Expansion joints, piping

Scenario 1—Compressor piping/expansion joint failure, with VCE consequences... 

If unbalanced layouts cannot be avoided, appropriate analytical methods must be apphed to assure adequate flexibility. If the designer determines that a piping system does not have adequate inherent flexibility, additional flexibility may be provided by adding bends, loops, offsets, swivel joints, corrugated pipe, expansion joints of the bellows or slip-joint type, or other devices. Suitable anchoring must be provided. 

The movement-absorbing devices used in semirigid and nonrigid piping systems are usually called expansion joints. Based on the method by which the pressure seal is effected, joints are either of the packed type or the packless or bellows type. Selection depends not only on the required movement but also on the severity of service in terms of pressure and temperature, tolerabiUty of leakage, and the number of service cycles. 

The selection and appHcation of an expansion joint is not as simple as selecting a pipe fitting or a valve and requires a sound understanding of the joint s capabihties and limitations. Improper appHcation of any type of joint can result in serious or damaging effects. However, when properly selected and integrated into the piping system, satisfactory service and safe operation can be expected. Selection and appHcation of beUows expansion joints require special attention to design and installation. 

Dual-beUows assembhes, ie, universal-type expansion joints, are particularly vulnerable to squirm, and can experience elastic squirm at one-fourth the pressure of an individual bellows. When large amounts of offset are encountered, as is often the design basis, a pinwheel effect occurs because of unbalanced pressure forces. This effect tends to rotate the center-spool pipe which may lead to bellows mpture. Eor this reason the center spool should always be stabilized by hinges or tie-rod lugs to prevent such rotation. 

The coefficient of linear expansion of these alloys in the temperature range of 21 to 100°C (70 to 212°F) is 12.2 X lO C (6.8 X 10"V°F), which is slightly above that of cast iron (National Bureau of Standards). Since these loys have practically no elasticity, it is necessary to use expansion joints in relatively short pipe hnes. Connections for flanged pipe, fittings, valves, and pumps are made to 125-lb American Standarci drilling. 

Bending or torsional flexibihty may be provided by bends, loops, or offsets by corrugated pipe or expansion joints of the bellows type or by other devices permitting rotational movement. These devices must be anchored or otherwise suitably connected to resist end forces from fluid pressure, frictional resistance to pipe movement, and other causes. 

Expansion Joints All the foregoing applies to stiff piping systems, i.e., systems without expansion joints (see detail 1 of Fig. 10-169). When space hmitations, process requirements, or other considerations result in configurations of insufficient flexibihty, capacity... 

FIG. 10-170 Hinged expansion joint. (From Kellogg, Design of Piping Systems, Wiley, New York, 1965.)... 

Pressure thrust, which is the product of the effective thrust area times the maximum pressure to which the joint will be subjected during normal operation. (For shp joints the effective thrust area shall be computed by using the outside diameter of the pipe. For corrugated, omega, or disk-type joints, the effective thrust area shall be that area recommended by the joint manufacturer. If this information is unobtainable, the effective area shall be computed by using the maximum inside diameter of the expansion-joint bellows.)... 

The force required to overcome the static friction of the pipe in expanding or contracting on its supports, from installed to operating position. The length of pipe considered should be that located between the anchor and the expansion joint. 

Double Layer Pipe expansion is a significant factor at tenmera-tures above 600°F (3I6°C). Above this temperature, insiilation should be apphed in a double layer with all joints staggered to prevent excessive heat loss and high surface temperature at joints opened by pipe expansion. This procedure also minimizes thermal stresses in the insulation. 

Thermal Expansion in Flare Header - Sliding-type expansion joints may be used in flare headers as an alternative to piping expansion loops, if required to achieve a reduction in pressure drop or where expansion bends may result in liquid surging, subject to the following conditions ... 

For a high-temperature system, a separate subheader may be run up to the point where the temperature drops down to the allowable limit of a less expensive material. It may then be connected to the main flare header (either low pressure or high pressure).To properly evaluate this a heat loss calculation is needed. As a rule of thumb a heat loss of 10 BTU/hr/ft may be assumed for a quick estimate for bare pipe. Consideration should also be given to the need for expansion joints. Main flare headers may be as large as 36 to 42 inches in diameter for a large-capacity plant. Expansion joints of such magnitudes may be so expensive as to call for a separate small header for the hot flare system. 

At the Flixborough plant there were six reactors in series. Each reactor was slightly lower than the one before so that the liquid in them flowed by gravity from No. 1 down to No. 6 through short 28-in.-diameter connecting pipes (Figure 2-5). To allow for expansion, each 28-in. pipe contained a bellows (expansion joint). 

Piping should also be arranged to allow Uie shortest, most direct route without congestion. They should be installed to prevent Uie trapping of liquids and should contain adequate valving and provisions for contracUon and expansion. Joints must be properly welded and secure. 

Efforts should be made to eliminate the use of expansion joints in process piping. However, if needed, the expansion joints are used to mitigate the pipe stresses caused by large thermal movements. Table 7-9 lists the recommended mechanical design criteria for expansion joints. 

Expansion Joint is a mechanical assembly designed to eliminate large thermal stresses in the piping. 

Tolerance Whenever possible, equipment should tolerate poor installation or operation without failure. Expansion loops in pipework are more tolerant of poor installation than are expansion joints (bellows). Fixed pipes, or articulated arms, if flexibility is necessary, are friendlier than hoses. For most applications, metal is friendlier than glass or plastic. 

Piping layout, anchors, restraints, guides, and supports for all types of expansion joints shall be designed in accordance with Appendix X, para. X301.2 of ASME B31.3. 

An expansion joint that depends on external main anchors to restrain pressure end load shall be tested in place in the piping system. 

A piping system containing expansion joints shall be leak tested without temporary joint or anchor restraint at the lesser of 150% of design pressure for a bellows-type expansion joint or the system test pressure determined in accordance with para. IP-10.6. In no case shall a bellows-type expansion joint be subjected to a test pressure greater than the manufacturer s test pressure. 

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