TL;DR
A bath temperature profile upset means one or more heater zones are not delivering the design heat input. Check the heater element continuity and thermocouple calibration before adjusting any setpoint.
What you might see
- glass thickness across the width outside target
- transverse temperature gradient above allowable
- heater zone controller showing full demand with no response
- tin bath temperature recovery after a stop taking longer than normal
Likely causes
Heater element burnout in one zone, leaving a cold streak across the ribbon
Thermocouple failure giving the controller a false low reading and causing overheating in one zone
Tin level below the design value, reducing thermal mass and amplifying local temperature variations
Heater power circuit fault limiting the zone output
Required tools
- Portable thermocouple for zone cross-check
- Replacement heater elements
- Preheated tin for level top-up
- Full radiant-heat PPE
Safety first
- The tin bath is above 600 degC. Full radiant-heat PPE is mandatory.
- Cold or wet tin added to the hot bath causes violent molten metal spatter. Preheat all tin additions before introduction.
- The bath atmosphere contains hydrogen. No ignition sources near open access.
Procedure
- 1
Read the temperature profile across all bath zones from the DCS and identify which zones are deviating.[1]
- 2
Compare the heater current draw in each zone to the rated value. A zone at full demand but below setpoint has a heater failure.[1]
- 3
Test the thermocouple in the suspected zone by comparing its reading to a portable thermocouple at the same position.
- 4
During a planned stop, inspect the heater elements in the deviating zone for burnout.
- 5
Check the tin level in the bath. Low tin level increases temperature profile sensitivity. Top up with pure tin if the level is below specification.
Warning: Adding tin to a hot bath must use preheated tin only. Cold tin added to the hot bath causes violent spattering of molten tin. - 6
Replace failed heater elements or thermocouples and re-verify the temperature profile before increasing glass production.
Sources
Pilkington (NSG) Custom-engineered tin baths up to 60 m long Float Glass Tin Bath general technical documentation, Pilkington (NSG)
General float glass tin bath thermal management and heater maintenance procedures (general)
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Stop fixing the same fault twice.
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