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Technology of Tomorrow
“The technology of tomorrow” has already been implemented at BORSIG in Germany by Dr.Ing Andreas Risch, Head of the Welding Engineering Dept. at BORSIG GmbH, Germany, and Mr Bengt Ekelöf, Senior Project Manager at ESAB Welding Equipment AB, Sweden
First published: Svetsaren 1-2/1999
Keywords: SAW, oxyfuel cutting, pressure vessels, heat exchangers, ABW, multi-functional gantry.
Segment: Process industry
Summary: BORSIG (a company within Babcock-Borsig AG), a famous supplier of pressure vessels and heat exchangers for the chemical and petrochemical industries, has optimized its welding and cutting production process by implementing fully-automated systems for submerged arc welding (SAW) and oxy-fuel cutting.
An advanced, fully-adaptive SAW process is used at BORSIG for butt welds of up to 120 mm in joint depth. The multi-layer tandem SAW process is controlled by intelligent software which makes its own decisions for the complete welding operation.
The holes for nozzle intersections in the shell of the pressure vessel are cut by an industrial robot. Off-line programming for cutting holes with constant weld bevel angles or constant joint volume is performed using different macros.
The advanced welding and cutting systems are mounted on a multi-functional gantry which travels on rails. The gantry works in conjunction with two anti-creep roller bed stations, see Figure 1. This type of installation was specially designed by ESAB for the requirements of the BORSIG product range.
BORSIG products
As one of the leading suppliers of complete process gas waste heat recovery systems and quench cooling systems for the chemical and petrochemical industries, BORSIG designs and fabricates different types of heat exchanger and pressure vessel.
The quench coolers are used for the rapid quenching of the gas effluent from cracking furnaces in ethylene plants. The main applications for the process gas waste heat recovery systems are ammonia, methanol, hydrogen and coal gasification plants.
Examples of components in these systems, manufactured by BORSIG, include:
• Process gas waste heat boilers
• HP steam superheaters
• HT shift waste heater boilers
• Boiler feed water preheaters
• Gas/gas heat exchangers
• Synloop waste heat boilers
• Steam drums
Fig 2 shows a typical configuration for a combined reformed and synthesis gas heat recovery system.
Almost every pressure vessel or heat exchanger is a unique application, specially designed according to the requirements of the specific process or client restrictions.
All these applications involve high gas inlet temperatures (up to 1,200°C), often accompanied by high process gas pressure up to 300 bar, as well as the generation of high-pressure steam (up to 140 bar).
Quality requirements
The design and manufacture of high-pressure equipment is strictly regulated in worldwide pressure-vessel codes like AD, ASME, BS, Raccolta, Codap, Stoomwezen, IBR, JS, AS and so on. The production quality, and the quality of the welding connections in particular, is very important because of the critical operating conditions of the pressure vessels. Imperfections in the welded zone are restricted to an absolute minimum (mostly min. Group B according to ISO 5817 or better).
Every pressure vessel containing longitudinal or circumferential joints in the shells or the inlet and outlet sections is subjected to complete non-destructive testing such as magnetic particle or dye penetrant checks, as well as 100% radiographic (RT) and/or ultrasonic (UT) examination. Nozzle welds are normally completely examined by UT.
Prior to weld production, a process qualification test (PQR), including intensive non-destructive and mechanical testing, has to be performed in order to verify that the properties of all welds match the requirements specified in the applicable codes and the base materials. The process which is going to be used in production is restricted to the qualified range of the PQR when it comes to base material group, thickness range, post-weld heat treatment (PWHT), range of welding parameters (e.g. pre-heating temperature, welding speed, voltage, amperage, interpass temperature and so on), for example.
