– B MALKANI

A technical paper on automatic welding of storage tanks with all position variable speed carriages using the MIG process.

With the large amount of welding required in the construction of storage tanks, contractors all over the world, including India, are resorting to automatic welding techniques to increase productivity, improve quality, and to meet the stiff deadlines. The acute shortage of skilled welders makes it more of a mandatory necessity to automate the welding process.

The objective of this paper is to give an overview of the types of automatic welding equipment being used worldwide for the fabrication of large storage tanks. The author has stressed more on the application of the equipment than the design, joint configuration, metallurgy, consumables etc. The paper has been “punctuated” with drawings and photographs of tanks being welded by the programmable, variable speed, all position welding carriages.

Equipment used for tank welding

The automatic welding equipment used for storage tank fabrication falls mainly into four categories:

1. Electro-gas welder for vertical welding on shell butt welds

2. “3 O’clock” submerged arc girth welder for welding of circumferential seams

3. Programmable, variable speed, all position welding carriages (used with the MIG welding process) for welding of vertical and circumferential seams of tanks, wind girders, floor plates, floor plates to vertical shell welding, etc., and

4. Auxiliary equipment, i.e., mechanical high speed bevellers, automatic gouging equipment, etc.

Electrogas welder

The electro-gas welder is an extension of the MIG process, and is designed for carrying out vertical welds in a single pass. It is an efficient process except that it can lead to high heat input and unrefined grain structure in special steels, not to mention distortion.

The arc creates a molten puddle between two sets of copper shoes (one on either side of the weld seam), which climb up (powered by a motorized trolley) at the same speed as the vertical travel carriage. These copper shoes are water cooled, and “freeze” the molten metal, creating a full penetration weld.

The equipment is a “dedicated type” (can only be used for this type of application), and may require long periods of inactivity till the next tank job. It would be useful for large contractors who have a steady flow of contracts to ensure minimum idle time of the equipment.

It can only be used for the vertical welds (3G) on storage tanks.

The “3 o’clock” girth storage tank welder

This equipment uses the submerged arc process. It can be a single or dual head type, where the latter is used with one head on one side of the circumferential seam, and the other head on the other side, so that the seam can be welded from both sides simultaneously. One, or both the heads are suspended from a motorized, variable speed trolley (providing the welding speed) running on the top edge of the tank shell. The heads are lowered by a telescopic structural arrangement so as to reach the various tiers of the shells in “jack-up” tank construction.

The equipment welds at high speeds, and reduces the number of passes. It is the most popular equipment used by large tank fabricators and contractors. However, it has its own disadvantages, such as: the equipment is dedicated, and once the job is over it may require long periods of inactivity till the next job!

This is a high speed and efficient process, but, at the same time, uses high currents, which means high heat input. This can lead to coarse grain structure on certain heat sensitive materials. It is very prone to slag inclusions if the welding parameters are not optimised.

The equipment is very bulky, and has a high capital cost. It uses dedicated power sources. This equipment is normally used by large fabricators of tanks, who have continuous jobs in hand to ensure minimum idle time of the equipment between projects. It can only be used for welding the circumferential seams (2G) of storage tanks.

Welding carriage

The programmable, variable speed, all position carriages run on rigid or flexible tracks fitted with magnets, which can be mounted parallel to the vertical and circumferential seams of the weld.

These “all position” sophisticated carriages have controls for oscillation of the welding torch, i.e., “dwell”, left-centre-right, programmed arc start, speed control, bead width and oscillation speed control, store and recall programmes, etc.

A MIG welding torch is clamped onto the torch holding device, which can position the torch at any angle. Vertical and horizontal rack arms and rack boxes are provided on the carriages for making minor adjustments for maintaining a consistent arc length, and following the weld seam. Idlers can be hooked on to the carriages, facilitating the towing of wire feeders.

Since all the functions are automatic, the operator has to only ensure that the arc length is maintained, and the MIG welding torch follows the weld seam.

These carriages can be used with any conventional MIG welding equipment, hence lending it flexibility.

There is very little, or virtually no idle time, as these carriages can be used for almost any type of welded fabrication.

The same carriages can be used for cutting “in situ”. An oxy-acetylene machine torch has to be mounted on a special cutting torch holder, which provides up and down movement to the torch by means of a rack and pinion arrangement to ensure a consistent gap between the torch tip and the work piece. The torch can be tilted at various angles for bevelling operations.

