Costing and design statements – Identify the incorrect one: Which of the following statements is wrong regarding equipment and cost indices for process design?

Introduction / Context:
Economic and fabrication choices in process design must be grounded in realistic cost relationships. Misconceptions about welding, geometry, and indices can lead to inaccurate capital estimates.

Given Data / Assumptions:

• Tubular exchanger economics typically favor fewer shells and longer tubes per unit area.
• Cost indices (e.g., CEPCI) benchmark cost levels over years.
• Turnover ratio commonly relates sales to fixed capital investment.
• Welding methods affect shop time, edge preparation, and inspection cost.

Concept / Approach:
Butt welds require precise preparation and alignment; they are generally not inherently “less expensive” than lap welds simply because of “no squaring.” The statement as given is flawed and reverses practical considerations. The other statements reflect standard economic definitions or rules of thumb.

Step-by-Step Solution:
Evaluate each statement against standard practice.(a) Fewer shells and heads often reduce cost/area → generally acceptable rule of thumb.(b) Correct definition of a cost index.(c) Turnover ratio definition is standard in chemical plant economics.(d) Welding claim is inaccurate; butt-welds typically require edge prep, fit-up, and inspection, and are not intrinsically cheaper for the reason cited.

Verification / Alternative check:
Fabrication standards note additional QA/QC for butt welds (e.g., radiography), while lap joints can be simpler in certain platework, depending on design codes and service.

Why Other Options Are Wrong (i.e., why they are not the incorrect statements):

• (a) Accepted heuristic for exchanger design cost per area.
• (b) Correct definition and usage of indices.
• (c) Common textbook definition for turnover ratio.

Common Pitfalls:
Overgeneralizing fabrication economics without considering code, thickness, NDT requirements, and corrosion allowances.

Final Answer:
Plates with butt-welded joints are less expensive than lap-welded joints because squaring of plates is not necessary.