Nnamdi Nwaokocha offers practical advice on pump selection
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.
In a spin: Slurry centrifugal pump with rubber lined casing and rubber impeller
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.
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: The relationship between the pressure drop and the slurry&#;s velocity compared to a pure liquid in horizontal pipework
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
Figure 2: Durand factor for minimum suspension velocity (from Govier and Aziz, the flow of complex mixtures in pipes, Van Nostrand Reinhold, New York, )
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:
The solid particle will play a crucial part in selecting the material of the wetted parts. The following, amongst other things, should be considered:
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.
The most common pumps generally in use are centrifugal pumps. When specifying this type of pump, as a minimum, the following must be considered:
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
Figure 3: Example operating curve and efficiency curve for a mixture of solids and water only
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.
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.
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 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.
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.
Cross section of an ECP pump: Learn how it works at https://bit.ly/2FXMGbU
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.
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.
SHANGLISHI contains other products and information you need, so please check it out.
1. Sinnot, R and Towler, G, Chemical Engineering Design, Fifth Edition, Elsevier, .
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, .
The slurry business is a complex industry filled with myriad physical, economic, and legal considerations. As a project manager, choosing a proven, American-made slurry system can go a long way towards preventing clogs and breakdowns, ensuring your system remains a profitable and resilient wastewater solution.
However, not every system is built to handle the same loads or manage slurry in the same way. Understanding the different slurry pump and filtration system options on the market will help you find the right system for your project.
Choosing a Slurry Pump
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Slurry pumps come in various lengths, elevations, and materials. Be sure the pump you choose will perform at your specifications and can integrate well into your existing system.
Types of Slurry Pumps
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Not all slurry pumps are used for the same purposes or at the same processing stages. The first thing to consider when browsing the pump market is what kind of pump you need&#;horizontal, vertical, or submersible.
Horizontal Pump
Horizontal slurry pumps move slurry parallel to the ground and handle highly abrasive or corrosive slurries with ease. Because they&#;re so durable, horizontal pumps are popular in heavy industry, including mining, metallurgy, and power generation.
Vertical Pump
Vertical pumps are used to transport materials to different elevations. Often (but not always), a vertical pump will be partially submerged in a slurry pool to transport its contents to different parts of the project site.
Submersible Pump
Submersible pumps are fully immersed in slurry. They are specially designed and built to handle the rigors of operating within a liquid, but they can degrade quickly, especially if your slurry is highly acidic.
Slurry Pump Materials
Consider what material you will be pumping. Some pumps are perfect for moving sand and mud, while others are more suited to processing sludge and other waste. Before making a purchase, ensure the pump&#;s material can handle the density, pH, viscosity, and temperature of the material you process.
Flow Rate
Different pumps process materials at different speeds, and the flow rate that&#;s best for your system will depend on what materials you process and during what stage of processing the pump will be used. For example, slurries that contain a lot of solids need to be pumped at a higher flow rate to prevent material from settling inside the pump and its connected hoses.
Discharge Head
Any slurry pump you choose needs to be able to exceed the discharge head of your entire discharge system; otherwise, the system will not operate at peak efficiency and may suffer abrasion.
Knowing the answer to these questions before you make a purchase will help you choose the best pump for your operation and avoid the costs associated with an inefficient or damaged slurry system.
Integrating Your Pump into a Wastewater Filtration System
After you&#;ve selected a pump, you can pair it with a new or existing filtration system to optimize your entire water treatment process. Full Circle Water offers many different water filtration systems, including industrial slurry silos, portable filtration plants, and rapid resettlement systems.
All-in-One Plants
All-in-One slurry silo filtration systems are turnkey solutions designed to provide efficient wastewater decanting regardless of the type of operation you run. Full Circle Water&#;s All-in-One system was designed based on feedback from over 600 stone fabricators and is among our smartest and most effective slurry silo plants.
Trailer Plants
Trailer filtration systems are designed to offer clean water on-the-go. Their compact, portable design makes them ideal for projects that need to set up or tear down quickly.
Rapid Settlement Systems
Wastewater decanting is one of the most important phases in any slurry management system. Rapid resettlement systems are designed to recycle up to 90 percent of wastewater and feed it back into production, ensuring your operation never has to wait for water again.
Find the Right Slurry System with Full Circle Water
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