The programmable variable speed, all position carriages can be used for almost all the continuous welding applications on storage tanks.

Auxiliary equipment

Mechanical high speed portable bevelling machine

All the advantages, such as increased arc-time and high deposition rates that the variable speed carriages ‘family’ can provide, will be ‘undone’ if the edge preparation is poor, leading to bad joint fit-ups.

High-speed portable bevelling machines are commercially available. They can accurately bevel carbon steel, alloy steel, stainless steel, titanium and aluminium etc; at speeds up to 10 ft/min at different bevelling angles on plates. Plates up to 50 mm thickness can be accommodated by the beveller. Bevelling is carried out with special cutters. This process is a noiseless and vibration free shearing process.

Mechanical bevelling, being a non-thermal process, has the important metallurgical advantage of not inducing any thermal effects like an HAZ, nor causing any thermal distortion. The machine is portable, and can be easily wheeled to the job without the help of cranes. No skill is required for operation. The beveller follows the edge while bevelling without the help of the operator. A spring loaded base allows the machine to run on an undulating floor.

The adjustment of the root face is with the help of a graduated scale on the machine. No further grinding is required prior to welding. Bevellers are also available for upside down bevels. For large scale production, one beveller can bevel the topside of the plate, while a second one follows by bevelling the under side of the plate. This way a double ‘V’ can be achieved without turning over the plate.

Automatic carbon arc gouging

This portable automatic gouging equipment can be interfaced to a programmable variable speed, all position carriage, and an automatic arc gouging torch. Arc gouging at high speeds can be carried out, resulting excellent and consistent grooves. Since the carbon arc gouging process is a non-oxidation process, the gouged grooves (with an accuracy of 0.25 inches), do not require grinding before welding.

The various diameter electrodes are copper coated carbon electrodes (male and female types). One electrode can be push fitted into the other one, hence making the gouging process to be continuous. The arc length is electronically controlled, and the electrode feed speed is controlled automatically by the arc length.

Ceramic backing strips

With the variable speed carriages and oscillators capable of high deposition welding, it is imperative to eliminate the expensive and slow process of back chipping/gouging, and re-welding, by using ceramic backing strips for one side welding.

For sound welds and excellent radiography results, the inherent characteristics of the ceramic backing strips, such as; halogen-free, non-hygroscopic (resistance to moisture pick-up), neutral, fibrous materials, granular density, etc., the refractory nature of the material to withstand high welding heat as a heat absorbing metal bar, the quality of the adhesive tape to resist preheat temperatures, and a host of other positive characteristics to increase productivity, has made this product very popular with fabrication workshops and shipyards.

These non-metallic weld backings are available in several segmented shapes and sizes, to suit almost any application requirement. The ceramic tiles are normally available in 2 ft strips that are held in position by a heat resistant adhesive coated aluminium tape.

Welding with programmable variable speed

The main areas for automation in the welding of large storage tanks with programmable variable speed, all position welding carriages, are:

*Vertical welding of shell plates

*Welding of circumferential tiers of shells

*Tank floor welding butt/lap welding

*Shell to floor plate (inside and outside) fillet welding

*Dome to curb angle, wind girders, etc

*Roof welding.

Vertical welding of shell plates

These shell plates vary from 2m to 2.5 meters, and have thicknesses; generally ranging from 6 mm to over 60 mm. Up to 15 mm plate thickness, the joint preparation is normally a single ‘V’ butt preparation, with an included angle of 60º.

The root run is welded by the MMAW process, or by MIG. The subsequent passes are MIG welded (FCAW 1.2 diameter wire) with the programmable variable speed, all position carriage, at welding currents of 180 to 200 amps, and an average carriage speed of 10 cm/min to 150 cm/min.

A highly skilled welder is not required for operation of the programmable, variable speed, all position carriage, as it welds automatically, oscillating (weaving) the torch as it climbs up the track, which is fixed parallel to the vertical weld seam by means of magnets (and vacuum cups on non ferrous weldments).

After automatic welding one side of the shell, the root run is gouged out on the other side of the plate, and the gouged area re-welded manually. For plates above 16 mm thickness, normally a double ‘V’ is required with a “? - ?” weld preparation, having an included angle of 60º on both sides. The root run is welded manually by MMAW or the MIG process. The variable speed carriages are used for the rest of the passes.

