Shoe press case study - Using nip scan as pre-shutdown tool
Aug 30, 2022
A mill had been having issues with widening the sheet after threading the machine. They suspected that some of the problems could be associated with the 3rd press. The mill brought in Valmet to perform a nip scan as a pre-shutdown tool to collect information as to what is going on with this press with respect to loading and possible alignment problems.
Onsite actions
Valmet arrived on site and performed the nip scan between midnight and 9:00am. The press was checked with a nip scan blanket with press loaded at 700Kn/m on the main loading and 650Kn/m at the edges.
Nip scan sensor positions in the nip
The results of the scan showed misalignment between the shoe on the SymBelt and the center roll. The loading on the lead-in side of the SymBelt shoe was much lighter on the center to back edge of the press. With the sheet wetter on the back half of the machine, it was more likely to stick to the center roll and cause sheet widening problems. The surface of the center roll was also in need of reconditioning and this was adding to the sheet-widening problem.
Valmet told mill personnel that this press was delivered with spare offset keys when it was installed and these would be needed after the upcoming rebuild to correct the misalignment problem. Results of the nip scan indicated that the front end of the press needed the center key changed to move the front to the wet, in order to square the press. The amount needed for the offset change would be decided after a mechanical or optical check of the press.
Results of the nip scan and possible upgrades were discussed with the mill the same afternoon, after the testing was completed. They were very interested in the cover upgrades of the journals to help reduce continual cleaning of the journals, and minimize problems with indexing heads.
Nip scan load impression
The large amount of water coming over the top of the SymBelt prior to the press shutting down brought up the issue of the blanket doctor. Mill personnel were interested in knowing if this machine could be engineered to accept this type of wipe and wanted to learn the expected gains.
Shoe Press Process & Condition Test
A well-functioning shoe press is necessary for good paper quality and excellent runnability. The shoe press process and condition test analyzes the operation and mechanical condition of the shoe press and the scope of the reconditioning needs. The objective of the process and condition test and service is to ensure optimum operation of the shoe press. This is achieved by the solid expertise of Valmet service engineers and state-of-the-art measuring technology. Condition testing determines the need for immediate service actions and provides the information necessary for drawing up a long-term service plan for all subprocesses.
A flawlessly operating and regularly serviced shoe press gives substantial economic benefit through reduced steam consumption and good runnability. A defective shoe press, on the other hand, may cause considerable economic loss over a short period of time.
An increase of one percentage unit in the web dry content after the press reduces the steam consumption in the dryer section by 4%, which decreases the heat energy used for making one ton of paper by about 80 to 176 MJ. For example, a 3% higher dry content enables an increase of up to 200 m/min in paper machine production speed. With the shoe press it is possible to increase the web dry content after the press by 3% - 7% compared with the conventional press concept. These benefits can only be achieved if the shoe press operates correctly and without problems.
Valmet – for shoe press expertise!
The Shoe Press Process & Condition Test is only one of many shoe press services offered by Valmet, including: NipScan load impressions, SymBelt improvements such as oil collector trays, mechanical overhauls, belt change supervision, and several Beloit ENP-specific services. The mill in the case study described above benefited greatly by trusting the expertise of Valmet’s shoe press service personnel.
Is it time you called Valmet to look at your shoe press? Contact your Valmet representative.