For a B16.11 plug included in a Section VIII-1 design, determine MDMT from the plug’s documented material and the applicable construction-code rules; do not assign a permitted temperature from the fitting standard designation alone. A design reaching -20°F needs a documented basis for the plug, or a boundary that ends before the threaded connection.
Does the pressure boundary include the plug?
Trace the pressure boundary first. If the vessel design terminates at the threaded connection and the plug is omitted, the plug is not part of that vessel boundary. If the plug closes the connection and contains pressure, it is part of the pressure-containing assembly and needs a supported MDMT basis in the design documentation.
Resolve this on the vessel drawing and design basis before selecting material. A plug omitted from the design cannot serve as a pressure boundary component; a plug that will be installed in service must be treated consistently with the design and inspection documentation.
| Design arrangement | MDMT decision |
|---|---|
| Vessel boundary ends at the threaded connection; plug is left out | Document the boundary termination and verify the design does not rely on the plug to contain pressure. |
| Plug is installed and closes a pressure-containing connection | Determine and document the plug material’s permitted MDMT under the applicable Section VIII-1 rules. |
Check: The approved drawing and pressure-boundary definition show whether the plug is included before material selection proceeds.
What material records connect the plug to its MDMT?
Identify the actual plug material, not just its B16.11 size or fitting designation. The B16.11 designation identifies a fitting standard; it does not by itself establish the material’s impact-test status or the assembly’s permitted MDMT.
Collect the material specification, grade, heat identification, and material test documentation for the plug. Compare those records with the applicable construction-code material requirements and the design temperature. A material specification that requires impact testing may provide a direct path; SA-420 and SA-350 were cited as examples of specifications to consider. Confirm the selected grade and supplied product records actually match the specification and required testing.
| Record or designation | What to verify |
|---|---|
| B16.11 plug designation | Fitting identity and configuration; not a standalone MDMT determination. |
| Material specification and grade | That the plug’s documented material is the material evaluated in the design. |
| Heat and test records | Traceability and any required impact-test documentation. |
Check: Material traceability links the installed plug to a specific specification and grade, with supporting records available for the design review.
Can the design use the UG-20(f) exemption?
Evaluate the proposed UG-20(f) exemption against the actual component and design conditions. The exemption is a code-based decision, not a general permission to treat an undocumented plug as acceptable at a lower temperature. Verify each applicable condition in the governing Section VIII-1 edition and record why the plug qualifies.
If the exemption conditions do not apply, select a material and testing path that satisfies the code rules for the specified MDMT. Do not infer eligibility solely because the plug is small, threaded, or commercially available. The engineering record should identify the basis used: applicable exemption, qualifying material and testing, or exclusion of the plug from the pressure boundary.
Check: The calculation or design review cites the governing rule and documents the conditions used to accept the exemption, or proceeds with an alternate material basis.
How should the design resolve a -20°F requirement?
At a stated design temperature of -20°F, compare that requirement with the plug’s documented material basis and the construction-code acceptance path. The available choices are to use a plug whose material and required testing support the design temperature, demonstrate a valid UG-20(f) exemption, or define the vessel boundary so the plug is not part of it.
- Confirm the governing Section VIII-1 edition and the design MDMT in the design documents.
- Determine whether the plug contains pressure in the approved boundary arrangement.
- Match the plug’s material specification, grade, and traceability records to the code evaluation.
- Document the applicable exemption or material/testing basis for the required temperature.
- Revise the design or material selection if the documented path does not support -20°F.
Impact-tested material may avoid uncertainty about relying on an exemption, but its use should follow the code evaluation and procurement specification rather than a blanket rule. Specify the exact material and documentation requirements in purchasing records so the delivered plug can be matched to the approved design.
Check: The calculation package, drawing, and purchase specification agree on the pressure-boundary arrangement, material, and MDMT basis.
What recurring errors undermine the determination?
Do not equate a fitting standard with impact qualification. Do not treat a suggested material family as proof that a specific plug’s supplied grade passed the required tests. Do not apply UG-20(f) without checking all governing conditions, and do not leave a plug’s pressure-boundary status ambiguous on the drawing.
When documentation is incomplete, resolve the uncertainty through the material records and applicable code evaluation—not by assigning the vessel’s temperature rating to the plug. If the manufacturer’s records cannot establish traceability, choose a documented acceptable material path or remove the plug from the pressure boundary by design.
Check: An independent design review can reproduce the MDMT decision from the drawing, material records, and cited code basis without relying on an undocumented assumption.
What happens if the plug is left out?
What happens if the vessel boundary ends at the threaded connection?
The plug is outside the defined vessel pressure boundary if the design genuinely terminates there and the plug is omitted. Confirm the drawing and service arrangement do not rely on that plug to contain pressure.
What happens if the B16.11 plug has no impact-test record?
Use its traceable material specification and grade to evaluate an applicable code path, including any claimed UG-20(f) exemption. If neither the material/testing basis nor the exemption supports the design MDMT, select a documented material solution or exclude the plug from the pressure boundary.
What happens if the design MDMT is -20°F?
The installed plug needs a documented acceptance basis for that temperature, such as qualifying material and testing or a verified applicable exemption. The specification designation alone does not establish that basis.
Final verification: Before release, confirm the approved boundary drawing, traceable plug material records, and Section VIII-1 calculation all document the same MDMT acceptance path.