Why Screens Fail
Reprinted from Powder & Bulk Solids
Lawrence H. Stone, President, Kason Corporation
Screens fail or break because of fatigue, shock, corrosion or abrasion. An examination of why these problems arise will help reduce screen breakage and maintenance costs.
The sign of breakage caused by fatigue is rounded wire ends with a dark color. Fatigue occurs for a variety of reasons, all associated with rapid repeat flexing. It will happen when there is not enough tension. One of the telltale symptoms of this problem is a floppy screen. Screens must be taut before long service life can be expected. Lack of adequate tautness may result from a cocked center post or improper clamping procedures.
Experience shows that there is only one correct way to install screens. First you tighten the screen ring clamps, then draw tension on the center post. This is especially important for fine wire screens. The reason for this procedure is to ensure that tension is equal along all radians or lines from the center to the screen edge.
The clamp rings must remain tight too, since a loose clamp ring can cause screen rotation that will lead to fatigue. This happens because the screen loses tension, leading to flexing during vibration which, in turn, leads to fatigue with resultant breakage.
In applications where Kleen-Screen Rings (open cylinders that move underneath a screen to minimize blinding), are used, jamming may occur. When it does, the screen vibrates over the ring until the flexing leads to breakage. The sign of this problem is a circular imprint on the underside of the screen.
A second device for preventing blinding of resinous or fatty materials is the Kleen Sweep rubber wiper that sweeps across the top of the screen. It helps maximize throughput when installed correctly. However, if the wiper starts to bounce, it can create a flexing action that leads to fatigue and breakage.
Another cause of fatigue is constant pounding by heavy accumulations of material moving around the screen periphery. This accumulation is sometimes referred to as rope.
Approximately 90% of breakage is caused by fatigue. This has been determined by visual inspection of broken screens over several decades.
The signs that indicate breakage caused by shock are rounded wire ends and a distorted, rather than smooth, surface.
Shock with resultant screen breakage occurs when substantial impact forces the wires to separate. Typically, shock follows when heavy items fall on the screen. These may be extraneous materials that enter with the feed or handtools that have slipped away during installation. People have even been known to accidentally step on screens, causing them to break.
Signs of breakage due to corrosion include wire ends showing a reduced diameter and discoloration. Breakage from corrosion can occur anywhere but it is most likely to happen near the edges in applications where material builds up near the screen edge. The best way to avoid the problem is to use the appropriate screen material for the material being screened.
Wires near tears caused by abrasion appear polished and shiny, and will have flat ends.
Abrasion results when materials being screened are abrasive and if the feed velocity is too high or if too much material accumulates at the center or at the periphery of the screen. Abrasion is especially severe when materials being screened are moist or wet. This mushy material will not fluidize the way dry material will.
One solution for rope is installation of an Internal Kascade Deck/Auxiliary High Capacity Discharge, which prevents rope from forming. But before taking that step, check to see if the eccentric weight setting can be adjusted to move material on the screening deck more efficiently. A plough at the spout entry also sometimes helps by preventing recycling of the rope. Occasionally, a screen may break if abrasive material becomes embedded on the top edge of a Kleen-Screen Ring. Periodic inspection of the screen ring helps minimize this problem.
Attention to basics such as proper screen installation, regulating the throughput, and knowing the problems that are likely to arise help prevent screen breakage. But if a screen does break in your facility, determine the cause to avoid repeated failure.
Actual screen break caused by abrasion due to accumulated material at the screen edge.
This large breaks could have been caused by sudden impact or improper tensioning when the screen was installed.