1. What is a hard seal?
A hard seal refers to a sealing structure in which both sides of the sealing pair are made of metal or other harder materials. Hard sealing uses seals made of hard materials (such as metal) to tightly connect the contact surfaces between the two components to form a reliable seal. These seals are usually gaskets, O-rings, packings, etc. For example, the full-function ultra-light regulating valve has wear-resistant alloy protection for sealing, high reliability, and a leakage rate of 10-7 . It can already meet the requirements of the cut-off valve. The cut-off valve requires sealing performance, and the leakage of the soft-sealed valve is the lowest. The cut-off effect is of course good, but it has the disadvantages of being unwearable and unreliable. From the dual standards of leakage and sealing performance, hard-sealed cut-off is better than soft-sealed cut-off.
2. Why can't the regulating valve-double-seat sealed valve be used as a cut-off valve?
- Structural characteristics
The force problem of the double-seat valve core
The double-seat sealed valve has two valve cores, located at the upper and lower positions of the valve seat respectively. This structure makes the valve core subject to a large unbalanced force. When the valve is in the closed state, it is difficult for the fluid to completely balance the force on the two valve cores, which easily leads to the valve core not being able to fit tightly on the valve seat.
For example, under some high pressure difference working conditions, the lateral force generated by the fluid on the valve core may cause the valve core to have a slight displacement, thereby destroying the sealing effect and failing to achieve complete cut-off.
Sealing surface wear problem
The seal between the valve core and the valve seat of the double-seat sealing valve is achieved by the close fit of metal and metal (or other sealing materials and metal). In actual use, due to the up and down movement of the double-seat valve core and the scouring of the fluid, the sealing surface is easily worn.
Once the sealing surface is worn, the leakage will increase, and it is difficult to ensure the sealing required for the cut-off function. Moreover, it is common for the two sealing surfaces of the double-seat valve core to wear at the same time, which further reduces its reliability as a cut-off valve.
- Fluid dynamics
Complex flow path leads to severe scouring
The flow path of the double-seat sealing valve is relatively complex, and the fluid will produce a more complex flow state when passing through the valve, such as vortex, turbulence, etc. This complex flow state will produce a large scouring force on the valve core and the valve seat.
Being in this kind of scouring for a long time will not only aggravate the wear of the sealing surface, but also may cause the valve core to loosen and deform, thereby affecting the closing performance of the valve and failing to effectively cut off the fluid.
- The influence of the regulating characteristics on the cutting function
The contradiction between the regulating characteristics and the cutting requirements
The original intention of the design of the regulating valve is to achieve precise regulation of the flow rate, and the opening of its valve core changes continuously within a certain range according to the control signal. The requirement of the cut-off valve is to work in a fully open or fully closed state to achieve the purpose of fully conducting or completely cutting off the fluid.
As a regulating valve, the double-seat sealing valve frequently switches between different openings during the adjustment process. This frequent action is not conducive to maintaining good cutting performance. Even in the closed position, due to the influence of its regulating characteristics, it may not be able to meet the zero leakage standard required by the cut-off valve.
3. Why is the regulating valve-double-seat valve prone to oscillation when working with a small opening?
- In terms of fluid mechanics characteristics
The flow field is unstable.
In the case of a small opening, the cross-sectional area of the channel through the double-seat valve is relatively small, and the flow rate increases significantly. At this time, the flow state of the fluid becomes more complicated and turbulence is easy to form. Turbulence has irregular pulsation characteristics, which can cause unstable forces acting on the valve core.
For example, the speed and direction of the fluid are constantly changing, generating periodic pressure fluctuations, which are transmitted to the valve core, causing the valve core to vibrate, and then causing valve oscillation.
Large changes in local resistance
When the double-seat valve is opened at a small degree, the gap between the valve core and the valve seat is very small, and the fluid will generate a large local resistance when passing through these narrow channels. Moreover, since the flow characteristics of the fluid at a small opening are very sensitive to factors such as the position of the valve core and the geometry of the valve seat, even a small change will cause a large change in local resistance.
This unstable change in local resistance will break the force balance of the valve system, causing the valve core to constantly adjust its action, thereby causing oscillation.
- In terms of the valve's own structural characteristics
The influence of unbalanced force
The double-seat valve itself has the problem of unbalanced force, and the influence of unbalanced force is more obvious when working at a small opening. Since the double-seat valve has two valve cores, the force exerted by the fluid on the two valve cores may not be completely symmetrical, and at a small opening, the direction and magnitude of the force exerted by the fluid on the valve core change more dramatically.
When the unbalanced force exceeds a certain limit, it will push the valve core away from its equilibrium position, and the valve control system will try to pull the valve core back to the equilibrium position. This repeated action will cause the valve to oscillate.
Valve core guide problem
The valve core of the double-seat valve needs to rely on the guide device to ensure its linear motion during operation. In the case of a small opening, the movement amplitude of the valve core is relatively small. At this time, the accuracy and stability of the guide device have a more prominent impact on the valve performance.
If the processing accuracy of the guide device is not enough, it is worn or poorly lubricated, etc., it may cause the valve core to become stuck or deflected during movement, thereby causing valve oscillation.
- Control system
Adjustment lag and overshoot
The control of the control valve is usually achieved through the control system. When working at a small opening, the valve response speed is relatively slow, and adjustment lag is prone to occur. That is, when the control system sends an adjustment signal, the valve cannot reach the required opening position in a timely and accurate manner.
At the same time, in order to overcome factors such as system inertia and interference, the control system may over-adjust, causing the valve opening to exceed the target value (overshoot). Subsequently, the control system will try to correct this overshoot, causing the valve to swing back and forth near the target opening, thereby generating oscillation.