Cellar Dweller
Proper care of winery tanks is essential to avoid corrosion and other problems.
M
ost of us are shielded from the technical details of the various grades and finishes available to the stainless steel fabricator, but a rudimentary knowledge of stainless steel standards can avoid future costly repairs.
For the wine industry, the standard was set by the dairy industry about 75 years ago. Type 304 stainless steel is an alloy that contains 18 percent chromium, eight percent nickel and very low carbon. It is the most popular grade of stainless steel used in tanks and equipment.
The chemical industry and some wine applications use 316 stainless steel, which contains molybdenum and is more corrosion-resistant. For example, the headspace of a wine tank often has acidic condensation from the presence of sulphurous acid. Type 316 is more resistant to acid pitting than type 304 and may be recommended for the uppermost sheet and the tank roof of a wine tank.
The corrosion resistance of stainless steel is provided by a thin, tenacious film of inert chromium oxide. If the protective surface is removed by abrasion or corrosive chemicals, the surface may be pitted. A new film will form spontaneously on exposure to air. Cleaning with steel wool can deposit iron on the surface and lead to pitting. A wrench or other metal object left on a wet stainless steel surface can enable electrolysis to pit the surface. If a surface is pitted often enough, it can actually develop a pin hole. Chlorinated cleaners are notorious for removing the protective film and damaging stainless steel surfaces if they are not rinsed away promptly after use. The construction design of equipment can assist its corrosion resistance. For example, if a tank roof is butt welded to the tank wall, it creates a 90 degree seam that can’t be polished. Welding slag remains and sediments collect on the rough surface. This is not only unsightly and susceptible to
By Gary Strachan Stainless steel ... isn’t
corrosion, but also impossible to disinfect. The better welded joint is to roll the edge of the tank roof with (say) a two-inch radius and then do a flat weld to the tank wall. A flat weld can be polished to the same level as the fabricated sheet.
There are a lot of finishes that can be applied to stainless steel. The dairy and food industry standard has long been a #4 brushed finish. This is commonly used on kitchen appliances as well because marks on the surface are less obvious than if the surface were more highly polished. A surface pattern that is even more concealing is the swirled pattern, often used on European equipment.
If the sheet is annealed through rollers to create a smooth, non- patterned surface, the process is less expensive than a brushed finish. One of the most common of these is the 2B bright finish, often used in domestic wine tanks.
There are also other grades of annealed finish such as 2M (mirror) or BA (bright annealed). Another type of less-common, highly-polished surface is a #8 polish.
There is anecdotal evidence that more highly polished surfaces are easier to
clean.It makes sense that surface pits could harbor bacterial deposits and processing soil. With an appropriate cleaning agent and warm temperature, the range of commercial stainless steel surfaces in common use may not have much bearing on whether soil is more quickly removed. The more important factor is whether tanks are cleaned by CIP (cleaned in place) sprayballs or manually brushed. There is no doubt that a well designed CIP system is more effective than manual cleaning and requires less labour.
In addition, the tank floor surfaces are likely to be scratched when someone enters the tank. CIP assures good stainless steel surface preservation. It also assures the continuing smile on the face of your local worker safety inspector. A well-designed stainless steel tank
British Columbia FRUIT GROWER • Summer 2011
Top: Type 304 stainless steel winery tanks with a 2B finish. Bottom: Type 304 stainless steel tanks with a swirled finish.
drains well, washes easily, and gives unsurpassed protection to its contents. Stains and various kinds of deposits on surfaces are readily removed by an appropriate cleaner. If the surface is scratched, it can easily be repolished. What’s not to like about that? — Gary Strachan can be reached at
gestrachan@alum.mit.edu .
Whoops!
In the spring issue I said the following: “I like cane pruning but it’s not for everyone. It can’t be mechanically pre-pruned as you can with spur pruning.“ Dave Wagner rightly took me to task on that because he and his neighbours regularly use a mechanical pruner for cane-pruned vines. The usual setting for pre- pruning spur pruned vines would leave only a short cane along the cordon, but if you set the lower blades high enough, you can leave a cane that is two feet or more. If your vines are close enough together, a couple of feet of cane is enough for a bilateral VSP. Dave says that it’s a big time-saver.
Thanks for the feedback, Dave. — G.S.
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