straight tube welder en

7/29/2019 Straight Tube Welder En

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The use of "straight tube welder" in boiler manufacturing

During the prefabrication process of boiler tubing for power plants an important

number of welds have to be carried out in the workshop. Special attention should

be paid to the features of the weld lathe at the beginning of the assembly line.

Here, different concepts and types are presented and discussed.

During energy production in fossil fuel-fired power plants, various types of fuel

are burned inside a combustion chamber. The released heat is conducted to thetube bundle inside the surrounding boiler shell; steam produced inside the tube

bundle is used to power steam turbine generators for electricity production.

The boiler erection operations can be divided into three main parts:

Making available the appropriate pipes with matching dimensions, made of the

requested material, corresponding to the specified quality level, shipped at the

desired date;

Prefabrication work such as cutting, joining and bending of the pipes in the

workshop;

Final boiler assembly with the prefabricated parts on site.

Pipes available on the market are generally of fixed lengths, which depend on

production methods, transport limitations etc.

During prefabrication, the pipes are welded together to get appropriate lengths

for the pre-assembled units.These are shipped to the site where they are used to construct the boiler.

The prefabrication can be organised in two different ways, which results in

different structures concerning the production line and the required equipment.

The first prefabrication technique consists of welding several pipes together

(depending on the length of the delivered pipes for example two or three

sections), the final shape of the pre-assembled unit is arrived at by joining themwith bends (Fig. 1).


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This prefabrication technique requires a welding machine for the joining of the

tubes at the beginning of the production line. At this stage of operation the

workpieces are still rotationally symmetrical, so the welds are carried out usually

on rotating tubes with the torch at a fixed position. On the commonlyimplemented welding lathe the tubes are clamped, centred and rotated at the

desired travel speed to carry out the weld. The proper welding operation is

performed by means of the welding torch which remains in a fixed position.

The welded tubes are taken out of the welding lathe and, after testing of the

welds and an in some cases necessary heat treatment, they are assembled

using corresponding bends and form pieces. A significant number of welds have

to be realised, either manually, or by means of orbital welding. The production

time of these pre-assembled units can be considerably influenced by the number

of welding machines and staff available to execute this work.

Fig. 1: Pipes which are already welded together are joined by bends and form

pieces in a pre-assembly line

Photo: Polysoude


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In the second method of prefabrication, many pipes are welded together. The

final length corresponds to the total length of the pre-assembled unit, which can

be up to 100 metres. Several bending operations are then carried out to

transform the pre-assembled unit to its final shape.

This type of prefabrication requires virtually all welds to be carried out by the

welding lathe or tube welder at the beginning of the assembly line. The total

production time of each pre-assembled unit is influenced strongly by the capacity

of this machine.

Due to the importance of these straight tube welders their design has been

continuously improved and adapted to the specific needs of production. With theincrease in efficiency of power plants, higher service temperatures became

necessary and new heat-resistant materials had to be developed, making

welding operations more and more delicate.

The suitability of materials under high mechanical stress and elevated

temperatures can be evaluated by means of a creep rupture strength diagram

(Fig. 2). Above the service temperature in degrees centigrade (abscissa) the

mechanical tension (Mega Pascal) which can be tolerated by the workpiece

during 1 x 105 h is indicated (ordinate). By agreement, its value for power plants

is set to 100 MPa.

Since 1950, the high temperature steel X 20 Cr Mo V 12 1 (X20) with 12 %

chromium has been used successfully in Germany and many other countries for

fossil fuel fired power plants of 150 MW in size. However, manufacturing and

welding of this material demands great care and has to be carried out

thoroughly, it has never been added to the ASME code (American Society of

Mechanical Engineers).

In 1980 the martensitic steel P91 with 9 % chromium was introduced in the USA.

Due to its increased creep rupture properties it allowed a rise in service

temperature from 540 C to 600 C. This steel is figuring on the ASTM-

specification A335 (American Society for Testing and Materials) and on the

ASME Boiler & Pressure Vessel Code, in the DIN standard it is specified as X 10

Cr Mo V Nb 9 1 under the material no. 1.4903.

