Control valve, also known as regulation valve, is the terminal control unit of industrial automation process control system. In the system, the control valve receives the control signal output by the instrument and drives the power operation to change the flow and pressure of the regulated medium. It can also be called a throttle component, which acts as a variable resistance element. Its core is a throttling area formed between a variable displacement valve core and a non-moving valve seat. The change of position results in the change of the resistance characteristics of the control valve, thereby changing the resistance characteristics of the process system, to achieve the purpose of adjusting the flow, and automatically adjust certain process parameters (such as flow, pressure, temperature, liquid level, etc.) in production to realize process control automation.
A control valve includes actuator, valve body, valve internal components, and various accessories.
Actuator: pneumatic, electric or hydraulic operation
Accessories: positioners, transmitters, converters, manual manipulators, solenoid valves, limit switches, etc.
According to the driving mode, it is divided into:
pneumatic control valve,
electric control valve,
hydraulic control valve,
self-operated pressure control valve,
self-operated temperature control valve;
According to the operation mode, it is divided into:
straight stroke control valve;
angular stroke control valve;
According to the structure, it is divided into:
double-seat control valve,
sleeve control valve,
angle control valve,
three-way control valve,
Flow coefficient: The measured large flow rate is the flow coefficient under a uniform and constant valve pressure drop. It is a constant related to the valve geometry and a given stroke. The flow coefficient is an important indicator that reflects the flow capacity of the control valve, and the flow coefficient is usually expressed by Cv and Kv;
Cv—The flow coefficient in English units. It is defined as the number of US gallons of water at a temperature of 60·F (15.6°C) that flow through the control valve per minute under a pressure drop of llb/in⒉ (7kpa).
Kv—The flow coefficient of the International System of Units (SI system). It has been widely used internationally and domestically. It is defined as the number of cubic meters of water flowing through the control valve per hour at a temperature of 5-40°C under a pressure drop of 100kpa (0.1mpa).
The relationship between C, Cv and Kv is: Cv=1.17CKv=1.01C
Rated flow coefficient: the flow coefficient of the valve under the rated stroke. The rated stroke is the distance that the valve shut-off element moves from the fully closed position to the rated fully open position. It is usually the large opening of the valve and expressed as a percentage.
Relative flow coefficient: the flow coefficient when specifying the stroke. The ratio between the flow coefficient and the rated stroke.
Flow characteristics: When the rated stroke changes from 0 to 100%, the relationship between the percentage of the large flow through the valve and the percentage of the rated stroke, expressed in a curve, is called the flow characteristic. This is also an important indicator to measure the flow characteristics and capabilities of the valve.
Inherent flow characteristics: when the pressure drop through the valve is constant, the flow characteristic curve between the flow and the stroke is obtained. It is only related to the valve geometry and stroke.
Characteristic of curve analysis
Regarding the valve flow characteristic curve measured by the constant valve pressure drop method, there are typically three types: linear, equal percentage and quick opening.
Quick opening characteristic: an inherent flow characteristic, in the position where the stroke percentage is very low, that is, in the position where the stroke of the intercepting element is small, and the opening degree is small, it provides a large flow change, that is, a large flow coefficient can be obtained.
Linear characteristic: an inherent flow characteristic. The equal increase of the rated stroke increases in proportion to the large flow, that is, the equal increase of the stroke provides the same increase in the flow coefficient. It can be represented on the flow characteristic graph with a straight line.
Equal percentage characteristic: an inherent flow characteristic, an equal increase in the rated stroke produces an equal percentage change in the large flow. An equal increase in the rated stroke produces an equal percentage change in the flow coefficient.
Comparison of flow characteristics
It can be seen from the characteristic curve diagram of the regulating valve that the quick opening can provide a large flow change at a low opening position, and the valve with this characteristic is usually used for switching. The linear characteristic is always above the equal percentage characteristic. Under the same opening degree, the linear characteristic flow is large and the pressure difference changes quickly, so the adjustment speed is faster than the equal percentage characteristic.
From the perspective of the relative change of flow, the linear characteristic is small, the opening changes greatly, and the large opening changes little, so that the adjustment is too fast and too strong when the opening is small, and it is easy to overshoot and cause oscillation; when the opening is large, the adjustment is too slow Too weak, not timely and sensitive enough. The equal percentage characteristic just compensates for this shortcoming. Its relative flow change is a constant. When the opening is small, the flow is small, and the flow change is small, so the adjustment is stable and gentle; when the opening is large, the flow change is also large, and the adjustment is sensitive and effective. , So its adaptability is also stronger than linear characteristics.
Because of the equal percentage characteristic, most of the flow is concentrated in the back. When the opening is 70%, the relative flow is only 36.2%, and at 90% it is 71.2%. Therefore, the capacity of the equal percentage valve is not easy to fully utilize, so the economy is poor. When selecting a valve, sometimes the equal percentage characteristic is one step larger than the linear characteristic diameter. Special materials should be considered when selecting valves.