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Orbital welding
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By Mahesh Ugale
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Orbital welding is Automatic Tunguston inert gas welding. Iteliminates chances of manual errors in welding. It producesidentical welds for hundred of times hence accuracy in welding.
Orbital welding was first used in the 1960's when the aerospaceindustry recognized the need for a superior joining techniquefor aerospace hydraulic lines. A mechanism was developed inwhich the arc from a tungsten electrode was rotated around thetubing weld joint. The arc welding current was regulated with acontrol system thus automating the entire process. The resultwas a more precision and reliable method than the manual weldingmethod it replaced. Orbital welding became practical for manyindustries in the early 1980's when combination power supply /control systems were developed that operated from 110 V AC andwere physically small enough to be carried from place to placeon a construction site for multiple in-place welds. Modern dayorbital welding systems offer computer control where weldingparameters for a variety of applications can be stored in memoryand called up when needed for a specific application. The skillsof a certified welder are thus built into the welding system,producing enormous numbers of identical welds and leavingsignificantly less room for error or defects.
Orbital Welding Equipment
In the orbital welding process, tubes / pipes are clamped inplace and an orbital weld head rotates an electrode and electricarc around the weld joint to make the required weld. An orbitalwelding system consists of a power supply and an orbital weldhead.
Power Supply: The power supply / control system supplies andcontrols the welding parameters according to the specific weldprogram created or recalled from memory. The power supplyprovides the control parameters, the arc welding current, thepower to drive the motor in the weld head and switches theshield gas (es) on / off as necessary.
Weld Head: Orbital weld heads are normally of the enclosed typeand provide an inert atmosphere chamber that surrounds the weldjoint. Standard enclosed orbital weld heads are practical inwelding tube sizes from 1/16 inch (1.6mm) to 6 inches (152mm)with wall thickness' of up to 0.154 inches (3.9mm) Largerdiameters and wall thickness' can be accommodated with openstyle weld heads.
The Physics of the GTAW Process
The orbital welding process uses the Gas Tungsten Arc Weldingprocess (GTAW) as the source of the electric arc that melts thebase material and forms the weld. In the GTAW process (alsoreferred to as the Tungsten Inert Gas process - TIG) an electricarc is established between a Tungsten electrode and the part tobe welded. To start the arc, an RF or high voltage signal(usually 3.5 to 7 KV) is used to break down (ionize) theinsulating properties of the shield gas and make it electricallyconductive in order to pass through a tiny amount of current. Acapacitor dumps current into this electrical path, which reducesthe arc voltage to a level where the power supply can thensupply current for the arc. The power supply responds to thedemand and provides weld current to keep the arc established.The metal to be welded is melted by the intense heat of the arcand fuses together.
Reasons for Using Orbital Welding Equipment
There are many reasons for using orbital welding equipment. Theability to make high quality, consistent welds repeatedly at aspeed close to the maximum weld speed offer many benefits to theuser:
? Productivity. An orbital welding system will drasticallyoutperform manual welders, many times paying for the cost of theorbital equipment in a single job.
? Quality. The quality of a weld created by an orbital weldingsystem with the correct weld program will be superior to that ofmanual welding. In applications such as semiconductor orpharmaceutical tube welding, orbital welding is the only meansto reach the weld quality requirements.
? Consistency. Once a weld program has been established anorbital welding system can repeatedly perform the same weldhundreds of times, eliminating the normal variability,inconsistencies, errors and defects of manual welding.
? Orbital welding may be used in applications where a tube orpipe to be welded cannot be rotated or where rotation of thepart is not practical.
? Orbital welding may be used in applications where access spacerestrictions limit the physical size of the welding device. Weldheads may be used in rows of boiler tubing where it would bedifficult for a manual welder to use a welding torch or view theweld joint.
? Many other reasons exist for the use of orbital equipment overmanual welding. Examples are applications where inspection ofthe internal weld is not practical for each weld created. Bymaking a sample weld coupon that passes certification, the logicholds that if the sample weld is acceptable, that successivewelds created by an automatic machine with the same inputparameters should also be sound.
Industries and Applications for Orbital Welding
Aerospace: As noted earlier, the aerospace industry was thefirst industry to recognize the requirement for orbital welding.The high-pressure systems of a single plane can have over 1,500welded joints, all automatically created with orbital equipment.
Boiler Tube: Boiler tube installation and repairs offer aperfect application for orbital welding. Compact orbital weldheads can be clamped in place between rows of heat exchangertubing where a manual welder would experience severe difficultymaking repeatable welds.
Food, Dairy and Beverage Industries: The food, dairy andbeverage industries require consistent full penetration welds onall weld joints. Most of these tubing / piping systems haveschedules for cleaning and sterilization in place. For maximumpiping system efficiency the tubing must be as smooth aspossible. Any pit, crevice, crack or incomplete weld joint canform a place for the fluid inside the tubing to be trapped andform a bacteria harbor.
Nuclear Piping: The nuclear industry with its severe operatingenvironment and associated specifications for a high qualityweld has long been an advocate of orbital welding.
Offshore Applications: Sub-sea hydraulic lines use materialswhose properties can be altered during the thermal changes thatare normal with a weld cycle. Hydraulic joints welded withorbital equipment offer superior corrosion resistance andmechanical properties.
Pharmaceutical Industry: Pharmaceutical process lines and pipingsystems deliver high quality water to their processes. Thisrequires high quality welds to ensure a source of water from thetubes that is uncontaminated by bacteria, rust or othercontaminant. Orbital welding ensures full penetration welds withno overheating occurring that could undermine the corrosionresistance of the final weld zone.
Semiconductor Industry: The semiconductor industry requirespiping systems with extremely smooth internal surface finish inorder to prevent contaminant buildup on the tubing walls or weldjoints. Once large enough, a build up of particulate, moistureor contaminant could release and ruin the batch process.
Tube/Pipe Fittings, Valves and Regulators: Hydraulic lines andliquid and gas delivery systems all require tubing withconnector fittings. Orbital systems provide a means to ensurehigh productivity of welding and improved weld quality.Sometimes the tubing may be welded in place to a valve orregulator body. Here the orbital weld head provides the abilityto produce high quality welds in applications with restrictedaccess to the weld joint.
A manual weld taken from an operating plant. This weld hasdefects that include lack-of- penetration, misalignment, a hugecrevice, and discoloration due to poor ID purge. This weld wouldbe considered unacceptable by any standard
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