A set of descriptions about zinc pots

Replacement timeframe for galvanized pots

The most important equipment in hot-dip galvanizing is the zinc pot, usually made of steel plate. For the galvanizing industry, it's like a human heart. However, prolonged use causes damage to the zinc pot, so it must be replaced after several years of use.

However, there is no clear standard for replacing zinc pots.

In the past, when companies replaced zinc pots, they generally considered production volume, years of use, and loss, and then based on each factory's own experience. Generally, blow galvanizing is replaced every 2-5 years, and the highest record for dip galvanizing in China is 11 years.

Zinc pot volume calculation formula

Volume calculation is performed by subtracting 200mm from the depth of the zinc pot.

Subtracting 200mm from the pot's depth is to prevent zinc liquid from overflowing when the workpiece is immersed in zinc.

The amount of lead added to prevent damage to the bottom of the zinc pot is not considered.

The calculation formula is as follows:

V = Zinc pot length L × (pot depth H - 0.2m) × zinc pot width

Another viewpoint expresses the scale of the zinc pot using the unit zinc volume production W/V (ton/m3), where the production volume represents the total weight of galvanized steel workpieces during the zinc pot's use. The zinc pot's usage period is from when the zinc is melted and galvanizing begins to when the galvanizing operation stops, represented by N (years).

Zinc pot loss and its measurement

Loss is calculated from the difference between the original plate thickness and the thickness at the time of the survey.

Different operating temperatures affect the loss of the galvanizing pot. Assuming that the operating temperature is not considered, although the galvanizing pots have different steel types, a temperature difference of 10℃ will result in a difference of about 10% in loss.

The zinc liquid temperature in the zinc pot is 450±10℃, the operating time is 8-10 hours/day, and the zinc liquid temperature at night is maintained at about 435℃ to prevent the zinc from solidifying.

Plate thickness is measured using an ultrasonic thickness gauge with an accuracy of ±0.1mm.

The measurement points for loss are divided into two areas: the general area and the heating area (where the loss is most severe).

The loss in the general area is analyzed.

When measuring the plate thickness from the outside of the zinc pot, remove the oxide on the surface of the measurement point before measuring.

When measuring the plate thickness from the inside of the zinc pot, remove the zinc-iron alloy layer first to expose the steel before measuring.

Generally, the general area of loss refers to the area 500mm above the zinc slag deposition line at the bottom of the zinc pot and 500mm below the zinc liquid level (liquid level fluctuation area). The area 500mm away from the center of the heating burner also has less loss.

The heating area, where the burner flame directly contacts, has a higher temperature and therefore greater loss than other areas. This area is generally within 500mm of the center of the burner due to the distribution of the heating air.

Conditions for replacing the zinc pot

Regardless of how many years a zinc pot has been used, if the plate thickness decreases by about 4-8mm, a new zinc pot needs to be replaced. Or, if the plate thickness decreases by about 8%-23%, a new zinc pot needs to be replaced.

Experience shows:

(1) Blow-galvanizing zinc pots generally need to be replaced after about 2-4 years, but this is not a universal rule. Different companies have different service lives, but dip galvanizing lasts longer, with many cases lasting ten or more years.

(2) When the zinc pot plate thickness decreases by about 8%-23%, it should be monitored and prepared for replacement (Source: Hot-Dip Galvanizing Magazine).

(3) During use, the production per unit volume is approximately 1000-3000 tons/m3 (Source: Hot-Dip Galvanizing Magazine).

(4) During operation, the production per unit volume is approximately 100-400 tons/m3 (Source: Hot-Dip Galvanizing Magazine).