PE Slurry Flow

PE Slurry Flow

General Design Considerations

The abrasion resistance characteristics and flexibility of Vinidex PE pipe make it ideal for slurry flow line applications, such as mine tailings, and such installations are in widespread use throughout Australia.

The transportation of Non-Newtonian fluids such as liquids or liquid/liquid, liquid/solid mixtures or slurries is a highly complex process and requires a detailed knowledge of the specific fluid before flow rate calculations can be performed.

As distinct from water, many fluids regarded as slurries have properties which are either time or shear rate dependent or a combination of both characteristics. Hence it is essential for the properties of the specific fluid to be established under the operating conditions being considered for each design installation. In addition to water flow, slurry flow design needs to take into account the potential for abrasion of the pipe walls, especially at changes of direction or zones of turbulence.

The most usual applications of Vinidex PE pipes involve liquid/solid mixtures and these must first be categorised according to flow type:

• Homogeneous Suspensions
• Heterogeneous Suspensions

Homogeneous Suspensions

Homogeneous suspensions are those showing no appreciable density gradient across the cross section of the pipe. These slurries consist of material particles uniformly suspended in the transport fluid. Generally, the particle size can be used to determine the flow type and suspensions with particle sizes up to 20 microns can be regarded as homogeneous across the range of flow velocities experienced.

Heterogeneous Suspensions

Heterogeneous suspensions are those showing appreciable density gradients across the cross section of the pipe and are those containing large particles within the fluid.

Suspensions containing particle sizes of 40 microns and above may be regarded as heterogeneous.

In addition to the fluid characterisations for both types, the tendency for solids to settle out of the flow means that a minimum flow velocity must be maintained.

This velocity, the Minimum Transport Velocity, is defined as the velocity at which particles are just starting to appear on the bottom of the pipe.

The flow in short length pipelines differs in that these lines may be flushed out with water before shutdown of operations. Long length pipelines cannot be flushed out in the same way and the selection of operating velocities and pipe diameter needs to address this aspect.

The design of slurry pipelines is an iterative process requiring design assumptions to be made initially, and then repeatedly being checked and tested for suitability. The specific fluid under consideration requires full scale flow testing to be conducted to establish the accurate flow properties for the liquid/ particle combinations to be used in the installed pipeline.

Without this specific data, the assumptions made as to the fluid flow behaviour may result in the operational pipeline being at a variance to the assumed behaviour. The principles of slurry pipeline design as outlined in the methods of Durand, Wasp, and Govier and Aziz are recommended in the selection of Vinidex PE pipes for these applications.

Note: The published Vinidex PE pipe flowcharts relate ONLY to water or other liquids which behave as Newtonian fluids. They are not suitable for calculating the flow discharges of other fluids, including slurries. For further information on slurry pipeline design, the designer is referred to such publications as Govier G.W. and Aziz K, The Flow of Complex Mixtures in Pipes. Rheinhold, 1972. and Wasp E.J. Solid Liquid Flow – Slurry Pipeline Transportation. Trans Tech Publications. 1977.

Pipe Wear

Polyethylene pipe has been a proven performer over many decades in resisting internal abrasion due to slurry. It is particularly resistant to abrasion from particles less than 500 microns in size depending on particle shape.

The abrasive wear of any slurry handling system is heavily dependent on the physical characteristics of the solids being transported. These characteristics include angularity, degree of particle attrition, angle of attack, velocity, and the concentration of solids in the transporting fluid.

With metal pipes, corrosive wear interacts synergistically with abrasive wear, producing rates of wear that can be many times greater than a simple combination of the two modes of wear. Corrosive attack on a piping material can lead to increasing roughness of the surface, loss of pressure and localised eddying, and hence increase the abrasive attack.

Factors Affecting Rates of Wear

The wall of polyethylene pipes are worn by contact with the solids particles. The principal causes of wear are as follows:

• Particle Size
• Particle Specific Gravity
• Velocity
• Angle of Attack

Particle Size

The size of the particle combined with the requisite velocity is one of the principal factors which contribute to wear. The rate of wear increases with particle size with very little wear occurring on polyethylene systems below 300 microns. Above this size the rate of wear will increase proportionally with particle size with the maximum practical D ₅₀ size around 1mm.

