Slurry can be an efficient tool for soil removal. However, one must first determine the correct size of slurry pump.
Every trenchless project needs a method to remove excavated soil from the bore. Some methods use augers, while others could even be hand excavated. But, many trenchless construction projects use slurry systems for this purpose.
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A slurry mixture – or mud containing water, bentonite and other additives – is pumped down the drill string. The mud pushes out of the drilling head itself, helping to lubricate the side walls of the bore. This makes it easier for the pipe to slide through the soil. Slurry also maintains a pressure inside the bore to withstand the effects of groundwater pushing in. A positive pressure inside the bore is important to prevent tunnel collapse. Microtunneling and HDD are typical trenchless methods which use a slurry system. (For more on mud, check out Bentonite and the Use of Drilling Mud in Trenchless Projects.)
Slurry pumps supply enough pressure to push the mud through the bore, carrying spoil from the excavation back to the surface with it. From there it enters a separation unit to remove the solids before being recycled in the system. These pumps operate in harsh conditions and often require both high pressures and high flow rates.
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Sizing a slurry pump correctly is a critical part of the pump selection process. This ensures that it will operate within its design parameters, avoiding premature pump failures and costly downtime.
How do you size a slurry pump to match your needs?
There are a wide variety of types of pump, including reciprocating, diaphragm and centrifugal pumps. Many slurry systems use centrifugal pumps because of their robustness and performance characteristics. A centrifugal pump works by rotating an impeller which pushes the slurry through the system.
Pump capacity is primarily defined by two parameters: head pressure and flow rate. Manufacturers supply each pump with a pump curve where pressure and flow are plotted against each other. It is this curve that we use to determine whether a pump is suitable for our application.
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Chemical Engineering describes a detailed process for engineers to follow when sizing a pump. The more complex the application, the more important it is to have expert help in establishing the pump design characteristics needed. Nevertheless, there are some fundamental steps that can be applied in every situation.
Dynamic head is effectively the pressure at the discharge of the pump in order to supply the energy needed to push the slurry through the system. A major factor that affects the dynamic head is changes in level from the slurry supply tank to the lowest point in the bore and back up again. The pump must overcome the forces of gravity to push the mud back out of the ground.
Another significant factor is the friction of the system. As the slurry passes through the drill pipe and back up to the surface, it encounters friction against the surface walls of the pipe. Slurry pumps must provide enough energy to overcome these forces of friction too.
Besides supplying sufficient dynamic head based on the calculations above, a slurry pump must also push the required volume of slurry through the system. This is defined in terms of flow rate. The flow rate required for a trenchless project is heavily dependent on the soil conditions and the size of the bore. Sandy soils need the lowest volume of slurry to carry the spoil to the surface in an HDD project. But, shale conditions may require up to 20 times as much. (To learn more about ground conditions, see When Ground Improvement is Needed During Trenchless Rehabilitation.)
No pump operates at 100 percent efficiency. In fact, efficiencies can range from as low as 50 percent up to over 90 percent. This means that from 10 to 50 percent of the energy you supply to the pump is not actually used to move the slurry through the system – it is lost. The better the efficiency of the pump, the lower your operating costs will be. However, efficiency is based on where on the pressure/flow curve the pump is operating.
A pump curve is essentially a graphic representation of the performance of a pump. If the head and flow rate of your application sits on or below the pump curve, then the pump is capable of performing that task. A steep curve means that it can generate a lot of head but limited flow rate. Flat pump curves represent pumps that can generate high flows but have a limited head pressure.
Pump manufacturers actually supply a series of curves for each pump that represent the pump’s performance for different diameter impellers. Efficiency curves can be overlaid on the pump performance curve to indicate where the pump is operating in its best efficiency range.
Selecting an efficient pump is important, as it saves on operating costs like electricity or generator fuel. One pump may be cheaper to buy but cost much more to run in the long term. It may be better to buy a more expensive pump up front, but one that has a good efficiency and lowers your operating costs.
Operating on the left side of the pump curve will lead to symptoms of temperature rise, noisy operation due to cavitation and vibration, which will ultimately result in low bearing and seal life and regular maintenance outages.
The same effects can occur if a pump is operated on the right-hand side of the pump curve. The best operating point for the pump is in the middle of the pump curve, where it is most efficient and least likely to have component failure.
When analyzing pump failures, it is important to look at the whole system and not just the pump itself. Monitor and record pressure readings in the mud circulation system and check friction calculations. An error in these calculations could mean that you have the wrong pump for your service and therefore have a solution that is prone to failure.
As in most process applications using pumps, it is often not the cost of a pump repair that is the primary problem. The problem is the interruption to the project while the repair is being executed.
Man hours lost and overheads are still incurred while there is no progress on the trenchless construction activity itself.
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Selecting a Right Slurry Pump:
1. Determine the flow rate
To size and select a pump, we first determine the flow rate. In an industrial setting, the flow rate will often depend on the production level of the plant. Selecting the right flow rate may be as simple as determining that it takes 100 m3/h (3.6 L/s) to fill a tank in a reasonable amount of time or the flow rate may depend on some interaction between processes that needs to be carefully analyzed.
