8.5 Piping Rerating and Alterations (API 570)
Key Takeaways
- Repairs restore damaged areas; alterations change design geometry, materials, or components; rerating changes the pressure-temperature rating or MAWP of existing piping.
- Rerating requires engineering calculations, updated nameplate or marking, documentation in inspection records, and often a pressure test at the new rating.
- Derating (lowering MAWP) is mandatory when calculated MAWP of corroded pipe or flange ratings falls below current operating pressure.
- The authorized inspector must authorize alterations and rerating work; the piping engineer approves design calculations and methods.
- Incomplete rerating documentation—including skipped pressure tests or missing flange verification—fails both API 570 exams and owner-user audits.
8.5 Piping Rerating and Alterations (API 570)
API 570 Section 8 governs repairs, alterations, and rerating of in-service piping. Inspectors who confuse these terms risk authorizing work that is code-compliant on paper but unsafe in operation. Repairs restore integrity to damaged areas without changing the system's pressure-temperature design basis. Alterations physically or metallurgically change the piping system—routing, diameter, material, or components—in ways that may require new calculations and testing. Rerating changes the pressure-temperature rating or maximum allowable working pressure (MAWP) assigned to existing piping, either upward (increase) or downward (derating).
Definitions: Repair vs. Alteration vs. Rerating
| Activity | Definition | Typical Examples | Engineering / Test Triggers |
|---|---|---|---|
| Repair | Work that returns a component to a sound condition consistent with original design | Insert plate, Type B sleeve, gasket replacement | Repair design by engineer; inspector authorization; NDE; local leak test |
| Alteration | Physical change to configuration, material, or design conditions beyond original scope | New branch connection, pipe size change, material upgrade spool | Full design calculations, inspector authorization, pressure test often required |
| Rerating | Change to stamped or documented MAWP / temperature rating without necessarily replacing major lengths | Increase MAWP after verifying thicker pipe was installed; derate after corrosion | MAWP calculations, flange/valve rating check, marking update, pressure test per code |
A weld repair of a thin spot is a repair. Installing a new 8-inch branch on a 6-inch line is an alteration. Raising the operating pressure from 300 psig to 400 psig after verifying wall thickness supports the new MAWP is a rerate—even if no new pipe is added.
Rerating Requirements Under API 570
Rerating must comply with the original construction code (typically ASME B31.3) and API 570 Section 8. Key elements:
Engineering Calculations
The piping engineer must calculate the new MAWP using the actual measured thickness (not nominal) and appropriate allowable stress at design temperature. Calculations must account for:
- Minimum wall thickness at the thinnest CML
- Joint efficiency and weld strength reduction
- Temperature derating of allowable stress
- External loading (weight, thermal expansion) if rerating increases stress levels
For increased rating (up-rating), every pressure-containing component in the bounded system—pipe, fittings, flanges, valves, blinds—must meet or exceed the new rating at design temperature.
Flange and Component Limits
A common exam scenario: wall thickness supports a higher MAWP, but Class 150 flanges remain in the circuit. Flange pressure-temperature ratings per ASME B16.5 (or applicable standard) may govern at elevated temperature. The rerate is invalid if any component becomes the limiting element.
Derating (Lowering MAWP)
Derating is required when:
- Calculated MAWP from current thickness falls below operating pressure
- Corrosion has consumed allowance such that retirement thickness is approached
- A component downgrade (e.g., temporary valve replacement) lowers system capability
- Flange rating at operating temperature is exceeded even if pipe wall is thick
Derating is not optional when math demands it. The owner-user must lower operating pressure, replace components, or repair/replace pipe to restore margin.
Pressure Testing After Rerating
When rerating follows physical alteration or when the construction code mandates, a hydrostatic or pneumatic test at the new test pressure is required. Test pressure follows B31.3 rules (typically 1.5× MAWP adjusted for stress ratio). If only a documentation rerate is performed on unchanged piping with adequate history, the code may allow engineering justification in lieu of retest—this is owner-user procedure dependent, but the exam default is: significant rerating after alteration = pressure test.
Marking and Documentation
After rerating:
- Nameplate or marking on piping or circuit documentation must reflect new MAWP and temperature limits
- P&IDs and isometrics updated
- Inspection records must show date, calculations, engineer approval, inspector authorization, test results, and component verification
Alteration Requirements
Alterations include adding branches, changing metallurgy, replacing long spools, or re-routing. API 570 requires:
- Design by piping engineer per construction code
- Authorization by authorized inspector before work
- Welding per qualified WPS and Section IX
- NDE per construction code and API 570
- Pressure test unless specifically exempted with engineering justification
- Material verification per API RP 578 when required
Closure welds (golden welds) on alterations may use 100% volumetric NDE + surface NDE when isolation for hydrotest is impractical—see Section 4.3—but this is not a free pass for rerating documentation.
Inspection Records Implications
Rerating and alterations permanently change the piping circuit file. Minimum record updates:
- Revised MAWP / design temperature table
- New thickness basis and CML locations if geometry changed
- Weld maps, NDE reports, MTRs, PMI records
- Pressure test report or approved exemption
- RBI revalidation if consequence or probability changed
Missing any element creates audit findings and invalidates interval extension arguments—the inspector cannot prove the system still meets code without a complete chain.
Practical Exam Scenarios
Scenario A: Corrosion Forces Derating
A Class 2 hydrocarbon circuit operates at 285 psig; original MAWP is 300 psig. UT shows minimum thickness yielding calculated MAWP of 260 psig. Action: Immediate derate to ≤260 psig or take out of service for repair/replacement. Continuing at 285 psig violates API 570. Inspector documents notification to operations and engineering evaluation.
Scenario B: Up-Rating Blocked by Flanges
Engineering calculates pipe wall supports MAWP 500 psig at 400°F. Circuit contains Class 300 flanges rated to 370 psig at 400°F per B16.5. Maximum rerate MAWP = 370 psig, not 500 psig, unless flanges are upgraded.
Scenario C: Alteration Without Test
A 12-inch hot tap branch is welded onto a live line; work is authorized and NDE completed. Engineer specifies hydrotest, but operations claims outage cannot be spared. Inspector cannot accept permanent alteration without test or code-approved alternative documented by engineer. Incomplete record = failed inspection.
Scenario D: Repair vs. Rerating Trap
A Type B split sleeve repairs localized thinning; original MAWP unchanged. This is a repair, not rerating—no MAWP recalculation unless the sleeve changes the design basis. Contrast with replacing entire spool with thicker Schedule 80 pipe to justify higher pressure—that is alteration + rerate.
Exam Tips
- Flanges often govern rerating—always check pressure-temperature tables.
- Derating is proactive integrity; ignoring low MAWP is a serious violation.
- Inspector authorizes work; engineer approves design—memorize the split.
- Pressure test after rerating/alteration is the default expectation unless a documented exemption exists.
A piping circuit's calculated MAWP based on current thickness is 220 psig, but the unit continues operating at 250 psig. According to API 570 principles, what is required?
An owner-user proposes increasing MAWP from 300 psig to 450 psig after verifying pipe wall thickness is adequate. Class 300 flanges in the circuit are rated to 385 psig at operating temperature. What governs the maximum rerate?
Which activity is classified as an alteration rather than a repair under API 570?