Operating Slurry Pipelines with a Deposit

Slurry pipelines are conventionally designed to operate without a deposit on the pipeline invert – i.e. the minimum operating velocity is greater than the deposition velocity. Largely this approach has been adopted due to concerns regarding potential blockage of a pipeline operating with a deposit.

A typical slurry pipeline design duty envelope is shown in the figure below. If the pipeline is designed to operate just at the deposit velocity for the lowest duty flow rate (4,480 m³/h), the operating velocity at the highest duty flow rate (6,590 m³/h) will be about 1.5 times the deposition velocity. Considering that the pipeline wear and energy requirements are roughly proportional to the square of operating velocity, this results in an operationally inefficient system when the pipeline is operated close the maximum design flow rate.

Typical Slurry Pipeline Operation Envelope

An alternate approach is to ensure the pipeline operates without a deposit for the nominal duty and accept that a deposit will form when operating at lower flow rates. This results in the selection of a larger diameter pipeline, but there are significant benefits in terms of reduced pump energy requirements and lower pipeline wear. The following table shows that the energy requirements are reduced by about 35% compared with the conventionally designed pipeline.

The design of slurry pipelines to deliberately operate with a deposit requires that several measures are considered to mitigate the risk of possible pipeline blockage:

• The designer needs to be able to accurately model the behaviour of the slurry pipeline operating with a deposit.
• Steep slopes (>15%) should be avoided when selecting the pipeline route. In cases where steep slopes are unavoidable, smaller diameter pipe should be considered in these sections to avoid the formation of a deposit.
• The design should limit the amount of material deposited onto the pipe invert. In establishing this limit, we suggest the designer consider the flow conditions if there is a process change which results in rapid remobilization (re-suspension) of the deposited material.
• Reliable operation requires that the system has good flow rate control (i.e. accurate flow rate measurement and variable speed pump drives).
• Ideally the pipeline should be equipped with instrumentation to detect the height of deposited material on the pipe invert.

There are several pipelines operating reliably with a deposit which achieve both reduced energy requirements and improved pipe wear performance. This new design approach, which is well suited to tailings pipelines due to their typically wide duty range, can reduce capital and operating costs of slurry pipeline systems.