GB2119687A

GB2119687A - Method of and apparatus for orbital welding

Info

Publication number: GB2119687A
Application number: GB08212276A
Authority: GB
Inventors: Martin Blake; David Robert Campbell
Original assignee: British Nuclear Fuels Ltd
Current assignee: British Nuclear Fuels Ltd
Priority date: 1982-04-28
Filing date: 1982-04-28
Publication date: 1983-11-23

Abstract

A problem that occurs when welding pipes (1, 2) is weld sag. The present invention overcomes the problem by providing a pressure of inert gas at a preselected value in the pipes, the pressure being below atmospheric, e.g. between 3 o'clock and 9 o'clock. A needle valve 9 controls exhaust conductance through the pipes to control pressure in accordance with a time cycle. <IMAGE>

Description

SPECIFICATION Method of and apparatus for orbital welding This invention relates to a method of and apparatus for orbital welding.

When welding pipes using automatic orbital welding techniques, a problem that can occur is that of sinkage of the weld, during welding, owing to the effect of gravity. A method for welding tube-to-tube sheet joints is known from an article in Welding Journal of January 1980 on pages 26 to 37. In this article, a higher pressure is provided on the same side of the weld as the welder.

An object of the present invention is to provide an improved method of and apparatus for orbital welding, which mitigates the above mentioned problem.

According to one aspect of the present invention, a method of orbital welding for pipes, comprises providing a sealed region with a joint to be welded, providing preselected pressures within the joint at preselected desired welding positions.

A reduced pressure is provided between three o clock and nine o'clock of the weld. The reduced pressure may be minus twenty five millimetres water pressure.

Advantageously, pressure within the joint is reduced by introduction of an inert gas.

According to another aspect of the present invention, apparatus for orbital welding of pipes includes means for altering pressure within a joint to preselected values at preselected points in the welding cycle.

Advantageously, pressure sensing means is provided for sensing pressure within the weld and the pressure sensing means controls conductance of an exhauster from within the joint control sensed pressure to a preselected value.

Advantageously, the apparatus comprises timing means for timing periods of preselected pressures.

An embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawing which is a schematic figure.

When welding together pipes by an automatic orbital weld, one problem that can occur is sinkage of the weld root owing to the effect of gravity at the six o'clock welding position.

In the figure, a pipe 1 is to be welded to a pipe 2. An end of the pipe 1 away from a joint between the pipes is sealed by a seal 3 and an end of pipe 2 away from the joint is sealed by seal 4. A gas inlet 5 is provided through seal 4 and a gas outlet 6 is provided through seal 3. A pressure sensing device 8, in the figure schematically indicated as a pressure switch is connected in the outlet pipe 6.

A servo motorised needle valve 9 is arranged in the outlet after the pressure sensing device 8. A rotary pump 10 is arranged to pump gas out from the pipe 6. The pump 10 has an exhaust 1 1.

In use of the invention, argon is fed in through pipe 5, through the pipes 2, 1 and out through the outlet 6, the needle valve 9, the pump 10 and its exhaust 11. Pressure within the outlet 6 is sensed by the sensor 8, which can be pre-set by an operator to desired values of sensed pressure. The pressure is sensed automatically and provides a feed back control to the motorised needle valve 9.

The argon or other suitable gas pressures is controlled by the electronically controlled servo needle valve 9 in response to a signal from the automatic pressure sensing device 8. The device 8 is controlled by an electronic timer 13 which causes the device to switch to a preselected desired value of sensed pressure at preselected desired times in the welding cycle.

In this example, the preselected pressure within the pipes is atmospheric from nine o'clock to three o'clock (i.e. including twelve) and minus twenty five millimetres water gauge from three o'clock to nine o'clock (i.e. including 6). This is in order to counteract any tendency of the weld to sink in the lower regions of the pipes because outside air pressure counteracts gravity. Normally the pressure backlash in the system would allow pressure changes to be effected within ten degrees of arc. Also any overlap where the weld begins at nine o'clock is at zero pressure. The pressures are calibrated for various argon inlet flow rates by adjusting the needle valve settings.

The timer 13 causes the pressure changes since the total welding time is known and the timer is started when the weld is initiated so that pressure can be changed after half the welding time has elapsed.

In the figure, orbital welding apparatus is indicated by 15 and an enlarged detail shows a welded joint between the pipes 1 and 2. The weld 16 can be seen to be uniform. In the absence of the present invention, the weld 16 would tend to sag downwardly in the direction of arrow x.

Operation of the apparatus is as follows.

A seal welded butt joint is made quickly by passing the orbital welding apparatus 15 around the joint. A desired pressure (e.g. nought and minus twenty five), based on experience and calculation, within the sealed chamber defined by the pipes 1 and 2 and seals 3 and 4, is chosen.

Also, a desired argon purge rate in the range of a flow of four to twelve litres per minute is selected.

The correct gas pressure is produced by adjustment of the needle valve 9 and the pressure sensing device 8. The timer control circuit 13 is then set. The welding cycle is then commenced by starting the welder, the timer control circuit being synchronised to the apparatus 1 5, so that the timer is actuated automatically and can cause a pressure change when desired at three o'clock.

It is possible to use the invention with manual orbital welding rather than automatic orbital welding in which case mechanical switches could be used to cause pressure changes. Also, in some circumstances, a higher pressure could be used with the pipes to counteract gravity at twelve o'clock.

It is also envisaged that a gradual change in pressure around the weld cycle could be effected to suit specific welding conditions/pipe orientation. A more complicated smooth change would be controlled by a programmed microprocessor.

From the above description, it can be seen that an improved method of and apparatus for welding is provided.

Claims

1. A method of orbital welding for pipes, comprising providing a sealed region within a joint to be welded, providing preselected pressures within the joint at preselected desired welding positions.

2. A method as claimed in claim 1, in which a reduced pressure is provided between three o'clock and nine o'clock of the weld.

3. A method as claimed in claim 2, in which the reduced pressure within the joint is minus twenty five millimetres water pressure.

4. A method as claimed in any one of the preceding claims in which pressure within the joint is reduced by introduction of an inert gas.

5. Apparatus for welding of pipes, includes means for altering pressure within a joint to preselected values at preselected points in the welding cycle.

6. Apparatus as claimed in claim 5, in which pressure sensing means is provided for sensing pressure within the weld and the pressure sensing means controls conductance of an exhaust from within the joint to control sensed pressure to a preselected range.

7. Apparatus as claimed in claim 5 or 6, in which timing means is provided for timing periods of preselected pressures.

8. A method of orbital welding substantially as hereinbefore described and as shown in the accompanying drawing.

9. Apparatus for orbital welding substantially as hereinbefore described and as shown in the accompanying drawing.