NEW LEAKAGE REQUIREMENTS FOR ASME B16.20 AND CURRENT GENERATION SPIRAL WOUND GASKETS

摘要

The ASME B16.20 Metallic Gaskets for Pipe Flanges standard was extensively revised in 2017 [1]. One of the significant changes is the introduction of maximum permissible leakage rate. This marks a landmark introduction of an actual leakage performance criterion into ASME B16.20, a most welcome advance. A common maximum permissible leakage rate of 0.0137 mg/s-m (7.67E-10 lb/s-in) is specified for all sizes and pressure classes of finished spiral wound gaskets, that is, including the windings and any or no gauge rings for that particular gasket. Test conditions are defined - ambient temperature and calibration gas with a known methane concentration and flow rate of 1 L/min. The test pressure is defined by the pressure class: 20 bar (290 psi) for Class 150 and 40 bar (580 psi) for Class 300 and above. The qualification parameters listed in B16.20-2017 include prescribed gasket seating stress targets which also vary by pressure class. These gasket seating stress requirements are defined as 35 MPa (5,000 psi) for Class 150, 56 MPa (8,000 psi) for Class 300 and Class 400, and 70 MPa (10,000 psi) for Class 600 and above. Three questions will be explored in this paper. First, to what tightness does the new B16.20 spiral wound gasket leakage rate criterion correspond? Second, do current generation spiral wound gaskets meet this criterion? Several commercially available spiral wound gaskets will be analyzed and compared to the new B16.20 requirements. Leak rates and tightness at the new B 16.20 performance qualification test conditions can be determined using publicly available, published Room Temperature Tightness (ROTT) test constants for these gaskets. Finally, an exploration of Assembly Tightness compared to Operating Tightness for a selection of spiral wound gaskets will be presented and compared to the new B16.20 Performance Testing requirements. This exploration of the new maximum leakage performance criterion in ASME B16.20 -2017 will help to familiarize the end user with a valuable new aspect of this gasket standard as well as how the current generation of spiral wound gaskets meets that criterion using publicly available ROTT performance data.