Tanks and enclosed vessels, should be designed to allow for acid cleaning solutions, fluxes and molten zinc to enter and flow upwards through the enclosed space and out through an opening at the highest point. This prevents air from being trapped as the article is immersed. The design must also provide for complete drainage of both interior and exterior details during withdrawal.
When both internal and external surfaces are to be galvanized, at least one filling and draining hole and a vent hole must be provided. The filling hole should be as large as the design will allow but at least 3 inches in diameter for each cubic yard (or 10 cm in diameter for each 1.0 cubic meter) of volume with a minimum diameter of 2 inches (50 mm). A vent hole of at least the same size should be provided diagonally opposite the filling hole. This allows the air to escape and facilitates draining.
In tanks, internal baffles should be cropped on the bottom or provided with suitable drainage holes to permit the free flow of molten zinc. Manholes, handholes, bosses and openings should be finished flush inside to prevent trapping excess zinc.
Openings must be placed so that the flux on the vessel can float to the surface of the bath. They will also prevent air pocket formations which would keep the acid bath from completely cleaning the inside of the vessel.
The diameter of the opening should be at least 1 inch per foot (83.3 mm per meter) of tank diameter. Minimum allowable diameter opening is 2 inches (50 mm). Tanks over 48 inches (1.22 meters) in diameter should have a manhole in one end and should have all six holes.
Items such as vessels and air receivers which are to be galvanized on the outside only must have “snorkel” tubes or extended vent pipes. These openings provide an air exit from the vessel above the level of molten zinc in the galvanizing kettle. The galvanizer should be consulted before using these temporary fittings.
It is always wise to have the galvanizer review the drawings of enclosed or partially-enclosed vessels before they are fabricated. He can advise you of any changes that would provide you a better product. If a change is needed to facilitate galvanizing, the least expensive time to make the change is before fabrication.