The development of E911 and P92 martensitic chromium steels allowed a further

rise in service temperatures and efficiency (Fig. 2). These materials can also be

used for repair and replacement of older boiler components; the lower requiredservice temperature leading to considerably decreased wall thicknesses of the


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parts. In addition to material savings, easier installation and less weld metal to

be melted, the reduced wall thickness brings down the mechanical stress which

results in longer life expectancy of the components.

Further enhanced service temperatures will only be possible if austenitic steels

or nickel-based alloys are used.

Fig. 2: 105 h creep rupture diagram of boiler materials used in fossil fuel firedpower plants

The increased creep rupture strength of these types of high temperature steel is

based on specifically conditioned micro-structure properties, which if possible,

should not be altered by the welding process. Changes to the micro-structure

and brittleness caused by a too rapid cooling of the workpiece can be avoided by

preheating; overheating and modifications of the micro structure due to

excessive welding temperature are excluded by limited interpass temperatures;and internal stress of the parts caused by welding can be removed by an

adequate post weld heat treatment. Preheating and heat treatments must be

carried out by resistance or inductive heating, operating with naked flames does

not provide the required precise control of the process and provokes partial

overheating and hence is not permitted.

Conditions to be respected when welding heat-resistant or high temperature

steel for steam boiler purposes are listed in Table 1.


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Table 1: Welding characteristics of heat-resistant and high temperature steel

for steam boiler construction

Tube welders must be designed to clamp tubes with diameters from

22 millimetres to approximately 76 millimetres. If finned pipes, i.e. pipes which

are equipped with baffle-plates, with diameters of more than 100 millimetres

need to be welded, special clamping units become necessary.


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Straight tube welders are commonly equipped for MIG/MAG or TIG-welding.

Despite its lower performance, TIG-welding is preferred in Europe because of its

increased weld quality. Due to improved construction of the machine, it has been

possible to drastically reduce the arc time of the TIG welding process. Thewelding torch has been fixed at the seven o'clock position, as seen at an old

VKW-machine (Fig. 3), so pipes with a wall thickness of 5 mm (and in some

cases even 7 mm) can be butt-welded in a single pass.

Fig. 3: Example of a "Tube Welder": the VKW-machine is equipped with a torch

fixed at the seven o'clock position; the filler wire is added from above

Photo: Polysoude

On some straight tube welders the pipes were joined by friction welding, but this

method did not gain acceptance for a long time. However, the excellent

concentricity precision of these machines was later used to position the tubes,

even though the joining was realised by orbital TIG-welding.

The design of the straight tube welders had been adapted to the harsh operating

conditions of the workshop, featuring rigid frame construction and heavy-duty


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mechanical components. For this reason it can become economical to overhaul

such machines (Retrofit) and add modern welding equipment. An older Straight

Tube Welder with recently installed state-of-the-art TIG-welding components is

shown in Fig. 4. The torch with the reddish ceramic nozzle at its end, and theprotruding tungsten electrode, is fixed in the twelve o'clock position. In front of it,

the hose for the filler wire with a nozzle and the positioning device are situated.

The microprocessor-based control of the weld cycle is then integrated into the

power source; the necessary commands are stored as a program in its memory.

To obtain a perfect synchronisation between the movement and the welding

operation, the rotation of the pipes is controlled by the power source as well. The

welding is started via the remote control pendant; the cycle is carried out

automatically without any further intervention from the operator. If necessary,

however, weld parameters can be corrected directly during welding.

Unlike MIG/MAG-welding, where current intensity, arc length and wire feeding

speed are interdependent, TIG welding allows adjustment of the wire feeding

independently from the arc length and current. The ignition of the arc, the pre-

melting at the beginning of the weld, and a perfect downslope at the end are

carried out without filler wire, so defects in the weld can be excluded in these

zones.

Fig. 4: Older tube welder with recently installed TIG-welding equipmentPhoto: Polysoude


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A production line using the second prefabrication method, where a very long

pipe is welded together at the beginning, requires significant investment. A

workshop of sufficient length and large surface area must be available to allow

the bending operations, as well as adequate bending machines and heattreatment equipment. Cost-effective production can only be achieved with

efficient workload of the production line, requiring its quick and easy adaptation

to various work orders.

