Another leading guideline is the ASME BPE standard, which advocates the use of 300 series stainless steel, and more specifically 316L. Indeed, for hygienic applications, 316L stainless steel is cleanable, cold-formable and contains 2 to 3% of molybdenum, which significantly increases its resistance to corrosion.
For the execution of branches, there are three principal methods:
- The installation of pipes on holes drilled in the manifold (Fig. 1 and 2)
- The installation of T-joints on lengths of pipe (Fig. 3)
- The forming of the branch connections directly from the run pipe (T-DRILL method) (Fig. 4)
Fig. 1: Drilled holes
Fig. 2: Fish-mouth connection
Fig. 3: Welded T-Joints
Fig. 4: Extruded collar (Source: T-Drill)
For pipes of a thickness of even up to 12,7 mm, the best and most effective method of manufacture of these manifolds consists of mechanically (T-DRILL) forming the branch connections directly on the run piping. This minimises the need for welding, which also minimises the possible locations of leaks or locations susceptible to trapping contaminants. If an additional polishing of the pipes at the welded points is required, it is more effective with mechanically formed branch connection, since there is minimum amount of butt welded and perfectly perpendicular connections.
In the field of the installation of pipes over hole, there are so-called “fish mouth” connections (Fig. 2).
Although the “fish mouth” connection has been the most widely used method for a very long time, it is nowadays the one which presents the greatest risks with regard to hygiene. Indeed, the shape is complex when it comes to welding and, for reasons of productivity, manual welding is widely employed. This method, even with the best manual welder, cannot guarantee a smooth, clean weld bead at the most tortuous areas. Moreover, this shape presents areas where cleaning becomes uncertain.
Where possible, one of the most effective methods consists of elliptical hole milling, mechanically extruding or shaping (collaring) the branch connection and trimming the face of the extruded portion, all without having to move the pipe. After many years, it is accepted that automatic orbital welding is the most recommendable solution for assembly due to the quality of the result, the gains in production, and the fact that it meets all the requirements of High Purity.
Fig. 6: Equipment for mechanical extruding and collaring of tubes
Another common aspect of these industries is that their systems are manufactured from polished austenitic stainless-steel pipe, generally assembled by autogenous butt welding or by fusion. When executed correctly, and when the sulphur content of the elements is very similar, this type of weld produces a highly solid join, with no cracks or porosity which might trap elements susceptible to subsequently contaminating the product. It might be necessary, in the field of Ultra-High Purity, to carry out an electropolish in order to optimise the flow.
To ensure correct assembly, a number of characteristics inherent to the execution of these manifolds with extruded holes must also be taken into consideration. Mechanically forming the branch connection systematically involves a very slight ovalisation which must always be included in the normalised values of the orbital welding.
Extrusion also involves some reduction in the thickness of the mechanically formed branch outlet; this is perfectly known and under control, depending directly on the ratio between the diameter of the branch and the diameter of the manifold. Knowing that the more similar the run and branch tube diameters are, the more important the thinning of the wall is, a ratio close to 2 is generally used to optimise the result of the assembly.
The height of the collars thus formed is determined by the material used, its elongation and the dimensions/ratio of the run and branch pipes. Typically, the collar height varies between 2 and 20 mm between the small and large pipe dimensions. This height is of course altered by the operation of trimming the face to guarantee an optimal contact surface with the branch pipe. It has been experienced that with the height of the mechanically formed collar it is perfectly possible to carry out the assembly via orbital TIG welding.
Of course, the height of the collars implies that in certain cases the welding head should feature specific equipment. It must have a system which tilts or skews the electrode to position it in the area to be welded, ensuring an effective gas shield in order to prevent any oxidation. According to the type of heads used, the gas shield will be either in a closed space (Fig. 7) or by diffusion (Fig. 8).
Fig. 7: Orbital TIG welding with a closed chamber welding head
Fig. 8: Orbital TIG welding with an open type welding head with shield gas diffuser
Evidently, an additional precaution, whichever type of head is used, will be to implement a gas system on the other side to prevent any oxidation of the internal weld bead as well. Nowadays, welding generators are equipped with dialogue systems which enable the selection of the best operating method according to various parameters such as the diameters implemented, the materials, etc. Assisted programming favours weld quality. Division into welding areas facilitates mastery of the weld pool in every respect, and more specifically, its position, its thickness and its shape within the joint.
In conclusion, technical breakthroughs and technological innovations in the mastery of welding materials and procedures currently guarantee the quality of products and installations while improving productivity. Mechanically shaping the collars on the manifolds minimises the number of secondary welds and operations, reduces the quantity of T-joints purchased and is perfectly compatible with an orbital TIG-welding equipment, ensuring weld repeatability in complete safety.