Galvanizing - Venting and Draining Design Guide

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One of the key challenges while designing fabrications for hot-dip galvanizing, is the correct venting and draining of the fabrication. It is essential to immerse all the steel to be galvanized in molten zinc. The zinc should be able to flow freely into and out of all hollow sections and corners.

One of the most important factors that determine the ultimate quality of the coating is the flow of molten zinc into, off, and out of the fabrication. Mentioned below are galvanized coating defects that may result due to the inadequate draining and venting process:

• Coating failures caused by airlocks prevent the molten zinc from contacting the surface of the steel.
• Thick zinc flows on the surface due to the freezing of zinc while draining.
• Puddling of zinc in corners leads to wastage of zinc and impedes subsequent assembly.
• Trapping of ash on the zinc surface results in surface defects.
• Any water trapped inside a hollow section will increase 1750 times its original volume of steam and create pressures as high as 7250 psi or 50 m Pa.
• Inconsistency in surface appearance is caused by irregular immersion and withdrawal; this occurs because either the item is floating or zinc is trapped internally.
• Since steel is only 15% heavier than zinc, a relatively small amount of air trapped inside a hollow section will prevent the section from sinking into the molten zinc.

Basic Venting Rules

• The ideal minimum size of the vent hole is 12 mm.
• The vent hole should be at least 8 mm.
• About 200 g of zinc ash will be produced for every square meter of the galvanized steel surface. Since ash is solid powder, it cannot enter through tiny openings. Large internal areas require larger vent holes for venting to allow the ash to escape. Hollow vessels require 1250 mm² of vent hole for every cubic meter of the enclosed volume. This means a 40 mm² diameter hole is needed for every cubic meter of volume.
• Vent holes must be distributed at the edges of hollow sections. Hollow sections like RHS, SHS, and tube require a least vent hole area that is equivalent to 25% of the diagonal cross-section of the section.

Basic Draining Rules

• The ideal minimum size of the drain hole is 25 mm.
• Drain holes should be distributed at the edges of hollow sections.
• The drain hole should be at least 10 mm. Hollow sections like RHS, SHS, and tube require at least a drain-hole area that is equal to 25% of the diagonal cross-section of the section. The best design option is to leave the ends of the RHS, SHS, and tubes open.
• Large hollow sections (like pressure tanks and vessels) require a 100-mm diameter drain hole for each cubic meter of closed volume.

Drain and Vent Hole Sizes for Hollow Sections

Table 1. Recommended minimum hole and vent sizes for various size hollow sections

Vent and Drain Hole Sizes for Tanks and Pressure Vessels

Table 2. Recommended vent and drain hole sizes for tanks and pressure vessels