What is the allowable misalignment for horizontal joints in plates that are equal to or greater than 5/16 in. thick?

• A. 10% of the upper plate thickness
• B. 15% of the upper plate thickness
• C. 20% of the upper plate thickness up to 1/8"
• D. 25% of the upper plate thickness

Answer: (C)

For horizontal joints in plates that are equal to or greater than 5/16 inches thick, the standard allows for a specified amount of misalignment to ensure the integrity and functionality of welded tanks. The correct option indicates that the allowable misalignment is 20% of the upper plate thickness, but it is limited to a maximum of 1/8 inch. This specification is critical because it balances the need for proper alignment, which is crucial for effective welding and overall structural integrity, while also providing a practical limit to accommodate manufacturing and assembly tolerances.

In practice, allowing misalignment aids in the alignment of plates during construction and minimizes potential issues during the welding process. By capping the allowable misalignment at 1/8 inch, the standard helps prevent excessive deviation that could lead to defects in the welding joint or potential weaknesses in the structure. Understanding these parameters is vital for ensuring the safety and reliability of welded tank constructions as outlined in API 650.