Flexible Discs versus Inflatable Seals
Despite the fact that professionally designed weld purging systems have been available for some time it's surprising that fabricators of stainless steel tube and pipe work still employ unreliable home-made devices or unsuitable products, to prevent oxidation of the weld root.
Even on prestigious and demanding work such as LNG storage and distribution the use of plastic, paper foam, cardboard and even screwed up newsprint proliferates.
It's a fallacy that such relatively archaic practices are more cost-effective than the use of widely available, proven, properly engineered systems.
Defining the problem
Joints of high quality between stainless steel cylindrical sections such as tubes, pipes and vessels can only be made by ensuring that atmospheric gases, and in particular oxygen, are eliminated.
The presence of oxygen, and to a lesser extent nitrogen, around the molten weld can lead to wide-ranging defects.
This is unsightly and in some instances might produce metallurgical imbalance, especially with some stainless steels.
This inevitably results in reduction of mechanical properties and can cause catastrophic loss of corrosion resistance.
Nitrogen gas in the weld may give rise to cracking during or after cooling.
Uneven weld root bead
These can cause entrapment of contamination and lead to bacteria build up and degradation of products.
The cost of removing these post-welding imperfections can be time-consuming and therefore expensive.
Pickling Processes through treatment with a mixture of acids are effective, but are often unsuitable for reasons of accessibility and environmental protection.
Electrochemical removal of discolouration requires manual operation and is slow; it is generally only practical for use with smaller products.
Grit blasting and the use of other mechanical techniques such as grinding bring with them the risk of leaving unwanted residual materials behind.
Basic Principles of Weld Purging
Weld root quality when making tubular joints can be best controlled by applying appropriate safeguards based on removal of air from the fusion zone and the provision of a blanket of inert gas around it.
This is achieved by inert gas purging.
Selection of the optimum gas or gas mixture will depend upon many factors but not least the materials being joined and the welding process employed. Argon is the most widely used as it is completely inert and, in most countries, the cheapest of all the inert gases.
Purge gas flow rate and pressure also need to be established and once selected they should be included in the formal welding procedure.
An examination of two types of Tube and Pipe Weld Purging Equipment
Several different principles have been used to provide inert gas coverage of the weld under-bead. We confine ourselves here to an examination of flexible discs and inflatable seals.
Various types of systems based on natural rubber, silicone rubber and other synthetic rubber discs have been developed specifically for use in weld purging.
Persuasive arguments are put forward by the manufacturers of these systems. They are claimed to be relatively cheap, can be deployed quickly and removal post-joining is easy since the assembly can be withdrawn past the weld.
The principles appear sound but the practical aspects render this type of purge equipment unreliable because disc to pipe sealing depends upon a very small contact area.
If we look at two typical disc concepts in current use it will become clear how unattractive they are in reality.
Rubber Gasket Dam
A rubber, foam or similar disc can be sandwiched between a pair of wooden or metal discs and some adjustment to diameter can be effected by applying axial pressure. This gasket technique is not fully collapsible and after welding the discs must be pulled out past the weld root, an operation that may cause difficulties. Cheap, yes ... Reliable, no!
Wooden/metal disc system sandwiching compressible seal
Multiple Floppy Discs
Improvements on the compressed gasket have been introduced as cost-effective solutions to weld purging. Nonetheless, they still only fit a very limited range of internal tube or pipe dimensions and sealing depends entirely on the disc edge being true - even minor variations in diameter and roughness cannot be accommodated without significant leakage of inert gas.
These disc systems also contain numerous metal parts that may scratch polished surfaces. This can lead to a loss of corrosion resistance. Furthermore, to purchase a disc system for just one internal diameter of tube or pipe becomes uneconomical compared to systems that cover a range of diameters, without having to make changes.
These disc systems also contain numerous metal parts that may scratch polished surfaces. This can lead to a loss of corrosion resistance. Furthermore, to purchase a disc system for just one internal diameter of tube or pipe becomes uneconomical compared to systems that cover a range of diameters.
The only totally reliable and sufficiently versatile purging systems are those based on inflatable seals.
Considerable design effort has been applied by the designers and manufacturers to these solutions over the past decade or so and currently available systems address the problems of:
• Controlled inert gas pressure and flow
• Easy and rapid deployment and removal
• Thermal resistance
• Leak-tight access for oxygen monitoring equipment
• Provision of a large pipe contact area and therefore excellent and reliable sealing.
Basis of operation of an inflatable weld purge system.
The large contact area ensures reliable and consistent sealing between the dams and the pipe wall. When deflated the dams can be inserted and removed easily and quickly without damage.
Pre-set valves control gas pressure and flow.
The flexible connecting section allows use either side of sharp pipe bends.
PurgeGate® is fitted to eliminate the risk of bursting due to over pressurisation.
