Venting and Draining Guide

  1. No vent hole should be less than 10mm in diameter unless otherwise agreed with the galvanizer (Note: the bigger the holes the better)
  2. Vent holes should not be located in the centre of end plates and connections
  3. Vent holes should be located as close as possible to the high and low points of the hollow section when hung and in the same plane as the fabrication to prevent air locks, entrapment of the pre-treatment solutions and zinc pooling
  4. Large hollow vessels require a vent hole with a diameter of 50mm for each half a cubic metre of enclosed volume
  5. Hollow sections (pipe, RHS, and SHS) require at least one vent and one drain hole, each with a diameter equivalent to 25% of their diagonal cross section length,  or multiple holes (for both venting and draining) of equivalent minimum cross sectional area. The preferred design option is to leave the ends of hollow sections completely open
  6. Hollow sections that are connected require external vent holes as close to the connection as possible. If internal vent holes are used, they must be a total of at least 50% of the internal diameter of the connecting section, although they are recommended to be the same size as the connecting section’s internal diameter where possible. A 10mm diameter external check hole must be included at each location where internal venting is specified to allow the galvanizer to check the part is safe for galvanizing
  7. Large seal welded overlapping surfaces will require a vent hole in one surface as the enclosed area may contain condensation or allow process chemicals to enter the overlap through weld pinholes during the galvanizing process. Very large overlapping areas should be avoided as an undesirable design for galvanizing or corrosion protection in general

a.       Overlaps under 10,000mm² generally do not require venting.

b.       Overlaps between 10,000mm² and 40,000mm² should be vented with a minimum 10mm vent hole.

c.       Overlapping areas larger than 40,000mm² require vent holes at least 12mm.

d.       When the overlapped surface area reaches 250,000mm², vent holes shall be minimum 20mm and progressively placed every 250,000mm².

Venting pic

Adequate hole sizes in sealed hollow sections in the correct locations ensure galvanizing quality.

For further information about designing for hot dip galvanizing, our Design Guide for Hot Dip Galvanizing including Best Practice for Venting and Draining is available in PDF format in our Technical Publications Guides section, as an iPad App, and on the Google Play Store.