Materials and welding technologies
Due to the service conditions of the equipment that is going to be manufactured, many different steels are used to manufacture the pressure vessels and heat exchangers. The following materials are examples for the main parts (hull) of the above-mentioned pressure vessels:
• High strength C steels (e.g. SA 516 Gr.70) for shells of steam drums and WHBs
• C-0.5% Mo steels (e.g. 15Mo3) for shells and nozzles of steam drums and quench coolers
• High strength, temperature-resistant steels (e.g. 15 NiCuMoNb 5 or SA 302 Gr.B/ Gr.C) for shells and nozzles of steam drums, process gas WHBs or synloop WHBs (steam side)
• C-1.25% Cr-0.5% Mo steels (e.g. 13 CrMo 4-5) for shells of WHBs (steam side) and for gas-inlet and gas-outlet sections, tube sheets or forged rings for process gas WHBs (gas side)
• C-2.25%Cr-1%Mo steels (e.g. 10 CrMo 9-10) and C-3%Cr-1%Mo steels (10 CrMo 9-10 mod.) for gas inlet sections, shell, tube sheets and nozzles of synloop WHBs (gas side)
The above-mentioned or comparable steels were welded to themselves (e.g. shell-shell) or were combined with one another (e.g. shell-tube sheet). The thickness of the shells for steam drums and WHBs (steam side) generally ranges between 40 and 120 mm, the gas-inlet sections of synloop WHBs increase up to 250 mm. The diameter of the vessels can differ from approx. 1,000 mm (e.g. quench coolers) to 3,000 mm (e.g. WHB). Typical applications are shown in Figs 3 and 4.
Due to the wall thickness and in order to produce the required mechano-technological properties, most of the applications, especially the higher alloyed steels, require pre-heating during welding and cutting, as well as controlled energy input and restricted interpass temperatures during welding.
In order to prevent cold cracking, the high strength steels need to be pre-heated to between 150 and 250°C. C-1.25%-Cr-1%Mo- and C-2.25%Cr-1%Mo steels also require pre-heating within 200-250°and 250-300°C.
For circumferential joints, a U preparation with a weld bevel angle of 8° is normally used. Cones are welded to cylindrical parts using a V-joint preparation (complete opening angle 50°). Nozzle welds (set-in nozzle) were performed with a half-V preparation (weld bevel angle 30-40°). The main welding technologies are GTAW and SMAW for the root pass and SAW for the fill/cap layers and back welding.
In order to optimize the production quality and the efficiency of the welding operation, BORSIG has incorporated a large number of automated welding and cutting processes in its production process.
Examples include fully-automated, tube-to-tube sheet welding for heat exchangers with computerised orbital GTAW welding machines (multi-layer TIG technology with filler wire), automated GTAW hot-wire overlay welding for critical dissimilar joints, GMAW robot welding of double pipe quench-cooler elements, robot welding of special stiffener systems to thin tube sheet and CNC plasma or oxy-fuel cutting of plates using CAD data and macro-programming systems.
BORSIG’s criteria for the choice of the advanced welding and cutting systems
Up to the end of 1997, the submerged arc welding of circumferential and longitudinal joints, as well as the cutting of nozzle holes in shells and gas inlet/gas outlet sections, was performed exclusively with operator-controlled welding and cutting equipment. The quality of these operations was therefore mainly influenced by the knowledge and experience of the operators.
The use of CNC-controlled machines which require absolute programming is not suitable because the actual geometry does not always correspond to the nominal geometry (base material thickness, joint condition, accuracy of shells, misalignments and so on) Additionally, pre-programming the bead placements for the multi-run sequence leads to a reduction in efficiency due to the increase in downtime.
So, more effective automation required the implementation of intelligent and adaptive software that makes its own decisions for the entire operation. This was verified in 1997 by installating the welding gantry, including the advanced ESAB ABW technology for the SAW process and the robot cutting system which can be programmed off-line on a macro base. Only the chosen type of software guarantees the flexibility which is necessary in pressure vessel manufacture which specializes in client-oriented solutions.
The system
The fully-automatic, multi-purpose gantry was developed by ESAB and the project was realised in close co-operation with BORSIG engineers. All the elements and programming units were designed with user orientation as the starting point.
The gantry has a fully-automatic, laser-supported, submerged-arc ESAB ABW welding system (see Fig 5), which works in conjunction with two 150-tonne roller bed systems equipped with anti-creep units. This permits the welding of longitudinal seams up to 4,200 mm in length and circumferential seams with a diameter of up to 3,500 mm.
The special narrow roller bed design permits the rotation of vessels with attached nozzles or flanges with a maximum projection of 750 mm and a minimum distance of 500 mm between two nozzles and the attached parts respectively.
At wall thicknesses of up to 120 mm, which covers 95% of all BORSIG applications, fully-automatic adaptive tandem welding can be performed. At thicknesses of up to 135 mm, mechanized tandem welding with the ESAB ABW head is possible. In the event of thicker wall thicknesses of up to 250 mm, a second conventional single-wire welding head can be used.
