Figure 2 . |
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Additionally , the challenge with the ‘ lowstress ’ nomenclature is that it lacks a governing standard to ensure consistency across manufacturers . Some might choose to offer styles that have low density windings , while others may offer styles that provide stress relief ports . In essence , ‘ low-stress ’ is generally very hard to define . What is the end user receiving ? Is it referring to low bolt stress , or low gasket stress ? How much is ‘ low ’? Is it an overall gasket stress value , such as 5,000 psi ? Is it dependent on pressure class ? How are the ‘ low-stress ’ gaskets constructed ? What are the winding element dimensions ? What variety can be |
expected between manufacturers ? What are the manufacturing and design tolerances ? And most importantly , how does a ‘ low stress ’ design impact the sealing performance and reliability ?
One common criticism of low stress designs is that the flange will go metal-tometal with the outer guide ring . This in turn results in an unloading effect on the intended sealing element of the gasket ( the winding ). Metal-to-metal sealing can be achieved in flange and gasket combinations that are designed for it . However , this is not the case for standard pipe flanges with spiral wounds , as they are not intended for metal to metal sealing . In
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fact , testing has shown that it is the paints and coatings on the outer guide rings that effects the seal ( at least , temporarily ). Why would one pay for a spiral wound gasket that ends up sealing on a painted metal outer ring when another option exists ? It seems like a funny scenario , but in fact is a common occurrence . What inherent dangers lay dormant in plants just waiting for the right circumstances to let loose a significant event ? |
Specification Changes
The previous ASME B16.20 , 2012 revision contained a provision on construction . This provision has since been updated , per the ASME B . 16.20 , 2017 revision to include a maximum compression requirement , as well as the gasket stress required per pressure class . Additionally , the 2017 revision removed the former 3.2.6 revision relating to gasket compression and replaced this with performance testing requirements .
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Figure 3 . |
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Figure 4 . |