Suction couch roll operation can do much to boost the overall efficiency of the paper machine press section. In fact, at its optimum efficiency, the suction couch roll can improve sheet dryness by as much as 3%, leading to longer pickup felt life and less wet-end sheet breaks for improved runability of the machine. However, when improperly set up or deprived of maintenance attention, the couch roll can be detrimental to machine operation, resulting in poor machine efficiency and diminished sheet quality. Optimizing couch roll efficiency begins with the proper setup of the suction box and couch components, followed by dedicated maintenance.
The drier, the better. The purpose of the couch roll is to strengthen the sheet as much as possible before it enters the press section. To that end, water is removed from the sheet as it passes over the vacuum area of the couch suction box.
Success in this dewatering process depends largely on the setting of the suction box. If the suction box is positioned too far toward the wet end, rewetting occurs. Vacuum can also be lost if the oncoming box is unwrapped by the wire. If the suction box is set too far toward the dry end, vacuum will definitely be lost since the box is not "sealed" by the wire on the off-going side.
Suction box setup is common to have couch boxes with either one or two vacuum zones. In either case, the suction box setup is basically the same on all machines with wire turning rolls.
It is a relatively simple process to set the suction box in the correct position. With the machine down and the wire off, the leading edge of the dry-end seal strip should be placed at the point where the forming fabric leaves the couch. The leading-edge position can be determined by probing through the couch's holes with a hooked-end welding rod, which cannot be accidentally dropped into the couch shell or box. A string can be used to indicate the wire run from the couch to the wire turning roll.
For maximum sheet dryness, the final adjustment of the suction box must be done with the machine running, a sheet on the wire, and the suction pickup in the operating position (if the machine is so equipped). Rotate the box toward the dry end until the vacuum starts to decrease.
Next, rotate the box toward the wet end until maximum vacuum is reached and the water being thrown off the couch roll is not following the fabric, causing rewetting of the sheet. A separation between the fabric and water throw-off should be evident.
Finally, match-mark the journal to the saddle for angular position. This will ensure that it will always be reset to the correct position.
To maximize couch roll performance, thorough cleaning of the holes with a high-pressure needle shower should be performed on a routine basis. Some couch rolls are also equipped with internal box flushing showers that should be operated as necessary to keep the suction box clean. Also, the internal shell flushing shower should be used whenever the toll is turned without a sheet.
Good maintenance records after each couch overhaul will also help determine the frequency required to ensure good performance. By paying close attention to the setup and maintenance of the couch roll suction box and its various components, operators can increase machine efficiency and improve paper quality.