Slurry Pipeline Changes: What Effect Can It Have On Your Operation?Learn what happens when changes are made to a slurry pipeline in the middle of an operation. Find out how these changes affect the pump and overall performance of the operation.
Slurry Pipeline Description:
Pipe abrasion and the associated erosion loss is a major concern in any slurry pipeline system. Efforts are constantly being made to improve pipeline structure and materials for a variety of industries.
Slurry pipelines can be made of many different materials such as carbon steel, alloy steel, hardened steel, stainless steel, abrasion resistant lined pipes, and non-ferrous pipes, HDPE etc. The material of the pipeline is generally selected based on the application and cost.
The use of non-ferrous slurry pipelines is increasing day to day around the world.
High-density polyethylene (HDPE) is extensively used for applications such as mine tailings due to its ultrahigh molecular weight. It has a much better life expectancy than unlined carbon steel pipes, especially when dealing with abrasive or corrosive slurries.
Polyurethane is generally used for in-plant slurry piping. It is flexible and resistant to heat and corrosion.
Polybutylene pipe is a flexible thermoplastic pipe and having the higher tensile strength and very good heat resistance.
Besides the above, other low wear resistance non-ferrous pipes for slurry are PVC (Polyvinyl chloride), PP (Polypropylene), ABS (Acrylonitrile-butadiene-styrene), fiberglass pipe with internal ceramic chips, etc.
For Internally Lined Steel Pipelines, non-ferrous materials are generally used as a liner inside the steel pipe to safeguard against erosion and corrosion.
For Situ Lining of Steel Pipelines, a plastic pipe of slightly smaller diameter inserted into the steel pipe, and then the annular space filled with cement. Another way is to take a slightly larger diameter high-density polyethylene pipe and compress it to reduce the outer diameter. This compressed pipe is then carefully pulled inside the steel pipe; when releasing the constricting force, the inner pipe firmly presses against the inside of the steel pipe.
Guidelines for an Effective Slurry Pipeline
- The slopes of the horizontal line should not exceed the angle of repose of slurry.
- Maintenance routines for flushing and draining of pipelines along with manual cleaning must be kept.
- Prone wear points must be identified.
- Use long radius bends.
- Use a valve with maximum port size.
- Use full port ball valves.
- Avoid use of globe valve (seat may be plugged by solid deposition).
- Provide flushing connection of valves.
- Steel pipes lined with abrasion resistant lining can significantly increase the life of pipes.
- To extend the service life of a pipe the lining methods can be used in an operating pipeline also.
- Generally, the bottom of slurry pipes wear out the quickest because it is most in contact with the abrasive slurry. This can be somewhat mitigated if the pipes have the ability to be rotated periodically and then replaced fully after 3/4 rotations.
What Happens When a Pipeline Diameter is Changed?
When talking about pipeline diameter, mainly it is referring to the internal diameter of the pipe through which the fluid will flow. If a discharge pipe is 11 inches in total diameter, but the inside diameter is 10 inches, this would be referred to as a 10-inch pipe.
To move a certain amount of fluid through a pipeline, energy is required. A portion of that energy is lost due to the friction between the fluid and the inner wall of the pipe. This loss is called head loss or pressure drop.
The velocity of fluid refers to the speed of a fluid flowing through a pipeline.
Now, let’s assume a slurry pump and pipeline has been set up for a certain flow of a slurry. If we somewhere down the line decrease the internal diameter of the pipe, the pipe will constrict, giving the slurry less space to flow. This results in the slurry coming into even more contact with the pipe’s inner wall. This will result in more friction, leading to a higher pressure drop or energy loss of pump. Hence, the overall operation cost will be higher as the energy needed for the pump will need to be increased.
Moreover, due to less space available in the pipe, the velocity of the slurry will be higher than the original setup was with the larger pipe diameter. If the velocity of the slurry is higher, the degree of erosion in the pipeline will also be higher.
Contrary, for the above case; if we increase the internal diameter of the pipe, the slurry will be allowed more space to flow, meaning less friction between the slurry and the pipe inner wall. This will result in a lower pressure drop, requiring more energy to meet the original flow rate. Additionally, with a wider pipeline, the velocity of the slurry has a higher chance of being below the critical line velocity. This means the solid particles of the slurry and its fluid will move separately, potentially causing a settling or deposition of the solids in the pipeline which will lead to pipeline clogging and other maintenance problems.
When Considering Pipeline Diameter Changes
The speed at which to run a pump will need to be calculated based on several factors including the type of material, distance from the material, pipeline diameter, pipeline distance, etc. By performing these calculations, you are determining the exact speed the material needs to move through the pipeline to make the most economic sense.
Optimum pressure drop also needs to be considered; meaning which pressure will be optimal, as far as energy consumption goes, to move the slurry from one place to another.
When selecting the size of the pipeline in a slurry system, ensure that the technical calculations have been performed in order to save on not only the present costs but the future costs of the system as well.
