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日期: 2019-03-20

Correct Selection of Electric Regulating Valve

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The flow characteristics of electric control valve are divided into ideal flow characteristics and working flow characteristics. Generally, the flow characteristics provided by manufacturer are ideal flow characteristics, while the actual application needs the flow characteristics of workflow. Because the pressure drop ratio S is less than 1, the workflow characteristics are convex. Therefore, when choosing the flow characteristics of the control valve, the selection should be considered first.

The flow characteristic of electric control valve directly affects the control quality and stability of the system, so correct selection is needed.

The flow characteristics of electric control valve are divided into ideal flow characteristics and working flow characteristics. Generally, the flow characteristics provided by manufacturer are ideal flow characteristics, while the actual application needs the flow characteristics of workflow. Because the pressure drop ratio S is less than 1, the workflow characteristics are convex. Therefore, when choosing the flow characteristics of the control valve, the workflow characteristics should be considered first. Then, the ideal flow characteristics are selected according to the practical application. The ideal flow characteristics commonly used in production are linear, equal percentage and fast opening characteristics. The quick-opening characteristic is mainly used in two-position control and program control, so the choice of flow characteristic of control valve usually refers to how to reasonably select linear and equal percentage ideal flow characteristic.

In practical use, the control valve is always installed in the process pipeline system. The pressure difference before and after the control valve varies with the resistance of the pipeline system. Therefore, when choosing the flow characteristics of the control valve, we should consider not only the process characteristics, but also the piping conditions of the system.

(1) According to the process characteristics, the flow characteristics of the valve are selected. The control law of conventional controllers is linear. After setting the parameters of the controllers, it is hoped that they can adapt to a certain range of work and do not need to be adjusted frequently. This requires that the generalized object is linear, that is, when the load, pre-valve pressure or setting value changes, the characteristics of the generalized object remain basically unchanged. Therefore, from the point of view of automatic control system, the principle of choosing the working characteristics of control valves is to make the whole generalized object have linear characteristics, that is, when the characteristics of other parts (transducer characteristics and process characteristics) are linear in the generalized object, the control valves with linear workflow characteristics (i.e. Kv is constant) should be selected; if the characteristics of the transducer are linear, and if the characteristics of the transducer are linear, then the control valves with linear workflow characteristics should be selected. When the magnification factor of the process characteristics decreases with the increase of the operating variables, the control valve should choose the same percentage of workflow characteristics. In a word, when the generalized object (except the control valve) has the non-linear characteristic, the control valve should be able to overcome its non-linear influence and make the generalized object approach the linear characteristic.

(2) According to the distribution situation, the ideal flow characteristics are deduced from the required workflow characteristics. In the production site, the control valve is always used together with the pipeline and other equipment, so there must be piping resistance, which makes the flow characteristics of the control valve different from the ideal flow characteristics. Therefore, when choosing the characteristics of the control valve, the process piping of the system should also be considered. After the pressure drop ratio S value is determined according to the process piping condition, the ideal flow characteristics can be determined from the selected working flow characteristics.

 


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