In any case, all the automatic or semi-automatic operations were controlled by the main control PC, Fig. 6. Two 250-litre pressure tanks located at the base of the gantry automatically supply the integrated continuous flux recovery system with new flux.
Alternatively, different fluxes depending on the procedure can be supplied from the pressure tanks at the base. The flux system is equipped with built-in electrical heaters. Minimum flux consumption is guaranteed by integral flux suction and circulation.
An industrial robot of the ABB IRB 2400/S4 type, equipped with a flame cutting (oxy-fuel) burner system, is mounted on a carriage which is installed perpendicular to another carriage at the main horizontal boom of the gantry, Fig 7. Both carriages are installed as linear external robot axes which are used to position the robot and, if required, as additional robot axes during cutting operations. The cutting of nozzle holes, programmed on an off-line, macro-supported basis, is possible at wall thicknesses of up to 150 mm.
The system permits the cutting of holes with a diameter of up to 1,500 mm. The maximum ratio between the hole cut-out and the diameter of the course is 0.68. This results in a minimum shell diameter of 2,500 mm if a hole of 1,500 mm is to be cut.
Additionally, the gantry can be used as a multi-functional platform for fitting and welding nozzles and the other attachments to the vessels. For this purpose, the platform can be flexibly modified using removable insulated floor plates. The whole gantry and the roller bed systems can be positioned free on rails over a length of 45 m. All the main energy and data transfer cables were installed under the floor in cable chains covered by movable floor plates.
The ESAB ABW adaptive joint fill program
The ESAB ABW adaptive tandem welding system mounted on the gantry can handle both circumferential and longitudinal welding in a fully-automatic joint filling procedure. This is possible thanks to the intelligent software in the system.
True measurement data from the joint profile measured by an optical sensor during welding determine both the required level of the welding parameters on a continuous basis, as well as the positioning of related axes for tracking and inter-run formation.
This means that both the bead size and bead placement are controlled by the system software in a self-adaptive manner for all the fill layers including the cap.
The system software influences the following four fill parameters:
• The weld speed
• The current
• The bead placement
• The number of beads in fill and cap layers
This unique feature enables ABW to adapt the parameters to match deviations in the cross-sectional area and geometry along the entire joint line.
Each of the four fill parameters influenced by the ESAB ABW system software performs different tasks in the adaptive ABW weld fill procedure.
• The weld speed controls the amount of weld metal deposited in different areas along the joint line.
• The current controls both the bead height and the amount of weld metal deposited in different areas of the joint.
• The bead placement influences the pattern of the inter-run formation in different areas of the joint.
• The selected number of beads in each layer determines the inter-run penetration, the shape of the actual joint bottom and the degree of weld fill.
The unique ESAB ABW weld technology is designed to give manufacturers dealing with high quality butt welding a 100% automatic multi-layer technology, thereby enabling them to produce a defect-free weld fill, even if the joint geometry deviates from the nominal configuration.
The configuration of the ESAB ABW system was specially modified according to BORSIG requirements for flexible production. The modified system can also verify joints between shells and thicker flanges (step on one side near the weld), as well as joints between shells and cones, with the automatic generation of a smooth transitional contour between the two parts.
Registration and documentation of the welding operation
Continuous, fully-automatic, multi-run tandem welding for many hours with limited operator surveillance requires not only excellent man-machine communication (MMC), during the weld fill operation, but also a report system which explains how the work has been accomplished.
The ESAB ABW operating system software installed at BORSIG includes a report system in which welding and positioning data from the operation are registered in two separate files – the weld report file and the log file.
In the weld report, all the installation parameters such as wire type and wire dimension, flux type and permissible interpass temperatures are stored, together with the specified process parameters such as welding voltage, welding current and welding speeds and their report, alarm and stop limits.
All the important events during welding, such as start, stop(s), re-starts, exceeded report limits and warnings for flux level, high or low interpass temperature, are stored in the weld report. All the events are stored together with the actual date, time, weld layer, weld bead and position in the joint. Should the event be an exceeded process parameter, the parameters at the time in question are also stored.
In the log file, the position and process parameters are continuously registered (every 20 mm). A normal log file report for a thick-walled welding object could fill 1,000 pages.
Robotic oxy-fuel cutting of nozzle holes
Due to the saddle contour of a tubular intersection in a cylindrical shell, the programming of the hole-cutting operation is mathematically complicated.
