This section presents the practices necessary to repair the body bore area, which serves as a corrosion-resistant sealing surface for the valve bonnet seal ring. This section also addresses the repair of the valve bonnet seal area (see Figure 8-2a).
Figure 8-2a
Pressure Seal Repair Location [18]
8.2.1 Repair Strategy
Leaking of a pressure seal joint can be the result of insufficient load on the pressure seal gasket; corrosion, chips, or other foreign matter on the sealing surfaces; damage to the pressure seal gasket itself; or surface imperfections in the body wall in the form of pin holes, cracks, erosion, or indentations from assembly. Only the surface imperfections require repair techniques. The other issues can be resolved with standard maintenance practices.
available from Sunnen) as shown in Section 9.4. If the defects are greater than .010 inch (.25 mm), a weld repair should be performed. If the leak is the result of a flaw on the bonnet sealing surface, the flaws can be removed by machining the angled sealing surface in a lathe and polishing it with an emery cloth. Larger defects up to .020 inch (.20 mm) might be scalable using a graphite pressure seal ring.
The following questions should be asked in developing a repair strategy for the repair of the pressure body bore:
• Does the bore have a welded inlay of corrosion-resistant weld metal? Many manu-facturers do not deposit an inlay on forged stainless steel valves but do on cast stainless steel.
• Is the flaw or crack localized such that it could be removed by grinding? If so, ma-chining might not be required.
• Does the flaw or crack encroach upon the retaining ring groove such that machining of the groove will be required after repair? If so, a boring bar will be required.
• Is the flaw or crack deep enough that weld repair would cause distortion to the valve body bore? If so, a boring bar will be required.
• Will removal of the crack or flaw violate minimum design wall thickness? If so, can a qualified base metal repair be performed? The base metal repair should be per-formed as presented in Section 8.1.
• Is machining and welding equipment available to perform the repair?
• Are sufficient qualified and experienced personnel available to perform this repair?
• Does the owner or repair vendor possess the proper NBIC repair certifications and ASME Section IX qualified welding procedures?
• Are sufficient rigging points and space available to move equipment in and out of the valve?
• Is equipment available to preheat the repair area? If required, can the welding and machining equipment withstand elevated temperatures?
8.2.2 Repair Prerequisites
The following should be considered prior to initiating a repair:
• Record the manufacturer’s name, size, pressure rating, valve type, body style, body material, and trim material data from the valve nameplate.
• Obtain drawings and material specifications from the manufacturer.
• Obtain the original design minimum wall thickness from the manufacturer.
• Obtain and review the maintenance and repair history to see if there have been any changes to the original design.
• Procure a replacement pressure seal ring and an oversize ring.
• Prepare a repair “traveler” or work instruction that outlines the repair sequence and inspection “hold points.” All measurements and findings should be recorded for a final report.
• Install clean plugs in the upstream and downstream bores of the valve to prevent chips and foreign materials from getting down the line.
• Install a protective liner over the in-body seats.
• Qualify the necessary welding procedures for hardfacing and base material buildup and repairs.
• Demonstrate the machining and welding sequence on a full-scale mockup to famil-iarize personnel with the work sequence and requirements.
• Measure and record critical dimensions, such as guide bores, distances between seats, retainer ring groove diameter and depth, diameter at the top of the body bore, and the pressure seal bore, to monitor distortion due to welding.
8.2.3 Flaw Removal
The flaw should be removed by grinding or machining.
If grinding is being utilized to remove a flaw, liquid penetrant should be applied to the area to help identify the crack location. After completely removing the flaw, the area should be smoothed and tapered to provide good welding visibility and access.
If the flaw distribution is extensive or if orbital welding equipment is going to be uti-lized for welding, a boring bar should be utiuti-lized to remove material about the com-plete bore area as shown in Figure 8-2b. A 2:1 or 3:1 transition should be employed at the upper and lower end of the weld preparation groove.
End View
Chuck Module
Tool Head
Stand Off
Figure 8-2b
Example of Pressure Seal Area Boring [23]
After completing the grinding or machining operation, an MT or PT examination of the weld preparation area must be performed to ensure that no unacceptable indications remain prior to welding.
8.2.4 Filler Material Selection
An austenitic stainless steel wire or electrode is the filler material for pressure seal inlays that is specified by the valve manufacturers. Alloys typically referred to by valve manufacturer include 18-8, Types 302, 304, 308, 309, and 316. The Type 309L is most commonly used by the valve manufacturers.
