a process expert such as CSI to address your specific situation.
Please contact CSI at (417) 831-1411 to discuss your specific needs and application.
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.