The inside root run is gouged out, and the groove welded manually, or automatically with the variable speed carriage, depending on the thickness of the plate and the number of passes required for the carriage to offset the “set up” time.

Manual back-gouging of the root run is a very slow and monotonous process. This can be totally eliminated by the use of ceramic backing strips which facilitate welding from one side only, allowing the formation of a bead on the other side. The weld satisfies all the metallurgical and radiographic requirements laid down by the respective codes.

Ceramic backing strips, when used with automatic welding carriages, can increase welding deposition rates tremendously, resulting in reduction of welders.

Welding of circumferential tiers of shells

Flexible tracks fitted with magnets are placed parallel to the circumferential joint, which is to be welded. The variable speed carriage (with an idler to carry the MIG wire feeder) run on these tracks with a MIG welding torch at a preset welding speed.

These carriages can have oscillators fitted on to them, for facilitating weaving of the weld to increase weld deposition rates. To increase productivity on large diameter storage tanks, 3 to 4 variable speed carriages can run simultaneously on one track around the circumferential seam, hence increasing productivity three to four fold. The circumferential seam of a tank can be divided into 4 quadrants, with one carriage and an operator in each quadrant.

The welding can be carried out in a mix of stringer passes and oscillation, in order to control the heat input. The root run is gouged out from the inside by the carbon arc gouging process, or grinding/back chipping, and the gouged groove welded manually, or, by automatic welding, depending on the number of passes required.

Ceramic backing strips can also be used (as explained earlier) to prevent back gouging/grinding. Welding automation, with programmable carriages and ceramic backing strips increase productivity substantially as the arc time lost by the grinding of the root is eliminated. The same carriages can be used for automatic butt welding of the vertical seams of shell plates, shell to floor annular plate fillet welding, floor butt/lap welding, roof plate welding etc.

Tank floor welding butt/lap welding

The floor plates are normally lap joints. In the case of butt joints, a backing strip is an integral part of the joint. The lap or butt joints on the floor plates can be welded by the same variable speed carriages by positioning the tracks on the magnets, parallel to the longitudinal joints to be welded.

The process is fast, clean, and the welds are of a very high quality. Weld metal deposition rates of 4 kg/hr on lap joints, and, up to 7 kg/hr for butt joints can easily be achieved depending on the thicknesses of the joints, or the size of the lap welds.

Shell to floor plate (inside and outside) fillet welding

These fillet joints (the outside joints of the shells are normally bevelled, as they are thick plates) can be welded by mounting the flexible tracks on the vertical shell parallel to the fillet welds, and the carriage running on the tracks with the MIG torch positioned for fillet welding. The torch can also be programmed for oscillation, if required.

A similar procedure can be used with the same carriages for welding the inner vertical shell to the floor plate or the annular plate. The welds are consistent (with minimal stops and starts) and of high quality.

Dome to curb angle, wind girders, etc

The wind girders consisting of angles or channels can be welded to the shell automatically with the programmable carriages in a similar manner to the floor plate to the vertical shell, i.e., the tracks will be mounted along the circumference of the vertical shell, and the torch placed in position for welding the continuous fillet weld on the angle/channel to be welded on to the shell. These can also be welded (running fillets, intermittent fillets and lap welds) with the trackless magnetic base fillets welders.

Roof Welding

Depending on the tank configuration, the roof plates are normally lap welded to each other. These can easily be welded with the same programmable, variable speed carriages, by placing the track longitudinally parallel to the weld seam.

The dome of the roof, which is welded to the shell (again, depending on the type of tank), can be welded with the same programmable, variable speed carriages, by placing the tracks on the circumference of the shell plate and positioning the torch to follow the weld seam.

The same carriages can be used for oxy-acetylene cutting with one or two torches, in all positions of the plane. They give consistent, excellent cuts, and can also be used for bevelling operations.

