Nnamdi Nwaokocha offers practical advice on pump selection

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PUMPS are the backbone of the process industry. In a process plant, it is necessary to move material from one point to another. In keeping with the laws of thermodynamics, fluids move from an area of high pressure to low, and depending on the plant layout often require the assistance of a pump to achieve this. With many different pump types available, selecting the right pump can be tricky, especially when slurries are involved.

This article will discuss some of the variables to consider when characterising a slurry and selecting a suitable pump for transporting those slurries in a plant. This is not definitive and is by no means a complete review of handling slurries by pumping but is meant to provide some useful information and a good starting point of what to consider.

Summary

Pumping of slurries can often lead to blockages or equipment failure. The job of the designer is to assess all the factors of each situation, including client and existing site preferences to design a system and select a pump which is robust enough to minimise blockages and makes maintenance for operators as easy as capital would permit whilst providing a safe system of work.

Slurry type

What is a slurry? Typically, the term slurry is used to refer to a mixture of a liquid and a solid or combination of solids. The liquid is often referred to as the carrier fluid and in most cases is water, although it can be anything from an acid solution (eg nitric acid) to a hydrocarbon (eg diesel).

Producing a slurry or maintaining solid suspension in static conditions is outside the scope of this article.

Slurries can broadly be broken down into two types: settling, and non-settling slurries. This characterisation is based on the nature of the solid(s). Non-settling slurries contain solids made up of fine particles, which largely remain in suspension when the applied mixing energy ceases. Settling slurries, as the name suggests, contain solids whose particles settle out when the applied mixing energy ceases. From a designer’s perspective, it is important to know the type of slurry. For example, non-settling slurries can be transported around under laminar flow conditions, whereas turbulent flow conditions are required for settling slurries, particularly in horizontal sections.

A useful rule of thumb provided in Sinnot and Towler’s Chemical Engineering Design states that solids with particles of less than 200 microns (0.2 mm) will usually be expected to produce non-settling slurries. Larger particle sizes will produce settling slurries.1

Before selecting the right pump, the first step is to determine the pressure drop requirements using the system characteristics. The parameters required are:

The following equations2 are useful in determining the slurry’s density:

For settling slurries, the velocity in the pipework is the key design criteria.

Perry’s Chemical Engineers’ Handbook

Figure 1, an extract from Perry’s Chemical Engineers’ Handbook, depicts the relationship between the pressure drop and the slurry’s velocity compared to a pure liquid in horizontal pipework. The important point to note is that the horizontal pipe velocity should be above the point labelled Vm2 (minimum transport velocity). This is the point at which the solids are fully suspended. This is determined using the Durand equation3. 

where:

Once the minimum transport velocity is calculated, it is common to add a safety factor, but care is needed. If the velocity is too high, the required pressure drop and the subsequent work required by the pump can increase significantly. For vertical flow, a good  starting velocity can be taken as twice the solid’s settling velocity. The main aim is to stop the solids from dropping out. Velocities in the range of 1-3 m/s is  a useful rule of thumb.3

The pressure drop for settling slurries can now be determined at the calculated velocity assuming pseudo-homogeneous behaviour, using the slurry’s density and the carrier fluid’s viscosity in established pressure drop calculation and applying a correction factor. A correction factor of 25% is suggested in Perry’s Chemical Engineers’ Handbook3.

Note that the above is for solids heavier than the carrier fluid. Depending on the solid particle size, at particular concentrations the particles begin to interact with each other and can start to affect the slurry’s viscosity. This is discussed further with calculations provided in the Processing of Solid-Liquid Suspensions, Chapter 112. Appendix 3, Warman Slurry Pumping Handbook4, also has useful calculations and correlations for water-based slurries.

Perry’s Chemical Engineers’ Handbook

For non-settling slurries, the resultant slurry typically displays non-Newtonian behaviour, and its rheology and behaviour must be determined empirically to ascertain the work required by the pump. The pressure drop for these can then be calculated using established pressure drop calculations depending on the slurry viscosity and density calculated.