Coupled with these advantages comes flexibility to allow access and removal through pipe bends, abrasion resistance and the use of materials that meet nuclear compliance standards.
The most advanced systems limit the use of metallic materials close to the weld zone. This allows for post-weld radiographic inspection whilst the purging system remains in place.
The inflatable systems designed and manufactured by Huntingdon Fusion Techniques Ltd have set a global standard that no other supplier can match. They are used by major international fabricators on the most demanding projects.
The HFT® Argweld® inflatable systems cover tube and pipe diameters from 25 to 2400 mm.
At the smaller end the PurgElite® concept (see Technical Note TN-10 PurgElite® Inflatable Tube and Pipe Weld Purging Systems) covers 25 to 600 mm. The 25 mm system is the smallest inflatable system available anywhere in the world. PurgElite® Systems can be fitted with PurgeGate® on request, to prevent bursting due to over inflation.
PurgElite® System of Inflatable tube and pipe weld purging systems for internal diameters of 1 - 24" (25 - 600 mm)
The HFT® QuickPurge® (see Technical Note TN-12 QuickPurge® Systems) can accommodate between 200 and 2400 mm pipes. This been specially developed for the welding of larger diameter pipes where welding time and gas consumption become significant factors.
Special materials are used to withstand the higher temperatures encountered when the inflatable components are placed closer to the weld zone to reduce purge time.
Thermally resistant versions of the QuickPurge® II Systems are available for use when weld pre-heating and post-heating are required. (Argweld® HotPurge®). These can accommodate temperatures up to 300oC for 24 hours. This allows these devices to remain in place in the pipe work, during preheating, welding and while controlled cooling is taking place to prevent undesirable metallurgical characteristics being developed.
Argweld® QuickPurge® II System with integrated preset gas purge and inflation valve, sintered metal exits to control gas flow and minimise turbulence. Also included is, the RootgloTM system for lighting the inside of the dark pipe to align the purge system correctly. Argweld® QuickPurge® and HotPurgeTM Systems are delivered with PurgeGate® to prevent systems from bursting due to over inflation.
In addition, materials with the lowest outgassing rates have been chosen to further reduce the amount of contaminating vapour released into the purge volume during welding. A further reduction of purge time has been achieved by the central collar that creates a small annulus for purging. Typical times would be 7 minutes to purge a 36" pipe joint to 100 ppm.
We commented earlier on the temptation to use ostensibly cheap solutions for purging. Judging by the number of pipe welds carried out where screwed-up paper, foam and cardboard discs are used to seal the volume beneath the joint line prior to admitting protective inert gas these crude techniques are still widely employed. Even worse than these primitive solutions is the practice of simply flooding the pipe with inert gas without using sealing dams at all in the hope of maintaining a working environment with a residual oxygen content that is compatible with the production of sound welds.
There is no sound technical argument for using such solutions. The claim that they are cost-effective is seriously flawed.
To illustrate this we can examine a comparison between a contemporary inflatable purge system such as HFT®'s 'QuickPurge® II and plastic foam dams for a range of pipe sizes.
Comprehensive Return on Investment chart, available from HFT® on request also in US Dollars
The figures should be regarded as nominal but they are based on typical inert gas costs and known welding and purging times The extent of the potential cost differences will come as a surprise, even to seasoned users of engineered purging systems.
The economic argument for using inflatable purge systems becomes stronger when other factors are taken into consideration:
Gas Flow Rates and Pressure
Using proprietary purge systems in conjunction with oxygen monitoring instrumentation the inert gas pressure and flow rates can be set for maximum efficiency. Indeed, with recently introduced innovative technology, flow rate is pre-set by the manufacturer. Compare this with the guesswork involved when using plastic foam and similar solutions - to be on the 'safe side'. Gas flow rates here are invariably set far too high to compensate for possible leaks and the additional gas cost can be very significant. To compound this practice, high flow rates cause turbulence and this leads to poor welding quality.
Monitoring of Oxygen Level in Purge gas
In order to ensure that the oxygen content is maintained at the appropriate level it is necessary to monitor it. A variety of instruments is available but few have been designed specifically for use as Weld Purge Monitors®.
Two essential characteristics of a suitable instrument are that it must have an adequate measuring range and it must sample the purge gas inside the pipe volume. The sensitivity should be such that an oxygen level as low as 10 ppm (0.001%) can be detected. Instruments that only display down to 1,000 ppm (0.1%) are totally unsuitable.
A typical high sensitivity instrument will include a sampling tube, gas extraction facility and sensing electronics that are reliable and repeatable. Such facilities are available with HFT®'s advanced PurgEye® monitoring systems. (See Technical Note TN-15 Weld Purge Monitors®).
The fact that even very small amounts of oxygen in the purge gas can cause problems makes it desirable to choose a purging system that ensures a high level of sealing reliability. In all respects the inflatable PurgElite® and QuickPurge® II Systems excel here.
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