Pressure vessel shell design: if weld joint efficiency is 1.0 for longitudinal seams (parallel to the axis) and 0.8 for circumferential (girth) seams, which efficiency should be used in the shell thickness formula for an internally pressurised cylindrical vessel?

Introduction / Context:
For internally pressurised cylindrical shells, the governing stress for required thickness is usually circumferential (hoop) stress. The associated limiting joint is the longitudinal seam because it is aligned parallel to the axis and is stressed by hoop tension. Codes therefore require the longitudinal seam joint efficiency in the thickness formula.

Given Data / Assumptions:

• Thin-wall cylinder under internal pressure.
• Joint efficiencies: longitudinal seam = 1.0; circumferential (girth) seam = 0.8.
• Standard code formula for shell thickness includes joint efficiency E.

Concept / Approach:
Hoop stress σh = p * D / (2 * t) acts to separate the shell along a longitudinal plane; thus the longitudinal seam carries hoop stress and dictates the shell’s pressure integrity. The joint efficiency E used in the formula must correspond to the seam stressed by hoop tension—i.e., the longitudinal seam efficiency. If longitudinal E is larger than the girth seam’s E, shell thickness is set by E = 1.0, while girth joints are checked separately for axial stress and quality.

Step-by-Step Solution:
Identify the governing stress: hoop stress.Map governing stress to seam orientation: hoop tension → longitudinal seam.Use E = E_longitudinal = 1.0 in the thickness formula; verify girth seam for axial stress and joint quality.

Verification / Alternative check:
Code commentary and textbooks align the weld efficiency in shell equations with the seam parallel to the axis because it restrains hoop separation.

Why Other Options Are Wrong:
(a) Uses the girth seam’s E, not governing for hoop. (b) and (d) mix efficiencies without basis. (e) ignores code requirements and reality of weld quality.

Common Pitfalls:
Confusing which seam is stressed by which principal stress; forgetting separate checks for heads and girth seams; neglecting corrosion allowance.

Final Answer:
1.0 (longitudinal seam efficiency)