2. Determine the static head
This a matter of taking measurements of the height between the suction tank fluid surface and the discharge pipe end height or the discharge tank fluid surface elevation.
3. Determine the friction head
The friction head depends on the flow rate, the pipe size and the pipe length. This is calculated from the values in the tables presented here (see Table 1). For fluids different than water the viscosity will be an important factor and Table 1 is not applicable.
4. Calculate the total head
The total head is the sum of the static head (remember that the static head can be positive or negative) and the friction head.
5. Select the slurry pump
Slurry Pump selection is based using the total head and flow required as well as suitability to the application.
Slurry Pump Calculations
The following parameters must be determined when calculating a slurry pump
Particle size and distribution
Particle size d50 (d85) is a measure of the percentage of particles in the slurry with a certain size or smaller.
The value is determined by sifting the solids through screens with varying mesh and then weighing each fraction. A sieve curve can then be drawn and the percentage of particles of different sizes read.
Ex: d85= 3 mm means that 85% of the particles have a diameter of 3 mm or less.
Mass fraction of small particles
The fraction of particles smaller than 75 µm. It is important to determine the percentage of small particles in the slurry. Particles smaller than 75 µm can to some extent facilitate the transport of larger particles. However, if the percentage of particles smaller than 75 µm exceeds 50%, the character of the slurry changes towards non-settling.
Concentration of solids
The concentration of particles in the slurry can be measured as a volume percentage, Cv , and a weight percentage, Cm
Density/Specific Gravity
Solids
The density of the solids is stated as the Specific Gravity. This value, SGs , is determined by dividing he density of the solid by the density of water
Water
The density of water is 1000 kg/m3 . The SG of water is 1,0 at 20°C. The value varies somewhat with temperature.
Slurry
The specific gravity of the slurry can be determined using a nomograph. Two of the values of SGs, Cv , and Cm, must be known
Particle shape
The shape of the particles is very significant for the behavior of the slurry when pumping and for the wear on the pump and the pipe system. The form factor denotes the deviation of the slurry particles from a perfect sphere.
Slurry characteristics
Slurries can be divided up into settling and non-settling types.
Non-settling slurry
A slurry in which the solids do not settle to the bottom, but remain in suspension for a long time. A non-settling slurry acts in a homogeneous, viscous manner, but the characteristics are non-Newtonian.
Particle size: less than 60-100 µm.
A non-settling slurry can be defined as a homogeneous mixture.
Homogeneous mixture
A mixture of solids and liquid in which the solids are uniformly distributed.
Settling slurry
This type of slurry settles fast during the time relevant to the process, but can be kept in suspension by turbulence. Particle size: greater than 100 µm.
A settling slurry can be defined as a pseudo-homogeneous or heterogeneous mixture and can be completely or partly stratified.
Pseudo-homogeneous mixture
A mixture in which all the particles are in suspension but where the concentration is greater towards the bottom.
Heterogeneous mixture
A mixture of solids and liquid in which the solids are not uniformly distributed and tend to be more concentrated in the bottom of the pipe or containment vessel (compare to settling slurry).
Liquid definitions
Except for density, the characteristics of a liquid are decided by its viscosity.
Liquids deform continuously as long as a force is applied to them. They are said to flow. When a flow takes place in a liquid, it is opposed by internal friction arising from the cohesion of the molecules. This internal friction is the property of a liquid called viscosity.
The viscosity of liquids decreases rapidly with increasing temperature.
Newtonian liquids
Newtonian liquids give a shearing stress that is linear and proportional to the velocity gradient, or the shearing rate. Water and most liquids are Newtonian.
Non-Newtonian liquids
Some liquids, such as water based slurries with fine particles, do not obey the simple relationship between shearing stress and shearing rate. They are referred to as
non-Newtonian liquids.
Some non-Newtonian liquids have a unique property of not flowing until a certain minimum shear stress is applied.
Slurry Pump performance
The performance of a centrifugal pump pumping slurry differs from the performance with clean water depending on the amount of solid particles in the slurry.
This difference depends on the characteristics of the slurry (particle size, density, and shape, as described in the previous chapter). The factors that are affected are the power (P), head (H), and efficiency (η). The differences between slurry and water are shown schematically in the curves below
System design
Static head
Static head is the vertical height difference from the surface of the slurry source to the discharge point.
Friction losses
When the liquid starts to flow through the discharge line and valves, friction will arise. When pumping slurry, friction losses caused by pipe roughness, bends and valves, are different compared to the corresponding losses when pumping water.
Total discharge head
This value is used for pump calculations and comprises the static head plus friction losses caused by pipes and valves, converted to meters of water.
Critical velocity
In general, the flow velocity in the pipes must be kept above a certain minimum value. If the flow velocity is too high, friction losses will increase. This may also increase the wear in the pipe system. Flow velocities that are too low will result in sedimentation in the pipes and, thus, increased losses.
