Galvanizing of hollow sections and pipes requires that the item be properly vented to allow for the free flow of Pre-treatment Liquids both inside and outside of the item. Also, when the item is dipped into the galvanizing bath it must allow for the ingress of the molten zinc to fully coat both the inside and outside surfaces while at the same time, allowing for the expulsion of the displaced air which will be superheated at the galvanizing temperature of approximately 450° C.
Ideally, if the ends are left fully open, venting will be at its optimum and will cease to be a concern, however, ends closed off by end-plates, base-plates or cap-plates will need to be adequately vented.
This is achieved by placing at least one hole each end – but a single hole should only be considered for small sections and should be placed as close as possible to the inside face of the profile. The hole should never be placed at the center of the profile as this will create an entrapment point for the molten zinc as the work of lifted out of the bath. It may also prevent the draining of pre-treatment liquids.
they will also not allow for any entrapment zones providing a more efficient free flow of zinc and pre-treatment fluids.
The holes should be as large as practically possible and be placed at each end of the article.
The recommendation is to place at least two holes placed diagonally opposite at each end of the piece – for larger sections 4 holes may be recommended.
Multiple holes are always preferable, it means the item can be dipped and removed from the galvanizing bath without having to worry about which is the top and bottom –
Vent Hole Sizes
The general guideline is that there should be at least one vent and one drain hole, each end 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. Whichever case, the holes should never be less than 10 mm Dia.
Refer to Tables 1 to 3 below for the minimum recommendations for standard hollow sections.
The length of the hollow section should also be taken into consideration for the required hole size. The hollow vessel rule (Table 4) may need to be applied for some longer lengths or larger volumes. Large hollow vessels require a vent and drain hole for every 0.5 m3 of enclosed volume, each being a minimum of 50mm in diameter. (See Table 4)
Table 1 - Selected Pipes / Circular Hollow Sections
Table 2 - Selected Square Hollow Sections
table 3 - Selected Rectangular Hollow Sections
Hollow Vessels
There are a few guidelines for venting of hollow vessels:
- Hollow vessels may require temporary stays during the galvanizing process to prevent distortion
- Hollow vessels should have at least 1 vent hole and 1 drain hole
- Standard minimum venting and draining for an enclosed volume is a 50mm diameter hole for each 0.5 m3 (see Table 4)
- Openings should finish flush inside the vessel
- Baffles inside vessels should have openings to allow free flow of liquid throughout. A minimum of 75mm snipes is required.
Table 4 - Hollow Vessels
