Slurry pump is an indispensable equipment of the sludge mining and transportation industry. During the operation of a plant, raw materials are always transported from one point to another. The law of thermodynamics states that the fluid only moves from a high pressure area to a low pressure area. Depend on the plant’s construction design, the pumps will be placed in appropriate locations to assist with fluid transport effectively. With so many different types of pumps and brands on the market, choosing the right pump can be very confusing, especially when transporting mixture of mud or water containing coarse solids.
This article will discuss about some necessary specifications when it comes to slurry characteristic and a proper pump solution for slurry and ore transfer in the plants and mining sites. This article is not a detailed instruction on how to transfer slurry using pump but an overview in the process of transporting sludge, ore by pumps.
Some features of slurry pumps
When operating pump sludge pumps, it is common to experience a blockage or damage to pump components. The engineers will coordinate with the pump supplier to evaluate all factors in all situations, including fluids of different nature, different transport piping system designs to find a suitable sludge pump that can minimize clogging, make maintenance easier and ensure operational safety.
What is sludge? Usually, the term sludge is used to refer to a mixture of liquid and solid particles or solids resulting from precipitation. The solution in a mixture is often called the loader and in most cases is water, although it can be other things, such as acidic solutions (ex. nitric acid) or hydrocarbons (ex. diesel).
Knowledge of the production of the sludge or maintaining the suspension under static conditions is beyond the scope of this article.
Sludge can be divided into two types: sediment and non-sediment. This property is based on the nature of the solid particles. Non- sediment sludge contains solid particles made up of fine particles, most of which remain in suspension when mixing stops. Sediment sludge, by the nature of the name, suggests it contains solid particles that settle down when the mixing stops. For the pump designer and manufacturer, sludge classification is a very important step. For example, non- sediment sludge can be transported under laminar flow conditions, while in contrast, the sediment requires fluid transport under turbulent flow conditions, especially in the horizontal transport pipes.
There is a useful rule is provided in “Chemical Engineering Design” by Sinnot and Towler. Solid particles that is smaller than 200micron (0.2 mm) will usually produce non-sediment sludge. Larger particle size will create sediment.
Before selecting the required sludge pump type, the first step is to determine the pressure relief requirements using the system properties. The required parameters are:
ρl: density of the liquid
d50: average particle size
ρs: density of solid particles in the fluid
ρsl: density of the sludge
%(w/w): mass concentration
%(v/v): volumetric concentration
The following equation can be used to determine the density of the sludge fluid:
For sediment sludge, the sludge velocity in the pipeline is the main factor in the design and selection of the equipmentSlurry pump performance curve
Figure 1: Relationship between pressure drop and sludge transport velocity compared to pure water transported in horizontal pipes
Figure 1 is taken from Perry’s Chemical Engineers’ Handbook, describing the relationship between the pressure drop and the velocity of the transported sludge compare to the pure liquid in the horizontal pipeline. The important point is the velocity when the sludge is transported in the horizontal pipe must be higher than the point Vm2 (minimum transport speed). This is the point at which the solids are in suspension. This velocity is determined by the Durand equation.
g: Gravitational acceleration
D: Pipe diameter
s = ρs/ ρl = Ratio between the density of solid particles and the density of the load
FL= The constant depends on the particle grade and the solid particle concentration.
Pump performance curve
Once the minimum transport speed has been calculated, it is also necessary to take into account other safety factors. If the speed is too high, the required suction pressure drops and the pump may have to operate beyond its design capacity. For a vertical flow, the discharge velocity when leaving the discharge pipe should be doubled the settling rate of the solid particle. The main purpose is to prevent the settling solid particles. As a rule of thumb, velocities in the range 1-3 m/s should be used. The pressure drop caused by the sludge can be determined by assuming and calculating based on a uniform velocity in the pipeline. This velocity is calculated based on the sludge density, the viscosity of the solution carrying solid particle and a correction factor. Correction factor of 25% proposed by Perry in Perry’s Chemical Engineers’ Handbook.
