TL;DR
Non-uniform hood temperature creates MD moisture streaks. Check burner flame pattern, air distribution nozzles, and hood sealing before adjusting zone setpoints.
What you might see
- MD moisture streaks in tissue product
- variation in hood temperature readings across zones
- some zones running at maximum gas to compensate for adjacent cold zones
- hot spots visible on hood exterior infrared scan
Likely causes
Blocked or misaligned impingement air nozzles in a hood zone
Burner flame impingement on hood panels rather than Yankee surface
Hood air seal gaps allowing cold infiltration at hood ends or splice joints
Imbalanced combustion air distribution between hood zones
Required tools
- DCS historian for zone temperature trending
- Compressed air for nozzle cleaning
- IR camera for hood exterior inspection
- LOTO kit for outage work
Safety first
- Yankee hoods operate with gas combustion above 400 deg C. Never open burner access doors or enter the hood during operation.
- Lock out all gas and air supplies before entering the hood for inspection or nozzle cleaning.
Procedure
- 1
Map the hood temperature zone readings from the DCS across the full machine width and identify the cold and hot zones.[1]
- 2
Compare zone gas consumption. A zone running at maximum gas with low temperature indicates air infiltration or blocked nozzles.
- 3
During a brief production pause, inspect hood end seals for visible gaps, wear, or missing seal material.
Warning: Yankee hoods contain hot gas above 400 deg C during operation. Do not open inspection doors or approach burner areas during operation. - 4
At the next outage, inspect impingement nozzles in the affected zones for blockage or misalignment. Clear any blocked nozzles with compressed air.[1]
- 5
Check combustion air duct balancing dampers for each zone. Confirm each zone damper is at its designed position.
- 6
Inspect the hood shell and end plates for gaps or failed sealing material. Replace seal material as needed.
- 7
After repairs, run a zone-by-zone temperature balance check at operating speed before returning to full production.
Sources
Valmet Valmet TurboDry Yankee Dryer general technical documentation, Valmet
Yankee dryer hood temperature balance and impingement nozzle maintenance, general tissue machine practice (general)
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