Dynamic stability of safety valve

When the pressure in the protected system increases, an additional force is generated on the sensing element, which moves the element to open the gap in the safety valve channel, and the medium is discharged through the gap. The discharge capacity of the safety valve and the service life of its parts , It is related to the dynamic stability of the safety valve. The vibration of the valve disc will reduce the channel cross-sectional area and discharge capacity of the safety valve, and the collision of the valve disc with the valve seat and the opening height limiter will damage the sealing surface of the safety valve. and other parts. The two-stage full-lift safety valve is most likely to produce disc vibration. The dynamic instability of the safety valve is mainly caused by the following reasons: the discharge capacity of the safety valve exceeds the medium flow into the safety valve, the spring If the rigidity is too large, there will be a large back pressure in the upper chamber of the valve disc and external dynamic interference when the valve is opened. 1. Influence on the discharge capacity In the following cases, the discharge capacity of the safety valve may exceed the flow rate of the medium entering the valve: 1 .When the cross-sectional area of ​​the selected safety valve exceeds the cross-sectional area required by the calculation: 2. When the safety valve is installed on a system with a change in emergency displacement: 3. When the cross-sectional area of ​​the inlet pipeline is not enough, etc. 2. The effect of spring stiffness is too large. It can also be the cause of valve flap oscillation. The full-open safety valve can achieve full opening and discharge depends on the recoil force of the medium, but when the spring stiffness is too large, this force cannot be reached by the spring force. The set opening height, so the safety valve cannot reach the full discharge, which will cause the safety valve to return to the seat as soon as it is opened, resulting in oscillation. If the rigidity is small, the safety valve can only return to the seat under very low pressure. To eliminate this phenomenon, a spring that conforms to the design should be installed, or several groups of springs should be tested during the design, and the most suitable spring should be determined by the test. Similarly, the back pressure generated by the body cavity above the valve disc when the safety valve is opened may also cause the valve disc to oscillate. In order to avoid large back pressure, the opening on the valve body is used for the open safety valve to make the body cavity above the valve disc and the atmosphere. Connectivity. For a closed safety valve, the upper body cavity of the valve disc can be connected to the valve body outlet or outlet branch pipe, and the suction effect generated can be used to reduce the pressure above the valve disc.