Book - School of Science and Technology

Expansion joints and loopsPipework expansion may be provided for in a number of ways:Provision for thermal expansion 257. By length and changes in direction as commented upon above. An example is providedin Figure 9.15(a) and it will be noted that certain points are designated as anchors orguides to ensure full control of the expansion. This is known as natural flexibility.. By purpose made expansion loops, set into the pipework of the rectangular shapeshown in Figure 9.15(b). This is known as induced flexibility.. By expansion joints manufactured from bellows inserts of spirally wrapped multi-plystainless steel which, although quite thin in individual section, become extremely robustand provide system security and personnel safety, Figure 9.15(c).. By sliding telescopic type joints, Figure 9.15(d), which rely upon a packed gland forintegrity and are restrained by bolts to prevent separation of the parts under pressure.Included here, but are no longer in common use since they could accommodate only acompletely axial movement and, in any case was prone to persistent leakage.A pipe stress evaluation of the complete system should always be considered. This will bedeemed a requirement from May 2002 to comply with the Pressure Equipments Directive(PED) for systems operating at temperatures in excess of 110 C.Manufacturers should be consulted to obtain the acceptable forces that their equipment,such as boilers, pumps, etc., are capable of absorbing. Any resultant stresses inbranches should be evaluated along with any building structural constraints as theseaffect transmitted forces. When designing any system the first approach must always benatural flexibility, but application of anchors and guides should be employed to fullycontrol the resultant expansion. Where natural flexibility is not practical induced flexibility(expansion loops), as Figures 9.15(b) and 9.17, would be the preferred option.Anchors and guides must always be applied to expansion loops to ensure correct operation.The lack of space in modern buildings often prohibits the use of loops.Expansion joints fall into two categories, unrestrained (axial bellows) and restrained(lateral and angular articulated bellows). Axials must always operate in a perfectlystraight line without any offset, but anchors and guides must always be installed correctly.The reason for this is that greater forces are exerted upon the anchors and guides due topressure thrust (the force necessary to restrain the effects of pressure cross sectional areaattempting to `straighten' out the convolutions that form the expansion joint) and inaddition there is a lateral force on guides equivalent to 15% of the axial force. Therestrained type of joint, Figure 9.16, can be introduced into an offset to eliminate the highforces evident in axial installations, since the tie-bars absorb the pressure thrust. The forcesexerted on the anchors are greatly reduced;the larger force being due to the frictiondeveloped by the guides and supports during expansion and contraction, Normally lateralexpansion joints can provide up to ‹100 mm movement, and for greater movementangular expansion joints are normally installed.Finally, the PED requirement will deem more use of pipe stressing software for systemswhere temperatures are in excess of 110 C. This could reduce the number of expansionjoints required, but normally a compromise is reached as expansion joints have theinherent benefit of de-stressing a piping installation.Cold drawIt is usual, in making calculations for dealing with the results of thermal expansion, totake account of what is called cold draw. In effect, this represents action taken during