Final answer:
Metallic gaskets other than ring type or spirally wound are not suited for Class 150 or lighter flanges as each class is designed to handle certain levels of pressure, with higher classes such as 600 and 900 requiring more robust gasket materials.
Step-by-step explanation:
Metallic gaskets other than ring type or spirally wound metal gaskets should not be used with Class 150 or lighter flanges. Flanges are rated based on their maximum allowable pressure and temperature. The Class number, including Class 150, Class 300, Class 600, and Class 900, denotes the maximum pressure in pounds per square inch (psi) that the flange can handle at the highest operating temperature for which the flange material is rated.
For higher pressure ratings such as Class 600 and 900, more robust gasket materials are typically required to withstand the increased pressure. Conversely, lighter flanges such as Class 150 are generally not designed to handle the same level of pressure and thus are typically paired with softer or less resilient gasket materials that are appropriate for lower pressure applications.