Variable diameter, a type of connecting device. Used when the nominal diameter of valves and pipelines (or pipelines and pipelines) is inconsistent (to save money or utilize existing materials), valves and pipelines (or pipelines and pipelines) cannot be directly connected or welded through standard flanges or threads. In this case, a type of pipe fitting (which can be self-made or purchased) is added, which can be directly connected to the valve at one end and directly connected to the pipeline at the other end. This type of pipe fitting is commonly known as the "big and small head" and changes the pipe diameter through a connecting pipe.