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The Characteristics and Performance of Cage Control Valve
Cage guided control valve is also called cage type globe control valve. The structure of the body is similar to the straight-through control valve and is also composed of the body, plug, cage, guide sleeve, bonnet, stem, and packing, etc. Its structural feature is the valve trim adopts plug and cage. The cage control valve is guided by the inner surface of cage, and the throttling hole of the cage is used to meet the required flow characteristics.
The characteristics of cage control valve are as below.
1. Easy to installation and maintenance. The valve seat is pressed tightly on the valve body through the bonnet, without thread connection, which is convenient for installation and maintenance.
2. Easy to change the flow characteristics. The fluid in the cage control valve flows outward from the cage, which is called the center outward flow direction, otherwise, it is called the external flow to the center. The figure shows the structure of the straight-through cage control valve that flows from the outside to the center. However, it usually flows from the lower part of the cage and flows out through the opening of the valve cage, which is opposite to the flow direction in the figure. 3, 4, or .6 throttling openings are symmetrically distributed on the cage. The shape of the throttling opening is related to the required flow characteristics. Therefore, the cage (throttling opening shape) can be easily replaced to change to control the flow characteristics of the valve.
3. Reduce noise and cavitation effects. To reduce the noise of the control valve, the cage control valve has a type with a noise reduction valve trim. The cage and the valve plug have multiple small holes. The small holes are used to increase resistance, convert the speed head into kinetic energy, and reduce noise. Generally, this type of cage control valve can reduce the noise by more than 10dB, so it is widely used in the occasions where noise reduction is required. In order to reduce the noise of the control valve, a multi-stage pressure reduction method can also be used. The total pressure drop at both ends of this type of noise reduction control valve is distributed to all levels so that all levels will not cause fluid flashing and cavitation, thus reducing the noise of the control valve, and can weaken and prevent flashing and cavitation erosion and wear caused. The bottom of the plug of the cage control valve is flat. If cavitation occurs, the impact of the bubble burst does not affect the plug but is absorbed by the medium itself. Therefore, the cavitation of the cage control valve has little effect and long service life.
4. The leakage rate is larger than that of single-seated control valves. Because there is a graphite piston ring seal between the cage and the valve plug, after long-term operation, the wear of the seal ring makes the leakage of the cage control valve larger than that of the single-seated control valve.
5. Large interchangeability and versatility: Replace different cages to obtain different flow coefficients and different flow characteristics.
6. Reduce the influence of unbalanced force. Generally, there are two types of cage control valves, one is a balanced valve and the other is an unbalanced valve. A balancing hole is opened on the valve plug of the balancing valve, which greatly reduces the unbalanced force on the valve plug. At the same time, it has a damping effect, which is beneficial to the stable operation of the control valve. Therefore, this type of control valve is often used in applications where the pressure difference is large and low noise is required.
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