Slurries are specialized compounds found in various processing industries, including sanitary sectors like mining, aggregates, mineral processing, dredging, tunneling, and water treatment, as well as other industries that involve slurry or sewage. Because slurries combine the properties of liquids and solids, special consideration must be given when determining the type and size of the slurry pump to use with them. Kingda Pump features several companies that specialize in slurry pumps capable of handling a wide range of loads, from light to heavy.
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Slurries are solid-liquid mixtures, with the liquid acting as a transport mechanism for the solid. The size of particles in slurries can range from a micron to thousands of millimeters. The ability of a pump to move a slurry through a process line is greatly influenced by particle size.
Aslurry pump is a centrifugal pump used to transport abrasive and frequently corrosive solid particles inside a carrier fluid. To minimize substantial particle velocity and thus wear rates, the slurry pump should have a low specific speed design that allows for slower operating speeds than ordinary water pumps. Depending on the application, wear-resistant metallic or elastomeric materials are used in slurry pump components that come into contact with the pumped medium.
Slurries are further classified into four classes by industry based on their aggressiveness, with Class 1 being the least aggressive and Class 4 being the most aggressive. Pumping slurries can wear out pumps and pipeline components in the following ways:
Several types of pumps are suitable for pumping slurries, as described below. However, we must address a couple of critical issues before deciding which technology to use.
The pump’s speed and shear the pump can cause damage to solids or slurry, which is a concern for centrifugal pumps. Twin screw pumps, generally, because of the minor slurry’s solids.
Because slurries weigh a lot and are hard to push, the pumps explicitly made for slurries will have heavier and more potent than pumps designed for liquids with less viscosity.
Slurry pumps tend to be larger than conventional pumps, have more horsepower, and are constructed with more robust bearings and shafts. The centrifugal pump can be described as the most popular kind of pump for slurry. These pumps employ an impeller rotating like water-like liquids flow through a centrifugal pump.
Pumps designed explicitly for slurry pumps will usually feature the following features compared to conventional centrifugal pumps. Centrifugal force can push it outward if something is spinning fast around a central point.
These pumps may also be made of specialized high-wear alloys for pumping abrasive slurries. Hardening stainless steel is another popular option for abrasive slurries.
Helpful displacement pumps may be better than centrifugal pumps for certain slurry pumping conditions.
Follow these four simple steps to determine what type of slurry pump is the best for your particular need or application.
Think about the following:
Are the design and material used to construct the impeller of a centrifugal pump suitable for pumping slurries?
It is also critical to consider the slurry’s chemical compatibility with any elastomers in the pump. Once the nature of the slurry and the components of various types of pumps have been addressed, we can choose a potential candidate slurry pump for the application.
The most crucial part of this equation is determining the required pump horsepower to supply a certain quantity of liquid at the needed differential pressure. Think about the following:
Most centrifugal slurry pumps operate at low speeds to reduce component wear — typically less than rpm. Find the ideal spot where the pump can run at a minimum speed without allowing solids to settle into the slurry and clog the lines.
Then, to reduce wear even further, reduce the pump’s discharge pressure to the lowest setting possible. Also, use proper piping layout and design principles to ensure consistent and uniform slurry delivery to the pump.
Thanks to features such as the large diameter of the impeller, shafts, bearings, and internal passageways, and their heavy-duty construction, Slurry pumps can withstand massive wear. Slurry pumps have more upfront operating costs than water pumps in the industrial industry. But, only slurry pumps are efficient in hydro-transporting solid materials. The longer-term advantages outweigh the initial cost.
The production of centrifugal force that causes the material to move outwards from the center of the pump is crucial for the efficiency of slurry pumps. However, the power of centripetal force pulls the material toward the center. Since the energies that impart the speed of the slurry increase, the process of transport of Slurry pumps works on the principle of centrifugal force. A centrifugal pump, however, is ineffective since the solids in the slurry would build up rather than flow freely.
Knowing the exact conditions required for each pump is crucial for anyone planning to set up a slurry pump.
Wet – the drive and slurry pump are entirely submerged in this setup. Certain types of slurry pumping, like underwater operations, will require this.
Dry — the setup protects the bearings and the pump’s drive free of the slurry. The wet part is freestanding and free of any liquid that surrounds it, comprised of the impeller, shell and suction liner, hub and shaft sleeve, and the stuffing box. The slurry pump technician puts most horizontal pumps in place using this method.
Semi-dry — this particular arrangement is used in horizontal pump dredging. The wet side and the bearings are flooded; however, the drive is dry. In this situation, bearings require a sealing arrangement.
This guide gives a brief overview of slurry pump types and their installation; however, there is more to be learned. Kingda will be there to help those looking to comprehend slurry pumps and applications or require help deciding what type of pump and installation is best for their needs.
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.
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:
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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’ HandbookFigure 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’ HandbookFor 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 HandbookOften, 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.
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.
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, .
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