Premature screen failure can be a frustrating and complicated occurance when dealing with pretentioned screens in a Round or Rectangular Separator. The reasons for premature screen breakage are numerous. Discounting any manufacturing defects or shipping damage (yes, that actually does occur), some of the more common causes for breakage can be attributed to things like poor installation, incorrect amplitude of vibration, material overload, screen blinding, and feed surges just to name a few. Here are a few ways you can deal with some of these issues.
Poor installation is generally a factor of either having the clamp ring on too loosely, neglecting to include or incorrectly installing the jam nuts in the center-tie-down, or tightening down the center-tie-down before putting on the clamp ring. If the clamp ring is not tightened adequately, the frame or frames above the loose clamp ring will tend to rotate with the motion of the motor. And since the center of the screen is stationary, the screen mesh will easily wrinkle or tear, causing premature screen failure. If the jam nuts are not installed properly into or are omitted from the center-tie-down, the tie-down will not effectively hold the screen in place. The center of the screen will bounce with the motion of the motor, stretching the screen mesh, causing undue stress and causing the screen to fail. Finally, if the center-tie-down is tightened down prior to the clamp ring, the clamp ring then being tightened can shift the screen and cause it to stretch in one spot and go slack in another. This will usually cause a fine screen to fail very quickly. The center-tie-down should be tightened down gently as the last step.
Incorrect amplitude of vibration or overloading the screen with material will cause the solids to bounce on the screen, also causing early failure. This problem can be addressed by adjusting the lead angle to lessen the vertical amplitude of the machine, or the installation of another screen deck to reduce solids load may help in this situation. Often, raising the center of a screen using a reverse tie-down will increase solids discharge rate and rid the screen of the pooling effect you can get in the center of the screen where there is little vibration or amplitude to move solids out. Once pooling starts, it creates a vicious cycle since the worse it gets the more it causes the screen to concave in the center, wearing the mesh down in that area and causing premature failure. By raising the screen from below, which creates a convex screen surface, this problem is usually eliminated.
Screen breakages from high vacuum have also been reported. A flexible connection from the table frame discharge spout into a closed pipe can create a "barometric leg" or high vacuum on a screen. This will be particularly severe if the pipe is below the liquid level in the receiving tank. Screens have been sucked out as a result. Cutting a hole in the top of the discharge spout to allow air in will solve this problem.
Premature Screen Failure
Premature Screen Failure
Premature screen failure can be a frustrating and complicated occurance when dealing with pretentioned screens in a Round or Rectangular Separator. The reasons for premature screen breakage are numerous. Discounting any manufacturing defects or shipping damage (yes, that actually does occur), some of the more common causes for breakage can be attributed to things like poor installation, incorrect amplitude of vibration, material overload, screen blinding, and feed surges just to name a few. Here are a few ways you can deal with some of these issues.
Poor installation is generally a factor of either having the clamp ring on too loosely, neglecting to include or incorrectly installing the jam nuts in the center-tie-down, or tightening down the center-tie-down before putting on the clamp ring. If the clamp ring is not tightened adequately, the frame or frames above the loose clamp ring will tend to rotate with the motion of the motor. And since the center of the screen is stationary, the screen mesh will easily wrinkle or tear, causing premature screen failure. If the jam nuts are not installed properly into or are omitted from the center-tie-down, the tie-down will not effectively hold the screen in place. The center of the screen will bounce with the motion of the motor, stretching the screen mesh, causing undue stress and causing the screen to fail. Finally, if the center-tie-down is tightened down prior to the clamp ring, the clamp ring then being tightened can shift the screen and cause it to stretch in one spot and go slack in another. This will usually cause a fine screen to fail very quickly. The center-tie-down should be tightened down gently as the last step.
Incorrect amplitude of vibration or overloading the screen with material will cause the solids to bounce on the screen, also causing early failure. This problem can be addressed by adjusting the lead angle to lessen the vertical amplitude of the machine, or the installation of another screen deck to reduce solids load may help in this situation. Often, raising the center of a screen using a reverse tie-down will increase solids discharge rate and rid the screen of the pooling effect you can get in the center of the screen where there is little vibration or amplitude to move solids out. Once pooling starts, it creates a vicious cycle since the worse it gets the more it causes the screen to concave in the center, wearing the mesh down in that area and causing premature failure. By raising the screen from below, which creates a convex screen surface, this problem is usually eliminated.
Screen breakages from high vacuum have also been reported. A flexible connection from the table frame discharge spout into a closed pipe can create a "barometric leg" or high vacuum on a screen. This will be particularly severe if the pipe is below the liquid level in the receiving tank. Screens have been sucked out as a result. Cutting a hole in the top of the discharge spout to allow air in will solve this problem.
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