Venting and draining basics for design

Basic venting rules

(Note: the bigger the holes the better)

1. No vent hole should be less than 10mm in diameter unless otherwise agreed with the galvanizer

2. Preferred minimum vent hole size is 12mm

3. Vent holes should not be located in the centre of end plates and connections

4. Vent holes should be located at the edges of hollow sections oriented in the same plane as the fabrication

5. Large hollow vessels require 1250mm² of vent hole area for each cubic metre of enclosed volume. This is equivalent to a 40mm diameter hole for every cubic metre of volume

6. Hollow sections (pipe, RHS, and SHS) ideally require vent holes equivalent to 25% of the sections’ cross section, made up of single or multiple vent holes. The preferred design option is to leave the ends of hollow sections completely open

7. Hollow sections that are connected require external vent holes as close to the connection as possible. If internal vent holes are used, they should be a total of at least 50% of the internal diameter of the connecting section

8. Large seal welded overlapping surfaces will require venting if the enclosed area may contain condensation or allow process chemicals to enter the overlap during the galvanizing process. Overlaps between 10,000mm² and 40,000mm² should be vented with a 10mm vent hole. Overlaps under 10,000mm² generally do not require venting.  Intermediate sized overlaps should be judged on the basis of weld integrity and residual welding heat in the joint to ensure total dryness at time of sealing.  Longer or larger overlapping areas require spaced holes for progressive venting. Very large overlapping areas should be avoided as an undesirable design for galvanizing or corrosion protection in general

9. Vent and drain holes must be located as close to the high and low points of the hollow section as possible to prevent air locks, entrapment of pre-treatment chemicals and zinc puddling. 


Basic draining rules

• No drain hole should be less than 10mm

• Preferred minimum drain hole size is 25mm particularly for items with a large internal volume

• Large hollow sections (tanks, pressure vessels) require a 10,000mm² diameter of drain hole area 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


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

Design for Hot-Dip Galvanizing

For a copy of the complete "Design Guide for Hot Dip Galvanizing - best practice for venting and draining" please click here to download a copy.