Editorial Feature

Galvanizing - Venting and Draining Design Guide

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When designing fabrications for hot-dip galvanizing, one of the key challenges is the proper venting and draining of the fabrication. It is necessary to immerse all to-be-galvanized steel in molten zinc. The zinc must be in a position to flow freely into and out of all hollow sections and corners.

One of the most crucial factors determining the final quality of the coating is the flow of molten zinc into, off and out of the fabrication. The galvanized coating defects mentioned below can be caused due to insufficient venting and draining:

  • Failures in the coating brought about by airlocks that prevent molten zinc from contacting the steel surface
  • Trapping of ash on zinc surface, leading to surface defects
  • Puddling of zinc in corners, resulting in wastage of zinc and hindering subsequent assembly
  • Thick zinc runs on the surface due to freezing of zinc while draining
  • Unevenness in surface appearance due to irregular immersion and withdrawal due to floating of item or trapping zinc internally
  • Any water entrapped within a hollow section will expand 1750 times its original volume as steam and create pressures as high as 50 mPa, or 7250 psi
  • Since steel is just about 15% heavier compared to zinc, a comparatively little amount of air trapped within a hollow section will avoid sinking of the section in the molten zinc

Basic Venting Rules

  • The ideal minimum vent hole size is 12 mm.
  • The vent hole should not be smaller than 8 mm.
  • Hollow vessels necessitate 1250 mm2 of vent hole for every cubic meter of enclosed volume, meaning a 40 mm2 diameter hole is required for every cubic meter of volume.
  • Nearly 200 g of zinc ash will be generated for every square meter of galvanized steel surface. The ash is a solid powder and cannot pass through small openings. Large internal areas necessitate larger vent holes for venting to enable ash to escape.
  • Vent holes must be distributed at the edges of hollow sections.
  • Hollow sections like SHS, RHS and tube necessitate a minimum vent hole area equivalent to 25% of the diagonal cross-section of the section

Basic Draining Rules

  • The ideal minimum drain hole size is 25 mm.
  • The drain hole should not be less than 10 mm.
  • Drain holes should be distributed at the edges of hollow sections.
  • Large hollow sections (such as pressure vessels and tanks) necessitate a 100 mm diameter drain hole for every cubic meter of enclosed volume.
  • Hollow sections like SHS, RHS and tube necessitate a minimum drain hole area that is equivalent to 25% of the diagonal cross-section of the section. The ideal design option is to leave the ends of SHS, RHS and tubes open.

Drain and Vent Hole Sizes for Hollow Sections

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

Pipe (dia) Hollow Section Min. Hole Size
SHS (mm) RHS (mm) Vent (mm) Drain (mm)
<32 8 10
40 <30x30 10 10
50 12 12
40x40 50x25 14 14
65 16 16
80 20 20
65x65 75x50 23 23
100 25 25
125 32 32
150 38 38
125x125 150x100 45 45
200 50 50
300 50 50

Vent and Drain Hole Sizes for Tanks and Pressure Vessels

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

Capacity (l) Drain dia. (mm) Vent dia (mm)
1000 112 40
3000 195 70
4000 225 80
5000 252 90
6000 276 98
7000 300 106
8000 320 114
9000 340 120
10000 356 125

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