Purging and Monitoring Systems for Welding Pipework in Power Stations

Thanks to the high temperature resistance of glass-coated cloth, Huntingdon Fusion Techniques, HFT® (HFT; Burry Port, Wales) now manufactures Argweld® Quick Purge Systems with glass treated silicone cloth that resist temperatures up to 760 deg C, which is particularly valuable for welding pipework suitable for power stations and other utilities that produce high pressure steam for the rotation of turbines to produce electricity for the national grid. Usually the materials specified for such pipework are chrome steels that need to be heated up to 300 deg C prior to and during welding, and 760 deg C for heat treating/annealing (for stress relief purposes) afterwards. For a number of technical reasons, the inside of the welds made to join the pipework must be covered in an inert gas to prevent oxidation and metallurgical defects. In order to provide the conditions for welding the joints described, a weld purging system is generally used to isolate a small volume either side of the weld, to create optimum conditions.

A pipe purging system will reduce the volume to be purged quite dramatically and in such large diameter power pipework the amount of argon gas saved could be sufficient to pay for the costs of such purging systems in just one weld. In addition, a professionally designed and manufactured system will ensure that the leaks found in lower quality products, can cause no oxidation or sugaring to the weld joints. High strength steel joints must pre-heated prior to welding and also require a post-weld heat treatment. Weld joints of this nature are often purged of oxygen and welded in an inert atmosphere to reduce the risk of ‘sugaring,’ an imperfection that could allow metal pieces to break off inside the pipework and cause damage to turbines, for example. With the high temperatures and length of time at such temperatures, most weld purging systems are unsuitable. The HotPurge® system developed by HFT is an inflatable pipe purging system for heat treated chrome and high strength stainless steel pipe joints where the temperatures might be as high as 760 deg C (1,400 deg F) for many hours.

“This system allows the purge to be safely continued for up to 24 hours while pre-heating, welding and post weld heat-treating with the purge system remaining in place,” said Luke Keane of distributor support for HFT. HotPurge includes PurgeGate®, a revolutionary device to prevent over-inflation and damage to the inflatable dams. No matter how high the user increases the pressure in order to deliver more gas flow, PurgeGate prevents too much pressure from being delivered to the inflatable dams, allowing an increased flow to achieve a lower oxygen level or faster cooling of the weld zone. HotPurge also ensures the purge space remains tightly sealed at all times as the inflatable dams maintain an even seal throughout the weld cycle, irrespective of pressure rises or excessive flow rates of argon gas. The large central collar dramatically reduces the space to be purged and makes it easier to reach lower oxygen levels faster than with conventional purging systems.

Each 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 of previous exposure to daylight. They are manufactured to suit pipe diameters from 6 in to 88 in (152 mm 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 inflatable pipe purging systems can be used repeatedly for weld purging and will keep the oxygen levels well below 100 parts per million (ppm) throughout the welding cycle, ensuring oxide and coke free welds, without the metallurgical imperfections caused by exposure to too much oxygen.

Welding stainless steel, titanium, zirconium, niobium and other metals sensitive to atmospheric gases requires oxygen levels to be as low as 10 ppm and, in some cases, knowledge of dew point level in the shielding and purging gas. Many welds of this nature are still welded with archaic or home-made items to try to shield the weld, but most fail to give satisfactory protection, resulting in oxidized joints that can lead to metallurgical failure at worst and the necessity for additional processes, such as mechanical or acid removal of the oxide film. These extra processes of cleaning/oxidation removal can be easily cancelled by using the PurgEye® 300 Plus Weld Purge Monitor® with PurgeNet™ that measures oxygen levels down to 1 ppm. This lightweight, portable gas analyzer indicates actual oxygen levels down to 1 ppm (accurate to 10 ppm) on a large alpha-numeric OLED display, perfect for achieving oxide-free, zero-color welds time and time again.

“The unique, highly advanced PurgeNet communicating system allows connection of accessories such as an automatic welding machine interface, a long distance visual alarm system and a dew point monitor,” explained Ron Sewell, the chairman of HFT. “Each of these allows control of a welding system by the weld purge monitor to avoid making faulty welds because of excessive oxygen being present.” The PurgEye 300 Plus is menu-driven by four push buttons on the front and has an internal audible alarm so acceptable and unacceptable oxygen levels can be set. A long-life oxygen sensor inside the monitor means there is very little maintenance requirement. As well as having superb shielding against high frequency electronic disturbances that are common with welding power supplies, the monitor is equipped with PurgeLog™ for quality control purposes, where data can be stored or printed and transferred to a PC. No longer do welders have to use oxygen monitors that are calibrated to be correct at atmospheric levels for health and safety purposes, as they can use a weld purge monitor that is calibrated down to 1 ppm to give welding level requirements and accuracy.

COB Industries, Inc., 6909 Vickie Circle, Melbourne, FL 32904, 321-723-3200, general@cob-industries.com, cob-industries.com.