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API Publ 937-1996: Evaluation of Erosion and Corrosion Damage in In-Service Piping Systems
Guidance for Non-Intrusive Inspection and Fitness-for-Service Assessment of Pressurized Piping
Scope and Application
API Publ 937-1996, titled Evaluating the Effects of Erosion and Corrosion on In-Service Piping, provides a structured methodology for assessing wall thinning due to erosion and corrosion in pressurised piping systems commonly found in petroleum refineries, chemical plants, and other industrial facilities. The publication addresses carbon steel, low-alloy steel, and stainless steel piping operating above atmospheric pressure.
Key scope elements include:
- Damage mechanisms: The document covers erosion caused by particulate-laden fluids, corrosion from process environments (e.g., sour service, acidic streams), and combined erosion-corrosion phenomena.
- Piping categories: It applies to Category D (severe cyclic) and Category M (moderate cyclic) services as defined in ASME B31.3, but the principles can be extended to other services with appropriate engineering judgment.
- Inspection strategy: Emphasis is placed on non-intrusive techniques (ultrasonic testing, radiography) to minimize process downtime while ensuring safety.
API Publ 937-1996 is not a mandatory code but a recommended practice. Its use is widely referenced in integrity management programs and by regulatory bodies (e.g., OSHA PSM, EPA RMP) as evidence of good engineering practice.
Technical Requirements and Assessment Methods
Damage Evaluation Procedure
The publication outlines a multi‑step approach:
- Data collection: Gather design records, operating history, process fluid characteristics (temperature, pressure, composition, particulates), and prior inspection results.
- Thickness measurement: Perform ultrasonic thickness (UT) scans at critical locations defined by flow patterns, pipe geometry (elbows, tees, reducers), and historical damage areas. A grid scheme is recommended to map minimum remaining thickness.
- Remaining life calculation: Use the measured minimum wall thickness and expected corrosion/erosion rate to estimate remaining life.
- Fitness‑for‑service evaluation: Compare the assessed condition with acceptance criteria from ASME B31.3 (or other applicable codes) for pressure containment and mechanical strength.
| Parameter | Requirement / Recommendation (API 937) | Typical Acceptance Criteria |
|---|---|---|
| Minimum remaining wall thickness | Must be ≥ 80% of nominal thickness for continued service without repair (subject to stress analysis) | ASME B31.3 Table 304.1.1 for straight pipe |
| Corrosion/erosion rate | Based on at least two UT surveys over a representative interval (recommended ≤ 3 years) | Rate ≤ 0.1 mm/year for low‑risk services; >0.3 mm/year triggers advanced analysis |
| Inspection interval | Half of the remaining life, not to exceed 5 years for moderate corrosion environments | API 570 Table 4 (thinning rates) |
| Maximum allowable working pressure (MAWP) | Recalculated using actual wall thickness per ASME B31.3 | Must equal or exceed design pressure |
| Geometric discontinuities (elbows, tees) | Minimum thickness measured at six points around circumference; local thinning accounting for bend extrados/intrados | Greater of 85% of straight pipe minimum or code‑specific |
Caution: The 80% thickness rule in API 937 is a screening threshold. For severe cyclic service, brittle materials, or high stress concentrations, a detailed stress analysis (e.g., FEA) per API 579 / ASME FFS‑1 is required before returning to service.
Non‑Intrusive Inspection Techniques
The publication recommends the following methods for data collection without disrupting operation:
- Ultrasonic thickness gauging (UT): Straight‑beam single‑element or dual‑element transducers for general wall loss; phased‑array for corrosion mapping.
- Radiographic testing (RT): Tangential RT for detecting pitting and localised corrosion, particularly in insulated piping where coating removal is impractical.
- Guided wave ultrasonic (GWUT): For rapid screening of long straight runs and unpiggable sections; indications are then followed by spot UT.
API 937 strongly advocates combining UT with profile radiography to distinguish general thinning from pitting. This hybrid approach improves the reliability of remaining life estimates and reduces the risk of unexpected failures.
