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Weld Purging Post Weld Heat Treated (PWHT) Pipework

Posted by Michaela Reay

13-Feb-2019

huntingdonfusion.com


Welding processes can weaken metals by imparting residual stresses into a material, leading to a reduction in corrosion resistance properties. Post Weld Heat Treatment (PWHT) is regularly used to retain the strength of a metal when welding certain materials including high strength stainless steel and chromium-molybdenum (chromoly) steel such as P91.

Temperatures during PWHT might reach as high as 760ºC (1,400ºF) for many hours, during which time the insides of the pipes need continuous purging using Pipe Weld Purging Products. Many purging materials cannot withstand these temperatures, nor resist damage during the time at temperature.

HotPurge® Pipe Purging Systems have been designed and developed by Huntingdon Fusion Techniques HFT® for Heat-Treated Chrome and High Strength Stainless Steel Pipe Joints. 

Ron Sewell, Chairman for HFT® said: “To ensure welds are free from oxidation which could otherwise lead to joint imperfections, they must be welded in an environment free from oxygen that has to be maintained during the lengthy high temperature post weld heat treatment process.”

“It is not practical or cost effective to fill whole pipe lengths with Inert gas and restricting the weld purging zone has proven most effective. Argweld® HotPurge® Systems allow the purge to be safely continued for up to 24 hours whilst pre-heating, welding and post weld heat-treating with the purge system remaining in place.” 

The Argweld® Tube and Pipe Purging Systems are positioned inside the pipe with one dam each side of the joint and inflated with an inert gas supply. Once a System is fully inflated and sealed in a pipe, the inert gas displaces the oxygen from within the weld purging zone ready for pre-heating and welding. Each HotPurge® System is manufactured as standard with a Weld Purge Monitor® connection for viewing the oxygen level throughout the whole heating and welding processes.

Each HotPurge® System includes PurgeGate®, a revolutionary device to prevent over inflation of, and damage to the inflatable dams. No matter how high the user increases the pressure, in order to deliver more gas flow, PurgeGate® prevents pressure rise in the inflatable dams but allows an increased inert gas flow, to achieve a lower oxygen level and/or faster cooling of the weld zone. 

In addition, each HotPurge® System incorporates an innovative central band for easy positioning inside the pipe. This band, known as RootGlo®, will illuminate inside the dark pipe, for up to 20 hours, with only 10 minutes previous exposure to daylight.

HotPurge® Systems are manufactured to suit pipe diameters from 6 to 88” (150 to 2,235 mm) and both inflatable dams have pull handles securely sewn with high temperature resistant Kevlar thread that will withstand a pulling force of 1,000 lb.

These HotPurge® Inflatable Pipe Purging Systems can be used repeatedly for weld purging and will keep the oxygen levels below 100 parts per million (ppm) throughout the welding cycle, ensuring oxidation and coke free welds, without the metallurgical imperfections caused by exposure to too much oxygen.



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