Other things to consider before moving onto pump selection are:

• slip conditions – when the solid and carrier velocities differ significantly;
• pipe size – ensure the pipe ID is considerably bigger than maximum particle size (6-10x is recommended2);
• piping design (using recirculation loops to ensure the slurry is constantly moving; using falls, so the  slurry drains to a safe point; using long radius bends; installing rodding or flushing points; minimising bends; minimising dead legs; minimising suction pipework);
• static head requirements; and
• available NPSH.

The solid particle will play a crucial part in selecting the material of the wetted parts. The following, amongst other things, should be considered:

• are the solids hard or soft? ie are they abrasive?
• will pumping the slurry cause erosion?
• are the solids corrosive? This applies to the carrier fluid.

Pump types

As with many pump duties, both rotary and positive-displacement pumps can be utilised. The following are some of the aspects to consider when selecting the type of pump for your slurry. However, always check with specialist pump suppliers before making a final decision.

Centrifugal pumps

The most common pumps generally in use are centrifugal pumps. When specifying this type of pump, as a minimum, the following must be considered:

• Impeller type – A recessed impeller type can be used, the design minimises contact between the particles and the impeller thereby minimising wear on the impeller whilst being gentle on the particles. Open impeller types can be used, as they are generally easier to clean and maintain. Closed impellers are often regarded as having the best efficiency but can be difficult to clean. The thickness should have suitable wear allowance. You should also consider any impact caused by the required impeller speed.
• Casing type – Metal casings can be used. These may be lined with rubber for added protection or as a sacrificial wear part. Split casings can also be considered, but these can be expensive. The thickness should have suitable wear allowance.
• Clearances – Slurry centrifugal pumps should have larger clearances than pure liquid pumps, to allow solids to pass through but also to reduce the velocity within the pump, thereby minimising wear.

These are just some of the things to consider when selecting a centrifugal pump for a slurry duty. In direct liaison with a pump vendor, the designer must choose the best options for their system. They should also consider any impact on the shaft and seals, and ensure there will be no issues with cavitation.

Centrifugal pumps are differential head devices and therefore, the head generated is based on properties of the fluid.

Warman Slurry Pumping Handbook

Often, the operating curve and efficiency curve provided by pump vendors are that of water, so a way of translating those figures is often needed. An example is shown in Figure 34. It also includes a ratio for the driver efficiency which would also assist in confirming the pump motor.

Note that the curve is only for slurries whose carrier fluid is water. Moreover, it is for Warman pumps. For similar correlations and fluids other than water, speak to your pump vendor.

Positive displacement pumps

There are various types of positive displacement pumps which may be utilised in pumping slurries: air-operated diaphragm pumps, peristaltic, rotary lobe, progressive cavity pumps, and piston diaphragm pumps to name a few. Assessing all of these to the same degree as the centrifugal pump above will be a significant undertaking and is outside the scope of this article. Instead, I’ve summarised different pump types used in my experience and highlighted specific things to consider in relation to handling slurries.

Positive displacement (PD) pumps are generally useful for fluids, which demonstrate pseudo-plastic behaviour. The pumps are better equipped to overcome that initial resistance to  flow. They generally run at lower speeds compared to centrifugal pumps and are therefore consequently gentler on the solid particles. However, some PD pumps are known to generate acceleration losses, which must be accounted for.

Air-driven diaphragm pumps

Generally, I have found air-driven diaphragm pumps to be suitable for handling slurries. However, as with centrifugal pumps, abrasion and erosion can be an issue, particularly with the balls and seats that form part of the check valve assembly. If the right material is not selected, the balls can be eroded to a point where they no longer seal properly, causing the pump to not operate efficiently. The same thoughts can be applied to piston diaphragm pumps.

Things to consider (specifically related to slurry handling) include material of check valve assembly; material of diaphragm; and clearances (the maximum particle size the pump can handle).

