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Title: The welding of hot tap connections to high pressure gas pipelines
Category: Pipeline Industries Guild technical library
Downloadable: Yes 
Project No.:
Research Agency:
Catalog No.: PIG0035
Date of Publication: October 22, 1974
Price: $25.00 US
Abstract: The function of the British Gas Corporation is to supply gas and it must be able to guarantee that supply. With the advent of Natural Gas and the consequent growth of the National Transmission System, that guarantee now depends upon keeping the pipelines in full operation at all times. However, during the life of a pipeline, circumstances may jeopardise that supply; it may be necessary to divert the pipeline, additional off-takes may be required or the pipe may be damaged and a section have to be replaced. All of these operations require cutting into a live line. The most obvious technique is to cut off the flow of gas, purge the line and then recommission the line after the alterations have been completed. As indicated, this may not always be possible or convenient and it becomes necessary to do all operations live.

Welding onto a live line presents a number of problems. Two of the most important are :-

a) Standard British Gas practice has limited the maximum internal pressure in a line undergoing hot tapping to 28 bar (400 lbf/in2 ) . Difficulty has been experienced in reducing pressure on some lines and anxiety has been expressed as to the safety of the welding operation at pressures above this. b) The faster flow rates have induced higher cooling rates in the weldments being attached to the line and this, in turn, has led to higher hardness in the weld joint, particularly in the Heat Affected zone (HAZ) of the pipewall.

It was thought that this high hardness could have a deleterious effect upon the performance of the fitting in service.

These problems have been studied at the Engineering Research Station of British Gas as described in this paper.

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