This is illustrated in the diagram below, in which the critical velocity (Vc) indicates the optimum velocity where losses are kept to a minimum.
When making calculations for a slurry pump for a certain flow, the desired flow velocity (V) must be compared to the critical velocity (Vc) for the slurry and the pipe system in question. As the figure below shows, the ideal velocity (marked green) is immediately above the critical velocity but with a margin for the extreme cases that can arise.
To determine the critical velocity, the pipe diameter and the particle size (d85) must be known. The value is then corrected with a factor, which depends on the specific gravity of the solids.
Sizing the slurry pump
The diagram above shows, schematically:
1. the pump curve for water
2. the reduced curve for slurry
3. the duty point for slurry, i.e. the point at which the pump system curve and the performance curve intersect.
System curve with correlation for settling slurry effects
Other considerations
Whenever pumps are used, it is important that the pump's inlet pressure exceeds the vapor pressure of the liquid inside the pump. The necessary inlet pressure that is stated for the pump, NPSH req must not be less than the available value in the pump system, NPSH.
The available value depends on the ambient air pressure (height above sea level), the vapor pressure of the liquid, the density of the slurry, and the level in the sump.
Example: Pumping a water-based slurry at a height of 1000 m above sea level. Liquid temperature 40°C, liquid level 2 meters above the pump inlet.
Formula:
NPSHa = air pressure – vapor pressure + level in sump inlet.
NPSHa = 9,2 – 0,4 + 2 = 10,8
Centrifugal Pumps for Slurry
Slurry is one of the most challenging fluids to move. It's highly abrasive, thick, sometimes corrosive, and contains a high concentration of solids. No doubt about it, slurry is tough on pumps. But selecting the right centrifugal pump for these abrasive applications can make all the difference in the long-term performance.
Slurry is any mixture of fluid and fine solid particles. Examples of slurries would include: manure, cement, starch, or coal suspended in water. Slurries are used as a convenient way to handle solids in mining, steel processing, foundries, power generation, and most recently, the Frac Sand mining industry.
Slurries generally behave the same way as thick, viscous fluids, flowing under gravity, but also pumped as needed. Slurries are divided into two general categories: non-settling or settling.
Non-settling slurries consist of very fine particles, which give the illusion of increased apparent viscosity. These slurries usually have low wearing properties, but do require very careful consideration when selecting the right pump because they do not behave in the same manner as a normal liquid does.
Settling slurries are formed by coarse particles that tend to form an unstable mixture. Particular attention should be given to flow and power calculations when selecting a pump. The majority of slurry applications are made up of coarse particles and because of this, have higher wear properties.
Below are common characteristics of slurries:
Many types of pumps are used for pumping slurries, but the most common slurry pump is the centrifugal pump. The centrifugal slurry pump uses the centrifugal force generated by a rotating impeller to impact kinetic energy to the slurry, similar to how a water-like liquid would move through a standard centrifugal pump.
Slurry applications greatly reduce the expected wear life of pumping components. It’s critical that pumps designed for these heavy-duty applications are selected from the start. Consider the following when making selections:
To ensure the pump will hold up against abrasive wear, the impeller size/design, material of construction, and discharge configurations must be properly selected.
Open impellers are the most common on slurry pumps because they’re the least likely to clog. Closed impellers on the other hand are the most likely to clog and the most difficult to clean if they clog.
Slurry impellers are large and thick. This helps them operate longer in harsh slurry mixtures.
Slurry pumps are generally larger in size when compared to low-viscosity liquid pumps and usually require more horsepower to operate because they're less efficient. Bearings and shafts must be more rugged and rigid as well.
To protect the pump’s casing from abrasion, slurry pumps are oftentimes lined with metal or rubber. Goulds Pumps, for example, lines their XHD (Extra Heavy Duty) slurry pump with rubber.
Metal casings are composed of hard alloys. These casings are built to withstand the erosion caused by increased pressure and circulation.
The casings are selected to suit the needs of the application. For instance, pumps used in cement production handle fine particles at low pressures. Therefore, a light construction casing is acceptable. If the pump is handling rocks, the pump casing and impeller will need a thicker and stronger casing.
Those with experience pumping slurries know it's not an easy task. Slurries are heavy and difficult to pump. They cause excessive wear on pumps, their components, and are known to clog suction and discharge lines if not moving fast enough.
It’s a challenge to make slurry centrifugal pumps last for a reasonable amount of time. But, there are a few things you can do to extend the life of your slurry pump and make pumping slurry less of a challenge.
Pumping slurries poses several challenges and problems, but with proper engineering and equipment selection, you can experience many years of worry-free operation. It's important to work with a qualified engineer when selecting a slurry pump because slurries can wreak havoc on a pump if not properly selected.
Check out the Must-Have Handbook for Centrifugal Pumps for more information on centrifugal pumps, including details about pumps specifically designed for slurry applications!
If you have any questions on slurry centrifugal pump. We will give the professional answers to your questions.