1、 PFI Standard TB-7 | Denotes Revision (Revised March 2004) GUIDELINES FOR FABRICATION fabrication, installation and final documentation required for stainless steel high purity distribution systems, for either liquid or gaseous products. 2. APPLICATION 2.1 Stainless steel high purity distribution sy
2、stems are typically specified for critical clean applications in the Semiconductor, Pharmaceutical, Bio-Pharmaceutical, Chemical, Fiber Optics, Food, and Dairy Industries. 2.2 This information provides a contractor the ability to review customers specifications from materials acquisition through sys
3、tem installation. This Technical Bulletin is not intended to take precedence over customer specifications or procedures. 3. REFERENCES 3.1 ASME, Section IX Welding however, manual welding may be used when it is not practical to use the automatic process. The piping system should be erected in a mann
4、er that will minimize manual welding. Inert backing gas shall be utilized for all welds. 6.2 Equipment: Automatic orbital weld equipment should include as a minimum, a constant current power supply, a control console with the capability of controlling the current and travel speed in at least four zo
5、nes, and an enclosed shielded orbital welding head with high frequency arc initiation. Where line voltage fluctuates, a constant voltage transformer may be required. 6.3 Manual GTAW equipment should include a constant current power supply, high frequency arc initiation, and a foot pedal current cont
6、rol or equivalent. The welding torch should be equipped with a gas lens. 6.4 Electrode: The electrode may be two percent thoriated or two percent ceriated tungsten, of size and configuration suitable for autogenous welds, and in accordance with the WPS and equipment manufacturers recommendations. El
7、ectrode should be sharpened to a pencil point having a flat end of 0.005“ - 0.010“. Grinding should be performed using a fine grinding wheel. 6.4.1 Tungsten lengths for orbital heads should be prepared per equipment manufacturer schedules. 6.5 Gas Tungsten Arc welds on high purity distribution syste
8、ms should be single pass, autogenous (no filler metal added). The maximum thickness of tube to be fusion welded is 0.125“ in a single pass. Typically, the use of a filler metal in any weld must have approval of the customer. 6.6 Shielding and backing gases should be procured to the required purity l
9、evel accompanied by actual analytical certifications. The fabricator or installer may be required to perform independent analytical certification of their gas delivery systems. Gas PFI Standard TB-7 | Denotes Revision (Revised March 2004) PFI Standard TB-7 | Denotes Revision (Revised March 2004) pur
10、ifying filters as required by the specification shall be used in all gas delivery systems. 6.7 After welding has been completed, a post weld shielding gas should remain flowing long enough to minimize discoloration of the weld. 6.8 Blind welds should not be permitted except as follows: An initial, i
11、dentical weld coupon must be welded using the same materials (heat number, dimensions etc.) as the blind weld and under the same welding parameters. If this weld is acceptable, the blind weld may be accomplished. After making the blind weld, a second weld coupon is made with the same materials and t
12、he same weld parameters. Only when the second weld coupon is acceptable should the blind weld be accepted. 6.9 Weld Repairs: 6.9.1 One re-weld over a weld bead rejected for incomplete penetration or excessive concavity is permissible. If the re-weld attempt fails, the weld must then be removed from
13、the system. 6.9.2 All other rejectable weld defects shall be corrected by removing the entire weld bead and heat affected zone. The joint is to be re-prepared for welding and a replacement weld installed. 7. WELD ACCEPTANCE CRITERIA 7.1 Welds should meet the following characteristics: A. Oxidation:
14、The interior surface of the weld shall be free of black, dark brown, dark blue, or grain type (sugaring) oxidation. Light straw, yellow, or light blue oxidation are acceptable. Discoloration of the interior weld and heat affected zone, should be minimized. B. Lack of fusion and/or incomplete penetra
15、tion is not permitted. C. Undercut: Undercut is not permitted. D. Misalignment: Misalignment in welds two inch in diameter, and smaller shall not exceed 10% of wall thickness. Misalignment in welds over two inches in diameter shall not exceed 15% of wall thickness (Note: Percentages may vary per cus
16、tomer specifications). E. Heat Cracking (checking): None is permitted. F. Porosity: No visually detected surface porosity is permitted. G. External Concavity: Concavity in welds two inch in diameter and smaller shall not exceed 10% of wall thickness. Concavity in welds over two inches in diameter sh
17、all not exceed 15% of wall thickness. H. External Convexity and Internal Protrusions: For welds under two inches in diameter - 0.005“ maximum. For welds over two inches in diameter - 0.010“ maximum. I. Meandering (Bead Wandering): Maximum amount allowable - 25% of the width of the interior weld bead
18、. Any amount of meandering that results in a lack of fusion is not acceptable. J. Inspections of the weld crater: The depth of the weld crater shall not exceed 10% of the weld thickness. When the end of the weld does not have full taper at its conclusion, the weld is subject to rejection. K. Weld Be
19、ad Width: The width of the weld bead on the interior surface shall be “T“ - “2T“ (where “T“ equals the wall thickness of the base metal). The weld bead uniformity shall not exceed plus or minus 1/2T. 7.2 Sample welds (workmanship welds) shall be made using the same heat No. of the material that will
20、 be used to install the system. The sample welds should be available for all personnel involved in welding and inspection. Sample welds should depict acceptable and rejectable criteria. The customer may require additional weld samples to establish a schedule of weld acceptance criteria for specific
21、projects. 7.3 Daily Weld Coupon: At the beginning and at the end of each shift (or as specified by the customer), each welder should produce one acceptable sample weld of the same size as the production welds which the weld machine is configured to produce. The welds should be logged on a daily weld
22、ing log after they are produced. 7.4 The cause of any defective weld should be determined and appropriate steps taken to correct the problem and its re-occurrence. 8. QUALIFICATIONS 8.1 Qualifications: Each welding procedure, welder, or welding operator should be qualified in accordance with ASME Se
23、ction IX. In addition, each welding operator should qualify separately on each size and wall thickness to be welded in production, by a visual examination of exterior and interior surfaces. 9. WELD DOCUMENTATION 9.1 A documentation program should be agreed upon before the project is started. A typic
24、al documentation file may consist of the following: A. Material Purchase Order (copy) B. Receiving Records PFI Standard TB-7 | Denotes Revision (Revised March 2004) C. Material Test Reports D. Receipt Inspection Reports E. Weld Procedure Specifications (WPS) and Procedure Qualification Records (PQR) F. Approved Weld Schedules (programs) G. Welder Qualification Records H. Daily Automatic Weld Log Sheets I. Weld Location Drawings J. Purge Gas Test Reports or Certification Documents K. Weld Inspection Reports L. System Testing, Flushing, or Passivation Reports
