Depending on the process situation, basically it’s safe to say that the default choice for pipeline system in food and beverage process industries is austenitic stainless steel, 304/304L or 316L. Comparatively, there is no difference in corrosion resistance between 304 and 304L, but low carbon content (0.03% max) plays an important role to the “after-weld condition” on the steel.
To connect various processing equipment to sanitary grade stainless steel valves, tubes and fittings, the most common method is by welding. Hence, by performing an excellent welding job on adequate material will bring higher level of hygiene qualities required to the process line. Heating stainless steel to temperature ranging 425°C to 860°C for several minutes inevitably lead to the formation of chromium carbide, which can be seen as yellowish heat tinting zone, that contributing to significant loss of corrosion resistance. By choosing 304L or 316L type of stainless steel tube and fittings, the risk of this intergranular corrosion will be lowered down to almost none.
Selecting an unsuitable material will bring defect on the weldment. There are some other reasons that act as the source of microbiological problem that occurred on the welding zone, for example misalignment, incomplete penetration by poor welding technique, lack of gas shielding, to name a few.
Note: Most of the time, tubes and fittings suppliers would be finger pointed for corrosion that took place at welding zone of food process pipeline. The main concern for user is if they have been cheated on the material supplied to them. This should not happen if users select a well-established brand name with proven quality record and support. Good supplier will provide material and inspection certificate on each and every piece of product based on the trace number printed on it. Minox provides all information required by user and help to analyze weldment defect together with third party laboratory if called upon.
Since the late 1940s’, austenitic grade of stainless steel has been widely used for food and pharma processing design, mainly due to it’s corrosion resistance level, strength, formability, machinability, weldability and most importantly, cleanability.
The EHEDG (European Hygienic Engineering Design Group) Guidelines, Doc.8: Hygienic Equipment Design Criteria highlighted the prevention of the microbial contamination of food products as the guideline’s fundamental objective. If equipment and process pipeline is of poor hygienic design, or produced not according to requirement, it will be difficult to clean. The property of austenitic stainless steel helps to create a smooth, easy to clean and non-absorbent surface for long period of time to resist accumulation of bio-films that present hygiene hazard.
As such, food grade or sanitary grade stainless steel valves and fittings must be inert both to the product and to detergents and disinfectants under the conditions of intended use. At the same time, they must also be corrosion resistant, non-toxic, mechanically stable, and their surface finish must not be adversely affected.
Most of the sanitary grade stainless steel valves, tubes and fittings are produced from two classic austenitic types, 304/304L (4301/4307) and 316L (4404).
Heavy welded section on grade 304 may require post-weld heat treatment (annealing) for maximum corrosion resistance. To reduce the risk of chromium carbide formation during welding process, low carbon series (0.03% max) of stainless steel, 304L or 316L are the solution.
Note: In Minox, all of our food grade stainless steel tube is produced from low carbon series, 304L (4307) and 316L (4404), and 90% of Minox sanitary valves, welding union (SMS, DIN, Tri Clamp, IDF, RJT, DS, Hygienic and Aseptic Flange and etc.) and weld end tube fittings (elbow, tee and reducer) are also produced with the same series. It has since helped users to reduce significantly on the risk of intergranular corrosion after the welding process.
Good TIG weld for hygienic requirement.
Intergranular corrosion caused by bad welding.
The characteristics and property of austenitic grade stainless steel has made it the most suitable material to be used in food and pharma processing design. As the internal surface of stainless steel valves, tubes and fittings is the most common contact zone between product and process pipeline, the inner surface finishing is therefore critical.
Dairy tubes or food grade tubes, depending on individual manufacturer, mostly are produced from cold-rolled 2B stainless steel plate, with bright and fine surface. The surface roughness of the 2B thin sheet (0.5 to 4mm thickness) is ranging from 0.1μm to 0.5μm. As recommended in EHEDG Doc.8, large areas of product contact surface should have a surface finish of Ra 0.8μm or better. With current much improved welding technology, most of the food grade tube will not require to be internal ground or polished to achieve Ra 0.8μm. Sometimes, user may prefer inner-polished dairy tube because they are “attracted” by the visual appearance, which is giving the “smoother” impression. Again, depending on individual tube manufacturer and their workers, some polishing techniques may damage the “ready to be used” 2B surface. Wrong speed and too great a pressure may create tiny pits on the surface which can’t be visually seen by user especially when it took place at middle of a standard 6-meter length food grade tube.
Note: Minox recommends EN 10357 tube which is widely use globally in food, beverage and brewery industries. Besides, Minox also supply A270/3A/ASME BPE inner polished tube for food and pharmaceutical industries.
Tube fittings like elbow and tee are generally produced from cut dairy tube, going through some reformation steps such as bending or hot extrusion process, which may damage the original surface of the short tube. As such, surface treatment like polishing would be applied for the inner surface of tube fittings to achieve designated surface finishing in order for sanitary use purpose. Comparatively, to polish a tube fitting either elbow or tee will be easier than polish a tube but still, a strict quality control policy have to be followed to ensure it achieved the requirement to be sanitary grade or for food process use.
All surface that are in contact with the product have an average surface roughness of Ra < 0.8 μm
Note: Minox sanitary stainless steel tube fittings are inspected in high AQL rate to ensure the products received by user is at the higher acceptance level of surface finishing requirement by standard.