1. PURPOSE
1.1 This global engineering specification defines the temperature monitoring required and the suggested cooling methods that may be used when welding transition joints (stainless steel to aluminum) into a pipe system or an assembly.
2. SCOPE
2.1 This specification applies to the installation of transition joints as supplied by RBDH Chaponnay and Asahi; and friction-welded joints as supplied by The Welding Institute and modified by Air Products. Any other transition joints shall be installed using the principles outlined in this specification or to the specific manufacturer’s recommendations. Caution: Transition joints can be seriously damaged during installation when they are allowed to overheat. For this reason, stringent precautions must and shall be taken during the installation of such joints.
3. RELATED DOCUMENTS
3.1 None.
4. GENERAL REQUIREMENTS
4.1 The temperature of the transition joint bond area shall be maintained as low as possible and continuously monitored. This shall be achieved by cooling the joint bond area during welding. When using wet rags to cool the joint, provision shall be made so that water does not drip or run into the weld area.
4.2 Whenever practical and the design permits, transition joints shall be welded into pipe assemblies or prefabricated sections outside the final cold box assembly. In some instances, this may result in the need for additional butt welds within the cold box assembly.
5. SPECIFIC REQUIREMENTS
5.1 RBDH Chaponnay
5.1.1 RBDH transition joints are generally supplied with a six- or an eight-level temperature indicator label graduated from 132° to 182°C (270° to 360°F).
5.1.2 To help the welder control the heat buildup, an additional eight-level temperature indicator label graduated from 116° to 154°C (241° to 309°F) should be added to the transition joint on both the aluminum and the stainless sides of the transition joint. The indicator shall be placed between the welding operation and the cooling media. It is recommended that 154°C (270°F) not be exceeded.
5.1.3 The maximum allowable weld temperature is 180°C (356°F). Under no circumstances shall the final panel on the temperature indicating panel added by RBDH [182°C (360°F)] be allowed to turn black. If this occurs, the joint is damaged and shall be removed and replaced.
5.1.4 During and after installation, transition joints shall have the manufacturer’s original temperature label securely attached or the joint shall be deemed unacceptable and shall be removed and replaced.
5.1.5 It is suggested that the fabricator monitor the heat input with a 121°C (250°F) Temp stick.
5.1.6 A visible gap between the aluminum to stainless steel interface may be seen on receipt or occur during welding. This gap primarily occurs in large sizes of transition joints and is caused by the differential thermal expansion between aluminum and stainless steel when they are manufactured or heated during welding. A slightly visible gap (hairline) is acceptable if the temperature indicating strip does not exceed 180°C (356°F) reading.
5.2 T.W.I./A.P. Friction Welded Joints
5.2.1 These transitions are less affected by temperature. A six-level temperature indicator label graduated from 241° to 290°C (466° to 554°F) shall be applied to the aluminum/stainless interface.
5.2.2 Welding shall be controlled to ensure that the middle indicator panel that is 260°C (500°F) is not allowed to turn black.
5.2.3 Under no circumstances may the final panel 290°C (554°F) be allowed to turn black. If this occurs, then the joint is damaged and shall be removed and replaced.
5.3 Asahi
5.3.1 Precautions shall be taken to avoid overheating the explosion bonded section of Asahi transition joints. It is recommended that the aluminum end of the joint is welded first before welding the stainless steel end. If necessary, preheat the aluminum using temperatures less than 80°C (176°F) to avoid overheating. A temperature indicator shall be pasted on the explosion bonded interface of the joint. The temperature indicator shall not be removed before weld completion. After welding is complete, discoloration of the temperature indicator must be checked.
5.3.2 Explosion bonded transition joints with a titanium interface (CCJ) shall not exceed a temperature of 350°C (662°F) during welding.
5.3.3 Explosion bonded transition joints with a silver interface (CCJ-AG) shall not exceed a temperature of 200°C (392°F) during welding.
6. RECOMMENDED COOLING METHODS
6.1 To prevent the transient joint from overheating during welding, it is recommended that wet rags or cooling coils be placed on the joint bond area to control the maximum temperature.
6.2 The rags shall be of industrial quality with high water absorbency and shall be kept dripping wet during the entire welding operation.
6.3 For larger transition joints, metal cooling coils with or without heat conductive putty may be used. Cooling coils shall be made out of metal tubing and shall be held tightly against the transition joint bond area while circulating water through the cooling unit. The bond area of the joint and cooling coil may be covered with heat conductive putty (McMaster Carr # 3561K1 or Bond Zone heat protection clay).
6.4 Alternative cooling methods may be agreed upon with the Air Products representative on a case-by-case basis.
6.5 Welding shall be done in small steps that balance the heat input and weld shrinkage.
6.6 The welding process shall be interrupted when the temperature approaches 121°C (250°F). Welding shall be stopped by ramping down or decreasing the welding current before stopping the weld. This decrease in the weld current may be accomplished by the use of a hand remote control, a ramp circuit built into the power source, or with a remote foot control.
6.7 Welding shall be performed in accordance with approved welding procedures, except that the heat buildup in the joint shall be minimized by using short weld run lengths followed by allowing the joint to cool before depositing the next run.
6.8 When the ambient temperature is below 5°C (41°F) or when there is moisture condensing on the welding surface, preheating before welding is recommended. In joints less than DN250 (NPS 10), the preheat shall be achieved by blowing with warm air and not by the use of gas heating torches.
7. MATERIAL SUPPLIERS
7.1 Approved suppliers are: Eight-Level Temperature Indicators: Part No. 5956K17 (range 241–309°F) Eight-Level Temperature Indicators: Part No. 5956K18 (range 320–390°F) Temperature Crayons: Part No. 3261K91 (225 or 250°F) Heat Conductive Putty: Part No. 3561K1 or K3 (1 or 5 gallon container) McMaster Carr PO Box 440 New Brunswick, NJ 08903-0440 Tel: 732-329- 3200 Fax: 732-329-3772 Email: nj.sales@mcmaster.com Eight-Level Temperature Indicators for the temperature range of 116° to 154°C (241° to 309°F): Ref. No. Type “C” Eight-Level Temperature Indicators for the temperature range of 204° to 260°C (399° to 500°F): Ref. No. Type “E” Six-Level Temperature Indicators for the temperature range of 241° to 290°C (466° to 554°F): Ref No. 06STHEOLM 8 Thermal Melt Crayons for the temperatures of 107 or 121°C (225 or 250°F): Ref. No. CMETHE United Kingdom United States Thermographic Measurements Ltd. Thermographic Measurements Riverside Buildins 1820 Pickwick Lane Unit 1 Dock Road Glenview, IL 60025 Connah’s Quay Tel.: 877-887-8367 Flintshire Fax: 847-998-6866 CH5 4DS Email: sales@thermax.com Tel: 01244818348 Fax: 01244818502 Email: sales@t-m-c.com