To understand this problem, we must first know two kinds of ball valve seat structures, SPE and DPE. SPE is a one-way sealing seat and DPE is a two-way sealing seat. In the API ball valve, the two valve seats of the ball valve are designed in different combinations and divided into three structures, namely DBB, DIB- 1, DIB-2 and DBB. Both seats are SPE seats, and DIB- 1 is an SPE seat and a DPE seat. DIB-2 has two seats with DPE. DIB- 1 has two structures, one is exhaust in front of the valve, and the other is exhaust after the engine.
The schematic diagram is as follows: