Pad flanges are nozzles in a vessel that allow the shortest possible projection outside and inside the vessel. This is critical in some applications for mounting certain equipment to the vessel. Often, equipment operators and engineers need to see inside a vessel during certain reaction or mixing processes. For many reasons, most of which revolve around safety, Sight Glasses are used to give a clear view from the safety of outside the tank or vessel. Sight Glasses are very specifically designed with special glasses that are designed to hold pressure.
It is typical to have at least one sight glass located on a vessel. Most often, they are found on the top head, but can also be found in the shell, bottom head, and manway covers. In some cases, the design will include two sight glasses in the same general area: One for projecting a light inside the vessel and the other for viewing.
There are multiple styles of sight glasses, but one of the most common is the direct-mount type. Other similar items are Level Gauges, and Flow Indicators, where a window allows visibility of a rotating vane to help understand when product is flowing through a pipe or small vessel.
Pad flanges are commonly utilized for mounting sight glasses to keep the lens as close as possible to the vessel/shell to avoid shadowing.
Self-Cleaning Sample Valves are also a common sight in chemical processing facilities. These are generally designed to give a sample when opened, but a piston clears any product left in the valve after the sample has been taken. The use of a pad flange ensures the length of the valve and piston are as short as possible and also makes it easier to have a precision-machined bore for the piston to travel in to minimize the possibility of any product build-up in the valve.
No matter what style of sight glass, sample valve, or other pad flange-mounted instruments your vessel requires, be sure to discuss your needs with your vessel design/fabrication partner so these important details are managed on the front end.