When the pressure in the protected system increases, an additional force acts on the sensing element, causing the element to move and open a gap in the Globe Valve channel, and the medium is discharged through the gap.
The discharge capacity of the Globe Valve and the service life of its parts are related to the dynamic stability of the work of the Globe Valve. The vibration of the valve disc will reduce the channel cross-sectional area and discharge capacity of the Globe Valve, while the valve disc has the same limit as the valve seat and opening height. The collision of the positioner will damage the sealing surface and other parts of the Globe Valve. The two-stage full-open Globe Valve is most likely to produce valve disc vibration.
The dynamic instability of the Globe Valve is mainly caused by the following reasons: the discharge capacity of the Globe Valve exceeds the flow rate of the medium entering the Globe Valve, the spring stiffness is too large, a large back pressure appears in the upper cavity of the valve disc when the valve is opened, and the external environment dynamic interference.
1. Impact on emission capacity
When the following conditions occur, the discharge capacity of the Globe Valve may exceed the flow rate of the medium entering the valve:
① When the passage cross-sectional area of the selected Globe Valve exceeds the cross-sectional area required for calculation;
② When the Globe Valve is installed on a system with changes in emergency displacement;
③When the cross-sectional area of the inlet pipe is not enough, etc.
2. Effect of spring stiffness
Excessive spring stiffness can also be the cause of valve disc oscillation. The ability of a full-open Globe Valve to achieve full-open discharge depends on the medium recoil force, but when the spring stiffness is too large, this force cannot be used by the spring force to achieve the set value. The opening height is high, so the Globe Valve cannot achieve full discharge, which will cause the Globe Valve to return to its seat as soon as it is opened, causing oscillation. If the stiffness is small, the Globe Valve will return to its seat at a very low pressure.
In order to eliminate this phenomenon, springs that comply with the design should be installed, or several groups of actual springs should be tested during design, and the most suitable spring should be determined through testing.
3. Impact of system back pressure
Similar to the impact caused by excessive spring stiffness, the back pressure generated in the body cavity above the valve disc when the Globe Valve is opened may also cause the valve disc to oscillate. In order to avoid the generation of large back pressure, for open Globe Valve, the valve body is used The opening connects the body cavity above the valve disc with the atmosphere. For closed Globe Valve, the body cavity above the valve disc can be connected to the valve body outlet or outlet branch, and the suction effect generated can be used to reduce the pressure above the valve disc.