Peristaltic pumps

Peristaltic pumps are alternatives to air-driven diaphragm pumps. Unlike the diaphragm pumps, there are no balls or check valves to maintain. To put it simply, the only things which require maintenance are the motor and the tube. The main advantage of this pump type is the capability to handle slurries up to 80% w/w solids (this is the highest value I’ve seen claimed, and should be confirmed with your pump vendor). A limiting factor in its selection is the maximum discharge pressure, and this is limited ultimately by the tube properties. Things to consider include tube material (hence tube life), and maximum discharge pressure.

Gear, lobe and ECP pumps

In these pump types, fluid is moved in the spaces between the teeth of the gear pump, lobes or pistons. They typically are specified for slurries with soft particles. ECP pumps are known  for dealing well with slurries that contain solids which settle readily, as they can be scooped up once flow is resumed. The clearances are usually quite tight in these pump types and any slurries which contain abrasive solids would cause excessive wear on these pumps5.

Things to consider include slurry type, and solid characteristics.

Progressive cavity pumps

Used extensively in the wastewater and process industries, this pump is well known for handling slurries. To improve wear resistance whilst pumping slurries, the rotor may be coated. The more abrasive the solids in the slurry the better it may be to operate the pump more slowly, ie select a larger pump and operate at a slower speed. Additionally however, if the pump is operated slower then solids may fall out of suspension and cause blockages within the pump. Be careful when looking to handle larger diameter solids. A limit of 45 mm is stated in Jones’ Pump Station Design.7

Things to consider include solid characteristics (size and abrasiveness), slurry type (do they settle easily?) and seal arrangement.

Conclusion

Selecting a suitable pump for a slurry application can be a tricky business. There are many variables to consider, some of which have been mentioned in this article. The overriding message  though is to ensure that the solids remain in suspension and to minimise wear and blockages. The above is provided for discussion and general guidance purposes only. For specific cases, you should gather as much information as possible on the carrier fluid and solids, and discuss options with a relevant pump vendor.

References

1. Sinnot, R and Towler, G, Chemical Engineering Design, Fifth Edition, Elsevier, .

Contact us to discuss your requirements of slurry pump impeller. Our experienced sales team can help you identify the options that best suit your needs.

2. Processing of Solid-Liquid Suspension, ed Ayazi Shamlou, P,   Chapter 11 by Shook, CA, Chapter 12 Etchells, AW, Butterworth-Heinemann, .

3. Green, DW and Perry, RH, Perry’s Chemical Engineers’ Handbook, Chapter 6, 8th Edition, McGraw-Hill, .

4. Warman Slurry Pumping Handbook, Warman International, Feb .

5. https://bit.ly/2Ud76ls

6. Coulson, JM, Richardson, JF, Backhurst, JR, Harker, JH, Coulson and Richardson’s Chemical Engineering Volume 1 - Fluid Flow, Heat Transfer and Mass Transfer, 6th Edition, Elsevier, .

7. Jones, GM, Pumping Station Design, revised 3rd edition), Elsevier, .

How to Choose a Slurry Pump 101

The purpose of this article is to provide an understanding of the important considerations for selecting a slurry pump for your application. There are many factors and considerations to ensuring the pump that your purchase is appropriately suited for your application needs. Proper pump sizing, motor power, performance requirements, slurry characteristics, and discharge head requirements are only a few of the things that must be factored into the slurry pump selection process.

This Article Includes

• Slurry Pump Selection Overview
• What is a Slurry Pump
• How to Choose a Slurry Pump
• EDDY Pump’s Premier Heavy-Duty Slurry Pumps

How to Choose a Slurry Pump – Overview Video

There are many different types of pumps available, and most pumps are designed for a specific purpose or type of application. Pumps that are designed to transfer thin fluids are different from pumps that are designed for thick, solid-laden, abrasive, heavy materials. Due to the difference in pump design between different types of pumps, it is very important that the most appropriate pump be selected for your slurry application. Selecting the right slurry pump to effectively pump slurry involves considering factors like material properties and pump specifications to avoid issues like clogging and excessive wear.

