DIN 11851, SMS, RJT and IDF Unions Compared: A Practical Guide to Sanitary Fittings
Sanitary unions exist because a hygienic line has to come apart. Every gasket, filter and pump has to be accessible for inspection and change, and a welded joint that cannot be opened is of no use in that position. The problem is that four union systems dominate the market, they look similar on a drawing, and they are not interchangeable. Mixing them in one line is the most common and most expensive mistake in hygienic pipework, because it is usually discovered on site with pipe already cut and welded.
What each standard actually is
DIN 11851 is the German threaded union, a male and female cone with an external nut, and it is the most widely used system in European dairy, brewing and food plants. SMS, originally Swedish, uses a three thread profile with a distinctive rounded nut and is common in Scandinavian and some food processing equipment. RJT, from the United Kingdom, is a ring joint union with an internal collar and a separate nut, and it is standard in British dairies. IDF is a European metric dairy standard with its own cone angle and nut. Each fixes its own thread form, cone angle, outside diameter and gasket profile.
Why mixing them fails
Two unions from different systems may share a nominal size such as 2 inch while having different actual diameters and different gasket profiles. Fitting them together either will not assemble at all or will assemble with a distorted gasket, which holds pressure in the workshop and leaks under CIP temperature cycling. The practical rule is to decide the union system at the start of the project, usually matching the equipment already installed, and then buy all pipe, fittings and valves to that one standard.
Fittings, materials and gaskets
Within each system the range includes elbows, tees, reducers, crosses, caps and the union itself, and the fittings are machined from bar or from forged stock in 304 or 316L stainless steel. Product contact surfaces are finished to a controlled roughness and the bore is matched to the tube outside diameter so that no step forms in the flow path. Gaskets are selected by duty, with EPDM for steam and general use, nitrile where oil resistance is needed and PTFE for aggressive chemistry.
Selection checklist for an enquiry
Union standard and nominal size, the tube outside diameter it must fit, material grade, gasket compound, surface finish and the quantity and type of fittings required. Providing the exact union system, rather than only a size, is the single most useful thing a buyer can send, because it removes the risk of a line that will not assemble.