Positive Material Identification (PMI) is a process of verifying that the nominal chemical composition of the alloy components and associated welds have been correctly supplied, installed and documented as specified, thus minimizing the risk of component failure and release of hazardous or toxic liquids and gases.
This article covers the guidelines for positive material identification (PMI) of pressure-retaining alloy material components, alloy bolting, welding consumables, welds, weld overlays and cladding. PMI examination may be carried out at the fabricators, vendors or material suppliers facility.
In the context of PMI, alloy material is defined as any metal containing alloying elements such as chromium, nickel or molybdenum that are added to enhance the mechanical properties and/or chemical resistance. It also includes filler metals used for welding alloy materials. Thus carbon steel, cast or ductile iron, aluminium and aluminium alloys, copper and copper alloys which are excluded from scope of PMI.
General Requirements
Testing performed by a manufacturer or supplier of raw material does not fall under the category of PMI examination.
- PMI testing should not considered as a substitute for the required Material Test Reports or Certificates.
- Material test reports should be considered as an acceptable alloy verification technique.
- The area to be tested should be solvent cleaned for removal of grease, slag (weld surfaces), oil or paint.
- The surface finish of the area to be PMI examined should be as per instrument manufacturers recommendation.
- The method and frequency of calibration and standardization shall be based on the instrument manufacturers recommendation.
- PMI testing should be conducted prior to post weld heat treatment, hydrostatic test, painting or insulation of any piping or equipment.
- PMI examination should be carried out on both the outside and inside of weld surfaces where accessible.
PMI Procedure
The PMI procedure should be approved by the Owner before starting any PMI examination and shall include the following as a minimum:
- Description of the testing method, testing equipment, minimum personnel qualifications, calibration verification and the required surface preparation.
- Description of the component, the quantities in each heat or lot and the total number of components subject to PMI examination.
- Identification and segregation of the rejected components.
PMI Instruments and Techniques
The instruments and methods used for PMI testing shall be suitable for identifying the material by quantitative measurement of the major alloying elements as required in the applicable material specification or welding procedure specification. The following equipment's can be used to carry out PMI testing:
- X-ray fluorescence (XRF) Spectroscopy or X-ray emission analysis with calibrated portable instrument. It may be noted that XRF analyzers do not detect light elements such as carbon, silicon, phosphorus and sulfur.
- Portable Optical Emission Spectroscopy (OES), also known as Spark Testing, is another method of positive material identification (PMI) to check for all the required elements including carbon. OES analyzers can detect and measure the percentage of light elements such as carbon, silicon, phosphorus, sulfur, magnesium and aluminum in part. Use of this equipment may require hot work permit. Any burn damage as a result of using optical emission spectroscopy should be removed by grinding.
- ICP-OES (Inductively Coupled Plasma Optical Emission Spectroscopy). ICP-OES takes advantage of the unique emission spectra of elements to identify and quantify them.
Personnel Competency
Personnel performing PMI examination should be trained, tested and certified in operation of the PMI Instrument and interpretation in conformance to an ASTM, ASME or API standard. They should be experienced and knowledgeable on all aspects of the PMI program. PMI procedure shall contain a statement of Personnel Competency or Qualification, and their experience in safe handling of PMI Instruments and various alloys.
Personnel qualification should be verified and those judged to be unqualified should be prohibited from performing tests. The Instrument operator should perform work in accordance with the written PMI procedure and should be trained to use the instrument in accordance with the procedure. Personnel records confirming the training, qualification and experience should be made available to the Owner for their review and approval.
PMI personnel should have a comprehensive understanding of the PMI requirements, the verification methods to be used, the period during fabrication that verification will be carried out, the method of marking the verified materials and segregation and reporting of rejected materials.
Items subject to PMI
The following items should be subject to PMI:
- Pipe, Fittings and Flanges
- Forgings
- Valves including Control Valves, Instrument Valves and Fully Cladded Valves
- Plates (heads, shells, tube sheets, etc) and internals such as trays
- Thermowells
- Furnace tubes and fittings
- Casings for pumps, compressors, turbines
- Alloy welds joining pressure parts to pressure parts
- Heat exchanger tubes
- Bolting
- Metallic alloy gaskets
- Equipment's, instruments, valves and bolting that are part of skid.
