Double nipples, tapered thread
Threaded fittings
Double nipples with a tapered thread carry a male R thread at both ends, joining two female threaded parts directly, in equal sizes or as a reduction. The range covers 99 versions from R 1/8" to R 3", rated PN 16 as well as PN 25.
The difference to the parallel design lies in the sealing principle: the tapered thread wedges itself in the thread flanks while being screwed in, sealing there, so the mating part needs neither a sealing face nor a gasket. In return the depth of engagement is variable, so the joint has to be sealed again after every removal. Materials are galvanised malleable iron, brass/gunmetal along with cast stainless steel 1.4408.
The tapered design is the right choice whenever the mating part offers a plain female thread without a sealing face, as usual on sockets, elbows plus tees in steel pipework. Take both thread sizes from the mating parts, then check the size across flats, because a spanner at 92 mm or 118 mm needs noticeable clearance. After every removal the joint has to be sealed again, since the preload in the thread is lost.
Screwed steel pipe installations carrying compressed air, water plus neutral gases, where sockets, ball valves and manifolds meet without an intermediate piece. Galvanised malleable iron rated 25 bar appears in building service lines, fire water plus heating pipework, brass/gunmetal in workshop air networks. Versions in 1.4408 serve process equipment as well as lines carrying aggressive fluids.
