What characterizes a longitudinal joint in a boiler?

A longitudinal joint in a boiler is characterized as a seam running the length of the shell. This type of joint is critical because it involves the structural integrity of the boiler, allowing it to contain high-pressure steam and withstand various operational stresses. These seams are typically welded or riveted to ensure that they provide a strong bond along the length of the boiler, which is essential for safety and efficiency in its operation.

The other options describe different aspects or types of joints and features in boiler design but do not accurately define a longitudinal joint. The horizontal joint mentioned runs around the boiler and is known as a circumferential joint. A joint found above the waterline does not specifically identify it as longitudinal, and connecting the boiler to external piping involves fittings and connections that are distinct from the longitudinal joints that run the length of the shell.