If a valve body of the channel is not the same diameter (often the seat is smaller than the diameter of the flange connection), known as the path contraction.
The size of the parts can be reduced, the required force of opening and closing can be reduced accordingly, and the application range of the components can be enlarged simultaneously. But after the path contraction. Fluid resistance loss increases.
Under certain operating conditions in certain sectors (such as the oil pipeline), the valve is not allowed to shrink through the path. This aspect is to reduce the pipeline resistance loss, on the other hand, in order to avoid the path contraction after the mechanical cleaning pipeline caused by obstacles.