In order to simplify the program procedure, a computer-based, off-line programming system of the ARAC type is used.
Two macros verify the calculation of the saddle contour and transfer it to robot co-ordinates. Cuts with either a constant groove opening or a constant weld volume can be made. The operator only puts the following data into the macro:
• Shell diameter
• Wall thickness
• Diameter of the hole/shell intersection
• Angle of the weld bevel
• Cutting parameters (e.g. pre-heating time, gas parameters, cutting speed and so on)
• Off-set for position of the cut-out and the cut width.
After transferring the program from the off-line PC to the robot control unit and putting the robot in the cutting position, a special measuring program is started prior to the operation.
A special measuring sensor mounted at the burner tip performs a stepwise control of the surface at the location of the cut-out in a test mode.
Deviations from an optimum cylindrical surface are corrected in the macro by setting an additional off-set in order to secure a constant distance between burner-tip and the metal surface on each occasion. This is a vital function for a good and reproducible cutting result.
The cutting operation can be started by a special remote-control unit, which has a user-friendly design for all the steps required during cutting. The welding taper is cut in one pass, see Fig 8. All the gas parameters of all the flames (pre-heating, stick-in and cutting flame) can be adjusted and changed at any time during operation using a digital gas mixture system.
Effect of the gantry installation in production
Immediately after installation, it was clear that the multi-purpose gantry improved productivity, as well as the quality level, very effectively. After one year of successful operation with the system, it can be established that many synergies are helping to increase the efficiency of heavy vessel production.
The anti-creep function of the specially-designed roller bed systems produces important advantages during welding start-up and actual welding.
Since all the industrially manufactured shells, rolled from plates, show deviations from an ideal cylindrical contour, it is impossible to avoid creep in the vessels or vessel parts without this function. In the past, the adjustment of the conventional roller beds in order to minimize creep required a great deal of time (sometimes more than one shift).
With the new system, the vessel only needs to be positioned on the rollers and, after some rotations which are required for synchronization, its horizontal position remains stable within a range of ±1 mm. The special narrow design of the roller beds in combination with the anti-creep sensor system reduces the restrictions relating to nozzle positions to an absolute minimum. This permits more flexibility in the pressure vessel design.
After a minimum of time for calibration and parameter setting, the welding process can be started directly. During welding, the operator only supervises slag removal and visually checks the weld quality from the bottom (floor). Downtime is reduced to an absolute minimum – interruptions are normally only required if the filler wire (100 kg wire coils) has to be renewed.
One of the most important points is the improvement in quality, which is no longer influenced by the operator’s practical experience and knowledge.
Due to the adaptive weld fill functions in the ESAB ABW system, the repair rate has been reduced dramatically in comparison with conventional semi-automatic SAW machines. After sufficient operator training, only defect-free joints have been produced.
Using the robot system to cut nozzle holes has significantly reduced the number of working steps that were previously necessary.
It is no longer necessary to mark the cut-out contour on the shell surface. The downtime produced by handling and positioning conventional mechanized cutting machines is avoided completely. Moreover, the effective cutting time has been reduced by half because the cut-outs are performed in one step instead of the normal two (straight cut and angle cut as separate operations).
Cutting is performed with high accuracy when it comes to wall thicknesses of between 50 and 150 mm. The maximum diameter deviations for the cut-out are ±2 mm and the weld bevel angle differs by no more than ±1°. This high accuracy influences the fit-up of the nozzles and the following welding operation performed using SAW nozzle welding machines very positively.
Another very important point is the human factor. Operators and welders are no longer exposed to high temperature radiation due to the necessary high pre-heating temperatures, because all the fully-automatic operations can be run from the floor. Moreover, if nozzle fit-up and nozzle welding or other operations are performed from the movable and flexible platform, the insulated floor plates protect the fitters and welders.
The quality result s produced by the fully-automatic welding and cutting operation are not dependent on the operator’s practical knowledge or his/her concentration. On the other hand, it has been found to be advantageous if experienced SAW operators and cutters handle the system, because of their “feeling” for the processes. Due to the user-oriented design of the process control units, only basic PC knowledge or basic experience of CNC cutting applications are required.
Conclusion
Installing the new technologies for adaptive welding and automatic robotic oxy-fuel cutting at BORSIG’s heavy-duty plant has clearly increased productivity. The high level of automation ensures a high degree of flexibility with a simultaneous high level of quality. Downtime is significantly reduced compared with similar plants and this reduces the number of hours spent on machining.
