TL;DR
Foam in the headbox causes formation defects and CD density variation. Check defoamer injection rate and stock system air entrainment sources before adjusting headbox consistency.
What you might see
- foam visible at headbox overflow or vent
- formation defects in finished paper
- CD density variation matching foam zones
- air alarms on stock flow meters
Likely causes
Insufficient defoamer dosing for the current furnish or broke content
Air entrainment from a pump cavitating or from a vortex at a chest inlet
High broke content with elevated resin and fatty acid causing foam
Stock temperature above the design range reducing surface tension
Required tools
- DCS operator station access
- Defoamer dosing pump controls
- Stethoscope or vibration meter for pump cavitation check
Safety first
- Defoamer chemicals may be irritating to skin and eyes. Use nitrile gloves and eye protection when adjusting chemical dosing equipment.
- Slip hazard from foam overflow around the headbox. Use non-slip footwear.
Procedure
- 1
Observe the headbox vent and overflow for visible foam. Note the CD location of formation defects on the sheet.[1]
- 2
Check the defoamer dosing rate against the current production recipe. Increase defoamer by 10-15% and observe formation after 15 minutes.
- 3
Check each stock pump for cavitation noise. A cavitating pump introduces air that persists to the headbox.
- 4
Inspect chest inlet pipes and suction boxes for vortex formation. A vortex can entrain air at the inlet.[1]
- 5
Review broke system content percentage. High broke, especially from coated or recycled grades, increases foam risk.
- 6
Monitor stock temperature at the fan pump inlet. Temperature above the design maximum reduces surface tension and worsens foam.
- 7
If foam persists after defoamer increase and air entrainment sources are addressed, consult the chemical supplier for a furnish-specific defoamer recommendation.
Sources
Voith Paper Voith MasterJet Pro Paper Machine / Headbox general technical documentation, Voith Paper
Paper machine headbox foam control and defoamer dosing, general paper machine wet-end practice (general)
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