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Mechanical Conveying Q&A

  •   I have a 16-in. diam by 18-ft long transfer screw conveyor in my plant conveying conditioned flyash from a pugmill. Water is sprayed on the flyash in the pugmill to reduce dust. The pugmill mixes the flyash and water to create a homogeneous mixture. The pugmill then discharges the mixture to a 16-in. diameter transfer screw conveyor. The transfer screw conveyor has a one-piece screw that is mounted on 5-in. schedule 80 pipe. The screw is very heavy-duty but we have continuous problems with it because the center pipe fails almost exactly at mid-span of the screw. The downtime is hurting my productivity. Why is this screw failing and how can I prevent it from happening again?

    Answered November 25th, 2014 by Expert: Bill Mecke, P.E.

    Excellent question! Your problem is not uncommon and there are solutions to prevent this problem from reoccurring. Most bulk materials are easily conveyed using screw conveyors. However, some bulk materials have a tendency to pack under pressure while being conveyed.  As a screw conveyor rotates, bulk materials are moved forward with each revolution of the screw.  There is a gap between the outside diameter of the screw and the inside diameter of the trough or housing. Bulk materials fill this gap and form a layer between the screw and housing. As the screw rotates, the screw flight shears through the bulk materials in the gap. This shearing action puts pressure on the bulk material. Bulk materials that have a tendency to pack will form a hard layer in the bottom of the trough. Most of the time the layer will break loose in chunks and be conveyed downstream. A new layer will form and then break loose again.

    In very rare applications a bulk material will form a permanent layer in the bottom of the trough. This layer will build up over time and cause the screw to deflect in the center. The screw is pushed up in the center during each rotation because each end of the screw is held fixed by the drive unit and end bearings. This condition puts enormous cyclical forces on the center pipe of the screw.Every revolution of the screw causes a complete reversal of the forces on the center pipe. The center pipe of the screw will fatigue and fail typically in the center. It’s very similar to taking a piece of wire and bending it back and forth until it breaks. In your application you are adding water to the flyash which is making the problem worse. The flyash/water combination is sluggish to convey and sticks to the bottom of the trough because it builds up and hardens like concrete. The outside diameter of the screw flights will become shiny as they wear against the layer of material in the trough. This is a key indicator that you have buildup. In some extremely abrasive applications the outside diameter of the flights will actually wear down, causing excessive horsepower draw and screw failure.

    Screw Conveyor Fatigue Failure

    Screw Conveyor Fatigue Failure

    There are multiple solutions to your problem. A liner can be added to the trough to reduce the gap between the outside diameter of the screw and the inside diameter of the trough. A smaller gap will cause the build up to break loose easier. We would recommend a liner made of abrasion-resistant material such as AR-235 for your application. AR-235 is an abrasion-resistant metal with an average hardness of 235-BHN. A second solution would be to add weld-on hardsurfacing to the outside diameter of the screw. The hardsurfacing will increase the outside diameter of the screw, therefore minimizing the gap between the screw and trough. The hardsurfacing is also very hard and rough and will act as a cutting edge to cut and break up the layer of material in the trough. A third solution would be to increase the stiffness of the screw by mounting the screw on larger pipe with heavier wall thickness. A 6 or even 8-in. pipe with wall thickness greater than ½-inch would create a very stiff screw assembly. The stiffer screw assembly will resist the buildup of product in the trough. Usually, a combination of any of the proposed solutions is required to permanently eliminate the screw failures.