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Product category: Machinery and Production Equipment
News Release from: Orbimatic | Subject: Welding stainless steel
Edited by the Engineeringtalk Editorial Team on 24 October 2002

How to achieve a clean process line

When planning the installation of a new stainless-steel process pipeline it is important to use the best possible materials and techniques to ensure it is clean when installed, says Steve Purnell.

The ideal process line will be designed to give smooth flow of the product To achieve this, the inside bore of the weld should be consistent through the entire system

The bore of the system should also be smooth with no marks or crevices which may cause potential contamination areas, know as "bug traps".

Bug traps are small voids in the material surface where product can catch and deteriorate over a period of time.

By designing a system with maximum welded joints the amount of designed problem areas is kept to a minimum.

Mechanical joints naturally have areas which cause potential bug traps so should be kept to a minimum.

Should the line not be installed to a satisfactory standard it may well not last as long as you hope and can cause serious infection in the product travelling through it.

The first and most important consideration for installation of a stainless-steel process line is to ensure that the stainless steel will not degrade during the installation operation.

Probably the biggest risk is the degrading of the material during the welding operation.

Stainless steels contain chromium and nickel which give the material the "stainless" properties.

If the material is not adequately protected from the oxygen in the atmosphere during the welding process the material will degrade dramatically.

Another serious risk is the pipelines which have had no post clean and treatment prior to service.

If a line is not cleaned prior to being put into production fabrication debris can be left in the system.

These debris will soon cause and infection of the product flowing through the system.

The main considerations to help create a clean process pipeline are: selection of tube and fittings; tube fabrication techniques; welding process and equipment; application and control of welding; and post cleaning and treatment of pipeline.

Each of these areas has its own important factors and we will explore each of these in details below.

To achieve a clean process pipeline it is important to select tube and fittings from a high quality source that can provide mill certification, chemical analysis and guarantee the quality of the tube finish.

Tubes should be supplied from a controlled stockholding in protective packaging to eliminate any chance of tube contamination before it is installed.

When selecting and procuring tube and fittings the following points should be considered: sulphur control; surface finish; manufacture specifications; batch control; and traceability.

We will look at each of these points in turn and investigate the importance of each area.

Stainless-steels have a trace of sulphur which is often not considered as a problem area.

But in practice the sulphur content has a drastic effect on the weldability of the material.

Only 0.001% difference in the sulphur content of the material can cause a vast difference in weld profile.

This is due to the Maragoni effect, which changes the surface tension temperature coefficient.

This means that the heat movement in material changes for different sulphur content materials.

As mentioned earlier it is important to have a smooth internal surface on the tube and fittings.

This includes the inside diameter of the tube and fittings selected and it is important that the tube supplier should be able to give a guarantee of the quality of the internal surface of the tube, especial in welded tubes where the internal surface will be effected in the weld zone.

There are many specifications to which tube can be manufactured but the most important regulation is ASME-BPE 1997.

It is important that the tube should be manufactured with a SPC (statistical process control) production line to give guaranteed quality tube.

All tube and fittings should be ordered in on lot to ensure that the tube and fittings delivered are from the same batch of material.

This is ensure that all materials supplied have an equal sulphur content, this in turn will eliminate any problems with weld quality.

All tube and fittings should be traceable to original source.

This allows any problem encountered during fabrication, installation or service to be traced and the problem identified and then the results of the problem rectified.

During fabrication it is important to follow recommended fabrication techniques to keep contamination of the tube bore and material degradation to a minimum.

Each stage of fabrication of a pipeline has potential to cause problems in the final process line.

The main fabrication techniques used are cutting, tacking and welding.

We will look at each stage of the fabrication process individually.

Lengths of tube will need to be cut to length during every stage of the installation.

In order to maintain the stainless properties it is important to control the heat input during the cutting process, this would require the use of an orbital cutting machine with a good condition cutting blade fitted.

The use of other methods or indeed an orbital cutting machine with a blunt blade allows heat to build up during the cutting process and can degrade the material being cut.

Over heating during the cutting process can be identified by the formation of oxides on the surface of the tube near to the cutting area.

When tubes are cut to length they will need to be tacked to fittings or other pipes to produce the pipe run required.

The atmosphere in the tubes internal bore should be have an oxygen content of less than 500ppm during tacking as well as welding so that the material does not degrade.

The tack should also be small, clean and should not fully penetrate the wall of the material.

Dirty tacks will have a derogatory effect on the quality of the final weld.

Dirty tacks will be dark blue or black and in extreme cases may have a crusty surface.

Welding has the most degrading effect on the properties of the material and is one of the most complex and critical operations in the fabrication process.

