What is the typical diameter of steel piping used for fill-pipe risers?

The typical diameter of steel piping used for fill-pipe risers is indeed 4 inches. This size is commonly chosen because it strikes a good balance between allowing for efficient fuel transfer during filling and ensuring that the system can handle the necessary flow rates without risking overpressure or spillage.

Using a 4-inch diameter allows for a relatively high volume of fuel to be moved quickly while also being manageable for installation and maintenance. Larger diameters, while they may seem advantageous for flow rate, can complicate installation, increase costs, and may not be necessary for conventional UST applications. On the other hand, smaller diameters could lead to restrictions in flow capacity, particularly in scenarios that require rapid filling.

In summary, the 4-inch diameter is a standard for fill-pipe risers because it effectively meets performance requirements while maintaining practicality in installation and operation within the regulatory framework for underground storage tanks.