Venting and Drainage of Steel
Venting of hollow sections in the cap plates
Where hollow sections are sealed they must be vented for reasons of safety so allowing the escape of air and the ingress and drainage of zinc during the dipping process.
Vent holes are required at each sealed end either in the end plate or the section.
The holes should be placed diagonally opposite one another, off centre and as near as is practically possible to the wall of the member to which the end plate is connected, which shows optimum venting with two sets of diagonal holes.
Where holes are provided in the member these should be diagonally opposite one another and as near to the end of the section as is practically possible.
Figure 2: Location of Venting and Drainage
Alternatives to vent holes
U or V notches or grinding off the corners of square or rectangular hollow sections as hollow sections provide ideal methods for venting and drainage. Where these options are used it is important that they have equivalent surface area to the vent hole that would otherwise have been provided.Â
Every sealed section of a fabrication must be vented for reasons of safety and to allow access and drainage of molten zinc. Holes diagonally opposite each other should be as close as possible to the sealed end (figure 3 and 4).
Venting
of sealed containers
1   large vessel

2   small vessel

a   inspection or access hole
b   lifting lugs

c   vents

d   internal baffles (cropped top and bottom)

e   man-way (large vessels) gs
Figure 5: Venting of sealed container with an internal diaphragm.
Design of tanks to avoid zinc traps and air locks
Venting of sealed containers or enclosed tanks is essential so as to allow a means of access and drainage for molten zinc and for the escape of gases from internal compartments so as to allow safe processing of work. Such venting will have the added benefits of ensuring complete coverage of internal surfaces so providing full corrosion protection as well as improving the quality of the galvanized coating and reducing cost.Â
"Due to the nature of internal venting, which is difficult to check, galvanizers will assume that internal venting has not been applied, once work is provided for galvanizing."
Number and Location of Holes or Crops
In addition, cropping of internal diaphragms is required while design should avoid potential zinc traps and air locks which might result in uncoated areas inside the tank. Design detailing for these issues is illustrated in figures 5-6.
Note 1 – The shaded holes or crops indicate the hole or crop in the opposite end of the hollow section.
Note 2 – The size of crop given in this table refers to the length of the adjacent side (not the diagonal length).
Note 3 – Table entries that are not applicable are designated by ‘-‘.
Venting of overlapping areas
Venting of a large 
overlapping area
- Key
-
Venting of overlapping surfaces by drilling holes
Holes should be drilled 300 mm apart with a diameter that is ≥ 12mm -
Venting of overlapping surfaces by intermittent welding

Welding is 300 mm wide with gaps of 25 mm
Guidance on the Number and size of holes for venting overlapping areas
An overlapping area of 100 cm2 means 10cm x 10cm or 20cm x 5cm etc.
Area of overlap ‘a’ represents areas shown in figure 8.Â
|  Area of Overlap ‘a’ |  Recommended Action |
|---|---|
| a ≤ 100 cm2 | Circumferential tight weld. The material should be dry for the welding process and overlapping parts should be smooth and assembled without gaps. |
| 100 < a ≤ 1000 cm2 | In diagonally opposite corners, either:
2x ≥ 12mm holes at corners or 2x ≥ 25mm intermittence of weld at corners |
| 1000 < a ≤ 2500 cm2 | Either:
4x ≥ 12mm holes at corners or 4x ≥ 25mm intermittence of weld at corners |
| a > 2500 cm2 | In diagonally opposite positions, either:
≥ 12mm holes at corners and circumferentially at least every 300mm from the corners or ≥ 25 mm intermittence of weld at corners and circumferentially at least every 300mm from the corners. |
Further guidance is given in EN ISO 14713-2:2020.
*Reproduced courtesy of ISO and/or based on EN ISO 14713-2:2020.
Please note, diagrams are not to scale.
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