What should be done if the thickness difference where the upper plate at a horizontal joint exceeds 1/8 in. compared to the lower plate?

• A. The upper plate must be left unchanged
• B. The upper plate shall have a 4:1 taper
• C. The lower plate should be adjusted
• D. An additional plate must be added to the joint

Answer: (B)

When the thickness difference at a horizontal joint between the upper plate and the lower plate exceeds 1/8 inch, a 4:1 taper is required for the upper plate. This specification helps ensure that the weld joint can accommodate the thickness variation without leading to potential stress concentrations or weld defects. The taper creates a gradual transition between the two plate thicknesses, promoting a more uniform distribution of stress across the joint. This is essential for maintaining the structural integrity of the tank and preventing failures in service.

In this context, simply leaving the upper plate unchanged does not address the issue of the uneven thickness, which could lead to inadequate weld penetration or other complications. Adjusting the lower plate may not resolve the discrepancy effectively, as it could still lead to similar issues. Adding an additional plate to the joint may introduce unnecessary complexity and weight without solving the underlying problem of the thickness difference. Therefore, creating a 4:1 taper on the upper plate is the most effective and compliant solution according to the standards set by API 650.