The stainless-steel finish of a headbox gives it a seemingly indestructible appearance, yet the slightest nick or scratch is enough to cause problems. Imperfections in a headbox's electropolished finish can interfere with the flow of fibers through its chambers, resulting in sheet uniformity problems and a poor end product.
Although there are different types of headboxes used to produce a variety of paper grades, there are a number of problems common to them all which are identified as part of headbox troubleshooting.
Corrosion is the single most destructive obstacle to optimal headbox performance. Once corrosion-resistant oxides protecting the stainless-steel surface are scratched or nicked, corrosion may begin and spread, and can ultimately diminish headbox effectiveness and sheet quality. In the closed loop environment of a mill, the residual chemicals circulating through the water can rapidly accelerate the corrosion process.
Headbox corrosion poses a number of problems that have a critical effect on sheet uniformity.
Fiber is extremely sensitive to rough surfaces. Corrosion on the inside surfaces of the headbox and stock piping create pitting and rough edges that snag fibers. Fiber hang-up often results in holes in the sheet that can cause a break, resulting in a loss of production.
Another result of fiber hang-up is nonuniform sheet profiles. The buildup of fibers, which have a natural tendency to flock, will form strings or clusters that affect the flow uniformity out of the slice, resulting in thickness variations across the sheet. In addition, these flocculations can occasionally release as lumps and be picked out in press and dryer nips to form holes in the sheet, resulting in a poor-quality product and lower machine efficiency.
Older headboxes, which are often made of mild steel with stainless steel cladding, are more prone to corrosion than their newer, solid stainless counterparts. The thin cladding corrodes, allowing moisture and chemicals to attack the mild steel underlay, resulting in cladding failure.
Cladding leaks lead to corrosion of the mild steel underlay, resulting in displacement or blistering of cladding itself due to "corrosion jacking." Such blisters or even bursts result in a loss of flatness and a disruption of stock flow, affecting the uniformity of sheet profiles.
The key to preventing headbox corrosion is to maintain a clean, smooth, scratch-free surface inside the headbox and piping, allowing corrosion-resistant oxides to protect the stainless-steel surface.
Not only does the headbox play a crucial role in sheet profiles, but it also determines paper characteristics. Control of fiber orientation depends heavily on proper headbox operation. Uncontrolled fiber alignment can result in undesirable sheet characteristics.
Fiber distribution and formation is achieved through microturbulences in the headbox. Prior to fiber discharge from the headbox onto the wire, turbulence disperses fibers to optimum orientation to form a uniform sheet with desired quality. If too much turbulence is applied, stock disturbances will discharge onto the wire, producing a sheet with an uneven profile. Correcting formation problems may require changing the length or thickness of the sheets to create the appropriate amount of turbulence for a flat jet delivery of stock.
To help troubleshoot a fiber orientation problem, mills can send sheet samples to a lab for analysis.
Machine limitations can also result in sheet profile and formation problems. Mills may have increased machine speeds beyond the design parameters of the headbox. If this is the case, structural members will typically deform, causing nonuniform profiles at higher speeds. Furthermore, the OEM designed stock flows through the headbox at specific rates and with certain fiber characteristics. If exceeded, the results may be undesirable. Operating and maintenance personnel should thoroughly inspect headboxes prior to changing operating parameters to avoid problems associated with machine design.
As a result of troubleshooting sheet quality and uniformity problems, the headbox may require design modifications. Mills may need to replace rectifier rolls on older headboxes with different open area rolls to accommodate the increase in stock flow.
"We've gone through our checklist and everything points to the headbox as a major source of the problem," the mill maintenance operator told the supervisor. The maintenance supervisor replied, "Well, let's bring in the OEM supplier to do a thorough inspection and see what they come up with."
The problem facing the specialty paper mill located in the Midwestern U.S. was a persistent one. The subject mill, which produced specialty papers, was experiencing holes in its sheet and less than desirable sheet formation. Runnability was also a concern with wet-end breaks increasing daily.
Because the mill had a good working relationship with the OEM supplier, it decided to call in their service personnel to perform a thorough troubleshooting analysis of the rectifier roll headbox.
An internal inspection of the headbox revealed the following problems:
Fiber hang-up was occurring at the holes in the floor, on the rough surface of the slice rectifier roll, and on the vertical wall of the slice body. Service personnel believed that this was a contributing source to the holes in the sheet. As for the poor fiber formation, the supplier noticed that the deterioration of the mild steel support beneath the cladding caused the floor to rise in areas beneath the slice rectifier roll. The inconsistent gap between these components was causing cross flows in stock delivery and adversely affecting formation.
To solve these headbox problems, the OEM supplier cut out the corroded areas in the floor cladding and recladded it with stainless steel 316L material. Moreover, the supplier reinforced the mild steel support to return the floor to a flat condition.
The supplier belt polished the slice rectifier roll and reset it parallel to the repaired floor. Then, due to the extent of the corrosion, the OEM manufacturer cladded the corroded interior vertical wall of the slice body surface, which was a solid plate, with stainless steel 316L material as opposed to a spot repair.
As a result of troubleshooting and repairing the headbox, the mill found that the holes in the sheet were completely eliminated. The mill also reported a substantial improvement in formation along with a significant reduction in wet-end breaks.
For more information on headbox troubleshooting and maintenance, contact your Valmet representative.