Cleaning up in the process pipeline

The ideal process line is designed to let the product flow smoothly. To achieve this, the inside bore of the weld should be consistent throughout the entire system.

The bore of the system should also be smooth, with no marks or crevices that may cause potential contamination areas or "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 which has maximum welded joints keeps the amount of problem areas as low as possible. Mechanical joints have areas which naturally cause potential bug traps and so these should be kept to a minimum.

Degrading If the pipeline is not installed to a satisfactory standard, it may well not last as long as you hope and can cause serious contamination of the product travelling through it. The most important consideration for installing 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 steel contains chromium and nickel, which give the material its "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 are the pipelines that have had no post clean and treatment before service. If a line is not cleaned before put into production, fabrication debris can be left in the system. This debris will soon infect the product flowing through the system. The main considerations to help create a clean process pipe line are: • selection of tube and fittings • tube fabrication techniques • welding process and equipment • application and control of welding • post cleaning and treatment of pipe line 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 tube contamination before installation. When selecting and procuring tube and fittings, consider the following:

• sulphur control • surface finish • manufacture specifications • batch control • traceability

Sulphur control – Stainless steels have a trace of sulphur which is often not considered a problem. 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. Surface finish – It is important to have a smooth internal surface on the tube and fittings, including the inside diameter of the tube and fittings. It is important for the supplier to be able to guarantee the quality of the internal surface of the tube, especial in welded tubes where the internal surface will be affected in the weld zone. Manufacture specifications – There are many specifications to which tube can be manufactured, but the most important regulation is ASME-BPE 1997. The tube should be manufactured with a SPC (statistical process control) production line to give guaranteed quality tube. Batch control – All tube and fittings should be ordered in on lot to ensure the tube and fittings delivered are from the same batch of material. This ensures all materials supplied have an equal sulphur content, which in turn will eliminate any problems with weld quality. Traceability – All tube and fittings should be traceable to the original source. This allows any problem encountered during manufacture, installation or service to be traced and the problem identified and rectified. During manufacture it is important to follow recommended fabrication techniques to minimise contamination of the tube bore and material degradation. Each stage of pipeline fabrication has the potential to cause problems in the final process line. The main fabrication techniques used are cutting, tacking and welding. • Cutting: the 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. Using an orbital cutting machine with a good condition-cutting blade fitted is the best way to achieve this. 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 which is being cut. Overheating during the cutting process can be identified by the formation of oxides on the surface of the tube near to the cutting area. • Tacking: 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 have an oxygen content of less than 500pip (parts per million) during tacking as well as welding, so that the material does not degrade. The tack should also be small and 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: this operation has the most degrading effect on the properties of the material and is one of the most complex and critical operations in the manufacturing process. The welding of pipelines will be discussed in detail below. It is widely accepted that the most suitable welding process 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 be much further enhanced by using a pulsed arc. To give high quality weld profiles and to keep material degradation to a minimum, 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 images show the difference between a weld made by a manual pipe welder and a weld made with an orbital welding system. 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.

Internal surfaces 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 breakdown 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 need to be certified. An approved weld procedure should be submitted with the certification and this weld procedure should be followed 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.

Quality Test coupons should be submitted and approved before the welder commences 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 welding 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 is another feature of modern orbital welding equipment, which can be connected to an oxygen analyser to prevent the operator starting the weld cycle before the oxygen level is acceptably low. As a result of the above manufacturing 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. This is the recommended procedure for post installation treatment. 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 inspected 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 electro polished, 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, which gives an intense blue colour in the presence of iron.