API 609 pdf download Butterfly Valves: Double Flanged, Lug- and Wafer-Type 3.3SEAT RATING 3.3.1For Category B valves the valve seat ratings for PTFEand RPTFE shall be at least equal to those listed in Table 3.For seating materials not listed in Table 3 , seat ratings shall bcagreed upon by the purchaser and the manufacturer. 3.3.2Valves furnished with internal liners， encapsulation,resilient...