Pipe schedules and dimensions: two steel pipes of the SAME nominal size but DIFFERENT schedule numbers will have the same _____ by definition of standard pipe sizes.

Introduction / Context:
Nominal Pipe Size (NPS) systems standardize pipe outside diameters (OD) so that fittings are interchangeable. The wall thickness varies with schedule number, which changes the inside diameter (ID) and flow area. Recognizing which dimension stays constant across schedules is fundamental for piping design and procurement.

Given Data / Assumptions:

• ASME/ANSI standard steel pipes.
• Same NPS (nominal size) but different schedules (e.g., Sch 40 vs Sch 80).
• Plain-end or threaded-end does not change the basic OD standard.

Concept / Approach:

For a given NPS, the OD is fixed (e.g., NPS 2 has OD 2.375 in). Increasing the schedule number increases wall thickness, thereby reducing ID and flow area, while keeping OD constant to maintain compatibility with fittings (flanges, couplings, valves) designed around the OD.

Step-by-Step Solution:

Verification / Alternative check:

Pipe dimension tables list a single OD per NPS with multiple wall thicknesses under different schedules, confirming the principle.

Why Other Options Are Wrong:

Inside diameter and flow area change with schedule; wall thickness explicitly changes; thread depth depends on schedule/ID and thread series.

Common Pitfalls:

Assuming “2-inch pipe” has exactly 2.000 in OD (it does not); confusing nominal size with actual OD.

Final Answer:

outside diameter