API 6D Low pressure seat test / Supplementary requirements

In low pressure or low pressure differential applications valves with seat supported balls (i.e. floating ball valves) have problems sealing. With the valve bore horizontal and the stem vertical the ball relies on the seats to keep it in position. As the valve size increases, so does the mass of the ball and with no pressure generated force to push it into the downstream seat the ball can drop down and create a leak path at the 12 O'Clock position on the seat. Consequently in these applications trunnion mounted valves (or other valve types!) are specified. However with proper design and ball support, floating ball valves can work satisfactorily. One application that I recall was for valves needing to meet Australian Gas Association requirements as these tend to be used in low pressure applications.