Benefits

The following are the benefits of using the programmable variable speed, all position carriage:

*Excellent consistent x-ray quality welds with controlled heat input and reduced HAZ

*Programmable, hence the parameters can be preset as per the WPS

*Highly productive and flexible. It can be used on a variety of applications

*It is very simple to use, and operator friendly

*No need of skilled welders for operation

*Can be used for butt, fillet, lap, and welding in “all positions”

*Same equipment can be used for “all position” cutting with oxy-acetylene, plasma and water-jet cutting processes. It can be used with multiple torches for cutting operations in all positions

*The microprocessor based controlled carriages with tacho-feedback controls, can tow wire feeders up to 45 kg in any position with no change in the pre-set carriage speed

*For large tanks, 3-4 carriages can be used in tandem, to speed up production, and

*The carriages can be used on rigid tracks or flexible tracks, which can be bent to take the shape of the profile of the tank.

9% nickel tanks, double walled tanks are beyond the scope of this paper. It is a fact that these carriages have been used for welding them. These carriages can be used for any type of storage tank, i.e., LPG bullets, spheres, vessels, etc; they can also be used for welding aluminium, stainless steel, and other non-ferrous materials by using vacuum suction cups for holding the track on to the shells.

Automatic welding of spheres

Spheres have been successfully welded by the programmable, variable carriage oscillator combos. A few self-explanatory photographs of the carriages welding the inside and outside of the spheres are shown.

When used with ceramic backing strips, the elimination of back chipping/grinding helps substantially in increasing weld deposition rates, and reduction of manpower. It is common practice to weld Spheres with these carriages and ceramic backing strips.

Conclusions

It is evident that welding and cutting automation is the answer to the fabrication of storage tanks and spheres. With the projects getting larger, and with tight completion schedules, it is getting more and more difficult for contractors to mobilise large numbers of skilled welders. Not only, there is a shortage of welders, it is their migratory tendencies of changing jobs that create hold- ups and delays in projects.

A typical example below clearly highlights the need for automation, due to its high efficiency, and the high quality of welding, with minimum flaws. It provides the answer to achieving low manhours per tonne, (which every project manager looks for), and quick turn-around cycles without compromising on high quality. One can see from the foregoing paragraphs the flexibility of these programmable, all position variable speed carriages. They can be used for welding almost all the long joints on the tanks with excellent, consistent bead appearance and x-ray quality welds at high deposition rates, and at welding ‘arc-on’ time cycles up to 80%.

The efficiency and productivity of the programmable, all position, variable speed carriage, using the MIG process against the MMAW process on a 22 mm thick and 2.5 m long vertical joint (3G) on a storage tank is shown below:

Comparison of MMAW versus MIG welding with the programmable, all position, variable speed carriage in the vertical (3G) position

*Wire diameter: 1.2 mm, flux-cored wire, considering a shift to be of 8 hrs

*Arc time for MMAW, MIG and mechanised welding are taken as 18%, 30% and 80% respectively

*A manual welder burns approximately 70-80 of 4 mm rods/shift. = 2 kg weld metal

*A MIG welder deposits 2.6 kg weld metal/hour

*A MIG welder deposits 6.25 kg of weld metal/shift at 30% arc time, and

*Mechanised FCAW deposits 16.6 kg of weld metal/shift at 80% arc time.

Summary of deposition rates:

*A MMAW welder deposits 2.0 kg of weld metal in an 8-hour shift (18% arc time)

*A MIG welder deposits 6.25 kg of weld metal in an 8-hour shift (30% arc time), and

*Mechanised FCAW deposits 16.6 kg of weld metal in an 8-hour shift (80% arc time).

Actual example of a vertical (3G) butt weld on a 22 mm thick shell, 2.5m long.

(MMAW = 2.5 mm & 4.00 mm rods. FCAW = 1.2 mm dia wire).

*Actual arc time with MMAW process = 9 hrs 45 min/per joint

*Actual arc time with mechanized FCAW = 3 hrs 24 min/per joint, and

&Total saving with mechanized FCAW = 6 hrs 20 min/per joint.

The above highlights the distinct advantages and economics for switching over to welding automation.

From the above summary, it is evident that welding automation, not only gives high arc times and high deposition rates, but also reduces the number of skilled welders and welding machines substantially. This generates appreciable savings in power, capital cost, maintenance, supervision, etc. In short, it reduces the manhours per tonne, which is the basic requirement for high productivity and profitability.

B. Malkani, HNC-Elect (U.K.), FEL. I. Inst. W., Technical Consultant, Gullco International Ltd