WP-43 Principles of Tube and Pipe Weld Purging

Principles of Tube and Pipe Weld Purging

29W PurgEliteWeldPurgeSystemsDespite the fact that professionally designed weld purging systems have been available for some time it's surprising that fabricators of stainless steel tube and pipework still employ unreliable home made devices to prevent oxidation of the weld root.

Even on prestigious and demanding work like LNG storage and distribution the use of plastic, 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.

Joints of high quality between stainless steel cylindrical sections such as tubes, pipes and vessels can only be made by ensuring that atmospheric gases are eliminated.

The presence of oxygen, and to a lesser extent nitrogen, around the molten weld can lead to wide-ranging defects.

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 are eliminated. The presence of oxygen, and to a lesser extent nitrogen, around the molten weld can lead to wide-ranging defects.

    • Discolouration is unsightly and in some instances might produce metallurgical imbalance, especially with some stainless steels.

    • Gross oxidation inevitably results in reduction of mechanical properties and can cause catastrophic loss of corrosion resistance.

    • Nitrogen contamination can result in brittleness. Gases in the weld may give rise to cracking during or after cooling.

    • Uneven weld root beads caused by gross oxidation can cause entrapment, leading to bacteria build up and contamination of product flow.

The cost of removing these post-welding imperfections can be time consuming and therefore expensive.

Pickling through treatment with a mixture of acids is effective but can often not be employed for reasons of availability, 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 mechanical techniques such as grinding bring with them the risk of leaving unwanted residual materials behind.

Basic Principles

Weld root quality when making tubular joints can be ensured by applying appropriate safeguards that are based on removal of air from the fusion zone and the provision of a blanket of inert gas around it. This is achieved by gas purging.

Purging Gases

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 the cheapest of all the inert gases.

Even though it is clear that trying to save money using home made devices to prevent oxidation, operators still attempt to reduce costs further by using a cheaper gases such as nitrogen in the belief that this too is inert. This rarely the case.

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 underbead. We confine ourselves here to an examination of flexible discs and inflatable seals.

Various types of 'flexible disc' systems based on natural rubber, silicone rubber and 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. Reliability is suspect 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 collapsible and after welding the discs must be pulled out past the weld root, an operation that may cause difficulties. Cheap, yes, Reliable, no!

Crude-Pipe-dams-WEBCustomer-Foam-Dams2-WEB


Multiple and Floppy Discs

Disc-Systems-All

Improvements on the compressed gasket have been introduced as cost-effective solutions to weld purging. None the less, 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.

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.

Inflatable Seals

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.
Currently available systems address the problems of controlled inert gas pressure and flow, the need for easy and rapid deployment and removal to limit overall welding time, thermal resistance and leak-tight access for oxygen monitoring equipment.
They also provide a large pipe contact area and therefore excellent and reliable sealing.

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.

29W-PurgEliteWeldPurgeSystemsThe most advanced systems limit the use of metallic materials. This allows post weld radiographic inspection whilst the purging system remains in place.

The Argweld® range of inflatable systems have set a global standard that no other supplier can match. They are used by major international fabricators on the most demanding projects.

Argweld® Inflatable Systems cover tube and pipe diameters from 25 to 1800 mm. At the smaller end the PurgElite® (see HFT® Technical Note TN-17) concept covers 25 to 600 mm.

The 25 mm system is the smallest inflatable system available anywhere in the world.

QuickPurge® II Systems (see Technical Note TN-12 QuickPurge® II Systems ) can accommodate between 200 and 1800 mm pipes. The Quick Purge® II System has been specially developed for the welding of larger diameter pipes where welding time and gas consumption become significant values.

Special materials are used to withstand the higher temperatures encountered when the inflatable components are placed closer to the weld zone for reduction of purge time. In addition, materials with low 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.

01W-QuickPurgePipeWeldPurgeSystemThermally resistant versions of the Quick Purge® II Systems are available for use when welding pre-heating and post-heating are required. They can accommodate temperatures up to 300oC for 24 hours. This allows these devices to remain in place in the hot pipe work while controlled cooling is taking place to prevent undesirable metallurgical characteristics being developed.

As mentioned previously, it's always tempting to use ostensibly cheap solutions to any problem when commercial pressures need to be considered. 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 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.

Let's look first of all at the comparison between using a contemporary inflatable purge system such as 'QuickPurge®II System' and plastic foam dams for a range of pipe sizes.

Cost Comparison between Argweld QuickPurge® II System (AQP) and Plastic Foam Plugs

Working Time  Total Cost (£)
        

Norminal
Pipe
Diameter
(mm)

AQP
Total
Working Time
(mins)

Foam Dams
Total
Working Time
(mins)

Time Savings
(mins) using
Argweld®
QuickPurge®II
 

Total Cost
using
Argweld®
QuickPurge®II
£UK Sterling

Total Cost using
Foam Dams
£UK Sterling
Savings PER WELD
using
Argweld® QuickPurge®II
£UK Sterling

 

1

 

400

 

123

 

130

 

7

 

 

198

 

718

 

520

2 500 154
160 8   248 894 646
3 650 192.5 200 9   310 1,112 802
4 700 207 220 11   334 1,206 872
5 1100 345 355 11   555 1,962 1,408
6 1300 382 395 13   615 2,180 1,565
7 1500 458
470 14   737 2,605 1,868


The figures should be regarded as nominal since they are based on average inert gas costs and typical welding and purging times but they are practical figures from real applications. The extent of the potential cost savings 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 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

Summary

05W-PurgEye100-IP65-WeldPurgeMonitor

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 Argweld® systems excel here.

In order to ensure that the oxygen content is maintained at the appropriate level it is necessary to monitor it.

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® Weld Purge Monitors® (See Technical Note TN-15 Weld Purge Monitors® ).


System Comparison Table (Graded 1= Poor to 10 = Excellent

 

Purge System Ease of Use Releability Gas Pressure Control Gas Use Sealing
Rubber Disc 5 7 2 6 6
PurgElite® Inflatable Purging System 9 9 10 8 9


A table showing these systems in comparison with water soluble dams is also available on request

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