8.2.5 Preheat and Post-Weld Heat Treatment Requirements
Preheating and PWHT should be performed as presented in Section 13. Preheating provides very positive benefits to the welding application and final product. Preheating is employed to reduce the tendency for cold cracking (or hydrogen-assisted cracking), to reduce the hardening of the heat-affected zone (HAZ), to reduce the residual stresses, and to decrease component distortion.
Because of the differences in coefficient of thermal expansion between the stainless steel filler materials and the carbon or low-alloy steel substrates, high residual stresses, which promote distortion and cracking, are very common. As a result, a 400˚F (204˚C) preheat is strongly recommended when performing this repair to P-Nos. 1, 4, and 5 valve bodies (see Table 11-1). If necessary, the preheat can be lowered to the tempera-tures suggested in Table 13-1. If a repair is made to a cast stainless steel valve body (CF3 or CF8), a minimum preheat of 100˚F (38˚C) is suggested.
Castings that have been inservice might have water in the defect and surrounding area.
This water/moisture needs to be driven off with local heating to eliminate the potential for porosity. This localized preheating can be performed with an oxyacetylene torch.
Except as required by Code, stress relieving should be avoided. Post-weld heat treating is required for all weld repairs performed on P-Nos. 4 and 5 substrates. Exemptions to PWHT are available for P-No. 1 materials, based on the thickness and preheat tempera-tures, as provided in Table 13-4.
8.2.6 Welding Repair
Repair welding of pressure seal bores falls into two categories; local cladding and base metal repair, and bore buildup. Local repairs are generally performed with the GTAW and SMAW processes where weld quality and control is most important. The GTAW, SMAW, GMAW, and FCAW processes have been used for full diameter bore buildups.
The automatic GTAW and SMAW processes are generally used for large repairs, or where radiation levels require remote welding. Several automated systems, as shown in Appendix B, are commercially available to perform overlay welding.
Localized base metal repairs and deposition of corrosion-resistant cladding require special attention in order to ensure a successful repair. The following steps should be considered when making these high stress weldments:
1. Establish welding parameters prior to initiating a weld in the component.
2. Establish the weld sequence such that the first layer of weld metal covers the total repair cavity.
3. If SMAW is employed, use a 3/32-inch (2.4-mm) electrode on the first two layers. If additional layers are required, larger electrodes can be used.
4. Use thin stringer beads to reduce heat input and lower residual stresses.
5. Deposit welding on second and subsequent layers in a manner that controls residual stresses. Weld beads should tie into previously deposited weld metal and not onto base material.
8.2.7 Final Machining
The extent of the weld repair dictates the type of machining to be performed. In the case of small localized repair, the controlled grinding approach can be utilized. A hand grinder and a thin template cut from shim stock of .010-inch (.25-mm) thickness or less, having the same radius as the body bore diameter, can be used to reduce the weld buildup to the height of the shim stock above the bore. A flapper wheel can then be used to blend the weld buildup to just below the body bore. A straight edge and feeler gauges can ensure that the repair area is no more than .010 inch (.25 mm) below the surrounding material. A hone can then be used to finish the bore and blend the repair.
Where major welding has been performed, or where the welding has impacted critical dimensions at the top of the bore or retainer ring, a boring bar similar to that shown in Figure 8-2b should be used. Regardless of the machining technique, a hone should be used to finish the sealing area (see Section 9.4). A finish of 32 rms or finer is recommended.
Dimensional tolerances of .005 inch (.13 mm) on the diameter are required to obtain a good seal with an iron seal ring. The recent development and use of graphite seal rings have provided tolerances of up to .010 inch (.25 mm). These new rings also provide some tolerance for finish and small scratches up to about .010-inch (.25-mm) deep.
8.2.8 Inspection
The ASME Section XI Boiler and Pressure Vessel Code and the ANSI/ASME B31.1 Power Piping Code provide guidance for the repair and inspection of valve materials defined as pressure-retaining. ASME Section XI, Subarticle IWA-4400 requires that the repair welding be performed by qualified welders and procedures in accordance with Section IX. It also requires that the repair cavity (pressure-retaining base material) shall be examined by the liquid penetrant or magnetic particle method. A final PT must also be performed after final machining.
8.2.9 Testing
Pre-service testing shall be performed in accordance with the requirements of ASME/
ANSI OM, Part 10, when the owner determines that the repair activity affects the valves performance or designed function. This could include a local leak rate test (LLRT), an integrated leak rate test (ILRT), or a operability/functional test. If the pressure bound-ary has been penetrated, a hydrostatic test is required.