Inspection of Internal Venting for Tubular Assemblies
What recommendations are there for internal venting inspection of tubular assemblies prior to galvanizing?
Tubular assemblies (handrails, piping, pipe trusses, sign bridges) are commonly galvanized because corrosion protection is provided to the interior and exterior of the product. To achieve an optimal galvanized coating, these hollow products require proper venting and draining holes designed in accordance with ASTM A385.
Where possible, the galvanizer should be involved in the review of drawings for enclosed or partially enclosed assemblies before fabrication begins. Additionally, internal and external venting should be verified visually upon receipt of the material at the galvanizing facility to ensure proper venting was followed. This inspection will not only help to ensure the internal portion of the material is successfully galvanized but also ensure the safety and protection of the kettle personnel and equipment. Any cleaning solutions or rinse waters trapped in a blind or closed weld connection can be converted to superheated steam or gas and develop a pressure of up to 3,600 psi (1,100 MPa) when immersed in molten zinc. Such pressures can lead to rupture of the material and serious harm to galvanizing personnel and equipment.
What happens if you are unsure whether internal venting has been performed, or whether a tubular assembly contains a solid section that will interfere with the flow of zinc internally? For simple designs, it may be possible to place a flashlight at the end of the pipe and look through the other end for the right source or pass a long wire through to the connection confirm ventilation. For more complicated handrail or tubular structures; however, these methods may not provide sufficient confirmation.
Instead, a fiber optic light cable or any cord/extension with a light source at the end can be used to confirm internal venting at each joining connection. As shown in Figure 1, pass the corded light through one pipe end that makes up a joint. As the cord is passed through each joint along the pipe, internal venting is confirmed when the light is observed from the end of the opposite pipe that makes up each connection.
When fabricating steel for hot-dip galvanizing, the designer/customer is responsible for ensuring the pieces are suitable for galvanizing by following the guidelines provided within ASTM A385, including recommendations for venting and drainage. Therefore, if internal venting of the assembly cannot be confirmed, or if there is any doubt, then it is in the galvanizers best interest to return the assembly to the customer and raise the issue as an education point for the designer. Taking a risk to galvanize such an assembly is not worth the hazard to kettle personnel or equipment.