This is the technical "how-to" manual. When the NBIC determines a repair is needed, PCC-2 provides the detailed engineering procedures. For our topic, Article 2.12 specifically gives the design and calculation rules for fillet welded lap patches. It includes the formulas for determining required patch plate thickness and the size of the attachment welds to manage bending stress from eccentricity.
| Mistake | Why it Violates ASME | | :--- | :--- | | | Stress concentration factor increases infinitely. ASME requires radiused corners (min 1 inch radius). | | Undercutting the weld | Reduces net thickness below required minimum. | | Using mild steel on a chrome-moly vessel | Galvanic corrosion or thermal expansion mismatch. | | Forgetting the vent hole | A full penetration weld on a patch traps gas. ASME requires a 1/8" weep hole to prevent pressure buildup between patch and shell. | | No post-weld heat treatment | Leaves residual tensile stress, leading to stress corrosion cracking. |
Used when a shutdown is not immediately possible or when the structural degradation is localized. asme standard patched
In ASME terminology, they are synonyms. However, a "repair plate" often implies a temporary fix, while an "ASME standard patched" repair is permanent.
The type of patch used is a critical engineering decision that dictates the safety and longevity of the repair. There are two main types, each with distinct characteristics, regulatory acceptance, and engineering considerations: This is the technical "how-to" manual
: Must be a full-penetration butt weld, often requiring double-welding where accessible.
It is critical to distinguish between the standards used for ASME Section VIII : Focuses on the new construction It includes the formulas for determining required patch
A plate is placed over the damage and welded with fillet welds.
A flush patch requires cutting out the defective, thinned metal entirely and inserting a replacement plate that aligns exactly with the outer diameter of the existing shell. PCC-2 - Repair of Pressure Equipment and Piping - ASME
Under standard code guidelines, not all "patches" are created equal. They are classified rigorously by how they transfer pressure loads and their long-term viability.