As the slurry is a mixture of liquid & solids, it requires a bit more space to flow inside the pipe over just liquid. Otherwise, the slurry can be chocked or become clogged in the pipeline. Due to this, a slightly larger pipeline size should be considered.
In the selection of outer diameter & thickness of the pipe the erosion loss, inner lining of the pipe etc. must be considered.
Effect of Changing Pipe Length
The energy loss, or pressure drop, will also depend on the length of the pipeline. If the total length of a pipeline is increased, the inner surface area of the pipeline will also increase.
If the inner surface area of a pipeline increases, the total friction between the slurry particles and the inner surface of the pipe will also increase. When a slurry is moved through a pipeline, it creates friction with the inner wall of the pipe. This friction causes a reduction of the pressure of the fluid, thus requiring more energy to move the slurry.
If the pipeline is longer than it needs to be, the total amount of friction will be higher. This will result in poor pressure at the end of the pipeline. Consequently, some energy loss of the pump will happen, meaning the need for a higher rated pump, higher total operation cost, higher materials cost etc.
On the other hand, if we decrease the length of a pipeline unnecessarily; the total frictional loss will be less and we will get an undesirable higher pressure at the discharge end of the pipeline.
Tips for Optimal Pipeline Size
- Optimum pressure drop needs to be calculated.
- Adopt the shortest route possible for the pipeline
- The pump must then be selected based on the final pipe length and its associated pressure drop.
Effect of Changing an Existing Pipeline
Sometimes, a pipeline may need to change during an operation. If this happens, the following changes may occur:
- Change in pipe diameter
- Change in pipe length
- Change in pipe elevation
- Change in pipe fittings, valves etc.
- Change in pipe materials
Earlier, the two effects we previously explained i.e. diameter change and the length change will also applicable here. Also, some other ways in which a pipeline can be altered will be explored like the effect of elevation change, changes in fittings & valves, and changes in pipeline material.
If the pipe elevation increases, an additional head or pressure will be required to compensating that.
When a liquid flow changes its direction, there is resistance. Moreover, the liquid will try to flow around the outer edge of the fitting. This reduces the effective area of the fitting. Hence, the velocity of the liquid will increase and the frictional loss or pressure drop will also increase. Therefore, any change in the pipe fittings will affect the pressure drop or frictional loss of the system.
A similar effect will also occur in valves, due to their non-linear and non-uniform passage. As a result, a pressure drop or frictional loss will also occur in case of any change in the valves. Additionally, valves are prone to water hammer which are high-pressure shock waves that are produced when a liquid is suddenly forced to stop in a pipe, either due to valves, or the pump ceasing operation.
Changes made to the pipe materials will change the pipe friction factor. This will also affect the result of pressure drop or frictional loss calculations.
A Case Study:
Suppose, we have an old slurry pipeline system; we need to change the following:
- Pipe diameter due to the requirement of higher flow
- Pipe length due to the relocation of equipment
- Pipe materials due to wear of existing pipe
Now, before we detail what these changes would do to an existing pump and pipeline system, we will review mathematically how fluids interact with different pipe configurations.
1) The fundamental relationship between fluid flow and the pipe diameter is:
Flow = Internal Diameter of Pipe x Velocity of Fluid
2) The fundamental relationship between the pipe friction loss or pressure drop and the pipe diameter and the pipe length is:
Pipe Friction Factor x Length of Pipe x Fluid Velocity 2
Pressure Drop = ————————————————————————————-
Internal Diameter of Pipe x Gravitational Force
3) The fundamental relationship of energy required with regards to the fluid flow rate and pressure is:
Flow Rate x Pressure
Pump Power HP = ————————————
Conversion Factor x Efficiency
Now, if we want to get a higher flow while keeping the same velocity:
- According to the 1st relationship we need to increase the internal diameter of the pipe.
- If the pipe diameter is increased, the pressure drop or pipe friction loss will be less, resulting in increased flow.
- However, if the flow is increased, the power to the pump will need to be increased to match the previous velocity of the smaller diameter pipeline.
Now, if we need to increase the pipe length while keeping other parameters the same:
- According to the 2nd relationship, the pressure drop or pipe friction loss will increase.
- If the pressure drop increases, the pump power will need to increase to account for the increased friction loss.
Now, if we need to change the pipe materials while keeping other parameters the same:
- The pipe friction factor will change, depending on the material. Additionally, the pressure drop or pipe friction loss will also change, if the other parameters remain unchanged.
- If the pressure drop changes, so will the amount of power needed to effectively move the material through the pipeline.
So, in our above case, changing a pipeline mid-operation can be a costly decision, affecting:
• Project cost:
Design engineering, Materials, fabrication/manufacturing, construction/installation, and the commissioning.
• Operation cost:
Energy costs – electricity, fuel etc., Manpower costs – operator, labor etc., and the increased cost of Utilities – water, gases, oil, grease etc.
• Maintenance cost:
Manpower – labor etc., Materials – spare parts, grease, oil etc., and the associated utilities.
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