Slurry pumps (and submersible slurry pumps) are designed for pumping thick materials that can consist of such things as slurries, sand, gravel, mud, crushed rock, muck, manure, and many other highly viscous materials. Typically, a slurry is a mixture of a given material such as crushed rock and water. A slurry pump is purpose built with robust pump components and mechanical seals for pumping abrasive slurries and viscous fluids from the material source, into and through the pump, and downstream to its final destination.

Slurry pumps are inherently rugged by the nature of what they do and are capable of pumping high viscous and high specific gravity solid-laden slurries without clogging. This is why a standard centrifugal pump will often not move highly abrasive slurries over long periods of time.

The non-clogging aspect of a slurry pump design is one of the most critical differentiating factors compared to other pump types. These pumps usually feature heavy duty construction and high efficiency impellers and perform in some of the harshest environments, including dredging, oil & gas, power generation, mining, wastewater treatment, frac sand pumping, and many other industries.

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How to Choose a Slurry Pump

The following list of 8 significant considerations is to help you understand your pump application necessities in relation to selecting the best-suited pump for your heavy duty abrasive applications. The greater the viscosity, particle sizes, specific gravity, abrasion, and other important fluid and non-fluid characteristics make selecting a pump more difficult because these factors play into the situation at a greater level of importance, making the following application specifics even more critical.  

Critical – Know Your Application!

1. Type of material will be pumped (abrasive materials, sand, sludge, mud, muck, slurry, etc.). Knowing the characteristics of the material is critical for choosing the most suited slurry pump.
2. Density of material to be pumped (specific gravity – 2.5 Sg, etc.)
3. Viscosity of material to be pumped (1,000 centipoise (CPS), etc.).
4. Operating temperature of material.
5. The pH level – pH is a measure of hydrogen-ion concentration. An easy way to remember pH is that a pH of 7 is neutral, below 7 acid, and above 7 alkaline. High chrome steel is well suited for pH levels between 4 and 9. Duplex stainless steel is ideally suited for pH levels less than 3 and above 10.
6. Flow rate required (example: 500 GPM or 100 cu. yards per hour [cu-yd/h]).
7. Discharge Head (example: 100 PSIG).
8. The pump chosen must be able to surpass the total discharge head for the application. This helps to reduce the potential abrasion to the pump and discharge pipeline.
9. Length of inlet and discharge pipeline (example: 1,000-ft.).
10. Diameter of inlet and discharge pipeline (example: 4-in., 10-in., 12-in., etc.).
11. Pipeline material of construction (example: steel, polyvinyl chloride, cast iron, etc.).
12. Pump elevation in relation to fluid source.
13. Flooded suction (pump positioned below fluid source).
14. Submersible suction (pump positioned within fluid source).
15. Elevated suction (pump positioned above fluid source, the pump must beself-priming).

Match the Best Suited Pump to Your Application Requirements

1. Pump size must effectively produce a greater flow rate than the critical flow rate of the pump.
2. Critical flow rate is important when pumping slurries with a concentration of solids. The critical flow rate is the flow rate required to keep the material being pumped from settling on the bottom of the volute. Due to this, the actual flow rate should always be higher than the critical flow rate.
3. Pump’s material of construction (cast iron, stainless steel, high chrome steel, etc.)
4. The chemical compatibility of the pump must be suitable for the material that is to be pumped. Also, choosing a material that withstands abrasion is very important. Replaceable rubber liners are available that can be installed internally to the pump to assist with reducing abrasion to the inside of the metallic volute casing.
5. Motor requirements (example: 250 HP).
6. The power rating of the motor must be higher than the power consumption required. Flow rate, specific gravity, and discharge head are used to determine power consumption.
7. Consider the economic impact to your profits of having a pump that is capable of producing the desired flow volume, but do it for a lot less cost. Usually, pumps that can move viscous materials with a very low amount of water can save a large amount of money over a short period of time due to reducing the accruing water volume used.

