MULTIPASS GTAW ORBITAL WELDING OF PIPE

First developed 30 years ago by the aerospace and nuclear power industries, orbital mechanized pipe welding is now a mature industry widely used in virtually every industry that has any need to weld tubes and pipes. Despite its widespread acceptance and long-term use by many customers, each year many companies with no experience begin the evaluation process to see if mechanical welding makes sense for their specific application. This decision may be due to a lack of skilled manual welders; customer-mandated higher quality standards; the need to improve productivity and throughput; or the desire to reduce defects and rework. Most management and engineering individuals have the same concerns and questions, and this article will attempt to address some of these common considerations and basic issues. Although there are GMAW/FCAW process pipe welding systems commercially available, this article will focus on GTAW (Gas Tungsten Arc Welding) equipment. The relevant applications will be those where the pipe cannot be rotated, but must be welded in a fixed position, whether 5G, 2G or 6G (pipe horizontal, vertical or at an angle). There have been a great many articles published discussing the requirement for successful sanitary/sterile tube welding. Much of this welding can be done by simple autogenous tube welding systems in a single pass, and the equipment and requirement for success are quite different from pipe welding where the wall thickness dictates multiple weld passes to make root, fill, and cap passes. The focus then will be on multipass welding. Who Is Using Orbital Welding and Is it Suitable for My Industry? Orbital mechanized welding is used in a broad spectrum of industries - from semiconductor manufacture to shipbuilding, power plant construction to marine gas pipelines, chemical plant maintenance to food processing. In summary, another company in your industry is likely already using the technology. Is Mechanized GTAW Pipe Welding Faster than Manual? There is a misconception that a machine can make a weld faster than a man. This is not so. The process, which involves a molten, fluid puddle of metal acted on by gravity and surface tension forces, is identical for manual and machine welding. Mechanized welding has the capability of higher deposition rates, higher torch travel speeds, etc., but ultimately the process limits any substantial improvement in these factors over manual welding. Productivity with mechanized welding is primarily the result of duty cycle and reduced defect rates. A machine cannot tire, take coffee breaks, or make mistakes. The Importance of Duty Cycle for Cost Justification. Is mechanized pipe welding guaranteed to increase productivity in your company? Not necessarily. Numerous considerations such as workflow and work piece geometry may simply not make mechanized welding cost effective. For example, can workflow be arranged to allow welding of a number of similar size pipes in an uninterrupted succession? Or does your product require an assembly sequence where the pipe size is different for each subsequent weld, which will result in possible time lost to readjust the weld Head for pipe size. Given a suitable application, mechanized welding should achieve a 70% duty cycle, and a productivity improvement of 300% is realistically achievable as compared to manual welding. This productivity improvement will not be reached unless the workflow can be arranged to maximize arc-on time. If only one weld can be made followed by an hour of other processing before welding can resume, mechanized equipment will do little to improve productivity.

This is sometimes the case in field fabrication, resulting from the time to move the equipment from one weld site to another weld site. Just the opposite is often true in field fabrication. For example, physical access to the weld may be a fundamental factor in productivity. Field pipe welds must frequently be made in power plants and chemical manufacturing facilities where a manual weld will take two hours that a machine can make in 10 minutes. In some cases, the manual welder is forced to make welds using a mirror, or with tight access preventing him from making the weld from one side. Quality and maintaining careful control of weld parameters may also override duty cycle considerations. If you don't have manual welders that can achieve the specified quality standards, duty cycle becomes a secondary issue. Many of today's alloys require tight control over weld parameters such as heat input, for example. Duplex stainless steel and Inconel® alloys require tight regulation of heat input to maintain desired metallurgical properties in the weld deposit and heat affected zone or to avoid defects such as cracking. The Importance of Required Weld Quality Standards: If weld quality is not a major concern for your product or service, (you simply have to make sure the pipes are stuck together), there is unlikely to be a sufficient payback on equipment investments. If your application requires welding to rigorous codes such as ASME section IX, B31.1, B31.3, etc., you may achieve a rapid payback with reductions in weld defects and subsequent rework. Most companies can achieve sustained reductions in defect rates to well under 1%. Customers frequently benefit from relaxed QC mandates using mechanized welding. For example, 100% radiographic inspection may be relaxed to a 10% inspection sampling. What Are the Components of an Pipe Welding System: The systems are on the market today generally have the following components:

· A weld Head which carries and manipulates the torch.

