Problematic retention aid system leads to press wreck, case study

Good machine runnability is determined by many factors. The machine has to be mechanically correct to guarantee web stability. All rotating and stationary equipment must be aligned to tight tolerances. Chemical processes need to be managed as well.

Papermaking is as much a chemical process as it is a mechanical one. There are many chemical additives that are used in the wet end to control retention, drainage, formation, biological deposits, pH, conductivity, air content, brightness, color, ash, opacity and more. The importance of reliable additive metering systems should be a high priority.

Problematic retention aid system leads to...

A fine paper machine lost retention aid flow when the pump shut down for some reason. Without retention aid in the wet end, the first pass retention quickly dropped. When retention drops, a couple of things start to happen. First, the basis weight of the sheet drops quickly. The sheet moisture and drying demand drop, which causes draw issues which can break the sheet. If you're lucky and no sheet break occurs, the automatic basis weight control will start adding thick stock flow to meet the target basis weight. Higher thick stock flow will increase the consistency of the headbox. Poor retention increases the consistency in the tray. Eventually, a new equilibrium is established at a lower retention and higher solids and the basis weight target is achieved. Operating like this will increase the demand on the saveall system which could affect its operation and associated water balances.

... a press wreck

After investigating the retention aid pump issue, no changes were made to the retention flow control system and the retention supply pump was restarted. Its flow output control drifted higher, trying to meet a flow setpoint. The sudden flow of retention aid to the system caused first pass retention to spike up, sending a heavy, wet, fines-rich sheet down the machine.

In this case, the press could not handle the extra water load and the sheet followed one of the press felts. The result was a sheet wrap on one of the press rolls. This ended up melting the poly cover on one of the press positions and damaging other press rolls and press fabrics.

Lessons

A couple of valuable lessons can be learned from this example:

• Everyone, especially operators, needs to respect the power that retention and drainage aids have on machine runnability.
• If retention aid is lost for whatever reason, always close the automatic flow control valve or decrease pump speed before starting the retention aid supply pump. Then slowly bring retention flows back up over the course of a couple of hours, allowing the retention balance to gradually come back to normal. Monitor the draws closely as retention comes back in line, especially the first dryer section draw.
• Wet end starch and cationic polymer flows can also create retention issues, but not as much as retention aid does.

For assistance with your retention aid system, contact your Valmet representative.