Internal seat seal degradation initially causes pressure accumulation in the valve body cavity that can be detected through body bleed connections, with continued degradation eventually compromising isolation integrity and allowing flow through the supposedly closed valve, defeating lockout-tagout safety procedures and potentially exposing maintenance personnel to unexpected pressuriz... https://globalsealstech.com/product-category/oil-gas-seals/api-6d-ball-valve-and-lng-seals/