· A power source that provides both weld head control and programming, as well as current output.

· A remote pendant for system control at a distance from the power source.

· A water recirculator to provide torch and possibly weld head cooling. What Weld Head Features/functions Are Required for Multipass Welding? The weld head should have the capability of the following:

· Torch rotation.

· Filler wire feed capability.

· Electronic control of arc length (Arc Voltage Control).

· Torch oscillation (weave) capability with programmable width, speed and independent end point (sidewall) dwell times. Selecting the Optimum Weld Head Type:

A variety of weld heads have been developed to meet specific application requirements and constraints. Weld heads fall into one of two categories: "full function-in-place"; or "orbital full function". Full Function in Place Heads: These Heads adjust to clamp on pipe of a specified O.D. range. An adjustable lever-actuated clamp holds the head on the pipe and remains stationary. A torch and other mechanisms mount on a rotating module. The filler wire feeder may be entirely mounted on the rotary portion of the head, or may be floor mounted with a conduit connected to the rotating module. These heads incorporate electronic arc-voltage control and electronic torch oscillation. (Some models may incorporate a mechanical follower to control arc length, but these technically are not "full function" and have limitations for multipass welding). Benefits include rapid installation on the pipe. Limitations of this type are that each head model can weld only a finite range of tube and pipe sizes and several models may be required to cover the necessary pipe size range. Radial clearance requirements depend on pipe OD. Head mechanisms cannot be water-cooled, limiting their use on alloys requiring preheat. Floor-mounted wire feeders allow the use of larger spools (lower wire costs), but may sacrifice wire-feeding accuracy. Full Function Orbital Heads: The Heads also incorporate torch rotation, filler-wire feed, electronic arc voltage control, and electronic torch oscillation. Unlike the "in-place type head”, however, the entire weld head mechanism rotates around the work piece. The head attaches to the pipe using a metal band or guide ring fabricated to match the size of the tube or pipe. Generally, the guide ring attaches to the pipe, then the head installs on the ring, although some models retain the guide ring and both are installed simultaneously. One limitation is that they require slightly longer to install on the pipe than the "in-place" type Head. This type may also require a longer straight length of pipe for mounting. Benefits include the ability of one model to cover a broad pipe size range. Radial clearance remains constant on all pipe sizes. Head design permits water-cooling of the body allowing use on alloys requiring preheat. Other Considerations in Weld Head Selection: Other factors may dictate the type of equipment selected such as:

· Welding in tight clearance conditions with limited axial and radial clearance.

· Welding of alloys requiring preheat, requiring water-cooling of the weld head body.

· Applications dictating a very limited range of pipe sizes (for example, small diameter boiler tubing used in heat exchangers).

· Businesses such as fabrication job shops and mechanical contractors that require the capability of welding many sizes of pipe, which are determined by present and future contracts. Power Supply Considerations: Consider the importance of power supply size to your application. Some systems are not very portable, but might be perfectly acceptable for a fabrication shop. For field use consider the need for portability. Many inverter-based power sources today can be moved by one or two men without equipment. Consider input voltage requirements for the power source. A fabricator will generally have access to any input voltage, but a field contractor may require a specific three-phase voltage at a job site. If a motor generator

is to provide power, a single-phase power source may be desirable. Some power sources operate on a specific input voltage only, while others can operate with a range of input voltages, single or three-phase. A power source output of 200-300 amps is generally sufficient for conventional mechanical welding. Frequently customers ask if they can use an existing commercial power source to operate the weld head. An orbital welding power source integrates the controls to both operate the various weld head functions and integrate them with the power source output in one "package". A standard power source can provide output power only and cannot be used. In addition, multipass welding requires multipass programming, which is usually done with an integrated microprocessor and custom software. Analog Versus Microprocessor-based Power Sources: There are still analog power sources available. These are programmed by entering the desired speed, amperage, etc., on dials and enabling functions using mechanical switches. The units on the market today will allow only one pass (orbit of the pipe) to be programmed. With the requirement for multiple passes, the welder must stop the machine, reset the dials for the following pass, and restart the weld. The alternative is to change dial settings "on the fly" - a risky procedure at best. Analog power sources have advantages: easy to learn operation; tolerance of environmental extremes; simpler service. Unfortunately, their downside is greater: the need to stop between passes; welder error in setting dials; no ability to "lock-out" unauthorized changes in critical parameters; no program storage. With apologies to the computer haters out there, this is one place where the microprocessor-based power source offers significant advantages. Microprocessor Band Systems Offer the Following Advantages:

· Many levels of programming for all parameters.