Note that the calculation above is used when the solid particles are heavier than the load containing them. Depending on the size of the solid particles, at a specific concentration, the particles begin to interact with each other and can begin to affect the viscosity of the sludge. This is discussed further with the calculations provided in Processing of Solid-Liquid Suspensions, Chapter 11. Appendix 3. Warman Slurry Pumping Handbook also contains calculations and correlation coefficients for fluids as well (water-based slurries)
For non-settling sludge, properties are generally not subject to Newton’s law (non-Newtonian fluids). The rheological and sludge properties often have to be determined empirically to determine the right pump for the requirements. The pressure drop in this case can be calculated based on the measured viscosity and sludge density.
Other factors to consider before choosing the right pump include:
- Slippage: when the velocity of a solid particle and the fluid contain it is significantly different;
- Pipe size: make sure the inside diameter of the tube is much larger than the largest particle size (6 to 10 times larger);
- Pipeline design: using recirculating sections to ensure sludge is always in motion; slope design, from which the mud can descend to the end point; The bends should be installed with a large radius, minimizing bend points, minimizing dead leg, minimizing suction pipe;
- Required static head;
- NPSH (Net Positive Suction Head)
Solid particles play an important role in the selection of materials that make up the pump head components. Other factors that should be considered include:
- Solid hardness?
- Abrasion of solid particles?
- Corrosion level when fluid is pumped?
- Corrosion level of the fluid (the solution contains such solid particles)?
Common slurry pump types on the market
For a variety of tasks and purposes, both the centrifugal pump and the volumetric pump are applicable. Here are some aspects to consider before choosing the right pump for your sludge transfer. However, always double-check and consult with the supplier’s experts / technicians before making the final decision.
- Centrifugal pump
The most commonly used pump series are centrifugal pumps. With centrifugal pumps, the following factors should be particularly noted:
- Wing type: Vortex type can be used, with eddy current design to minimize contact between solid particles and impeller thereby minimizing wing wear. Open wing styles can also be used, as they are often easier to clean and maintain. The sealed wing will give the best performance, but the disadvantage is that it can be difficult to clean during maintenance. The wing thickness corresponds to the appropriate liner. Also we should consider any impact caused by the speed of the impeller rotation.
- Case type: Metal housing can be used. The housing can be internally lined with rubber for added protection against wear. A loose case may also be considered, but the cost of a loose case can be high.
- Clearance: The slurry centrifugal pump must have a larger clearance between the impeller and pump body than the pure liquid pump This allow solid particles to pass through and also to reduce the speed in the pump, thus minimizing the corrosion.
Above are some things to consider when choosing a centrifugal pump for sludge transportation. When working directly with suppliers, the engineers must choose the most suitable solution for the operating system. The team should also consider any factors affecting the shaft and seal, ensuring there will be no problem with cavitation when the pump is put into service.
Centrifugal pumps creates pressure differentials. Therefore, pressure on the fluid flow is generated based on the properties of the fluid.
Figure 3: Example of a performance curve for a mixture of water and solid particles
Normally, the pump performance curves provided by the manufacturers apply only to the aqueous fluids. Therefore, it is necessary to convert these curves when the fluid is slurry or ore. An example is shown in Figure 3, which includes conversion ratios for the efficiency curves, which in turn assist in the selection of the right motor for the pump.
Note that the chart above is for fluid carrying sludge only and represents the Schurco series of pumps. For the correlation coefficients for other brand pumps and for other fluids please contact your supplier.
There are a variety of volumetric pumps that can be used in sludge transport such as pneumatic diaphragm pumps, peristaltic pumps, lobe pumps, and screw pumps. Evaluating all these pumps in detail at the same level as the above mentioned centrifugal pumps will be an important undertaking and is beyond the scope of this article. Instead, I will briefly summarize the volume pumps based on my experience and highlight specific points that need to be awared of in the sludge transport.
Positive Displacement (Positive Displacement) pumps are often predominant in handling high viscosity, dense liquid transport. Volumetric pumps are designed and equipped with better components to overcome fluid flow resistance. Volumetric pumps typically run at lower speeds than centrifugal pumps and therefore act more lightly on solid particles. However, some volumetric pumps can generate acceleration losses, which should be carefully calculated.