Piping standards and nominal size: For a given nominal pipe size (NPS), which dimensional feature remains the same across different schedules (wall thicknesses)?

Introduction / Context:
Piping selection often uses nominal pipe size (NPS) with various schedules (e.g., 40, 80). Knowing which dimension stays constant simplifies fittings compatibility, insulation, and support design.

Given Data / Assumptions:

• Standard steel piping to ASME/ANSI dimensional practice.
• Comparisons at the same NPS but different schedules.
• Room-temperature geometric considerations only.

Concept / Approach:
By convention, pipes of a given NPS share a standard outside diameter so that flanges, couplings, and fittings match universally. Changing the schedule alters the wall thickness, which in turn changes the inside diameter (ID). Therefore, OD remains fixed, while ID varies with schedule.

Step-by-Step Solution:

Verification / Alternative check:
Dimensional tables show identical OD for a given NPS across multiple schedules; IDs differ according to wall thickness entries.

Why Other Options Are Wrong:

• Inside diameter: varies with schedule.
• Wall thickness: schedule changes wall thickness; not constant.
• None of these: incorrect since OD does remain constant.

Common Pitfalls:
Confusing tube sizing (often by OD) with pipe sizing (nominal). Also, assuming NPS equals actual ID for larger sizes—only approximately true for some ranges.

Final Answer:
outside diameter