· Multiple passes possible without stopping.

· Key switch or password authorized access to modify programs.

· Supervisor-defined override limits on each parameter provides process control.

· Weld program storage (most units store 100 programs internally).

· Integral printers provide hard copy of weld parameters; with ability to print weld number, date, etc.

· Solid-state data cards allow program transfer between systems or "offline programming" on personal computers.

· Weld head selection allows calibrated readouts in proper engineering units.

· QC monitoring programs.

· Selectable language and inch/metric operation. Microprocessor-based units do have a longer learning curve. Evaluate models from various manufacturers on their ease of programming. Most weld programming is done by the welder using the equipment, and the program should use simple "prompts" and not require computer literacy.

Unlike the pro and con argument that can be made for analog programmable power sources for fusion welding, microprocessor-based systems offer clear advantages. Is it Necessary to Create New WPS/PQR's?: There is a misconception by many people that the substitution of mechanized pipe welding for manual techniques requires that a new Procedure Qualification Record (PQR) be done (as well as the Weld Procedure Specification which it supports). This is not the case. The ASME code, section IX, defines welding variables under section QW-250. Nonessential variables are subsequently defined in section QW-251.3. "Nonessential variables: Nonessentials variables are those in which a change, as described in the specific variables, may be made in the WPS without requalification." Variables for gas tungsten arc welding (GTAW) are listed under QW-256. Nonessential variables are listed under subheading 256.2. QW-410.25 below is listed under 256.2 nonessential variables. QW410.25: A change from manual to semi-automatic to machine automatic welding and vice-versa". Joint design including bevel geometry and fit-up gap are nonessential variables for the GTAW weld process as defined under section QW-415 on welding variables. How Important Is Bevel Geometry? Very frequently customers will make the capital spending decision to purchase an orbital system, but will balk at spending the additional money to purchase pipe-prepping tools, to guarantee a repeatable, accurate bevel geometry. Although computerized, the pipe welding systems available are essentially "dumb" machine tools. They will unerringly repeat the programmed motions and precisely change functions such as current at the proper moment. They cannot, however, compensate for a changing bevel geometry and poor fit-up. If a user is unwilling to spend the money on a pipe end-prep tool, they will not succeed in achieving code-acceptable machine welds. There are a large number of manufacturers of portable pipe prepping tools, with electric, pneumatic, and hydraulic drive options. It is important to think of mechanized welding as if you were installing a new multi-component system:

· Pipe bevelling equipment to ensure repeatable pipe end geometry.

· Fit-up tools to ensure reasonable fit-up for tack welding.

· Mechanical pipe welders to ensure a repeatable weld process. What Bevel Geometry Should Be Used?: Pipes with wall thickness up to approximately .125" (3mm) can be welded in a single pass. Above this, a multipass technique will be required. For smaller pipes and boiler tubing, a standard V-bevel with a slight gap is adequate, if not optimal, for welding most materials. Generally, the most suitable prep for automatic welding is a J-prep with the pipes ends butted together. Is it not possible to use a standard v-bevel with an orbital head? Yes, it can be done with perfect fit -up and an extremely skilled and attentive welder. This defeats the most common rationale for mechanized welding - lack of skilled welders. Equipment manufacturers will strongly recommend use of a J-prep.

What Training Is Required and What Is the Typical Learning Curve Duration?: All manufacturers strongly recommend training. Typically three to five days of direct instructions, by a factory technical specialist is adequate. Welders will still need several weeks to reach full proficiency. One of the most common mistake is to purchase equipment as a "solution" to meet an ongoing contract deadline and to expect welders to become immediate experts. Recommendations: Get "buy-in" from your welding staff, invite welders to supplier demonstrations. They will be using the tool and can provide valuable input in equipment selection. Make them understand that this is a tool to make their job easier. (Often welders can feel threatened by mechanized equipment, and require reassurance that this is a tool to make them more productive and more valuable.) Make the cost/benefit decision the same way you would if buying a new computer system, or a new machine tool. Don't wait for the start of a big job as the impetus to evaluate mechanical welding. You cannot bypass the learning curve. Unfortunately, many first time users do not realize the importance of training and the length of the associated learning curve before welders can become proficient and achieve the productivity improvements possible with these welding tools.