While working in the Kalgoorlie gold fields in the 1930’s a young Australian engineer named Charles Warman invented and patented a more effective and reliable centrifugal slurry pump.
By the 1970’s the company he built became a global market leader. Today pump manufacturers around the world replicate the design principals of the Warman slurry pump.
The beauty of the modern slurry pump is in its robustness and simplicity. The slurry pump often operates in remote and harsh environments, so it must be reliable and easy to maintain.
The worldwide market for the slurry pump alone was US$1140m in 2017. However, the total capital and operational spend is significantly more than this.
Engineers who select and maintain slurry systems must understand the basic principles of slurry handling. The consequences of slurry handling problems can include long down times, and leakage in sensitive areas.
This article identifies the key considerations every engineer needs to know, from pipe friction losses and settling of solids, to the wear and performance of pumping systems.
What Is A Slurry?
A ‘slurry’, in the context of a slurry pump, is a liquid containing solid particles. It can look like a thick red mud, or it can be a thin liquid with large lumps - which in the case of gravel can be up to 10-20 mm in size.
Centrifugal pumps are widely used to transport slurry, usually over relatively short distances, but sometimes in long pipelines which run for hundreds of kilometres.
Slurry transport is common within the mining industry as part of minerals processing or coal washing. The solids can include anything from raw or final product to fine tailings. Other applications for slurry handling include power generation, water and sewage, sand and gravel, and even agriculture.
To simplify classification, slurries for transportation can be one of two things:
Homogeneous, also known as non-settling slurries, often appearing to have a wet cement consistency. An example being Red Mud. The solid particles are small and will not easily settle out, though in reality the concentration of solids at the bottom of a pipe is usually higher than at the top.
A widely accepted approach for a non-settling slurry design is to model the fluid as a non-Newtonian (also known as a Bingham Plastic) fluid. If such a fluid were released on the ground, it would not flow like water, but would slump slightly (i.e. spreading a little). The ‘less’ the fluid slumps, the higher its values of Shear Stress and Shear Rate, and the more difficult it is to pump.
At some point a centrifugal pump will not be able to handle a homogenous slurry. A typical example is coal tailings delivered by a deep cone thickener. In this case the operator can use a positive displacement pump which are comparatively expensive to buy and maintain, or a co-disposal approach such as adding the tailings to coarse reject on a conveyor belt, where it will be disposed of in pit or underground before rehabilitation.
Settling slurries with larger (and often denser) solid particles. d50 particle sizes can range from between usually over 0.02 mm up to around 150 mm – one example being gravel.
The ‘effect’ of handling a settling slurry is often quantified using an empirical approach. Theoretical approaches are inconsistent, in part due to their inability to model turbulence. Empirical approaches use a significant amount of data, and depend on factors such as:
- d50 particle size,
- particle specific gravity,
- solids concentration, and
- the ratio of particle size to the pump impeller diameter.
Why Is Slurry Difficult To Handle?
There are five things you need to consider when handling slurry. These are:
1. Pump Wear
The most obvious problem is the erosive wear of pump and pipes though abrasion and corrosion. For some pump materials the pump casing can hole in days, often with environmental and safety consequences, as well as plant down time. The original Warman slurry pump increased wear life by using ‘thicker’ components, and reduced the down time problem by incorporating liners which were easy to change out.
While the modern slurry pump still incorporates these features, a great deal of effort has gone into developing materials that are more wear resistant. These include:
- Rubbers and polyurethanes, which offer advantages for chemical resistance and give reduced wear when handling fine particles.
- Hard metals such as white iron alloys, some of which have patent protections. These materials work well with hard and sharp particles, such as for hard metal minerals processing.
- Ceramics, which are less successful as they are hard to manufacture and expensive. They can also fail by shattering.
In addition, a pump operating between 70% and 110% of the pumps best efficiency flow rate will exhibit less wear than outside this range. This is due to increased turbulence and recirculation.
2. Pump Sealing
An engineer must consider pump sealing. The cheapest and most effective sealing method is by packed gland sealing. However, this requires the addition of clean gland sealing water, a commodity that is not always available. One alternative is the use of centrifugal shaft sealing, which removes the need for gland water.
Another option is the use of mechanical seals, which can be more expensive than the pump itself. Seal failure becomes a costly exercise in terms of down time, part replacement and the need to carry expensive spares. When specifying a mechanical seal it pays to be cautious.
3. Pump Performance
Manufacturers publish performance curves for each of their pumps, usually complying with ISO 2548. Engineers use these curves to select the ‘best’ pump for their application. However, regardless of slurry ‘type’, when compared with water, the slurry will reduce both the pressure generated by the pump and the efficiency of the pump.
These reductions are often summarised as Head Ratio (HR) and Efficiency Ratio (ER), for a given slurry. Again, engineers use empirically based methods to estimate these values, often applying an additional contingency factor.
4. Pipe Friction Losses
When compared to pumping water, pipeline friction losses can increase when handling slurry. For settling slurries, the increase is greatest when pipe flow is laminar rather than turbulent.
For homogenous slurries, the calculation of pipeline friction loss can use computational packages, which assume inputs from a Bingham Plastic model. In this case, engineers may use slurry rheology testing to establish input values.
5. Pipe Blockage
All slurries, but particularly non-settling slurries, are subject to solid particles settling within a pipe. A sliding bed scenario can occur, with solid particles sitting-on and moving along the base of the pipe (not unlike sand shifting on the floor of the ocean), but if too much settling takes place the pipe will block, requiring down time to clear.
The goal is for pipeline velocity to be high enough to reduce the risk of solids settling and pipe blocking. However, as friction loss is proportional to the square of pipeline velocity, too high a pipeline velocity will result in high friction losses and require large pump motors. The designer must be careful to balance these competing criteria.
This article has outlined key considerations when selecting and operating slurry pumps. The engineer must select a pumping option with sufficient power and pressure to transport the slurry. They must also consider pump sealing, wear of components, and pipe blockage.
The engineer must consider upfront capital costs. They must resist the temptation to over design pump and motor size and consider overall lifetime costs.
Effective slurry system design requires expert advice. Engenium has extensive slurry transport experience, and are happy to assist.
Contact one of our local offices across Australia to discuss your slurry system design requirements.
Principal Engineer, NSW – Engenium Pty Ltd.