The GYL-20900 series is a kind of multi - level control valve of the cage type, which is another effective design method for the process of overcoming the easy to produce the blocking flow and cavitation. According to the medium conditions, the design of a number of different antihypertensive cage cylinder composed of a multi-stage step-down in pieces, starting from the first fluid contact pressure cylinder through a step-down cage, cage cylinder gradually reduce fluid pressure to prevent flash, cavitation occurred, the unbalanced type, ensure high grade seat leakage. At the same time, the large diameter size is required to adopt a balanced internal part due to unbalanced force.