Saturday, December 7, 2013

Types of 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 re-torquing of the bolted connections to reestablish the required seating pressure on the gasket face can minimize these problems. Creep-damaged bolts in high temperature 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 dis-assembly 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 deadends 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.

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