Maintenance recommended bolt load for the cool flange when the operation has not yet started . The crucial hot torque load value is rarely available . Successful flange hot torquing is a disappearing art because of reductions in senior maintenance professionals who have had many years of experience performing the task . Maintenance workers torque flanges before plant start-up , but when operational , flanges expand because of process heat . Personnel must also perform a hot torque process to prevent leaks . Torquing practice in the field is based on skills and judgment . Maintenance team members sometimes hot-torque a flange without a proper torque value for that temperature , and leakage persists . In this situation , maintenance personnel often continue to tighten the bolts until the leak stops . The gaskets are usually crushed at this point . Successful flange hot torquing is a disappearing art because of fewer senior maintenance staff who have had many years of experience performing the task .
Gasket Design
Designing better gaskets is a much easier solution in practice than attempting to deal with torquing procedures and the art of hot torquing . Spiral-wound and camprofile gaskets are the most commonly used metal gasket types . The compressibility and elasticity of spiral-wound gaskets are useful
Figure 4 . Spiral-wound gaskets have tendency to unwind . Graphics courtesy of AIGI Environmental Inc .
sealing characteristics . When paired with the strength that a camprofile gasket achieves through its unitary metallic structure , gaskets start to approach optimal design . These gasket types also have negative features – such as sealing issues – that should be addressed to improve gasket design . Both types also provide variable performance . Spiral-wound gaskets have a tendency to unwind , and camprofile gaskets do not recover well . An improved design would also eliminate the flange-damaging characteristics of both types by removing any sharp edges or points . The optimal gasket would be a solid , unitary metallic structure to prevent crushing . It would incorporate a mechanism for compressibility and elasticity for improved sealing ( see Figure 1 ). To achieve a crush-proof gasket , a simple feature called a stop-step can prevent over-compression of the flexible rib elements and eliminate the possibility of destructive compression of the overlaid sealing element . These stop-steps can be located outside the sealing area but between the flexible ribs for additional support ( see Figure 2 ). An internal rounded stop-step , which is integral to the design , provides a positive stop that prevents
Figure 5 . Sharpen edges in a camprofile gasket can lead to damage . Graphics courtesy of AIGI Environmental Inc . over-stressing of the ribs and the graphitesealing element . The stop-step ensures that the ribs remain elastic and functioning in their selfenergising capacity because it prevents them from being stressed beyond their elastic limit . The same is true for the graphite-sealing element . The ribs , stop-step and graphite-sealing element perform as an integrated system to ensure maximum sealing performance under bolt load . A gasket with this design is more difficult to make than constructing the concept . Designers cannot assume that any design element on its own will work . They must find a balance between the step height , rib angle or curvature , and thickness to achieve optimal sealing while eliminating gasket crushing and sealing element over-compression . The gasket prototype designs shown in Figures 1 and 2 were tested for crush resistance and sealing . The results confirm the effectiveness of the overall design and the specific utility of the stop-step ( see Figures 3 and 6 ). The stop-step , which prevents gasket over-compression , can be applied to other types of gaskets . Both spiralwound and camprofile gaskets can incorporate this design feature ( see Figures 4 and 5 ). Gasket innovation is often a difficult and slow process because of the limited number of gasket features that can economically be incorporated in a design that must fit the space constraints between two flanges . These improvements are important as gaskets move toward the ultimate sealing reliability . The stop-step design is one small step on the path to a new standard for metal gasket reliability .
1 0.5 0
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Style I Fishbone Gasket 8mm Width , 3.5mm thick
Leakage Rate ( 1 × 10 - 3 cubic centimeters per second )
Kammprofile 8mm Width , 3.2mm thick
Spiral Wound Gasket 8mm Width , 3.5mm thick
Figure 6 . A leak test compared the three gasket types . The gasket stress was 30MPa ( 4.351 psi ), and the nitrogen pressure was 4MPa ( 560 psi .) Graphics courtesy of AIGI Environmental Inc .
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First published in Pumps & Systems : Issue , December 2014
About the author Justin Zhao is the patent inventor of Fishbone Gaskets . He received a Master of Science from University of Chicago and an MBA from Rutgers University . For more information , visit www . aigienvironmental . com . www . valve-world . net Valve World April 2023
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