In the thickness calculation, which factor does the symbol Eff represent?

• A. MAWP
• B. SF
• C. RAD
• D. Joint Efficiency

Answer: (D)

Eff is the joint efficiency—the usable strength of the welds or joints compared to the base metal. In thickness calculations for boilers and pressure vessels, the design stress is effectively S times Eff, so the joint’s quality directly influences how thick the wall must be. A higher Eff means the joints are strong enough to carry the load like the parent metal, allowing a thinner wall; a lower Eff means weaker joints and a thicker wall is needed to meet the same design pressure. Eff depends on the weld type and workmanship (for example, shop welds with good quality can approach 1.0, while field welds or less robust joints are lower). This factor is distinct from MAWP (the rated design pressure), SF (a separate safety factor), or RAD (related to testing methods), which is why Eff specifically represents joint efficiency.