The welding of pipelines will be discussed in detail below.

It is widely accepted that the most suitable process from welding of stainless-steel tubes is TIG (tungsten inert gas) welding.

This process uses a non-consumable tungsten electrode and is shield from the atmosphere by and inert gas, usually argon.

This process gives a controlled heat input which can much further enhanced by the used of a pulsed arc.

To give high quality weld profiles and to keep material degradation a consistent weld speed should be used.

The most effective way to control weld speed is by the use of and automatic orbital welding system.

The difference between a weld made by a manual pipe welder and a weld made with an orbital welding system is easily identifiable.

It can be seen that the manual welding is significantly more oxidised and less consistent than the automatic weld.

It is now considered standard practice to use orbital welding equipment with fully enclosed weld heads for pharmaceutical process line installation.

Fully enclosed weld heads encapsulate the entire outside surface of the weld area.

This area is then filled with an inert gas, usually argon, before the weld cycle starts.

This gives a guaranteed clean outer surface and should the weld joint open slightly during the welding cycle it is inert gas drawn into the internal bore and not air.

As well as protecting the outside diameter of the material it is also essential that the inside surface is also protected.

If stainless steel is not protected from oxygen during the welding process then the chromium and carbon in the material will react with the oxygen to for chromium carbides, commonly know as coking.

Once the chromium is burned out of the material in this way it loses it corrosion resistance.

The result of this will be the break down of the material in a short period of time.

This reaction can be avoided by filling the internal bore of the tube with an inert gas, this method is called back purging and a range of special systems are available to insert into the tube to localise the area at which the gas is applied and to avoid filling large lengths of tube.

These systems not only save on the amount of gas used but also the amount of time taken to get the oxygen level of the back purge to a suitably low level.

All welding on the systems should be carried out to suitable standards and each weld should be tested and certified to that level.

If orbital welding equipment is used the equipment and operators will be to be certified.

An approved weld procedure will be submitted with the certification and this weld procedure should be adhered to at all times.

Modern orbital welding equipment has the facility to store the weld procedure and limit the amount the operator can change to parameters to the level indicated on the standard.

Test coupons should be submitted and approved prior to the welder commencing work on the installation.

Further test coupons should be submitted to ensure that quality is being maintained over the duration of the contract.

All welds carried out on the systems should be uniquely identifiable on the system drawing with a log of when welds were carried out and by whom.

This helps identify the cause if a problem should occur on a particular weld.

Ideally a data log will be kept for each weld carried out on the project.

This should identify the current used, weld travel speed and date.

Again modern orbital welding equipment can print or store this information to standard PC cards.

Each log file will include date, time, weld number, welding current, travel speed and average arc voltage.

In addition the log files will detail exact deviation from the programmed parameters.

The internal bore of the tube should also be filled with a suitable inert gas to give a clean internal finish.

It is commonly accepted that the oxygen level should be below 500ppm before welding commences.

This in another feature of modern orbital welding equipment, which can be connect to a oxygen analyser to prevent the operating starting the weld cycle before the oxygen level is acceptably low.

As a result of the above fabrication techniques debris will remain in the system and these will need to be flushed out.

In addition all weld areas should be passivated as the metal crystalline configuration at the surface changes during cooling after welding resulting in depletion of the chromium oxide layer.

The recommended procedure for post installation treatment is detailed below.

The system is flushed with water to remove debris, swarf etc.

A detergent is then circulated to remove oil, grease and organic muck.

The system is flushed with water again to remove all traces of detergent.

Valve bodies, diaphragms and other areas of potential debris accumulation are swabbed and is then inspection under UV and white light to demonstrate that organics have been removed.

Nitric acid is then circulated to dissolve iron oxide and other contaminants adhering to the steel.

This is particularly important at weld areas where the metal profile is relatively rough compared with polished internals and where there is a possibility of iron inclusions.

Stainless steel is naturally resistant to nitric acid and is not attacked by this chemical.

Nitric acid is an oxyacid which has the property to "oxidise" the chromium layer at the metal surface to resistant chromium oxide.

Even in the case of electropolished so called "bio tubes" with the use of closed cup automatic welding it is essential that the system is passivated after erection to enrich the chromium oxide layer at the metal surface of the weld.

Where nitric acid is unavailable or is to be avoided for safety or environmental reasons other chemicals can be used involving mixtures of citric acids and organic chelants.

The system is finally flushed with de-mineralised water to remove all traces of nitric acid.

On completion a certificate is issued for validation purposes and ferroxyl tests, if required, are carried out at weld areas.

The ferroxyl test is an extremely sensitive test which will detect very low levels of iron giving an intense blue colour in the presence of iron.

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