Centrifugal Slurry Pumps For Abrasive Slurries

The key reason these specialized centrifugal pumps offer reliable performance lies in their ability to transport large volumes of slurry with relatively low energy consumption. The centrifugal action, where a rotating impeller creates a flow by converting mechanical energy into kinetic energy, allows for continuous movement of slurries over long distances with reliable operation. Unlike other pump types (including twin screw pumps and other positive displacement pumps), centrifugal slurry pumps can handle large particles, high solids concentrations, and highly viscous fluids without clogging or excessive wear.

Their robust design, including materials like hardened steel or elastomers for critical wear pump components (such as pump shaft, inlet valve, outlet valve, suction hose and pumping chamber), ensures longevity and minimizes downtime, which is crucial in industrial operations where any disruption of fluid flows can lead to significant financial loss. Centrifugal slurry pumps also have the flexibility to be customized for specific applications, such as varying impeller designs or the use of liners to match the slurry’s characteristics (including corrosive liquids or abrasive particles), ensuring superior performance and extended service life.

In dredging, for example, these pumps help transport large volumes of sediment, sand, and rocks, perfectly suited to heavy duty applications, land reclamation and navigation maintenance. In mining, they are used for transporting ore slurry in mineral extraction processes. Their reliability, versatility, and ability to move challenging materials make centrifugal slurry pumps indispensable in any slurry applications.

Understanding Slurries

Slurry Composition

A slurry is essentially a mixture of fluid and fine solid particles, and its composition can vary widely depending on the specific application. Generally, slurries are classified into two categories: non-settling and settling. Non-settling slurries consist of very fine particles and have low erosive wear properties, making them easier to pump. On the other hand, settling slurries are formed by coarse particles and have higher wear properties, which can pose more challenges during pumping. The composition of a slurry can include a diverse range of materials such as manure, cement, starch, or coal suspended in water or other liquids. Understanding the specific composition of your slurry is crucial for selecting the right pump and ensuring greater efficiency slurry pumping.

Slurry Properties

Slurries possess several distinct properties that significantly affect their behavior and handling, making standard pumps inappropriate. These properties include:

• Viscosity: This measures the resistance of the fluid to flow. Higher viscosity means the slurry is thicker and more challenging to pump.
• Specific Gravity: This is the ratio of the mass of a unit volume of slurry to the density of water at a specific temperature. It indicates how heavy the slurry is compared to water.
• Particle Size: The size of the particles in the slurry can range from one micron in diameter up to hundreds of millimeters. Larger particles can increase wear on the pump.
• Density: This refers to the weight of the slurry per unit volume. Higher density slurries are heavier and more difficult to pump.
• pH: The measure of the acidity or alkalinity of the slurry. The pH level can affect the material compatibility of the pump.

Understanding these properties is essential for selecting the right pump and designing an effective slurry pumping system. Each property influences the choice of pump type, material of construction, and overall system design.

Centrifugal Slurry Pump Design and Construction

Centrifugal slurry pumps are specifically designed to handle the heavy and abrasive nature of slurries. Key features of these pumps include:

• Robust Design: They typically feature a heavy-duty casing and impeller to withstand the harsh conditions of slurry pumping.
• Large Internal Passage: A large, open internal passage helps reduce wear and tear on the pump by allowing larger particles to pass through without causing damage.
• Slow-Turning, High-Efficiency Impeller: This design reduces internal velocities, maximizing wear life and improving efficiency.
• Patented WRT Technology: This technology enhances wear performance and reduces the consumption of wear parts, extending the pump’s operational life.
• Cartridge-Style Bearing Assembly: This feature allows for easy maintenance and prolongs bearing life, reducing downtime and maintenance costs.
• Ductile-Iron, Fully-Lined Casing: Robust pump casing and lining materials offer enhanced durability, strength, and safety, ensuring the pump can handle the demanding conditions of slurry pumping applications.