The most recent types of Straight Tube Welders are designed to offer highest

flexibility; improved automation and integration into the production chain to

guarantee reliable and reproducible welding results.

These machines generally offer a choice between MIG/MAG and TIG cold wire

or hot wire welding. The TIG hot wire process allows a reduction in the weldingtime, so it matches approximately the time required for MIG/MAG-welding. The

pipe end preparation (V-preparation) is also identical for both processes (Fig. 5).

However, problems at the beginning and the end of the weld can be solved more

successfully with TIG-welding, allowing a zero defect result level to be obtained.

Fig. 5: The V-preparation of the pipe ends allows MIG/MAG or TIG hot wirewelding

Photo: Polysoude

The welding equipment for MIG/MAG and for TIG-welding are each mounted on

a mobile carriage each; one carriage always occupies one of the parking

positions (Fig. 6), while the second one is fixed in the working position. The

carriages are moved manually; collisions are avoided by pneumatic locking. To

change between MIG/MAG and TIG welding, the required carriage must bemoved into the working position, no mechanical modifications or changes in the


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electrical or pneumatic connections are required.

Fig. 6: Modern welding lathe "Straight Tube Welder". The MIG/MAG welding

equipment, shown here in working position, and the TIG-welding equipment in

parking position in the background, are both mounted on mobile carriages

Photo: Polysoude

Once the welding process has been selected, MIG/MAG for example, the

required carriage is fixed in its working position. For MIG/MAG welding, the

machine offers the choice between two types of wire. Two complete welding

units including two separate wire spools, two wire feeding devices and two

torches are installed on the carriage. The desired torch can be indicated in the

weld program and is positioned automatically.

The TIG-process can be carried out with three different types of filler wire. Also

controlled by the program, the nozzle which is guiding the selected wire is

moved to its position in front of theTIG-torch (Fig. 7).

Fig. 7: Three different types of

wire can be selected for the TIG-

process, controlled by the

software the wire guide is moved

in position automatically

Photo: Polysoude


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The general adjustments of all torches are carried out manually, tilted positions

allow welding with the torch directed towards the finished part of the weld or

towards the part of the weld still to be made; to reach an additional offset

position the torches can be displaced on a slide.

For the acceptance procedure of a modern straight tube welder, test welds on

different pipes for boiler construction have been carried out. Characteristic

welding parameters are listed in Table 2, the sectioned joints are shown in

Fig. 8.

Pipe Filler wire

Pipe

material 1

Pipe

material 2

O.D.

(mm)

Wall

thickness(mm)

Type

Diameter

(mm)

SA213T9112

CRMoVG54 8

ER90S-

B90.8

TIG Hot Wire Welding

Shielding gas: Argon

Complete time per weld: 260 seconds

Welding speed Wire speed Weldingcurrent

Voltage Oscillationwidth

1st

layer

140 mm/min. 1,500

mm/min.

155 A 8.6 V 0.8 mm

2nd

layer

135 mm/min. 4,300

mm/min.

190 A 9.4 V 3.3 mm

3rd

layer

130 mm/min. 5,300

mm/min.

180 A 9.6 V 5.0 mm

Table 2: Parameters of TIG hot wire tube-to-tube welding


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Fig. 8: Test welds by MIG/MAG (left) and TIG hot wire (right) show welding for

the acceptance procedure of a modern straight tube welder

Photo: Polysoude

It is the client's responsibility to specify the mode of acceptance of the welds

based on the relevant technical regulations such as the ASME-standard IX, theASTM-code or the DIN standards. Depending on the regulations, visual

inspections as well as destructive and non-destructive test methods have to be

applied.

In Table 3, an extract of the Chinese Steam Boiler Safety Technology

Supervisory Regulations J B/T 2636-94 including the acceptance criteria for the

evaluation of a destructive test of the welds is given. In this case the surface of

the weld is ground to the level of the tube O.D. and a notch is machined in themiddle of the weld seam. The joint is then broken at the notch, so any weld

defects become visible and can be classified corresponding to the listed criteria.


