Alternative Materials Not Suitable
S31600 is ideal for pressure vessels designed to filter swimming pool water. By Carroll McCormick
Nickel Magazine, November 2004 -- Filtration & Pumping Commercial Ltd. (FPC) in New Zealand has been manufacturing S30400 pressure sand filter vessels for public swimming pools for 30 years. But when New Zealand began experiencing epidemic levels of cryptosporidium oocysts, there was a move to the use of pressure diatomaceous earth (DE) filter vessels, which trap smaller bacteria.
In 1998 FPC manufactured about 20 DE vessels out of mild steel with an epoxy coating (a materials choice driven by a concern for low initial cost), but with unsatisfactory results. "If the epoxy coating is not absolutely perfect, it fails in a very short time and the steel tank begins to corrode," explains David Cameron, contacts manager for FPC. "If the coating has a blemish or broach, the best we get is three to five years.
"Chloramines are very aggressive," he continues, "and it seems any chemical imbalance in the water -- pH, total alkalinity, hardness and chlorine residual -- has a detrimental effect on the steel and the coating." A competing importer of pressure DE filters made of epoxy-coated mild steel has also experienced many vessel failures.
For these reasons, FPC investigated the cost of manufacturing the DE tanks with S31600 stainless steel. S31600 was recommended by consulting engineers in New Zealand as an alternative material to mild steel and epoxy because of its superior corrosion resistance in the pool environment.
"We've never looked back," says Cameron. "The material cost is obviously higher, but when you factor in sand blasting, priming, undercoating, top coating, and the labour and material cost for the coating, it equals the increased cost for stainless sheet and plate. We no longer make sand filter vessels in mild steel at all."
FPC makes the tanks using 2.5- to 5-millimetre (mm) sheet for the domes (heads) and vertical sides. The pressure and tube plates are made from 8- to 16-mm plate, depending on the tank size. Pipework uses tubes of nominal sizes varying from 10.16 mm to 15.24 mm to 20.32 mm with wall thickness typically 2 mm. The tanks vary in size from 800 mm in diameter and 2 metres tall to 1,800 mm in diameter and 2.3 m tall.
The domes at each end of the filters are hydrostatically formed. All welds are two-pass and passivated.
"In nearly every instance where we remove an old steel pressure sand filter from a school or public pool, we install a stainless steel pressure DE filter," says Cameron.
Carroll McCormick is a Montreal-based freelance writer.
PHOTOS: Filtration & Pumping Commercial Ltd.
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