Many researchers have attempted to develop relationships between particle size and rates of wear, however, these have not proven to be accurate due to the wide variation of slurry characteristics. The wear mechanism involved is not thoroughly understood, however, it is believed the higher impact energy resulting from a combination of particle mass and the high velocity required to transport this larger particle are the principal contributing factors.

Particle Specific Gravity

Similarly, the specific gravity will increase the mass of the particle resulting in increased wear. This is a result of the increased impact energy from the mass of the particle combined with the faster carrier velocity.

Velocity

A minimum velocity is required to provide the necessary uplift forces to keep a solid particle in suspension. This velocity also increases the impact energy of the particle against the wall of the pipe.

Angle of Attack

There are essentially two modes of wear, impingement and cutting. Cutting wear is considered to be caused by the low angle impingement of particles. In practice, cutting wear comprises a cutting action, and the accommodation of some of the energy of impact within the matrix of the material being worn. Hence, cutting wear also incorporates a component of deformation wear. The requirement for wear is that some of the solid particles must have sufficient energy to penetrate and shear a material, perhaps gouging fragments loose. As a result, a low modulus material such as polyethylene has very good resistance to cutting wear due to the resulting deformation upon impact. In the case of angular particles the cutting action is increased resulting in increased pipe wear.

The simple theory of abrasive wear suggests that specific wear (wear per unit mass transported) is proportional to normal force at the pipe wall. Therefore, the wear rate will increase as the angle of attack to the pipe wall increases. The increase in angle will also increase the amount of energy with which the particle strikes the pipe wall. It is for this reason that accelerated wear is caused by:

1. Fittings which effect a change in the angle of flow such as tees and bends.
2. Butt weld joints. Butt weld internal beads will cause eddying which will result in increases in angle of attack of the particle to the pipe wall. As a result accelerated wear generally occurs immediately downstream of the bead. This is usually prominent in D ₅₀ particle sizes over 300 microns. For coarse particle slurries the internal bead should be removed.
3. Fittings joints. At connections of mechanical fittings some misalignment of the mating faces may occur resulting in increased angles of attack of the particles.
4. Change in velocity. Some compression fittings cause a reduction in the internal diameter of the pipe under the fitting resulting in turbulence. A mismatching valve bore will also cause turbulence. It is for this reason that the use of clear bore valves such as knife gate valves is preferred for slurry pipelines.
5. Increased velocity. High velocities are required to create sufficient turbulence for the suspension of heavy particles. This turbulence increases the angle of attack to the pipe wall, resulting in increased wear for large particles.
6. Insufficient velocity. When a system is operated near its settling velocity, the heavier particles migrate towards the lower half of the pipe cross section. This will cause a general increase in pipe wear in this area. If saltation/moving bed occurs, then the heavy particles will impact against the pipe bottom, causing an accelerated wave profile wear. Should deposition occur on the floor of the pipe, then the particles above this deposition will cause the maximum amount of wear as they interact with the flow. This is characterised by the formation of wave marks on the 5 and 7 o’clock position of the pipe.

Maintenance and Operation

To reduce the cost of wear on a pipeline asset it is general practice to rotate the pipes at the appropriate intervals, this is particularly important when transporting sand slurries. In this respect mechanical joints are useful, although re-welding of pipes over 500mm has been preferred in some cases to reduce capital costs. These mechanical joints are usually installed at every 20m pipe length to assist the pipe rotation process and also permit clearance of blockages.

Slurry pipelines are usually operated as close to the critical settling velocity as practical to reduce operating costs. Unfortunately, if an increase in particle size occurs, then saltation will commence increasing friction loss eventually resulting in a blockage. Other factors that cause blockages are increases in solids concentration, loss of pump pressure due to power failure, or pump impellor wear.

Polyethylene pipelines may be cleared of blockages by clear water pumping provided they have been installed on flat even ground. Sudden vertical ‘V’ bends with angles over 10°may cause an accumulation of solids in the bore, preventing clearing by clear water pumping. If vertical bends are unavoidable then they should be installed with mechanical joints to permit their easy removal for clearing.

Fittings

A range of mechanical joints are available for polyethylene slurry pipelines. They include stub flanges and backing rings, compression couplings and rubber ring joint fittings.

References

1. The Transportation of Flyash and Bottom Ash in Slurry Form, C G Verkerk
2. Relative Wear Rate Determinations for Slurry Pipelines, C A Shook, D B Haas, W H W Husband and M Small
3. Warman Slurry Pumping Handbook, Warman International Ltd.