A damaged headbox apron lip is a common sight on a papermachine and a frequent cause of paper production quality problems.
Apron lip damage usually comes in the form of nicks, scratches or dents caused by the accidental dropping of tools, machine parts carelessly dragged across the surface, or by rotating members with foreign material. These minor abrasions can trap fibers from the pulp that flows over them or cause distortions in the flow of the headbox jet, which, in turn, jeopardizes sheet formation. Fortunately, in most cases, a qualified technician can repair this damage.
In situations where the gouges or scratches are a little deeper, a weld repair is usually necessary. A specialized welding technician with experience in headbox repair is required in these cases.
Under normal circumstances, and with preparation of the work area, an apron lip repair can take as little as six hours to perform if it involves simple honing or blending of damaged areas. However, if the proper steps are not taken by the mill prior to service, the same six-hour job can escalate to a 12-hour ordeal many thousands of dollars an hour in downtime and hourly repair charges and can create dangerous working conditions for the technician.
In the past, apron lip damage was determined using optical tools and straight edges. However, with advances in technology and expertise, headbox technicians can now make a topographical map of the apron surface to accurately determine the areas that require repair. Whereas the old method of using optics only considered deviations near the apron tip, today's technology allows the technician to find and repair problems originating well behind the tip of the apron by using highly accurate LVDTs and computer programs.
Common apron lip repairs include flattening and polishing the flow surface, sharpening the edge on the apron lip, smoothing over nicks and cuts, filling pinholes and gouges with weld, and blending the repairs to ensure a perfectly smooth operating surface.
In most cases, a pre-service visit by the technician to observe the machine in operation and "map" the apron will help to detect the areas that are damaged and require repair. Typically, the extent of damage can be established, which, in turn, can be used to determine the manpower and materials needed, and the time frame for the repair.
Before service work is performed, a few hours of preparatory work can greatly reduce apron lip repair time. The steps to prepare for an apron lip repair are the same standard procedures for replacing the apron lip. The aim is to remove obstacles for ease of maintenance in the apron lip area.
Preparations the mill should make to improve accessibility during an apron lip repair on a fourdrinier headbox include:
Preparations the mill should make for an apron repair on a gap former include:
Proper preparation for apron lip repairs makes a long and difficult process faster and easier to complete. By investing a couple of hours, mills can shorten service time, reduce downtime and lessen the risk of hazardous working conditions for the technician. For more information on apron lip repair, contact your Valmet representative.