The design and construction of a centrifugal slurry pump are critical to its performance and longevity. By incorporating these robust features, centrifugal slurry pumps can effectively handle the challenges of pumping abrasive and heavy slurries, ensuring reliable and efficient operation in various slurry pumping applications.

The EDDY Pump Corporation

The EDDY Pump Corporation is a leading provider of unique slurry and dredge pumps and manufactures products that provide superior value versus other pump types. The pump is not a centrifugal, vortex, or positive displacement pump, but rather a pump harnessing the fluid dynamics of an eddy current. This enables the EDDY pump to outperform conventional pumps in applications that involve high viscosity, large weight of solids and particulates, high specific gravity, corrosive and abrasive material, work well in applications that are susceptible to clogging, and do it in an environmentally friendly manner.

EDDY Pump Product Categories

• Self Priming Pumps
• Industrial Heavy-Duty Slurry Pumps
• Dredging Equipment
• Excavator Pump Attachment
• Dredge Sled
• Autonomous Dredge
• Cable Deployed Dredge
• Hydraulic Power Units (HPU)

If you are pumping slurry, brines, high solids, extremely viscous material, paste, high abrasives (sand & gravel), and material filled with solids, then you found the best-suited pump for the job.  Go to:  https://eddypump.com/  or Call Us!

Pump Orientation Options

When choosing a slurry pump for an application, there are two categories of slurry pumpsbased on product orientation; vertical pump and horizontal pumps.  Vertical or horizontal refers to the shaft orientation.  Both pump orientations have their advantages and disadvantages.  Vertical pumps help to reduce the space required for pump location and can be maneuvered into smaller spaces, whereas horizontal pumps are the most common pump orientation. 

EDDY Pump’s Premier Heavy-Duty Slurry Pumps

The EDDY Pump is considered one of the premier self-priming high viscosity and high specific gravity pumps in the world and has been designed specifically for demanding applications that require the transfer of thick, abrasive, heavy solid laden materials.  The non-clogging design of the pump includes an enlarged wetted path and recessed rotor.  This allows the pump to move large volumes of viscous material through the pump without clogging and causing downtime to your application.  

Premier Pump Design

The unique design of the recessed rotor creates an Eddy Current as it spins inside the volute.   This method of displacing fluid via the uniquely designed recessed rotor creates a vortex (a circular current of fluid) at the pump inlet that assists with drawing the material to be pumped into the volute, exiting the pump discharge, and pumping the material downstream of the pump to the desired location.  The Eddy Current also keeps particles and solids maintained in suspension which is critical when pumping high viscous, solid-laden material.  The EDDY Pump’s ability to pump concentrated solid matter is perfect for moving viscous materials with as much as 40-70% solids by volume; an impossible task for a lot of traditional pump types.  The EDDY Pump can also pass extremely large solids that are almost the size of the liquid inlet and liquid discharge of the pump; these results are beyond the scope of what most other traditional pumps can achieve.  

Low Cost of Ownership

An important factor when choosing the best pump for your application goes beyond whether or not the pump can meet the application requirements; there is also the subject of Cost of Ownership and how much it costs you to operate your pump over the course of the pump’s lifetime.  Due to the EDDY Pump’s ability to pump high viscous and high specific gravity materials with the volume of very high solid and extremely low water content, the cost to pump the material is drastically reduced.  The accruing cost of water that goes along with using pumps that require a high water content to move viscous materials dig very deep into your profits.   

Materials of Construction

The EDDY Pump is made of the highest quality materials and is extremely rugged, abrasion resistant, chemically compatible with most fluids, and capable of withstanding a great deal of punishment that is typically inherent in abrasive, high viscosity, and high specific gravity, slurry applications.  For more information on the EDDY Heavy-Duty High Viscosity and High Specific Gravity pump go to https://eddypump.com/ or give us a call today!

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If you need help with pump selection, sales or engineering support, call 619-404-.

EDDY Pump vs. Centrifugal Pumps

How EDDY Dredge Pumps Work Vs. Centrifugal pumps. EDDY Pump technology explained. Learn more about EDDY Pump Technology.

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