Stainless Steel and Hygienic Design

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Geoffrey Christian Gray
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1 Stainless Steel and Hygienic Design Claus Qvist Jessen & Erik-Ole Jensen; Damstahl a/s Hygienic Design? Designing the equipment to ensure maximum cleanability Materials, steel grade, corrosion resistance, surface condition etc. Physical design of process equipment, tanks, pipe systems, drainability etc. 1

We cannot see what s happening and have to rely on various systems.

2 Why Hygienic Design? It s relatively easy to keep things clean in the household. We can see what we are doing Much more difficult in a production line. We cannot see what s happening and have to rely on various systems. Why Hygienic Design? Denmark, July-August 2014: A Listeria outbreak from contaminated pork roll costs 18 lives! 2

3 Commonly Seen Types of Corrosion; Stainless Steel General corrosion Intergranular corrosion Pitting corrosion Crevice corrosion Stress corrosion cracking The Stainless Steel Surface Cosmetic Function Surface Roughness 3

4 Surface Profiles Normally, Ra is used to describe the surface. FDA, 3A and EHEDG all love Ra. Ra 0.8 µm has become a kind of official standard. Ra is hard to avoid, but it s like trying to describe a car with a single number. SEM Photos: Ground, 2B, Glass Bead-blasted and Electro-polished Ground, SiC, k mm, 2B 2 2 Glass bead-blasted Electro-polished Corrosion resistance Damstahl Group: Germany, Denmark, Sweden, Norway, the Netherlands, Slovenia andarchitects Finland 1 and designers 4

5 Micro Section; 4404, P80 Roughness measurements may produce an artificially positive result. Crevices like these are often not included. Real surface profile Profile using a very fine roughness meter Profile obtained when using a real roughness meter Micro section through a ground (P80) surface, 4404, t = 2 mm. Photo: Rasmus Lage Surface Project; 2012 Micro Section; 4404, P80 Real surface profile Profile using a very fine roughness meter Profile obtained when using a real roughness meter Roughness measurements may produce an artificially positive result. Crevices like these are often not included. Serious risk of crevice corrosion and bacteria, however, this may not be reflected in the roughness test or corrosion testing: Roughness testing does not include the narrow crevices, and crevice corrosion takes time. Corrosion testing almost always makes the ground surface perform less bad than they do in practice. Micro section through a ground (P80) surface, 4404, t = 2 mm. Photo: Rasmus Lage Surface Project;

6 Cleaning Rough Surfaces Cleaning Rough Surfaces Very small area for the CIP agents = poor circulation at the bottom Large area for the CIP agents 6

7 Above Water: Ground Surface vs. 2B Ground vs. 2B The ground surface has a poor corrosion resistance The rough surface implies an increased contact time Corrosion resistance (E pit, mv SCE ) 4301 specimens in diluted NaCl Sand-blasted Brushed Ground, P36 Ground, P120 Ground, P220 Ground, P360 Rust Salt vessels (4404) from Danish dairy plant. No corrosion problems have ever occurred from the inside of the tanks not even when using Rust 7

8 Above Water: Ground Surface vs. 2B 4404; 2.0 % Mo PREN 23.1 < 4432; 2.5 % Mo PREN B (cold-rolled) Ground Superficial pitting corrosion above water; a salt vessel and fire tube on a cheese dairy. The vessel is made of 4404, the tube 4432 (both within the 316 group). Why Does a Ground Surface Pass the Corrosion Testing? Corrosion testing of stainless steel is almost always performed as pitting corrosion testing (CYP, 2 hrs.). Crevice corrosion is normally a larger risk, but it takes 1-2 weeks to develop. Crevice corrosion is not included in a twohour laboratory test. TIME is an important factor but it s hardly ever considered. Lab testing is always done 100 % under water. Any difference in contact time is not taken into consideration. 8

9 Surface Recommendations R a is far from ideal, however, it s better than nothing at all. R a is easy to measure using cheap equipment. R a is good when comparing a ground surface with another ground one and 2B against 2B. Not ideal when comparing two very different surfaces. Suggestion: Different R a for different types of surfaces: Ground: 0.8 µm 2B: 1.2 µm Pickled: 1.2 µm Weld: 3.0 µm (pickled) Macroscopic Design Ensure that the equipment can be properly cleaned! Drainability. The simpler, the better! Corrosion may lead to hygienic problems. In order to deliver the best performance, the designer and constructor need to know the production methods. Pick the best surface for the job not necessarily the one with the lowest R a. Optimize the welding procedure; no weld defects are permitted crevice corrosion and bacterial growth may take place. Good manufacturing practice, please. 9

Communication Designers vs. Craftsmen? The designer and the builder close to each other. Hopefully, the proximity ensures a good level of communication both ways. Physically (and mentally!

10 Communication Designers vs. Craftsmen? The designer and the builder close to each other. Hopefully, the proximity ensures a good level of communication both ways. Physically (and mentally!), the designer and the builder are very far from each other. One-way communication from designer to builder, but no response the other way. The designer never gets to know when he s doing something silly. Communication Designers vs. Craftsmen Reality How the designer sees the welder How the welder sees the designer 10

11 Herring Bones from a Danish Dairy 300 mm 30 x 15 mm 4301 rectangular bars. Impossible for any normal-sized worker to weld the 30 angle properly. Bad design, both from a hygienic and corrosion point of view. By bending the lower bone a little, it s suddenly possible for the welder to do a good job. Much better for everyone. Change of steel from EN to 4307 due to risk of IC. Herring Bones from a Danish Dairy 11

12 Herring Bones from a Danish Dairy Flow Systems Membrane Filtration Unit; Danish Dairy Problem: Two extra 90 elbows Effect: Extra cost of buying the two extra elbows Extra cost of installing them Increased pressure loss during operation = extra energy costs 12

13 Flow Systems Membrane Filtration Unit; Danish Dairy Problem: Four excess elbows! Effect: Extra cost of buying the four elbows Extra mounting costs Increased pressure loss during operation = extra energy costs Solution: Move the T (blue arrow) to the positon of the green arrow Address Damstahl a/s Danmarksvej 28; 8660 Skanderborg, Denmark (+45) Technical consulting: Claus Qvist Jessen (chemical engineer, PhD, steel geek) cqj@damstahl.com (+45) (+45) Erik-Ole Jensen (mechanical eng., welding, inspection) eoj@damstahl.com (+45) (+45)

European Hygienic Engineering & Design Group

Why EHEDG? European Hygienic Engineering & Design Group Food-born diseases (1990 10Mio) as a result of not hygienically designed food processing equipment, process lines or plants forced the EU to react