Reliability of Plumbing Fittings – Threaded vs. Compression

Wherever there are liquids there are leaks – it’s inevitable.  Leaks, of course cost money in downtime and repair of industrial cleaning systems.  So, you ask, what is the best defense against leaks.

Most leaks occur where one piece of plumbing connects with another.  A pipe to a valve, unions, connections to pumps and filters and connections to tank fittings.  Other than connections that are metallurgically or adhesively bonded by techniques such as welding or brazing in the case of metals or adhesives in the case of polymer materials, liquids are contained simply by two surfaces wedged against each other.  Let’s use threaded fittings as a starting example.

Threaded Fittings

Threaded fittings serve well on materials that are somewhat malleable like iron, copper, aluminum, brass and polymers.  As the fittings are tightened, the metal or polymer deforms to fill in all crevices to effect a leak-tight joint.  A threaded fitting on harder materials like stainless steel, however, is very difficult to make leak free.  The problem is that because the material does not deform readily, there is nearly always a small opening around the root of the thread.  In many cases, trying to stop a leak by further tightening the mating pieces may make result in a larger leak as the roots of the thread separate further as the metal is deformed along its length.  Teflon tape and pipe “dopes” are helpful but seldom provide a reliable and permanent solution.  These materials may also be a source of contamination.

Alternatives to threaded fittings include a variety of compression fittings including those with a collar that is compressed around the circumference of the male pipe as it is seated into the mating female part.

In a typical compression fitting, a ferrule (often made of a softer material) is squeezed to make a tight seal between the tube and the ID of the fitting.
In a typical compression fitting, a ferrule (often made of a softer material) is squeezed to make a tight seal between the OD of the tube and the ID of the fitting.

Although there are threads involved, their only purpose is to provide the force required to compress the ferrule.  The actual seal is separate from the threads.

compression fitting schematic

Seating of the ferrule on the tube is a non-reversible process although the connection can be disassembled and reassembled repeatedly if desired.  In many cases, leaks can be repaired by simply tightening the nut more firmly.  Compression fittings can be used in high pressure applications and may even become more reliable as pressure is increased as the pressure tends to expand the tube against the ferrule.

Although compression fittings are generally considered more reliable than threaded fittings, there are some potential problems.  In general, compression fittings are not as resistant to vibration as soldered or welded fittings.  Repeated bending may cause the ferrule to lose its grip on the tube.  Also, for a reliable seal, the surface of the tube being joined must be reasonably round and free of longitudinal scratches.

Compression fittings can be applied to materials ranging from plastic and rubber to the hardest metals.  It is important to choose the appropriate fitting for the application.  In the case of softer materials, the fitting may include an insert to support the inside diameter of the tube thereby assuring a good seal to its outside diameter.  Other special adaptations are common to accommodate the wide variety of applications for compression fittings.

–  FJF  –

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