The hydraulic press loading valves in the manifold located in the basement below a paper machine in the northern USA were due for replacement. But when mill stores went to purchase new valves, they found that the model they needed was now obsolete and no longer available.
The mill called in Valmet to help determine a good path forward. It was noted in discussions that not only were the valves of 1970's vintage, but the entire press loading control system for the paper machine was of the same vintage. That realization helped to raise the next logical question, were there other press loading components that should be considered for upgrade?
A Valmet engineer experienced in press sections and related controls came to the mill to examine the press and help to determine the scope of the upgrade necessary for the machine's press loading hydraulics. The mill maintenance manager expressed concern that by simply replacing the hydraulic valves, the mill would only "be doing half the job."
Machine operators were having a number of problems that could be directly linked to the 1970's era press hydraulic control system. Not only were the valves obsolete but the existing original hydraulic control system was quite obsolete as well.
Due to limitations of the existing control system, proper and reliable nip impressions could not be obtained. In addition, during machine start-up, when the operators had the former running and then applied loading to the presses, the former stretcher pressure would decrease causing instability in the system and sheet breaks. The operators were forced to make continuous adjustments to the press loadings because the set point would not remain steady. And the 1970's era control system had no troubleshooting capabilities.
After further analysis and discussion with the mill, Valmet upgraded the hydraulic control system. After completing the upgrade, a new custom valve rack makes maintenance and troubleshooting much easier. All obsolete components are upgraded to current with the latest control technology and safety features.
The mill has a new Human/Machine Interface control featuring 1st out logic and user friendly control screens offering operators enhanced and simplified control of the system. A duplex filter system was also installed for the manifold supply oil to ensure a good clean supply of oil.
The mill changed out a sticky proportional valve after the upgrade and the procedure which used to take up to 4 hours or more, took less than 40 minutes. Nip impressions of the various press nips are now readily done and reliable. Breaks due to stretcher unloading have been eliminated and the entire system is now easy to troubleshoot, thus reducing down time. Press loading is now steady running with closed loop control and the mill has set a production record after the upgrade was installed.
Overall, mill personnel are very happy with the results of the upgrade to their press section. According to the maintenance manager, the equipment is easier to access, operate and maintain – and that makes for happy operators and maintenance personnel!
For more information on whether your controls are in need of an upgrade, contact your Valmet representative.