Extent of Examination for Piping Bulk Materials
The extent of examination applies to piping bulk components of corrosion resistant alloys such as stainless steel and nickel alloys. The table below provides recommended extent of examination.
| Sr. No | Items | Extent of Examination at Vendor's shop | Extent of Examination at site after piping pre-fabrication |
|---|---|---|---|
| 1 | Pipes, fittings, flanges and other pressure containing parts and welds | 5% to 10% | 5% to 10% |
| 2 | Valves including Instrument isolation valves | 5% to 10% | 5% to 10% |
| 3 | Bolting | Random inspection of 5% to 10% of each lot | Random inspection of 5% to 10% of each lot |
| 4 | Control Valves and fully cladded valves | 100% | Not Applicable |
| 5 | Pressure vessel: pressure retaining parts and welds | 100% | Not Applicable |
| 6 | Pressure vessel: internals | 10% | 10% (if supplied separately) |
| 7 | Fabrication welds including tube to tubesheet welds | 10% | Not Applicable |
| 8 | Rotating equipment: pressure retaining casing and welds | 10% to 100% | Not Applicable |
| 9 | Piping system including welds after field assembly at site or yard | Not Applicable | 100% |
| 10 | Instrumentation: Pressure retaining housing | 10% | Not Applicable |
Extent of Examination for Welding Consumables
Before opening the welding consumable packages, the labels shall be visually examined to ensure that they comply with the correct material type. Electrodes shall be properly stored and segregated to avoid mix-up. Different welding consumable types should be stored in separate ovens. The fabricator should ensure that adequate inventory control is established to account for all consumables. For welding consumables, it is recommended to perform PMI on two samples per delivered batch prior to commencement of work.
Acceptance Criteria
For the PMI test to be acceptable, the alloying elements should not deviate by more than ±10% of the standard ASTM chemistry ranges. If the PMI test results falls outside the acceptable range, the fabricator may perform a chemical analysis in an independent testing laboratory to determine the component acceptance. If the required alloying elements are not detected in the PMI test, the material should be rejected, quarantined and entered into the non-compliance record system.
Material that is rejected should be segregated from acceptable material and identified by markings or color-coding to avoid mix-up and reuse. Small items which cannot be marked or color-coded may be wire tagged. The material used for marking should not have a deleterious effect on the alloy material. The marking material should be free of lead, sulfur, zinc, cadmium, mercury and halogens. The fabricator should a submit a non-conformance or non-compliance report, identifying the root cause and a plan for corrective action.
If an item fails PMI, the remaining items from the affected heat and supplier shall be subject to 100% PMI. Similarly, if a weld fails PMI, remaining welds in the piece of equipment or piping shall receive PMI. Additionally, other equipment or components being fabricated by the contractor using the same welding procedure qualification on a project shall be reviewed for additional PMI requirements.
PMI Report Documentation
The PMI report should include the following:
- PMI report number and date
- PMI method and Instrument used
- PMI Instrument calibration details including date of latest calibration
- PMI Machine output
- Name and address of fabricator, vendor or material supplier
- Purchase Order number
- PMI Procedure number
- Component or weld number
- Component description
- Material Test Report
- Total numbers inspected from heat or lot
- PMI result: Acceptance or Rejection
- Signature of inspector
References
- API RP 578 - Material Verification for New and Existing Alloy Piping Systems
- ASTM A193 - Standard Specification for Alloy-Steel and Stainless Steel Bolting Materials
- ASTM A751 - Standard Methods, Practices and Terminology for Chemical Analysis of Steel Products
- ASTM E62 - Chemical Analysis of Copper and Copper Alloys (Photometric Methods)
- ASTM E322 - X-Ray Emission Spectrometric Analysis of Low-Alloy Steels and Cast Irons
- ASTM E327 - Optical Emission Spectrometric Analysis of Stainless Type 18-8 Steels by The Point-To-Plane Technique
- ASTM E350 - Chemical Analysis of Carbon Steel, Low-Alloy Steel, Silicon Electrical Steel, Ingot Iron, and Wrought Iron
- ASTM E352 - Chemical Analysis of Tool Steels and Other Similar Medium- and High-Alloy Steels
- ASTM E353 - Chemical Analysis of Stainless, Heat-Resisting, Managing, and Other Similar Chromium-Nickel-Iron Alloys
- ASTM E354 - Chemical Analysis of High-Temperature, Electrical, Magnetic, and Other Similar Iron, Nickel, and Cobalt Alloys
- ASTM E478 - Chemical Analysis of Copper Alloys
- ASTM E527 - Numbering Metals and Alloys (UNS)
- ASTM E572 - Test Method for Analysis of Stainless Steel and Alloy Steel by X-Ray Emission Spectrometric Analysis of Stainless Steel
- PFI ES-22 - Recommended Practice for Color Coding of Piping Materials
- PFI ES-42 - Positive Material Identification of Piping Components Using Portable X-Ray Emission Type Test Equipment