闸阀的通径收缩
Diameter shrinkage of gate valve
如果一个阀体内的通道直径不一样（往往都是阀座处的通径小于法兰连接处的通径），称为通径收缩。通径收缩能使零件尺寸缩小，开、闭所需力相应减小，同时可扩大零部件的应用范围。但通径收缩后。流体阻力损失增大。在某些部门的某些工作条件下（如石油部门的输油管线），不允许采用通径收缩的阀门。这一方面是为了减小管线的阻力损失，另一方面是为了避免通径收缩后给机械清扫管线造成障碍
If the channel diameter in a valve body is different (usually the diameter at the valve seat is smaller than that at the flange connection), it is called diameter shrinkage. The diameter shrinkage can reduce the size of parts, reduce the force required for opening and closing, and expand the application range of parts. However, after the diameter is reduced. The loss of fluid resistance increases. In some departments under certain working conditions (such as oil pipeline in petroleum sector), it is not allowed to use the valve with reduced diameter. On the one hand, it is to reduce the resistance loss of the pipeline, on the other hand, it is to avoid the obstruction of mechanical cleaning pipeline after the diameter shrinkage