Background One of the most common issues in designing fabrications for hot dip galvanizing is ensuring that fabrications are vented and drained correctly. All steel to be galvanised needs to be immersed in molten zinc and the zinc needs to be able to flow freely into and out of all hollow sections and corners.
The flow of molten zinc into, off, and out of the fabrication is one of the most important factors in determining the final quality of the coating. Inadequate venting and draining can cause the following galvanised coating defects:
· Misses in the coating caused by air locks preventing molten zinc contacting the steel surface.
· Puddling of zinc in corners, wasting zinc and interfering with subsequent assembly
· Ash trapped on zinc surface causing surface defects
· Irregularities in surface appearance caused by erratic immersion and withdrawal because of item floating or trapping zinc internally
· Thick zinc runs on surface caused by zinc freezing during draining
· Steel is only about 15% heavier than zinc. A relatively small amount of air trapped inside a hollow section will prevent the section from sinking in the molten zinc
· Any water trapped inside a hollow section will expand 1750 times its original volume as steam and generate pressures as high as 50 mPa (7250 psi).
Basic Venting Rules · No vent hole should be smaller than 8 mm
· The preferred minimum size is 12 mm
· About 200 grams of zinc ash will be produced for each square metre of steel surface galvanised. This ash is a solid powder and will not pass through small openings. Venting large internal areas required larger vent holes to allow ash to escape
· Hollow vessels require 1250 mm2 of vent hole for each cubic metre of enclosed volume. This means that a 40 mm2 diameter hole is required for each cubic metre of volume
· Hollow sections such as tube, RHS and SHS require minimum vent hole area equivalent to 25% of the section’s diagonal cross section
· Vent holes should be at the edges of hollow sections
Basic Draining Rules · No drain hole should be less than 10 mm
· Preferred minimum drain hole size is 25 mm
· Large hollow sections ( tanks, pressure vessels) require a 100 mm diameter drain hole for each cubic metre of enclosed volume
· Drain holes should be at the edges of hollow sections.
· Hollow sections such as tube, RHS and SHS require minimum drain hole area equivalent to 25% of the section’ diagonal cross section. The preferred design option is to leave the ends of tubes, RHS and SHS open.