The flow characteristics of a control valve refer to the relationship between the flow of the control valve and the stroke of the valve stem. The ideal flow characteristics can be divided into linear, equal percentage, parabolic, hyperbolic, quick opening, square root and other types. The ideal flow characteristics commonly used in China are linear, equal percentage, and quick opening.
Linear flow characteristics
The relative flow of the regulating valve has a linear relationship with the relative opening, that is, the relative flow change caused by the unit relative stroke change is a constant. When the opening is small, the flow change is large, when the opening is large, the flow change is small; when the load is small, the adjustment is too sensitive and easy to oscillate, and the adjustment is gentle and not timely when the load is large, and the adaptability is poor.
Quick opening flow characteristics
When the valve stroke is small, the flow rate will increase greatly and reach the maximum quickly. When the opening is small, the flow is large, and with the increase of the stroke, the flow of the valve opening quickly reaches the maximum. Commonly used in-position and program control occasions.
Equal percentage flow characteristics
The percentage of flow change caused by unit stroke change is equal. The work is relatively stable in the full range of travel, especially when the opening is large, the magnification is large and sensitive. Wide application and strong adaptability.
Parabolic flow characteristics
The characteristic is between the linear characteristic and the equal percentage characteristic, and the equal percentage characteristic is often used instead. The adjustment performance is ideal.
In practical applications, the pressure difference between the inlet and outlet of the regulating valve changes. At this time, the relationship between the relative flow and the relative opening of the regulating valve is called the working flow characteristic. It can be divided into the working flow characteristics of the pipeline in series and the pipeline in parallel.
When the regulating valve is connected in series with the pipeline, the change of the regulating valve opening will cause the change of the flow, the flow change, and the system resistance will also change accordingly. Due to the pressure drop distribution of the valve, the ideal linear characteristic will be distorted into the quick opening characteristic. The equal percentage characteristic is distorted into a linear characteristic. Distortion of flow characteristics will affect the quality of control.
When the regulating valve is connected in parallel with the pipeline, due to the bypass function, although the flow characteristic of the regulating valve does not change, the adjustable range of the system is reduced, and the range of flow change that the regulating valve can control is also greatly reduced. In severe cases, it can even fail to adjust. effect. Therefore, the general bypass flow cannot exceed 20% of the total flow.
In order to achieve control, the regulating valve changes the flow rate by adjusting its opening, and the change of the flow will change the pressure drop at the inlet and outlet of the regulating valve. If the above process is drawn as a curve corresponding to the flow rate and the opening degree, a curve can be obtained by using the trajectory where the flow rate and the pressure drop intersect. This curve represents the actual flow characteristics of the valve. If the corresponding relationship between flow and opening is understood as the relationship between gain and sensitivity, assuming that the pressure drop △P is constant, it can be seen from the figure below that the sensitivity of linear valves is constant; while the sensitivity of quick-opening valves and equal percentage valves is Is changing. In order to make the control system stable, it is always hoped that the gain of the system is constant. The regulating valve is an integral part of the system, of course, the constant gain is the best. But the gain of the system in practical applications is mostly not constant. From a theoretical point of view, the flow characteristics of the control valve are selected to compensate for the non-linear gain in the control system. The theoretical analysis and calculation are very complicated.
Selection of flow characteristics of regulating valve
The selection of the flow characteristics of the regulating valve generally adopts the empirical method, which is to consider from the aspects of control quality, working conditions, load and characteristics:
1. Select the flow characteristics of the regulating valve according to the control quality of the system
According to the characteristic compensation principle of control, in order to achieve good control quality, the product of the open-loop amplification factor and the amplification factor of each link should be kept constant. Therefore, select the flow characteristics of the regulating valve, and use the change of the valve's amplification factor to compensate for the change of the object's amplification factor, so as to achieve the goal of keeping the system's total amplification factor unchanged.
2. Select the flow characteristics of the regulating valve according to the working conditions
The specific working condition is the pressure drop change of the pipeline system, and the flow characteristics of the regulating valve are closely related to the pressure drop ratio. Therefore, the flow characteristics of the valve should be selected according to the pressure drop ratio of the pipeline system.
3. Select the flow characteristics of the regulating valve according to load changes
The linear valve has a large flow change when the opening is small, the adjustment is too sensitive, and it is easy to oscillate. When the opening is wide, the adjustment effect appears weak again, causing the adjustment not to be timely and sensitive. Therefore, linear valves should not be used in situations where the pressure drop is relatively small and the load changes greatly. The equal percentage valve works smoothly and smoothly when it is close to the closed state, and when it is close to the fully open state, the amplification factor is large and the work is sensitive and effective, so it is suitable for use in the occasion of large load changes. The quick-opening valve has a large flow when the stroke is small. With the increase of the stroke, the flow reaches the maximum quickly. It is often used in in-position and program control occasions.
4. Select the flow characteristics of the regulating valve according to the controlled object
For control objects with self-balancing capability, a regulating valve with equal percentage flow characteristics can be selected. For control objects without self-balancing capability, a regulating valve with linear flow characteristics should be selected.