As the slurry is highly abrasive, thick, corrosive, and contains a high concentration of solids, it is very challenging to move it. For pumps also, it is very tough. But with a proper selection of slurry pumps, the operation can be smooth for long-term performance. In this article, we will discuss the basics of slurry pumps, their working, types, and selection.
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What are Slurry Pumps?
Slurry pumps are heavy-duty and robust pumps (majorly centrifugal) that are capable of handling tough and abrasive fluids like slurries.
Some examples of industries that handle slurries are mining, dredging, steel processing, foundries, power generation, drilling mud, pulp and paper, wastewater treatment, mineral processing, etc. Due to the presence of solid particles and their highly thick and viscous nature, slurry movement requires more power than normal fluids. So, specially designed heavy-duty pumps are required to work as slurry pumps.
Selection of Slurry Pumps
There are various types of pumps that can pump slurries. However, the selection of exact slurry pumps depends on some critical considerations as listed below:
Type of Slurry to be handled including the size and nature of the solid particles present.
Corrosive property of the slurry mainly to decide pump material for the service.
Pipe size – the pipe ID must be considerably bigger than the maximum particle size.
Static head requirements; and
Available NPSH
Length of slurry pipe or pipeline
Pump operating parameters specifically discharge pressure and speed, the lower the better.
Cost
Slurry Pump Components
There are six basic components in a slurry pump. They are:
Impeller: Two options to choose from, closed impeller or open impeller.
Shell: Two options to select from; Solid single-piece or split shells.
Drive Control: Three types of drive control; Belt drive, gearbox drive, or direct connection of the motor with the shaft.
Suction plate liner
Shaft seals: Three design options; Stuffing box, mechanical seal, or expeller.
Bearing Assembly
Types of Slurry Pumps
Based on the working methodology, there are two types of pumps that are used to handle slurries.
Centrifugal slurry pump and
Positive displacement slurry pump
Centrifugal Slurry Pumps
The most common type of slurry pump is the centrifugal pump with a larger impeller, thicker vanes, and more horsepower. The working of the centrifugal slurry pump is quite simple. They use the centrifugal force generated by a rotating impeller which pushes the slurry to move through the discharge.
When choosing a centrifugal slurry pump the following should be decided:
To minimize the wear of the impeller, a recessed type large and thick open impeller can be used. Closed impellers should be avoided.
Metal casing with proper thickness and rubber lining to be considered.
Cavitation issues.
Positive Displacement Slurry Pumps
When a low slurry flow rate with improved flow control and greater efficiency is desired, a positive displacement slurry pump is more suitable. Common positive displacement pumps used for slurry service are
Rotary lobe pumps
Screw pumps
Diaphragm pump
Peristaltic pumps
Gear pumps, and
Progressive cavity pumps
Click here to learn the main differences between centrifugal and positive displacement pumps
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Again, depending on the installation of the slurry pumps, they are categorized as follows:
Horizontal slurry pumps
Vertical slurry pumps
Submersible slurry pumps.
Horizontal Slurry Pumps
Horizontal slurry pumps have their hydraulic end and the drive unit is located outside the sump. This group of slurry pumps is manufactured for a wide range of head, flow conditions, and material options. Horizontal slurry pumps usually use standardized electrical motors and seals. They are not suitable for plants where there is a risk of flooding.
Vertical Slurry Pumps
There are two types of vertical slurry pumps:
Tank pumps
Cantilever/sump pumps
Tank pumps are dry-installed pumps. The sump is incorporated into the pump. Their open sump and vertical inlet prevent air blocking, which provides a smooth operation. They don’t have submerged bearings or shaft seals, but quite a long shaft overhang from the lower bearing to the impeller.
Cantilever/sump pumps are considered semi-dry installed, as the hydraulic end is lowered into the slurry, but the drive unit and support structure are dry installed. There are no submerged bearings or shaft seals similar to tank pumps, but they have a long shaft overhang.
Submersible Slurry Pumps
Submersible slurry pumps are usually positioned at the bottom of a tank, pond, or lagoon. The slurry materials are taken in at the pump suction and passed through a hose connected to the discharge valve.
Submersible pumps provide a lot of benefits as listed below:
As they directly operate in the slurry, they do not need an additional support structure. Hence, submersible slurry pumps occupy less space.
The motor and volute of submersible slurry pumps are integrated into a single unit. Hence, they are compact and easy to install.
As they operate underwater, they generate low noise and so silent operation.
The surrounding fluid cools the motor which results in smaller and more efficient sumps.
Several installation modes, all of which are either portable or semi-permanent. So, they are enough flexible.
There are other types of classification of slurry pumps as well like Self-priming slurry pumps and flooded suction slurry pumps.
Self-Priming slurry pumps operate from land. A hose is connected to the pump’s intake valve through which the pump draws the slurry to discharge the material.
The flooded suction slurry pump is connected to a tank or hopper. It uses gravity force to move slurry and liquid from the enclosure. They are placed at the bottom or below the water and use the gravity force to continuously fill the pump and then pass the material out through the discharge valve.
Slurry Pump Installations
Depending on the environmental condition, there are three types of slurry pump installations:
Wet Environment– This type of slurry pump installation involves submerging the product fully for underwater operations.
Dry Environment– In a dry environment, the pump drives and bearings are kept away from the abrasive slurry. It calls for a horizontal pump as the shell, impeller, suction liner, and shaft sleeve have to be on the wet side.
Semi-Dry Environment– Since it’s an unusual scenario, a special type of horizontal slurry pump installation is preferred.
