Which of the following pipes should be avoided in a settling slurry pipeline?
The answer is . . . C!
Surprised? Most people assume vertical pipes to be the most difficult angle for slurry pumps. But inclined pipes of 30° to 45° cause more flow problems than either horizontal or vertical pipes.
Pumping solids through vertical pipes means pushing against the full force of gravity. This may seem like a worst-case scenario, but horizontal pipes present an altogether different challenge. If the flow velocity and resulting turbulence are not high enough, solids settle to the bottom of the pipe and stop moving. To remain in motion in a horizontal pipe, solids must be continuously “lifted up” by fluid. The flow velocity necessary to prevent settling in a horizontal pipe is frequently higher than the velocity required to lift the same solids through a vertical pipe.
In an inclined pipe, both dynamics operate together. Solids must be both lifted against gravity and supported by turbulence to prevent settling — thus requiring higher velocities than either vertical or horizontal pipes. If a bed does form, it can slide backwards down the incline and cause flow instabilities and even pipeline blockages.
To demonstrate this effect, GIW constructed a clear loop demo unit in its Grovetown Hydraulic Test Facility. The unit allows us a view of the action of flow in an inclined pipe, how solids collect along the bottom, and the periodic surges and instabilities which occur if velocity drops too low.
For a visual model of the inclined pipe problem, watch the above video of a clear pipe demonstration. Pipeline size is a significant factor, so the dynamics seen in the video — with small pipe — occur at much higher velocities in industrial sized pipelines.
For inclined pipelines transporting settling slurries, remember the following tips:
- For optimal system stability, avoid inclined sections in your pipeline designs. This is almost always possible for in-plant designs. Vertical and horizontal sections may require longer pipelines, but losses do not usually increase. Solids in slurry have little effect on vertical pipeline friction, and inclined pipe friction is typically higher than in horizontal pipe.
- In cases where inclined pipes cannot be avoided (e.g., a long-distance pipeline running up a steep slope), account for the incline by calculating the correct operating velocities and friction losses.