types of piping joints.docx

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TYPES OF PIPING JOINTSPosted byAntony Thomasat Friday, August 14, 2009

Read more:http://piping-info.blogspot.com/2009/08/types-of-piping-

joints.html#ixzz3FNoGKI5U

Joint design and selection can have a major impact on the initial installed cost,thelong-range operating and maintenance cost, and the performance of the pipingsystem. Factors that must be considered in the joint selection phase of theprojectdesign include material cost, installation labor cost, degree of leakage integrityrequired, periodic maintenance requirements, and specific performancerequirements.

In addition, since codes do impose some limitations on joint applications,joint selection must meet the applicable code requirements. In the paragraphsthatfollow, the above-mentioned considerations will be briefly discussed for anumberof common pipe joint configurations.

Butt-welded Joints

Butt Welded Joint

Butt-welding is the most common method of joining piping used in largecommercial,institutional, and industrial piping systems. Material costs are low, but laborcostsare moderate to high due to the need for specialized welders and fitters. Longtermleakage integrity is extremely good, as is structural and mechanical strength.The interior surface of a butt-welded piping system is smooth and continuouswhichresults in low pressure drop. The system can be assembled with internal weldbacking rings to reduce fit-up and welding costs, but backing rings createinternalcrevices, which can trap corrosion products. In the case of nuclear piping
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systems,these crevices can cause a concentration of radioactive solids at the joints,whichcan lead to operating and maintenance problems. Backing rings can also leadto

stress concentration effects, which may promote fatigue cracks under vibratoryorother cyclic loading conditions. Butt-welded joints made upwithout backing ringsare more expensive to construct, but the absence of interior crevices willeffectivelyminimize crud buildup and will also enhancethe piping systems resistance to

fatigue failures. Most butt-welded piping installations are limited to NPS 21(DN65) 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

(DN15). However, economic penalties associated with pipe end preparation and fit-up,and special weld procedure qualifications normally preclude the use of butt-weldedconstruction in sizes NPS 2 (DN 50) and under, except for those special caseswhereinterior surface smoothness and the elimination of internal crevices are ofparamountimportance. Smooth external surfaces give butt-welded construction highaestheticappeal.

Socket-welded Joints

Socket Welded Joint

Socket-welded construction is a good choice wherever the benefits of highleakageintegrity and great structural strength are important design considerations.Constructioncosts are somewhat lower than with butt-welded joints due to the lack of

exacting fit-up requirements and elimination of special machining for butt weldend
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preparation. The internal crevices left in socket-welded systems make them lesssuitable for corrosive or radioactive applications where solids buildup at thejointsmay cause operating or maintenance problems. Fatigue resistance is lowerthan

that in butt-welded construction due to the use of fillet welds and abrupt fittinggeometry, but it is still better than that of most mechanical joining methods.Aestheticappeal is good.

Brazed and Soldered Joints

Soldered Piping Joint

Brazing and soldering are most often used to join copper and copper-alloy

pipingsystems, although brazing of steel and aluminum pipe and tubing is possible.Brazingand soldering both involve the addition of molten filler metal to a close-fittingannular joint. The molten metal is drawn into the joint by capillary action andsolidifies to fuse the parts together. The parent metal does not melt in brazed orsoldered construction. The advantages of these joining methods are highleakageintegrity and installation productivity. Brazed and soldered joints can be madeupwith a minimum of internal deposits. Pipe and tubing used for brazed and

solderedconstruction can be purchased with the interior surfaces cleaned and the endscapped, making this joining method popular for medical gases and high-puritypneumatic control installations.Soldered joints are normally limited to near-ambient temperature systems anddomestic water supply. Brazed joints can be used at moderately elevatedtemperatures.Most brazed and soldered installations are constructed using light-wall tubing;consequently the mechanical strength of these systems is low.

Threaded or Screwed Joints
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Threaded or screwed piping is commonly used in low-cost, noncriticalapplicationssuch as domestic water, fire protection, and industrial cooling water systems.Installation

productivity is moderately high, and specialized installation skill requirementsare not extensive. Leakage integrity is good for low-pressure, low-temperatureinstallations where vibration is not encountered. Rapid temperature changesmaylead 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 highstressintensification effects caused by the sharp notches at the base of the threads.Screwedfittings are normally made of cast gray or malleable iron, cast brass or bronze,orforged alloy and carbon steel. Screwed construction is commonly used withgalvanizedpipe and fittings for domestic water and drainage applications. While certaintypes of screwed fittings are available in up to NPS 12 (DN300), economicconsiderationsnormally limit industrial applications to NPS 3 (DN 80). Screwed pipingsystems are useful where disassembly and reassembly are necessary toaccommodatemaintenance needs or process changes. Threaded or screwed joints must beused

within the limitations imposed by the rules and requirements of the applicablecode.
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Grooved Joints

The main advantages of the grooved joints are their ease of assembly, whichresultsin low labor cost, and generally good leakage integrity. They allow a moderateamount of axial movement due to thermal expansion, and they canaccommodatesome axial misalignment. The grooved construction prevents the joint from

separatingunder pressure. Among their disadvantages are the use of an elastomer seal,which limits their high-temperature service, and their lack of resistance totorsionalloading. While typical applications involve machining the groove in standard wallpipe, light wall pipe with rolled-in grooves may also be used. Grooved joints areused extensively for fire protection, ambient temperature service water, and lowpressuredrainage applications such as floor and equipment drain systems and roofdrainage conductors. They are a good choice where the piping system must be

disassembled and reassembled frequently for maintenance or processchanges.
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Flanged Joints

Flanged connections are used extensively in modern piping systems due totheir

ease of assembly and disassembly; however, they are costly. Contributing tothehigh cost are the material costs of the flanges themselves and the laborcosts forattaching the flanges to the pipe and then bolting the flanges to each other.Flangesare normally attached to the pipe by threading or welding, although in somespecialcases a flange-type joint known as a lap joint may be made by forging andmachiningthe pipe end. Flanged joints are prone to leakage in services that experience

rapidtemperature fluctuations. These fluctuations cause high-temperaturedifferentialsbetween the flange body and bolting, which eventually causes the bolt stress torelax, allowing the joint to open up. Leakage is also a concern in high-temperatureinstallations where bolt stress relaxation due to creep is experienced. Periodicretorquing of the bolted connections to reestablish the required seating pressureon the gasket face can minimize these problems. Creep-damaged bolts inhightemperatureinstallations must be periodically replaced to reestablish the required

gasket seating pressure. Flanged joints are commonly used to join dissimilarmaterials,
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e.g., steel pipe to cast-iron valves and in systems that require frequentmaintenancedisassembly and reassembly. Flanged construction is also used extensivelyin lined piping systems.

Compression Joints

Compression sleeve-type joints are used to join plain end pipe without specialend

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-

diametercompression fittings with all-metal sleeves may be used at elevated
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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.

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