304L and 316L Stainless for Sanitary Duty

Selecting the stainless for a sanitary exchanger

Where 304L is the right answer

Milk and cream, whey, juice without added salt, wort and beer, wine, sugar syrup, edible oil, plant hot water, chilled water and steam are all mild to 304L. A standard CIP program of caustic followed by nitric or phosphoric acid does not attack it. It is the correct default for food and dairy shell-and-tube and tube-in-tube exchangers, and it is what most 3-A units are made from.

The one caveat is the sanitizer. A plant that finishes its CIP with a hypochlorite rinse is putting chloride on the hot surface every day. On 304L that produces pitting at the tube welds within a few years. Peracetic acid sanitizers avoid the problem; if hypochlorite is the plant standard, buy 316L.

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Where 316L is required

Chlorides in the product make the decision: brine and pickle liquors, tomato and citrus concentrates, sauces with added salt, sports drinks with electrolytes, and any seawater-derived ingredient. So does hot purified water: WFI at 80 C and hot purified water loops rouge 304L quickly and every BPE specification calls for 316L. For orbital welding on BPE work the tube and fittings should be sulfur-controlled 316L, with sulfur held between about 0.005 and 0.017 percent, because sulfur at the low end of the ordinary range gives shallow, wandering weld penetration and mismatched sulfur between two parts steers the weld pool toward one side.

PREN, the pitting resistance equivalent number, is the shorthand: chromium plus 3.3 times molybdenum plus 16 times nitrogen. 304L at about 19 and 316L at about 24 sit close together; the practical rule is that 316L tolerates roughly twice the chloride at the same temperature before pitting begins.

• 316L for any product or CIP step with chlorides, and for all pharmaceutical water
• Sulfur-controlled 316L on orbitally welded pharma tube and fittings
• Match the sulfur range between tube and fitting heats on a BPE weld

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Above 316L: duplex, superaustenitic, nickel alloys

When chlorides are high and hot together, brine chillers below 30 F running 20 percent sodium chloride, seawater cooling, hot concentrated pickle liquor, the PREN of 316L is not enough and stress corrosion cracking joins pitting as a failure mode. 2205 duplex at PREN 35 resists both, and its higher yield strength allows a thinner tube wall for the same pressure, which partly offsets its cost. AL-6XN, a superaustenitic grade with 6 percent molybdenum, handles seawater and the hottest brines and polishes and welds much like 316L. Hastelloy C-22 is a nickel-base alloy reserved for cleaning or process chemistry that mixes acids and oxidizers, as in some biotech decontamination systems; it is not a food alloy in any normal sense.

Each of these alloys is ordered for a named chemistry, never as a general upgrade.

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Tube specifications and what they mean

A sanitary exchanger tube is bought to ASTM A270, the sanitary tubing standard, which calls for polished surfaces and controlled dimensions, in 304L or 316L, welded or seamless. The bundle's utility-side tube on a double-tubesheet or an industrial-utility unit may be to A269 or A249, the general and heat-exchanger tube standards, where a polished bore is not needed. The mill certificate for each heat states the standard, the grade, the chemistry and the mechanical properties, and it is the document that proves the tube is what the drawing says.

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Ordering the grade

State the product-contact grade, the shell-side grade if different, the tube standard, and whether seamless or welded tube is required. Add the chloride source if there is one, so the choice can be confirmed against the actual chemistry. Call and talk it through with an engineer: 1-805-484-2992