Piping terminology: the “nominal size” (nominal bore, NPS/NB) of a pipe primarily indicates which diameter for design and specification purposes?

Introduction / Context:
Engineers commonly specify piping by its nominal size (NPS/NB) and schedule. While the exact inner diameter varies with schedule (wall thickness), the nominal size historically tracks the inside diameter for small-to-midsize pipes, serving as the primary flow-carrying indicator, whereas the outside diameter is standardized by NPS series.

Given Data / Assumptions:

• Standard steel pipe per common specifications (e.g., ASME B36.10M/B36.19M).
• Focus on what the “nominal size” conceptually indicates.
• Schedules (wall thicknesses) modify the precise ID.

Concept / Approach:
For most sizes, the NPS is a name that historically approximated the inside diameter. Although OD is fixed for each NPS, the nominal designation guides designers about the flow capacity, which relates to ID. Hence, when asked which diameter the nominal size indicates, the answer is the inner diameter (not exact, but the indicated one).

Step-by-Step Solution:

Verification / Alternative check:
Pipe charts show constant OD for a given NPS and changing ID with schedule; the term “NB” literally means nominal bore (inside diameter).

Why Other Options Are Wrong:

• Outer diameter is standardized but not what “nominal size” indicates conceptually.
• “Approximate” without identifying which diameter is ambiguous.
• “None” is incorrect; the convention exists.

Common Pitfalls:
Assuming nominal size equals exact ID for all schedules; confusing tubing (often sized by OD) with pipe (by NPS).

Final Answer:
inner