Pipe schedule number is a conventional index of wall thickness used in process piping. Which expression (using allowable internal working pressure P′ and allowable stress S, both in consistent units) defines the schedule number?
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A1000 P′ / S
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B100 P′ / S
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C1000 S / P′
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D10000 P′ / S
Answer
Correct Answer: 1000 P′ / S
Explanation
Introduction / Context:Pipe “schedule” standardizes wall thickness across nominal sizes. Understanding its definition helps engineers relate design pressure and material allowable stress to expected wall thickness selection and mechanical robustness.
Given Data / Assumptions:
- P′: allowable working pressure (e.g., psi).
- S: allowable stress of pipe material (same unit basis).
- Schedule is a non-dimensional index used in ASME/ANSI standards.
Concept / Approach:
Historically, schedule number ≈ 1000 P′/S (with both P′ and S in psi). Larger schedule indicates thicker wall for a given nominal pipe size. The factor 1000 is conventional for scaling to convenient integers in catalogs (e.g., Sch 40, 80).
Step-by-Step Solution:
Relate pressure capacity to material strength → thicker wall needed as P′ increases or S decreases.Adopt standard relation → Schedule ≈ 1000 P′/S.Interpret: higher pressure rating or lower allowable stress → higher schedule number.Verification / Alternative check:
Pipe tables and design references commonly cite the 1000 P′/S rule-of-thumb for schedule index derivation in legacy sizing frameworks.
Why Other Options Are Wrong:
100 P′/S or 10000 P′/S: Incorrect scaling. 1000 S/P′: Inverse trend (would imply thinner walls at higher pressure), which is illogical.
Common Pitfalls:
Assuming schedule directly gives thickness independent of diameter; actual wall thickness also depends on nominal size and schedule per tables.
Final Answer:
1000 P′ / S