Pipe work may be designed either for rapid regular dismantling to permit cleaning (sterilization takes place after reassembly), or for CIP without dismantling the plant. In the first case, couplings can be used, while in the second case preference is given to welding. It is strongly recommended to minimize the number of joints, whether welded or detachable. Cold bending of pipes is highly preferable to the use of prefabricated bends that have to be installed using couplings. Pipes may be joined together either by welding or by couplings, which again should meet the highest standards, both technically and hygienically. It is important to avoid crevices and gaps where product residues can accumulate and potentially begin to decompose. Although more hygienic, welded joints may be the weaker places in a process system, depending on the quality of the weld.
Permanent Pipe Joints (Welded Joints)
Welding is the preferred method of joining, provided that it is done correctly. Stainless steel hygienic tubing joints should be made by automatic orbital welding (Fig. 7.35) where possible and hand welding in those places that are difficult to access. However, those welds that are difficult to access should, wherever possible, be completed in the workshop prior to installation on the plant. The applied materials should be easily weldable, and higher alloyed filler metal in comparison to the welded material should be used to improve the corrosion resistance. Piping with the correct interior diameters should be applied because any mismatch in diameters or thickness may result in misalignment,
FIGURE 7.35 Stainless steel hygienic tubing joints should be made by automatic orbital welding where possible. Orbital welding provides smooth and crevice-free junctions (Kopitzke et al., 2006).
introducing a step in the wall or bore. If the diameters of the pipes to be joined are not the same, then the smaller pipe should be expanded to match the larger one. Misalignment also can be due to incorrect fitting up (missed coincidence between the axes of the two coupled components) prior to welding. Alignment and clamping tools are available to ensure accurate alignment. Misalignment tolerance must be limited to less than 20% of the wall thickness.
For proper welding, the parts to be welded should be adequately prepared. Cutting should be done with a mechanical mill or saw to ensure that the cut face is exactly at right angles to the longitudinal axis of the pipe. Any burrs must be removed with either a file or emery paper. Care must be taken not to remove the corner edges of the pipe, as this can give rise to problems with fusion of the root of the weld. The pipe surface 25 mm either side of the weld should be roughened up with a stainless steel wire brush, or emery paper. Then both pipe ends and the roughened surface area should be degreased with a solvent and cleaned from contaminants, because any organic substances remaining on the metal surface are vaporized during the welding process and form bubbles (porosity) in the weld metal. Pores in welds may trap product residues.
After two deburred pipe ends are aligned and butted together to a gap of less than 0.25 mm between both pipe faces, a butt weld joint is made by fusing together the two stainless steel edges with the aid of filler material. If the gap during the joint preparation is too wide, a crack running along the weld metal itself may be the result (center line cracking). Full penetration welds should be used whenever possible to avoid pockets where volumes of gas or contaminants can be trapped. Single pass welds should be utilized instead of multipass welds to avoid trapped volumes. The weld metal should exactly fill the joint and remain flush with the surface. Underpenetration leaves a crevice at the joint, while excessive overpenetration can give rise to hold up of product in pipework once taken into service. The weld metal in the joint must be fully fused to the parent metal; otherwise a crevice will form at the interface between weld and plate. Weld zones should be continuous, smooth, and flush with this parent metal. The welding process always should occur with sufficient weld seam protection, because insufficient inert gas shielding or no internal purge will result in roughened welds of lower corrosion resistance, which are prone to increased adhesion of soiling and are difficult to clean. Typically, where inert gas shielding was inadequate, significant discoloration or carbonization in the heat-affected zone is observed.
Weld slag and debris generated within the pipe must be removed from the inside and outside of the weld by proper maintenance and cleaning practice with an alkaline detergent solution prior to the start of the production process. This is followed by rinsing with water of good microbiological quality, usually chlorinated water to 2 ppm available chlorine maximum. After draining, the access points should be covered and sealed. In some circumstances there is an additional requirement to passivate the weld area on the product contact side. The welds may be mechanically polished (outside) or electro-polished (inside and outside), but air leakage should be monitored after the polishing procedure.
Weld seams finally should be visually inspected for any discoloration and surface breaking defects, usually by endoscopy and aided by dye penetrant tests that highlight these defects. Inspection personnel should be trained and act with caution to avoid internal surface damage while handling endoscopic tools (Hauser et al., 1993; Kopitzke et al., 2006).