First published: Svetsaren 1-2/1999
Keywords: SAW, oxyfuel cutting, pressure vessels, heat exchangers, ABW, multi-functional gantry.
Segment: Process industry
Summary: BORSIG (a company within Babcock-Borsig AG), a famous supplier of pressure vessels and heat exchangers for the chemical and petrochemical industries, has optimized its welding and cutting production process by implementing fully-automated systems for submerged arc welding (SAW) and oxy-fuel cutting.
An advanced, fully-adaptive SAW process is used at BORSIG for butt welds of up to 120 mm in joint depth. The multi-layer tandem SAW process is controlled by intelligent software which makes its own decisions for the complete welding operation.
The holes for nozzle intersections in the shell of the pressure vessel are cut by an industrial robot. Off-line programming for cutting holes with constant weld bevel angles or constant joint volume is performed using different macros.
The advanced welding and cutting systems are mounted on a multi-functional gantry which travels on rails. The gantry works in conjunction with two anti-creep roller bed stations, see Figure 1. This type of installation was specially designed by ESAB for the requirements of the BORSIG product range.
BORSIG products
As one of the leading suppliers of complete process gas waste heat recovery systems and quench cooling systems for the chemical and petrochemical industries, BORSIG designs and fabricates different types of heat exchanger and pressure vessel.
The quench coolers are used for the rapid quenching of the gas effluent from cracking furnaces in ethylene plants. The main applications for the process gas waste heat recovery systems are ammonia, methanol, hydrogen and coal gasification plants.
Examples of components in these systems, manufactured by BORSIG, include:
• Process gas waste heat boilers
• HP steam superheaters
• HT shift waste heater boilers
• Boiler feed water preheaters
• Gas/gas heat exchangers
• Synloop waste heat boilers
• Steam drums
Fig 2 shows a typical configuration for a combined reformed and synthesis gas heat recovery system.
Almost every pressure vessel or heat exchanger is a unique application, specially designed according to the requirements of the specific process or client restrictions.
All these applications involve high gas inlet temperatures (up to 1,200°C), often accompanied by high process gas pressure up to 300 bar, as well as the generation of high-pressure steam (up to 140 bar).
Quality requirements
The design and manufacture of high-pressure equipment is strictly regulated in worldwide pressure-vessel codes like AD, ASME, BS, Raccolta, Codap, Stoomwezen, IBR, JS, AS and so on. The production quality, and the quality of the welding connections in particular, is very important because of the critical operating conditions of the pressure vessels. Imperfections in the welded zone are restricted to an absolute minimum (mostly min. Group B according to ISO 5817 or better).
Every pressure vessel containing longitudinal or circumferential joints in the shells or the inlet and outlet sections is subjected to complete non-destructive testing such as magnetic particle or dye penetrant checks, as well as 100% radiographic (RT) and/or ultrasonic (UT) examination. Nozzle welds are normally completely examined by UT.
Prior to weld production, a process qualification test (PQR), including intensive non-destructive and mechanical testing, has to be performed in order to verify that the properties of all welds match the requirements specified in the applicable codes and the base materials. The process which is going to be used in production is restricted to the qualified range of the PQR when it comes to base material group, thickness range, post-weld heat treatment (PWHT), range of welding parameters (e.g. pre-heating temperature, welding speed, voltage, amperage, interpass temperature and so on), for example.
Materials and welding technologies
Due to the service conditions of the equipment that is going to be manufactured, many different steels are used to manufacture the pressure vessels and heat exchangers. The following materials are examples for the main parts (hull) of the above-mentioned pressure vessels:
• High strength C steels (e.g. SA 516 Gr.70) for shells of steam drums and WHBs
• C-0.5% Mo steels (e.g. 15Mo3) for shells and nozzles of steam drums and quench coolers
• High strength, temperature-resistant steels (e.g. 15 NiCuMoNb 5 or SA 302 Gr.B/ Gr.C) for shells and nozzles of steam drums, process gas WHBs or synloop WHBs (steam side)
• C-1.25% Cr-0.5% Mo steels (e.g. 13 CrMo 4-5) for shells of WHBs (steam side) and for gas-inlet and gas-outlet sections, tube sheets or forged rings for process gas WHBs (gas side)
• C-2.25%Cr-1%Mo steels (e.g. 10 CrMo 9-10) and C-3%Cr-1%Mo steels (10 CrMo 9-10 mod.) for gas inlet sections, shell, tube sheets and nozzles of synloop WHBs (gas side)
The above-mentioned or comparable steels were welded to themselves (e.g. shell-shell) or were combined with one another (e.g. shell-tube sheet). The thickness of the shells for steam drums and WHBs (steam side) generally ranges between 40 and 120 mm, the gas-inlet sections of synloop WHBs increase up to 250 mm. The diameter of the vessels can differ from approx. 1,000 mm (e.g. quench coolers) to 3,000 mm (e.g. WHB). Typical applications are shown in Figs 3 and 4.
