Dynamic stability of safety valves

When the pressure inside the protected system rises, an additional force is generated that acts on the sensing element of the safety valve. This force induces the sensing element to move, thus opening a gap in the passage of the safety valve through which the medium can be discharged from the system.
The discharge efficiency of a safety valve, as well as the service life of its components, is closely related to the dynamic stability of its operation. The vibration of the valve flap will lead to a reduction in the passage area of the safety valve, thereby reducing its discharge capacity. At the same time, collision of the valve flap with the valve seat and the opening height limiter may also damage the sealing surface and other key components of the safety valve. Especially the two-stage action of the full-opening safety valve is more likely to have the problem of flap vibration.
The dynamic instability of the safety valve work, mainly caused by the following factors: the discharge capacity of the safety valve exceeds the flow of the medium into its internal, the spring stiffness is too large, the valve opens when the upper cavity of the valve flap generates a large back pressure, as well as from the outside world’s dynamic interference.
First, the impact of the discharge capacity
In the following cases, the discharge capacity of the safety valve may exceed the medium flow into the valve:
When the selected safety valve passage area is larger than the actual need;
When the safety valve is installed on a system where the emergency discharge will change;
When the cross-sectional area of the inlet piping is insufficient, and so on.
Second, the impact of spring stiffness
Spring stiffness is too large is also a cause of valve vibration. Full-opening safety valve can completely open and discharge the medium, which depends on the recoil of the medium. However, when the spring stiffness is too large, this recoil force may not be able to overcome the resistance of the spring to allow the valve flap to reach the preset opening height. As a result, the safety valve will not be able to reach full discharge and may seat back immediately after just opening, creating a vibration. If the spring stiffness is low, the safety valve may seat at a very low pressure.
In order to solve this problem, we should install the spring in line with the design requirements, or in the design phase to prepare a few more sets of springs for experimentation, through the experiment to determine the most suitable spring.
Third, the impact of the system back pressure
Similar to the effect caused by the spring stiffness is too large, the back pressure generated by the body cavity above the valve flap when the safety valve is opened may also lead to flap vibration. In order to avoid generating a large back pressure, open safety valves will utilize the valve body opening, so that the body cavity above the valve flap is connected to the atmosphere. For closed safety valves, we can connect the cavity above the valve flap to the outlet or outlet branch of the valve body to reduce the pressure above the valve flap by utilizing the suction effect generated.