The fundamental reason, nothing more than the earth's surface is subject to two forces, one is the external force, the two is the internal force.
其根本原因，不外乎是地球的表面受了两种力，一是外力，二是内力。
Most of the valve is sealed using force, that the valve is closed, forced to rely on external ram pressure to the valve seat to ensure the sealing surface of the seal.
大部分闸阀是采用强制密封的，即阀门关闭时，要依靠外力强行将闸板压向阀座，以保证密封面的密封性。
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