Due to the wall thickness and in order to produce the required mechano-technological properties, most of the applications, especially the higher alloyed steels, require pre-heating during welding and cutting, as well as controlled energy input and restricted interpass temperatures during welding.
In order to prevent cold cracking, the high strength steels need to be pre-heated to between 150 and 250°C. C-1.25%-Cr-1%Mo- and C-2.25%Cr-1%Mo steels also require pre-heating within 200-250°and 250-300°C.
For circumferential joints, a U preparation with a weld bevel angle of 8° is normally used. Cones are welded to cylindrical parts using a V-joint preparation (complete opening angle 50°). Nozzle welds (set-in nozzle) were performed with a half-V preparation (weld bevel angle 30-40°). The main welding technologies are GTAW and SMAW for the root pass and SAW for the fill/cap layers and back welding.
In order to optimize the production quality and the efficiency of the welding operation, BORSIG has incorporated a large number of automated welding and cutting processes in its production process.
Examples include fully-automated, tube-to-tube sheet welding for heat exchangers with computerised orbital GTAW welding machines (multi-layer TIG technology with filler wire), automated GTAW hot-wire overlay welding for critical dissimilar joints, GMAW robot welding of double pipe quench-cooler elements, robot welding of special stiffener systems to thin tube sheet and CNC plasma or oxy-fuel cutting of plates using CAD data and macro-programming systems.
BORSIG’s criteria for the choice of the advanced welding and cutting systems
Up to the end of 1997, the submerged arc welding of circumferential and longitudinal joints, as well as the cutting of nozzle holes in shells and gas inlet/gas outlet sections, was performed exclusively with operator-controlled welding and cutting equipment. The quality of these operations was therefore mainly influenced by the knowledge and experience of the operators.
The use of CNC-controlled machines which require absolute programming is not suitable because the actual geometry does not always correspond to the nominal geometry (base material thickness, joint condition, accuracy of shells, misalignments and so on) Additionally, pre-programming the bead placements for the multi-run sequence leads to a reduction in efficiency due to the increase in downtime.
So, more effective automation required the implementation of intelligent and adaptive software that makes its own decisions for the entire operation. This was verified in 1997 by installating the welding gantry, including the advanced ESAB ABW technology for the SAW process and the robot cutting system which can be programmed off-line on a macro base. Only the chosen type of software guarantees the flexibility which is necessary in pressure vessel manufacture which specializes in client-oriented solutions.
The system
The fully-automatic, multi-purpose gantry was developed by ESAB and the project was realised in close co-operation with BORSIG engineers. All the elements and programming units were designed with user orientation as the starting point.
The gantry has a fully-automatic, laser-supported, submerged-arc ESAB ABW welding system (see Fig 5), which works in conjunction with two 150-tonne roller bed systems equipped with anti-creep units. This permits the welding of longitudinal seams up to 4,200 mm in length and circumferential seams with a diameter of up to 3,500 mm.
The special narrow roller bed design permits the rotation of vessels with attached nozzles or flanges with a maximum projection of 750 mm and a minimum distance of 500 mm between two nozzles and the attached parts respectively.
At wall thicknesses of up to 120 mm, which covers 95% of all BORSIG applications, fully-automatic adaptive tandem welding can be performed. At thicknesses of up to 135 mm, mechanized tandem welding with the ESAB ABW head is possible. In the event of thicker wall thicknesses of up to 250 mm, a second conventional single-wire welding head can be used.
In any case, all the automatic or semi-automatic operations were controlled by the main control PC, Fig. 6. Two 250-litre pressure tanks located at the base of the gantry automatically supply the integrated continuous flux recovery system with new flux.
