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For fully welded ball valves, how can one determine the cause of DIB-1 failure? Definition of DIB in API6D: A single-valve with two valve seat sealing surfaces; when the valve is closed, either sealing surface can seal fluid from a single pressure source, and the valve is equipped with mechanisms to relieve the pressure in the space between the sealing surfaces. The operation of the DIB function relies on a dual-direction valve seat: when the valve is in the closed position, the pressure in the pipes upstream and downstream of the valve, along with the fluid trapped within the chamber, pushes the valve seat against the ball, thereby achieving isolation. This effect is known as the “Double Piston Effect” (DPE); therefore, the two-way valve seat is also referred to as a Double Piston Effect valve seat. If dual valve seats are installed both upstream and downstream of the valve, when the fluid trapped in the chamber becomes overpressured (1.33 times the design pressure) due to factors such as an increase in external temperature, the pressure within the chamber still pushes these two valve seats toward the ball rather than away from it; this action enables the valve to perform its isolation function. At this point, an external relief mechanism is required to release pressure outside the valve body in order to protect the valve from damage caused by overpressure; this is what is referred to as a DIB-Ⅰ type valve as defined in API6D.