World Pipelines Contribution
Joints of high quality between cylindrical sections such as tubes and pipes can only be made by ensuring that: atmospheric gases are eliminated and positive, smooth weld reinforcement is provided.
The presence of oxygen, and to a lesser extent nitrogen, around the molten weld can lead to wide ranging defects.
Discoloration is unsightly and in some instances might produce metallurgical imbalance, especially with some stainless steels.
Gross oxidation inevitably results in reduction in 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.
It is clear that a reduction in weld section at the root, as evidenced by a concave geometry, will reduce the joint strength. Perhaps not so evident, but in many applications of crucial importance is the presence of notches or cracks which tend to appear at the weld/parent material interface.
These can propagate in service and cause failure.
Weld root quality when making tubular joints can be ensured by applying appropriate safeguards which are based on removal of air from the fusion zone and the provision of inert gas. This is achieved by gas purging and the general principles are shown in Figure 1.
The most commonly used purging gas in Europe is commercial quality argon; in the USA helium is in more general use, being less expensive. For specialised applications, purging techniques using argon/hydrogen and helium/argon mixtures have been developed.
Selection of the optimum gas or gas mixture will depend upon many factors, but not least the materials being joined and the welding process used. Purge gas flow rate and pressure also need to be established and once selected they should be included in the formal welding procedure.
Variation in purge gas quality may arise during welding and it may be desirable to apply continuous gas monitoring, especially to control oxygen and moisture content. For this purpose dedicated oxygen analysers are available commercially.
The first requirement is to provide gas entry and exit points. Gas is fed through one end seal with an exit hole at the other end to prevent an undesirable build-up of pressure. Argon has a greater density than air and the gas inlet should be at a lower elevation than the bleed end so that air is expelled effectively from the pipe bore.
Inflatable Bladder Dam
By far the most efficient purge gas containment method is to use inflatable dams such as the Argweld1 system. This has been developed specifically to provide a re-usable solution to gas purging which is easy to use and economical when several similar joints need to be produced.
The bladder, which has sufficient length to ensure sound sealing, is manufactured from rubber with a protective canvas cover. One is placed on each side of the joint and inflated using either compressed air or the purge gas itself. The latter is much preferred since it overcomes any problems which might arise from leakage of the bladder. Variations on the basic equipment are commercially available;
Purge inlet and outlet pipes can be incorporated in the bladder to allow the full circumference to seal against the pipe wall.
High temperature covers can be provided to afford protection during weld pre-heat cycles.
Single bladders can be used for closed end joints
Inflation and purging gas pressures can be separately controlled
Longer or shorter spinal connecting tubes are available
Provision can be made for continuous alteration in gas flow rate up to 20 l/min
The Pre-Purge Process
A pre-purge is used to displace air present in the pipework system or dam volume. Numerous factors control the pre-purge time such as pipe diameter, purge volume and maximum permitted oxygen level. A common misconception is that increasing the purge flow rate will reduce the purge time. This is falacious. Increase in flow rate increases turbulence and results in unwanted mixing of purge gas and air and can actually extend the purge time. As a general rule the pre-purge flow rate and time should allow for about five volume changes in the pipe system or dam volume but a typical gas flow rate will be in the region of 20 l/min.
Figure 5 is an illustration of the relationship between pre-purge time and pipe diameter based on a pipe purge length of 300 mm. For different purge lengths it is reasonable to use a pro-rata calculation. Table 1 presents example of purge times for different pipe diameters and flow rates.
Weld joints which require a root gap or which exhibit bad end matching, both of which characteristics provide an unwanted leak path for the purge gas, can be sealed by taping.
Oxygen and moisture levels in the purge gas should be checked using appropriate equipment with checking taking place at the outlet point. Where dam inserts are being used the outlet point needs to be extended with a flexible pipe to a convenient access position. If this is impractical a system which has the purge inlet and outlet in the same dam unit should be used.
Figure 6 gives times to reduce the dam volume oxygen content to below 1% using inflatable bladders. Note that whilst 0.1% residual oxygen is a suitable working level for materials such as stainless steels, the level needs to be as low as 20 ppm when welding the more sensitive alloys based on titanium and other reactive metals.
The Weld Purge Process
Once the quality of the gas in the dammed volume has reached the required level, gas flow can be reduced to about 5l/min for the welding operation. On a more practical level it should just be possible to feel the gas flow from the exit point. Excessive flow can cause the internal pressure in the pipe to rise and create concavity in the weld root geometry and in more extreme cases can cause complete ejection of the molten weld pool.
On joints which are not fully sealed, a higher flow rate may be necessary to compensate for losses and avoid contamination. Towards the end of the weld run however, as the joint becomes permanently sealed, the gas flow rate will need to be reduced to avoid overpressurisation.
Major Cost Savings at LNG Terminal
The strategically significant multi-million pound facilities currently under construction near Milford Haven in the United Kingdom underpin a major project to receive gas in liquid form from Qatar and distribute it throughout the UK via a 128 km long pipeline link between the port and a terminal near Gloucester. A receiving station will transport liquified gas from the ships to massive storage tanks each holding 160,000 cu m. Handling the liquid and transporting the gas places the total project among the biggest global engineering achievements being currently undertaken.
An integral part of the project involves welding of large diameter stainless steel pipework which is used between the unloading jetties and the storage tanks. Two leading international engineering construction companies are currently undertaking the fabrication work.
The liquid methane transit and storage pipework is all fabricated from stainless steel to grade 316 and to pipe standards between schedule 10 and schedule 100. With pipe diameters ranging from 200 to 750 mm and typical lengths of 10 m the demands on handling and welding to meet the stringent joint acceptance levels impose particularly high quality control standards for which specific procedures have been developed.
The original welding procedure called for gas purging of the pipe diameter prior to and during welding to be undertaken using proprietary foam dams. The engineers noted that the dams were taking a long time to locate in position and to remove after welding, but of even more concern was the very high consumption of argon gas during the purge process, caused by leakage. Specialist weld purging company Huntingdon Fusion Techniques Ltd (HFT), conveniently located in Burry Port in South Wales, were contacted and technical staff examined the problem. An innovative and more eco-friendly solution was developed and the HFT Weld Purge Monitor® ‘Quick Purge’ system was selected to reduce gas wastage and reduce overall welding time.
The benefits were noted immediately. Gas consumption dropped by 90%, saving over £2500 per month for the contractors.
Providing precise data on cost comparisons between the different purging techniques is difficult not least because the pipe diameter and wall thickness have a profound influence on the cost.
It is clear from the Milford Haven experience and several other major pipe welding applications however that where several welds have to be made on similar pipe diameters there can be genuine cost savings when using inflatable bladders as the sealing medium. Add this to the technical advantages of reliable sealing and ease of use and the inflatable purge bladder concept can be seen to offer significant attractions.
By Drs Michael Fletcher and Ron A Sewell