Alternatively, different fluxes depending on the procedure can be supplied from the pressure tanks at the base. The flux system is equipped with built-in electrical heaters. Minimum flux consumption is guaranteed by integral flux suction and circulation.
An industrial robot of the ABB IRB 2400/S4 type, equipped with a flame cutting (oxy-fuel) burner system, is mounted on a carriage which is installed perpendicular to another carriage at the main horizontal boom of the gantry, Fig 7. Both carriages are installed as linear external robot axes which are used to position the robot and, if required, as additional robot axes during cutting operations. The cutting of nozzle holes, programmed on an off-line, macro-supported basis, is possible at wall thicknesses of up to 150 mm.
The system permits the cutting of holes with a diameter of up to 1,500 mm. The maximum ratio between the hole cut-out and the diameter of the course is 0.68. This results in a minimum shell diameter of 2,500 mm if a hole of 1,500 mm is to be cut.
Additionally, the gantry can be used as a multi-functional platform for fitting and welding nozzles and the other attachments to the vessels. For this purpose, the platform can be flexibly modified using removable insulated floor plates. The whole gantry and the roller bed systems can be positioned free on rails over a length of 45 m. All the main energy and data transfer cables were installed under the floor in cable chains covered by movable floor plates.
The ESAB ABW adaptive joint fill program
The ESAB ABW adaptive tandem welding system mounted on the gantry can handle both circumferential and longitudinal welding in a fully-automatic joint filling procedure. This is possible thanks to the intelligent software in the system.
True measurement data from the joint profile measured by an optical sensor during welding determine both the required level of the welding parameters on a continuous basis, as well as the positioning of related axes for tracking and inter-run formation.
This means that both the bead size and bead placement are controlled by the system software in a self-adaptive manner for all the fill layers including the cap.
The system software influences the following four fill parameters:
• The weld speed
• The current
• The bead placement
• The number of beads in fill and cap layers
This unique feature enables ABW to adapt the parameters to match deviations in the cross-sectional area and geometry along the entire joint line.
Each of the four fill parameters influenced by the ESAB ABW system software performs different tasks in the adaptive ABW weld fill procedure.
• The weld speed controls the amount of weld metal deposited in different areas along the joint line.
• The current controls both the bead height and the amount of weld metal deposited in different areas of the joint.
• The bead placement influences the pattern of the inter-run formation in different areas of the joint.
• The selected number of beads in each layer determines the inter-run penetration, the shape of the actual joint bottom and the degree of weld fill.
The unique ESAB ABW weld technology is designed to give manufacturers dealing with high quality butt welding a 100% automatic multi-layer technology, thereby enabling them to produce a defect-free weld fill, even if the joint geometry deviates from the nominal configuration.
The configuration of the ESAB ABW system was specially modified according to BORSIG requirements for flexible production. The modified system can also verify joints between shells and thicker flanges (step on one side near the weld), as well as joints between shells and cones, with the automatic generation of a smooth transitional contour between the two parts.
Registration and documentation of the welding operation
Continuous, fully-automatic, multi-run tandem welding for many hours with limited operator surveillance requires not only excellent man-machine communication (MMC), during the weld fill operation, but also a report system which explains how the work has been accomplished.
The ESAB ABW operating system software installed at BORSIG includes a report system in which welding and positioning data from the operation are registered in two separate files – the weld report file and the log file.
In the weld report, all the installation parameters such as wire type and wire dimension, flux type and permissible interpass temperatures are stored, together with the specified process parameters such as welding voltage, welding current and welding speeds and their report, alarm and stop limits.
All the important events during welding, such as start, stop(s), re-starts, exceeded report limits and warnings for flux level, high or low interpass temperature, are stored in the weld report. All the events are stored together with the actual date, time, weld layer, weld bead and position in the joint. Should the event be an exceeded process parameter, the parameters at the time in question are also stored.
In the log file, the position and process parameters are continuously registered (every 20 mm). A normal log file report for a thick-walled welding object could fill 1,000 pages.
Robotic oxy-fuel cutting of nozzle holes
Due to the saddle contour of a tubular intersection in a cylindrical shell, the programming of the hole-cutting operation is mathematically complicated.
In order to simplify the program procedure, a computer-based, off-line programming system of the ARAC type is used.
