The American standard for high-pressure systems

More and more often we hear about threaded and socket weld fittings in stainless steel and carbon steel series 3000: but what are they exactly?

The standard that dimensionally defines these fittings is the ASME B16.11 standard, created specifically for forged fittings by the American Society of Mechanical Engineers (ASME), which is responsible for defining engineering standards, particularly for everything related to pressure vessels.

ASME B16.11 regulates dimensional standards, temperature-pressure ratings, materials and markings for high-pressure fittings, which are forged and then machined to obtain the desired product. These fittings fall into two categories:

  • Threaded fittings with NPT conical thread defined by the ASME B 1.20.1 standard (sealing thread due to compression of external male conical thread with internal female conical thread).
  • Socket Welding fittings, aka Socket Welding SW,connection used at high pressures where the line pipe is welded directly into the socket at the end of the fitting.

These products are then further defined according to their rating, which is according to their maximum working pressure in psi:

  • NPT fittings: series 2000, series 3000 and series 6000.
  • SW fittings: series 3000, series 6000 and series 9000.

Inox Tecnica s.r.l. can supply, upon request, the two most widely used series, i.e. the 3000 series and the 6000 series, suitable for working pressures of 210 and 420 bar respectively in standard conditions.

Nominal sizes are expressed in NPS inches, and range from 1/8″ to 4″ for the 3000 series fittings, and from 1/8″ to 2″ for the 6000 series. To express the female or male in the case of NPT fittings, it is usual to express them as NPT(F) and NPT(M)respectively.

There is also a correlation between these series and the thicknesses of the pipes in theschedule which these fittings are usually combined to, for example, the 3000 series of NPT fittings corresponds to the 160s schedule (schedules defined by the standard ASME B36.10), while the 3000 series of S.W. fittings corresponds to the 80s schedule.

However, it is not mandatory to match fittings and pipes in this manner, any change to a lower rating will simply bring the maximum line resistance pressure to the more stringent of the two ratings.

The articles listed below can be supplied in ASTM A105 carbon steel, in ASTM A182 F304/304L stainless steel and ASTM A182 F316/316Lstainless steel:

  • 90° elbow (90° elbow) – NPT (F/F) and SW
  • 90° elbow M/F (street elbow) – NPT (M/F)
  • 45° elbow (45° elbow) – NPT (F/F) and SW
  • Tee – NPT (F/F/F) and SW
  • Cross (cross) – NPT (F/F/F/F) and SW
  • Pipe nipple (pipe nipple PE x THRD) o Socket – NPT (PE x NPT(M))
  • Pipe nipple (THRD x THRD) – NPT (M/M)
  • Hex nipple (hex nipple) – NPT (M/M)
  • Reducing hex nipple – NPT (M/M)
  • Coupling (coupling) – NPT (F/F) and SW
  • Half coupling (half coupling) – NPT (F/F) and SW
  • Reducing coupling (reducing coupling) – NPT (F/F) and SW
  • Reducer M/F (hex head bushing) – NPT (M/F)
  • Union (union) – NPT (F/F) and SW
  • M/F union (M/F union) – NPT (M/F)
  • Hex head plug (hex head plug) – NPT(M)
  • Female cap (cap) – NPT(F)

Upon request, we can also manage the articles that are part of the MSS SP 95 and 97 standards, such as the Sockolet, the Threadolet, etc. and the fittings listed above but with ISO 7/1 tapered gas thread.

Our sales team is available for any clarification: contact us!