New Mexico Pipe Fitter Journeyman (JPF) Practice Exam

For any given steel pipe size, how do the OD and ID relate?

1. OD is constant, ID varies

2. OD varies, ID is constant

3. Both OD and ID are constant

4. Both OD and ID vary

The correct answer is: OD is constant, ID varies

For any given steel pipe size, the outside diameter (OD) is standardized and remains constant across the pipe manufacturing process. This consistency provides a reliable specification for connecting pipes — components and fittings are designed to fit the standardized OD of pipes. However, the inside diameter (ID) varies based on the wall thickness of the pipe and the specific schedule of the pipe being produced. For example, a pipe designation like "2-inch pipe" refers to a constant OD of 2.375 inches, regardless of whether it is schedule 40, schedule 80, or any other schedule. As the wall thickness increases in heavier schedules, the ID decreases accordingly. Therefore, while the OD remains fixed and uniform, the ID can change to accommodate the different strengths and applications that various wall thicknesses deliver. This relationship is crucial in pipe fitting and installation as understanding it allows a fitter to make accurate assessments for flow capacity, pressure ratings, and ensuring compatibility with other components in a piping system.