TL;DR
Coating weight deviation from the air knife is usually a gap or pressure imbalance between the two knives. Verify pressure symmetry and clean the knife lips before adjusting the setpoint.
What you might see
- coating weight above or below specification
- air knife pressure inconsistent between operator and drive side
- strip surface showing coating streaks parallel to the line
- zinc spatter at the air knife gap
Likely causes
Air knife lip contaminated with solidified zinc reducing the jet uniformity
Knife gap asymmetric between operator and drive side from a worn positioning mechanism
Air supply pressure below the required value for the coating weight target
Knife angle misaligned from the strip surface, changing the wiping effectiveness
Required tools
- Feeler gauge for knife gap measurement
- Brass scraper for zinc lip cleaning
- Coating weight sampling kit
- Full heat-resistant PPE
Safety first
- The air knife area is directly above the molten zinc pot. Full heat-resistant PPE is mandatory.
- Dry all tools before bringing them near the zinc pot. Moisture causes violent zinc splash.
Procedure
- 1
Read the air pressure at the operator and drive side of the air knife separately. A side-to-side pressure difference above 5% causes coating weight asymmetry.[1]
- 2
Measure the knife gap at both ends using a feeler gauge. Adjust the gap to equal the design specification.[1]
- 3
Inspect the knife lips for solidified zinc buildup. Clean the lips with a brass scraper. Do not use a steel tool that could score the knife lip.
Warning: The air knife is located directly above the zinc pot. Full heat-resistant PPE and a dry zinc-free work area are required. - 4
Verify the knife angle relative to the strip surface. Consult the line setup documentation for the correct angle for the current product.
- 5
After any knife adjustment, run a sample length and measure the coating weight at operator side, center, and drive side.
- 6
Adjust the air pressure setpoint to bring the average coating weight to the target.
Sources
Tenova Tenova LOI Continuous Galvanizing Line (CGL) general technical documentation, Tenova
General continuous galvanizing line air knife setup and coating weight control procedures (general)
More guides for Tenova Tenova LOI
How to respond to a furnace atmosphere upset on a Tenova LOI galvanizing line
Oxygen ingress into the galvanizing furnace oxidizes the strip surface and causes coating adhesion failures. Find and seal the oxygen ingress point and restore the protective HNX gas ratio before resuming production.
How to manage zinc pot dross buildup on a Tenova LOI continuous galvanizing line
Dross in the zinc pot causes coating defects. Control pot temperature precisely within the specified range and schedule dross skimming at the correct interval to keep dross below the coating quality threshold.
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