TL;DR
Glass surface staining in the lehr means the protective atmosphere has been disrupted by air ingress or a gas control fault. Check the lehr seal curtains, gas supply, and O2 sensor calibration.
What you might see
- glass surface staining or discoloration at the exit
- lehr atmosphere oxygen level outside the specification
- combustion atmosphere sensor alarm active
- coating adhesion failure on coated glass products
Likely causes
Lehr entry or exit curtain worn, allowing air ingress into the controlled atmosphere zone
Protective gas supply pressure drop reducing the positive pressure barrier against air
O2 sensor failed or uncalibrated, giving incorrect atmosphere feedback
Lehr seal door held open during changeover longer than the safe exposure time
Required tools
- O2 sensor calibration gas
- Replacement curtain strip material
- Gas flow meter
- LOTO kit for gas supply isolation
Safety first
- If the protective atmosphere contains nitrogen or forming gas, it displaces oxygen. Use a personal O2 monitor near any open lehr access.
- Lock out the lehr heating power before replacing any internal seal or curtain material.
Procedure
- 1
Read the atmosphere O2 level from each zone sensor. Identify which zones are outside the target range.[1]
- 2
Verify the protective gas supply pressure and flow. Confirm the gas flow is sufficient to maintain positive pressure in the lehr.[1]
- 3
Inspect the lehr entry and exit seal curtains for wear or physical damage. Replace any curtain strip that is cracked, missing, or not covering the opening fully.
- 4
Calibrate the O2 sensor using a certified calibration gas according to the sensor manufacturer's procedure.
- 5
After repair, purge the lehr with the protective gas and confirm the O2 readings return to the target range.
- 6
Run sample glass through and inspect the surface for staining. No staining confirms the atmosphere is restored.
Sources
Pennekamp Pennekamp Profile / Mosaik Annealing Lehr general technical documentation, Pennekamp
General glass lehr atmosphere control and seal maintenance procedures (general)
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