PIPING JOINTS

PIPING JOINTS

Joint design and selection can have a major impact on the initial installed cost, the
long-range operating and maintenance cost, and the performance of the piping
system. Factors that must be considered in the joint selection phase of the project
design include material cost, installation labor cost, degree of leakage integrity
required, periodic maintenance requirements, and specific performance requirements.
In addition, since codes do impose some limitations on joint applications,
joint selection must meet the applicable code requirements. In the paragraphs that
follow, the above-mentioned considerations will be briefly discussed for a number
of common pipe joint configurations.

Butt-welded Joints

Butt-welding is the most common method of joining piping used in large commercial,
institutional, and industrial piping systems. Material costs are low, but labor costs
are moderate to high due to the need for specialized welders and fitters. Long term
leakage integrity is extremely good, as is structural and mechanical strength.
The interior surface of a butt-welded piping system is smooth and continuous which
results in low pressure drop. The system can be assembled with internal weld
backing rings to reduce fit-up and welding costs, but backing rings create internal
crevices, which can trap corrosion products. In the case of nuclear piping systems,
these crevices can cause a concentration of radioactive solids at the joints, which
can lead to operating and maintenance problems. Backing rings can also lead to
stress concentration effects, which may promote fatigue cracks under vibratory or
other cyclic loading conditions. Butt-welded joints made up without backing rings
are more expensive to construct, but the absence of interior crevices will effectively
minimize ‘‘crud’’ buildup and will also enhance the piping system’s resistance to
fatigue failures. Most butt-welded piping installations are limited to NPS 21⁄₂ (DN
65) or larger. There is no practical upper size limit in butt-welded construction.
Butt-welding fittings and pipe system accessories are available down to NPS 1⁄₂ (DN
15). However, economic penalties associated with pipe end preparation and fit-up,
and special weld procedure qualifications normally preclude the use of butt-welded
construction in sizes NPS 2 (DN 50) and under, except for those special cases where
interior surface smoothness and the elimination of internal crevices are of paramount
importance. Smooth external surfaces give butt-welded construction high aesthetic
appeal.

Socket-welded Joints

Socket-welded construction is a good choice wherever the benefits of high leakage
integrity and great structural strength are important design considerations. Construction
costs are somewhat lower than with butt-welded joints due to the lack of
exacting fit-up requirements and elimination of special machining for butt weld end
preparation. The internal crevices left in socket-welded systems make them less
suitable for corrosive or radioactive applications where solids buildup at the joints
may cause operating or maintenance problems. Fatigue resistance is lower than
that in butt-welded construction due to the use of fillet welds and abrupt fitting
geometry, but it is still better than that of most mechanical joining methods. Aesthetic
appeal is good.

Brazed and Soldered Joints

Brazing and soldering are most often used to join copper and copper-alloy piping
systems, although brazing of steel and aluminum pipe and tubing is possible. Brazing
and soldering both involve the addition of molten filler metal to a close-fitting
annular joint. The molten metal is drawn into the joint by capillary action and
solidifies to fuse the parts together. The parent metal does not melt in brazed or
soldered construction. The advantages of these joining methods are high leakage
integrity and installation productivity. Brazed and soldered joints can be made up
with a minimum of internal deposits. Pipe and tubing used for brazed and soldered
construction can be purchased with the interior surfaces cleaned and the ends
capped, making this joining method popular for medical gases and high-purity
pneumatic control installations.
Soldered joints are normally limited to near-ambient temperature systems and
domestic water supply. Brazed joints can be used at moderately elevated temperatures.
Most brazed and soldered installations are constructed using light-wall tubing;
consequently the mechanical strength of these systems is low.

Threaded or Screwed Joints

Threaded or screwed piping is commonly used in low-cost, noncritical applications
such as domestic water, fire protection, and industrial cooling water systems. Installation
productivity is moderately high, and specialized installation skill requirements
are not extensive. Leakage integrity is good for low-pressure, low-temperature
installations where vibration is not encountered. Rapid temperature changes may
lead to leaks due to differential thermal expansion between the pipe and fittings.
Vibration can result in fatigue failures of screwed pipe joints due to the high stress
intensification effects caused by the sharp notches at the base of the threads. Screwed
fittings are normally made of cast gray or malleable iron, cast brass or bronze, or
forged alloy and carbon steel. Screwed construction is commonly used with galvanized
pipe and fittings for domestic water and drainage applications. While certain
types of screwed fittings are available in up to NPS 12 (DN300), economic considerations
normally limit industrial applications to NPS 3 (DN 80). Screwed piping
systems are useful where disassembly and reassembly are necessary to accommodate
maintenance needs or process changes. Threaded or screwed joints must be used
within the limitations imposed by the rules and requirements of the applicable code.

Grooved Joints

The main advantages of the grooved joints are their ease of assembly, which results
in low labor cost, and generally good leakage integrity. They allow a moderate
amount of axial movement due to thermal expansion, and they can accommodate
some axial misalignment. The grooved construction prevents the joint from separating
under pressure. Among their disadvantages are the use of an elastomer seal,
which limits their high-temperature service, and their lack of resistance to torsional
loading. While typical applications involve machining the groove in standard wall
pipe, light wall pipe with rolled-in grooves may also be used. Grooved joints are
used extensively for fire protection, ambient temperature service water, and low pressure
drainage applications such as floor and equipment drain systems and roof
drainage conductors. They are a good choice where the piping system must be
disassembled and reassembled frequently for maintenance or process changes.

Flanged Joints

Flanged connections are used extensively in modern piping systems due to their
ease of assembly and disassembly; however, they are costly. Contributing to the
high cost are the material costs of the flanges themselves and the labor costs for
attaching the flanges to the pipe and then bolting the flanges to each other. Flanges
are normally attached to the pipe by threading or welding, although in some special
cases a flange-type joint known as a lap joint may be made by forging and machining
the pipe end. Flanged joints are prone to leakage in services that experience rapid
temperature fluctuations. These fluctuations cause high-temperature differentials
between the flange body and bolting, which eventually causes the bolt stress to
relax, allowing the joint to open up. Leakage is also a concern in high-temperature
installations where bolt stress relaxation due to creep is experienced. Periodic
retorquing of the bolted connections to reestablish the required seating pressure
on the gasket face can minimize these problems. Creep-damaged bolts in hightemperature
installations must be periodically replaced to reestablish the required
gasket seating pressure. Flanged joints are commonly used to join dissimilar materials,
e.g., steel pipe to cast-iron valves and in systems that require frequent maintenance
disassembly and reassembly. Flanged construction is also used extensively
in lined piping systems.

Compression Joints

Compression sleeve-type joints are used to join plain end pipe without special end
preparations. These joints require very little installation labor and as such result
in an economical overall installation. Advantages include the ability to absorb a
limited amount of thermal expansion and angular misalignment and the ability to
join dissimilar piping materials, even if their outside diameters are slightly different.

Disadvantages include the use of rubber or other elastomer seals, which limits their
high-temperature application, and the need for a separate external thrust-resisting
system at all turns and dead ends to keep the line from separating under pressure.
Compression joints are frequently used for temporary piping systems or systems
that must be dismantled frequently for maintenance. When equipped with the
proper gaskets and seals, they may be used for piping systems containing air, other
gases, water, and oil; in both aboveground and underground service. Small-diameter
compression fittings with all-metal sleeves may be used at elevated temperatures
and pressures, when permitted by the rules and requirements of the applicable
code. They are common in instrument and control tubing installations and other
applications where high seal integrity and easy assembly and disassembly are desirable
attributes.