Two macros verify the calculation of the saddle contour and transfer it to robot co-ordinates. Cuts with either a constant groove opening or a constant weld volume can be made. The operator only puts the following data into the macro:
• Shell diameter
• Wall thickness
• Diameter of the hole/shell intersection
• Angle of the weld bevel
• Cutting parameters (e.g. pre-heating time, gas parameters, cutting speed and so on)
• Off-set for position of the cut-out and the cut width.
After transferring the program from the off-line PC to the robot control unit and putting the robot in the cutting position, a special measuring program is started prior to the operation.
A special measuring sensor mounted at the burner tip performs a stepwise control of the surface at the location of the cut-out in a test mode.
Deviations from an optimum cylindrical surface are corrected in the macro by setting an additional off-set in order to secure a constant distance between burner-tip and the metal surface on each occasion. This is a vital function for a good and reproducible cutting result.
The cutting operation can be started by a special remote-control unit, which has a user-friendly design for all the steps required during cutting. The welding taper is cut in one pass, see Fig 8. All the gas parameters of all the flames (pre-heating, stick-in and cutting flame) can be adjusted and changed at any time during operation using a digital gas mixture system.
Effect of the gantry installation in production
Immediately after installation, it was clear that the multi-purpose gantry improved productivity, as well as the quality level, very effectively. After one year of successful operation with the system, it can be established that many synergies are helping to increase the efficiency of heavy vessel production.
The anti-creep function of the specially-designed roller bed systems produces important advantages during welding start-up and actual welding.
Since all the industrially manufactured shells, rolled from plates, show deviations from an ideal cylindrical contour, it is impossible to avoid creep in the vessels or vessel parts without this function. In the past, the adjustment of the conventional roller beds in order to minimize creep required a great deal of time (sometimes more than one shift).
With the new system, the vessel only needs to be positioned on the rollers and, after some rotations which are required for synchronization, its horizontal position remains stable within a range of ±1 mm. The special narrow design of the roller beds in combination with the anti-creep sensor system reduces the restrictions relating to nozzle positions to an absolute minimum. This permits more flexibility in the pressure vessel design.
After a minimum of time for calibration and parameter setting, the welding process can be started directly. During welding, the operator only supervises slag removal and visually checks the weld quality from the bottom (floor). Downtime is reduced to an absolute minimum – interruptions are normally only required if the filler wire (100 kg wire coils) has to be renewed.
One of the most important points is the improvement in quality, which is no longer influenced by the operator’s practical experience and knowledge.
Due to the adaptive weld fill functions in the ESAB ABW system, the repair rate has been reduced dramatically in comparison with conventional semi-automatic SAW machines. After sufficient operator training, only defect-free joints have been produced.
Using the robot system to cut nozzle holes has significantly reduced the number of working steps that were previously necessary.
It is no longer necessary to mark the cut-out contour on the shell surface. The downtime produced by handling and positioning conventional mechanized cutting machines is avoided completely. Moreover, the effective cutting time has been reduced by half because the cut-outs are performed in one step instead of the normal two (straight cut and angle cut as separate operations).
Cutting is performed with high accuracy when it comes to wall thicknesses of between 50 and 150 mm. The maximum diameter deviations for the cut-out are ±2 mm and the weld bevel angle differs by no more than ±1°. This high accuracy influences the fit-up of the nozzles and the following welding operation performed using SAW nozzle welding machines very positively.
Another very important point is the human factor. Operators and welders are no longer exposed to high temperature radiation due to the necessary high pre-heating temperatures, because all the fully-automatic operations can be run from the floor. Moreover, if nozzle fit-up and nozzle welding or other operations are performed from the movable and flexible platform, the insulated floor plates protect the fitters and welders.
The quality result s produced by the fully-automatic welding and cutting operation are not dependent on the operator’s practical knowledge or his/her concentration. On the other hand, it has been found to be advantageous if experienced SAW operators and cutters handle the system, because of their “feeling” for the processes. Due to the user-oriented design of the process control units, only basic PC knowledge or basic experience of CNC cutting applications are required.
Conclusion
Installing the new technologies for adaptive welding and automatic robotic oxy-fuel cutting at BORSIG’s heavy-duty plant has clearly increased productivity. The high level of automation ensures a high degree of flexibility with a simultaneous high level of quality. Downtime is significantly reduced compared with similar plants and this reduces the number of hours spent on machining.