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Table 3: Extract of the Chinese Steam Boiler Safety Technology

Supervisory Regulations JB/T 2636-94 for the acceptance of welded joints for

steam boiler pipes

In general, a straight tube welder is equipped for three kinds of operation. The

tube handling, i.e. the supply and removal of the workpieces and their

positioning, is carried out via a control panel (Fig. 9). A console installed at the

top allows to adjust parameters for MIG/MAG welding and to control the process.

A remote control pendant (Fig. 12) is used for TIG-welding and, for example, to

modify the torch position or welding speed during a welding operation.

For the handling of the pipes to be welded, a control panel is installed (Fig. 9).

The end of the first pipe is moved through the hollow shaft of one chuck to rest

against a retractable stop (Fig. 10) and then clamped in this position. The

second pipe is also clamped in a specified position against the retractable stop,

and once the stop has been retracted, it is shifted towards the end of the first

tube. The torch is moved automatically until its distance to the workpiece

corresponds to the programmed value, in the case of TIG-welding a program-

controlled centring of the tungsten electrode in the middle of the weld gap can be

carried out.


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Fig. 9: Control panel for the handling of the tubes; the console for MIG/MAG

welding is at the top

Photo: Polysoude

Fig. 10: Retractable stop for the positioning of the pipe ends

Photo: Polysoude


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The microprocessor-based sequencer controls the entire Straight Tube Welder

and is integrated into the power source of the TIG-welding equipment. All safety-

relevant parameters are monitored; for example all welding operations are

blocked if the carriages or torches are not fixed in their appropriate positions orthe welding gas supply is interrupted. A continuous data exchange is maintained

with subsequent machines, during a weld cycle no tubes can be loaded or

unloaded and if an X-ray test is executed, all pipe movements remain blocked.

An X-ray test can be carried out after each weld. If a serious defect is detected,

the weld can be completely cut off and replaced by a new one.

The weld cycles for the different workpieces are programmed on a PC with the

easy-to-understand Windows-based welding software POW (Fig. 11) and

transferred to the memory of the power source. All essential weld parameterssuch as current intensity, arc length, travel speed and wire feed speed as well as

up and downslope can be precisely reproduced in each weld cycle. The

commands to set-up the equipment can be given via the remote control pendant;

necessary parameter corrections can be carried out online during welding

(Fig. 12).

Fig. 11: Programming of a TIG-weld cycle by means of a PC

Photo: Polysoude


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Fig. 12: TIG hot wire welding, the left wire feeding unit is used; the remote

control pendant for parameter corrections during welding is in the left foreground

Photo: Polysoude

Conclusion

During the construction of boilers for fossil fuel-fired power plants a significant

quantity of the required pipes are welded at the prefabrication stage in the

workshop. The production time of the pre-assembled pipes is strongly influenced

by the capacity of the weld lathe installed at the beginning of the production line.

Various designs of these straight tube welders have been developed. Increased

flexibility can be achieved if a modern type is used, while MIG/MAG as well as

TIG cold wire and hot wire welding can be selected without manual conversion of

the machine. The TIG hot wire process guarantees high productivity, a zero

defect result weld level can be achieved. The machine allows the selection

between three different types of filler wire, in case of MIG/MAG welding process

two different wire types can be chosen. The change between the different wires

is controlled by the program and carried out without any intervention by the

operator. With this type of straight tube welder, work orders with dissimilar tubes

and materials can be treated without mechanical modifications to the machine.

Older tube welders in existing production lines should be checked formodernizing possibilities. Reconditioning and installation of state-of-the-art


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welding equipment (Retrofit) can help to improve productivity and reliability even

with a modest budget.

Dr.-Ing. J rgen Krger, Lippstadt

Contact :

Andrea HussonCommunication Dept.

POLYSOUDE S.A.S.

2, rue Paul Beaupre

F-44300 Nantes

France

Tel. + 33 (0) 2 40 68 11 74

[email protected]

straight tube welder en

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