Threaded Connections in Piping: Threaded Pipe Fittings

Threaded Connections are widely used for small bore piping having nominal diameters NPS 2 or smaller. This is the oldest pipe joining method and still highly popular. Threaded pipe fittings are used for non-critical applications with lower temperatures and pressure services. The majority of threaded fittings are used in the plumbing industry. The American Standard ASME B1.20.1 serves as the dimensional standards for taper pipe threads providing a number of threads per inch, pitch diameter, and normal engagement lengths for all pipe diameters. Threaded steel fittings are made by forging. They are available in various thread types like NPT, BSPP, BSPT, PF, PT, MPT. Even though threaded fittings are mostly used for small bore piping connection, threaded fittings are available up to 4” (NPS 4) sizes and sometimes used.

Threaded Fitting Types

ASME B 16.11 threaded fittings are available in Class 2000, 3000, and 6000. The schedule of pipe corresponding to each Class designation of threaded fitting for rating purposes is given below.

Threaded Fitting Types, Classes and Sizes
Fig. 1: Threaded Fitting Types, Classes and Sizes

Use of Threaded Fittings

Threaded fittings are not suitable for higher pressure and temperature and cyclical operations. Hence these are used for less critical, low-pressure applications as listed below:

  • Fire Protection
  • Water Distribution
  • Cooling Systems, etc

Fittings for Threaded Pipe systems

Various threaded fittings are available for threaded piping systems. They are:

Threaded Elbow:

Two types of threaded elbows (Fig. 2) are available as mentioned below:

  • Threaded 90° Elbow: These Elbows are used for a 90° changes of direction in the run of pipe.
  • Threaded 45° Elbow: These threaded Elbows help the main pipe to make a 45° direction change.
Threaded Elbow
Fig. 2: Threaded Elbow

Threaded Tee Connection:

Threaded Tee (Fig. 3) as pipe fitting makes a perpendicular branch from the main pipe run. They are available in two types

  • Equal Tee: Main pipe and Branch pipe size are equal.
  • Reducing Tee: Branch Pipe Size is smaller than main pipe

Threaded Cross:

Threaded crosses or cross tees are also known as four-way fittings. It has one inlet and three outlet connections. So the flow in pipe crosses (Fig. 3) is distributed in three directions. Threaded crosses make two 90° branches from the main run pipe direction. Crosses have female threads and make secure pipe connections with male piping elements.

Threaded Tee and Cross
Fig. 3: Threaded Tee and Cross

Threaded Couplings:

Threaded couplings are forged fittings for joining pipes. They are available as full couplings or half couplings.

Threaded Coupling
Fig. 4: Threaded Coupling

Threaded Cap:

Threaded caps are used to close or cover piping ends. So threaded piping caps (Fig. 5) are used for sealing purposes.

Threaded Plugs:

Threaded plugs are also used for sealing or blinding purposes. they are available in three variants:

  • Threaded Square Head Plug: The shape of the head is square.
  • Threaded Hex Head Plug: The shape of the head is hexagonal.
  • Threaded Round Head Plug: The shape of the head is circular (Fig. 5).
Threaded Cap and Threaded Plug
Fig. 5: Threaded Cap and Threaded Plug

Threaded Bushings:

Threaded bushings have hexagonal heads and are used for joining to threaded pipes of different size. So, threaded bushing can aid in size reduction.

Threaded Unions:

Threaded unions are designed based on MSS-SP 83. Threaded unions consist of three interconnected elements and used for installation and maintenance purposes. Unions are available in the male to female, female to female types, lug nut, and Rockwood design. The lug nut connected both the pieces.

Threaded Bushing and Threaded Unions
Fig. 6: Threaded Bushing and Threaded Unions

Standards for Thread connections

Following Standards are used for designing threaded pipe fittings

  • ASME B16.11: Forged Fittings, Socket Welding and Threaded.
  • MSS-SP-83: Class 3000 Steel Pipe Unions, Socket-Welding and Threaded
  • ASME B16.3: Malleable Iron Threaded Fittings
  • ASME B16.4: Gray Iron Threaded Fittings
  • ASME B16.39: Malleable Iron Threaded Pipe Unions
  • ASME B16.34: Valves — Flanged, Threaded, and Welding End
  • ASME B16.39: Malleable Iron Threaded Pipe Unions, Class 150, 250, and 300

Materials for threaded fittings

Threaded fittings can be manufactured from following materials

  • Carbon Steel (A-105)
  • Alloy Steel (A-182)
  • Stainless Steel (A-182)
  • Low Alloy Steel (A-350)
  • Duplex Stainless Steel and
  • Cooper Nickel

Advantages of Threaded Fitting Connections

The main advantages of threaded fittings are

  • Quick installation.
  • Suitable for low-pressure applications, leakage integrity is good.
  • Installation is easy, Special installation skill is not required.
  • Less force is required for joining.
  • Parts can be detached.
  • Economic as cheaper.

Disadvantages of Threaded Connections

However, there are few disadvantages of threaded piping connections as listed below:

  • Not suitable for high temperature-pressure applications.
  • Threads may suffer from corrosion in a corrosive environment.
  • Temperature changes may cause leakage problems.
  • Fatigue damage can occur for cyclic services.
  • Strength is less as compared to welding.

B31.3 recommendations for threaded connections

  • Threaded joints should be avoided in corrosive and erosive environments or where cyclic loading may occur.
  • The layout of piping employing threaded joints should minimize stress on joints, giving special consideration to stresses due to thermal expansion and operation of valves.
  • Threaded components of a specialty nature that are not subject to external moment loading, such as thermometer wells, may be used under severe cyclic conditions.
  • Table 314.2.1 of ASME B 31.3 provides the minimum Schedule of Components With External Threads

Threaded Fittings vs Socket Welded Fittings

The major differences between threaded and socket welded fittings are tabulated below:

Threaded FittingsSocket Welded Fittings
Threaded fittings are screwed into piping or piping components. Socket welded fittings are joined by fillet welding.
Less reliableMore reliable
Strength comparatively less; prone to leakageMore Strength and long lasting connection.
Available in pressure class 2000#, 3000# and 6000#Additional pressure class 9000# available
Threaded fittings vs Socket Welded fittings
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Anup Kumar Dey

I am a Mechanical Engineer turned into a Piping Engineer. Currently, I work in a reputed MNC as a Senior Piping Stress Engineer. I am very much passionate about blogging and always tried to do unique things. This website is my first venture into the world of blogging with the aim of connecting with other piping engineers around the world.

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