Choosing a Slurry Pump
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
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: 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, 1972)
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
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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
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.
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.
Cross section of an ECP pump: Learn how it works at https://bit.ly/2FXMGbU
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, 2009.
2. Processing of Solid-Liquid Suspension, ed Ayazi Shamlou, P, Chapter 11 by Shook, CA, Chapter 12 Etchells, AW, Butterworth-Heinemann, 1993.
3. Green, DW and Perry, RH, Perry’s Chemical Engineers’ Handbook, Chapter 6, 8th Edition, McGraw-Hill, 2007.
4. Warman Slurry Pumping Handbook, Warman International, Feb 2000.
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, 1999.
7. Jones, GM, Pumping Station Design, revised 3rd edition), Elsevier, 2008.
Top 8 Considerations for Selecting a Slurry Pump
Top 8 Slurry Pump Considerations
The following is a list of 8 considerations for choosing a slurry pump best suited for a slurry application. Selecting a pump for a slurry application is more difficult than for an application involving thinner fluids. If a mistake is made in the pump selection process, the pump chosen will most likely not work well, or will not pump the higher viscosity, abrasive, heavy, solid laden fluid at all, which renders the new pump useless!
Know the Material or Fluid Being Pumped
The fluid or material type and its characteristics are among the most important considerations. Is it a slurry, mud, sand, etc.
Fluid viscosity of the material, usually measured in centipoise (CPS).
Density of fluid, usually measured as specific gravity (Sg)
The pH level, which is the measure of hydrogen-ion concentration.
Static and operating temperature of the fluid.
Pump Flow Rate
Flow rate is another important factor for selecting the best-suited pump for a slurry application. The pump must be capable of exceeding the required flow rate to ensure desired flow rates are achievable (example of flow rate: 350 GPM or 200 cu. yards per hour {cu-yd/h}).
The flow rate of the pump must not only achieve the required flow rate of the application, but it must also be more than something called the critical flow rate. The critical flow rate is the constant flow rate required to maintain the suspended particles and solids in the slurry. Maintaining suspension of particles and solids helps to avoid the heavy portion of the fluid from settling at the bottom of the wetted path, as well as from settling at the bottom of the discharge piping.
Flow velocity is a critical consideration; the material must move at a consistent velocity through the piping to keep the slurry, particles, and solid-laden material suspended so it does not settle and cause clogging.
Materials of Construction
The materials that the pump is made of are necessary because the pump must be chemically compatible with the fluid being pumped. If the pump’s materials of construction and the liquid are not consistent, it can cause the pump to either melt down or crack, resulting in catastrophic failure of the pump, and can also cause damage to the immediate area surrounding the application and cause injury to workers.
The pump must also handle the abrasive characteristics of the fluid being pumped. If not, abrasive fluids can scour through the pump casing and cause premature wear of the internal pump components such as the rotor or impeller.
Inlet & Discharge Pipe Considerations
Pipe length, diameter, and the type of material of the piping are essential factors that are often not strongly considered when constructing a pumping system.
Pipe length is essential because the more significant the size of the pipe, the more fluid or material build-up will occur, requiring a more substantial amount of motor power to enable the pump to continue pushing the fluid or material to its final destination.
Pipe diameter should be sized considering two factors, reducing discharge head pressure and maintaining sufficient fluid or material velocity to avoid clogging of the discharge pipe. Regarding both reducing discharge head and maintaining adequate fluid velocity, the rule of thumb is to go more extensive on the pipe diameter, which will help to alleviate the adverse effects of both factors.
Pipe material should not only be chemically compatible with the fluid or material being pumped, but when selecting piping that has a reduced surface finish at the inside of the piping, it can also minimize pipe friction loss which can result in less energy required to pump the fluid or material to its final destination. The surface finish measure is denoted as Ra, which stands for Roughness Average.
Motor Power
Motor power, usually indicated by horsepower (HP), is important on any pump but wildly when pumping slurries and fluids with high specific gravity and viscosity because thicker, heavier fluids require a more significant amount of power and force to move the fluid or material to the desired final location.
The motor power must also be sufficient enough to overcome any forces within the discharge piping downstream of the pump. These forces within the discharge piping could be a result of pipe components such as tees, bends, and upward grades that create something that is referred to as discharge head pressure which is measured in PSIG.
Pump Operating Cost
Another important consideration that most pump user does not think about is the cost and economic impact of the pump. Having the best-suited pump for an application also includes how much money it requires to keep that pump running for whatever duration it is in service. It not only has the energy the motor uses but also involves selecting a pump that can move viscous material with low amounts of water or accompanying fluids.
Water and accompanying fluids used to make pumping viscous solid laden material possible can cost a lot of money. If these fluids can be reduced, it can save thousands of dollars on operating expenses.
Pump Elevation
The pump must be located in a manner that does not hinder its operation of the pump. In applications where the pump is positioned above the fluid to be pumped, the pump cannot be located higher than the pump’s ability to draw the liquid into the pump intake. If the pump is positioned at an elevation that is greater than the pump’s ability to remove the fluid into the pump, the result will be that the pump will not achieve prime, and the desired flow rate will not be reached, or even worse, the pump will not pump the fluid at all.
Pump Orientation
One last point to mention is pumped orientation. Pumps can be purchased with several different orientation options. The most common are vertical and horizontal, which refers to pump shaft orientation. Depending on the specific application, vertical and horizontal pump orientations can be the better choice. Horizontal orientation is the most purchased orientation, but vertical orientation can be better suited when a pump is used in a smaller space.