Implementation and Inspection Planning
Effective deployment of the API 937 methodology requires a systematic plan:
Circuitisation and Risk Ranking
Piping is divided into corrosion circuits — segments with identical process conditions, material, and damage mechanisms. Each circuit is assigned a risk level based on corrosion rate and consequence of failure. High‑risk circuits are inspected at twice the frequency of moderate‑risk circuits.
Documentation Requirements
- Baseline thickness measurements
- Inspection history with trends
- Calculated remaining life
- Rationale for acceptance or repair decisions
- Records of any changes in process variables
To maximise the value of inspections, API 937 recommends conducting UT scans at the same grid points during each assessment to build accurate corrosion rate trends. Electronic data management systems (e.g., PCMS, RBI software) are highly beneficial.
Compliance and Documentation
While API Publ 937 does not carry the mandatory status of a code such as ASME B31.3 or API 570, its use is widely accepted as meeting the “good engineering practice” requirement of several regulatory frameworks:
- OSHA Process Safety Management (29 CFR 1910.119): The publication supports mechanical integrity programs by providing a systematic method for assessing piping condition.
- EPA Risk Management Plan (40 CFR Part 68): Documented use of API 937 can be part of the risk assessment for accidental release prevention.
- API 570 (Piping Inspection Code): API 937 is referenced as a guideline for evaluating thinning damage and setting inspection intervals.
Compliance with API 937 is demonstrated through:
- Written procedure describing the damage evaluation method.
- Training records for inspection personnel (must be certified to SNT‑TC‑1A or CP‑105).
- Complete inspection reports with thickness data, rates, remaining life calculations, and repair/replacement decisions.
- Management of change (MOC) documentation for any deviation from the publication’s recommendations.
Non‑compliance risk: Failure to follow recognised practices like API 937 can lead to regulatory citations under OSHA’s general duty clause if a piping failure occurs. Insurance carriers may also deny claims if damage evaluation was not performed to accepted industry standards.
Frequently Asked Questions
Q: Is API Publ 937 still current?
A: The 1996 edition remains in use as a reference document. While API has not issued a formal revision, its principles are incorporated into newer integrity management documents (API 570, API 579‑1/ASME FFS‑1). Operators should check with API for any reaffirmation or replacement notices.
A: The 1996 edition remains in use as a reference document. While API has not issued a formal revision, its principles are incorporated into newer integrity management documents (API 570, API 579‑1/ASME FFS‑1). Operators should check with API for any reaffirmation or replacement notices.
Q: Can API 937 be used for piping in non‑petroleum services (e.g., chemical plants, power generation)?
A: Yes, the methodology is generic and can be applied to any pressure piping where erosion‑corrosion is a concern. The user must adapt the acceptance criteria and inspection intervals to the applicable codes (e.g., ASME B31.1 for power piping, BS EN 13480 for European installations).
A: Yes, the methodology is generic and can be applied to any pressure piping where erosion‑corrosion is a concern. The user must adapt the acceptance criteria and inspection intervals to the applicable codes (e.g., ASME B31.1 for power piping, BS EN 13480 for European installations).
Q: How does API 937 relate to API 579 (Fitness‑for‑Service)?
A: API 937 provides a simpler screening assessment for wall thinning. If the damage exceeds the 937 thresholds, a detailed Level 2 or Level 3 assessment per API 579 is required. The two documents are complementary in a staged integrity evaluation.
A: API 937 provides a simpler screening assessment for wall thinning. If the damage exceeds the 937 thresholds, a detailed Level 2 or Level 3 assessment per API 579 is required. The two documents are complementary in a staged integrity evaluation.
Q: What training is needed to apply API 937?
A: Inspectors should be qualified in NDT methods (UT, RT) and have a working knowledge of ASME B31.3 and API 570. Engineering personnel performing remaining life calculations should be familiar with basic stress analysis and corrosion mechanisms. API offers training courses that cover the publication.
A: Inspectors should be qualified in NDT methods (UT, RT) and have a working knowledge of ASME B31.3 and API 570. Engineering personnel performing remaining life calculations should be familiar with basic stress analysis and corrosion mechanisms. API offers training courses that cover the publication.
© 2026 Technical Review. This article is based on API Publ 937 (1996) and reflects current industry practice as of 2026. Always verify the latest edition with the American Petroleum Institute.