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Reducing elbow wear in pneumatic conveying
Q: My pneumatic conveying system, which transports an abrasive mineral, suffers from extreme bend wear problems. We're never done welding up the elbows—some fail every week or two—and the worst always seem to be the ones at line's end and the silo's top, which present a safety hazard to access. Would it help us to change to a longer bend radius?
Mike Bradley, of
The Wolfson Centre for Bulk Solids Handling Technology, says: Anywhere that you have hard, angular particles, there's potential for them to cause bend wear due to the particles' impact on the bend’s inner radius. It's tempting to think that a longer bend radius will allow the particles to follow the flow more easily and will reduce collisions with the wall. However, this is a fallacy—even in a very long radius bend, the particles still travel in virtually a straight line from the inlet to the impact point. A longer radius bend will give some extension to wear life because the impact zone is longer in respect to the arc’s length. Tests in our laboratory have shown roughly a doubling in bend life when the ratio of radius to diameter increases from 6 to 12. However, if you have a serious wear problem, you need much more than a doubling of bend life!
A key to one of the most important matters lies in your observation that the bends at the line's end wear out much faster. People think it's Murphy's Law that the most difficult bends to replace—those on top of the silo—wear out most often, but actually it's not Murphy's—it's Boyle's law! Boyle's law states that when a gas's pressure falls, it expands. System pressure drops along a pneumatic pipeline, which causes the gas to expand and therefore the gas velocity to increase toward the pipe's end. As gas velocity increases, so does the particle velocity, and research has shown that erosion rises dramatically with velocity:
Erosion= k • velocityn
where 2.2 < n < 2.7 depending on the material.
This has a drastic effect; doubling air velocity means erosion goes up between 5 and 7 times. This is why the bends at the system's end wear through much more often! In a system working at 15 psi, the air velocity at the line's end is double that at the inlet. The higher the system pressure, the stronger this effect.
Conversely, small reductions in air velocity give a big reduction in wear: A 25 percent reduction in air velocity will give a doubling of wear life, and a 50 percent reduction would return a wear life 5 to 7 times longer.
There are several ways to reduce air velocity. One is to make sure that the airflow is only just above the minimum value required for successful conveying, avoiding excess air flow. Another is to increase the pipeline's bore along its length to stop the air velocity from becoming excessive. If it's a blow tank system, avoid the "blowdown" along the pipeline at the conveying cycle's end by venting the tank, because this is when most of the wear will occur.
Reducing air velocity will have other benefits—reduced energy consumption, greater transport capacity, less particle damage, and less dust generation—so it's a no brainer. However, it has to be done with care, keeping in mind the material’s minimum conveying air velocity, to avoid blockages. Off-line material characterization in a test center helps in this regard.
Wear-resistant linings can also be very useful to extend bend life, and in many instances can deliver great economic benefit. The key is to understand different linings' wear life extension so that a sensible economic choice can be made. This potential can be measured with an erosion tester.
Certain bend geometries such as blind tees and specialty elbows such as the Vortice-Ell are also more wear-resistant and can be a useful choice, but most of these increase pressure drop and energy consumption and can seriously compromise the transport rate. Thus, they should be applied with care, especially if you have more than two bends in the pipeline. Before choosing these, conduct a quick test on an industrial-scale test system to know the effects in advance.
Mike Bradley is the director of The Wolfson Centre for Bulk Solids Handling Technology at the University of Greenwich.
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