parameter

The so-called small flow control valve, as the name implies, is a small flow capacity of the control valve.
The flow capacity of the valve is the valve capacity index under unified conditions. C value is used in China. It is defined as: when the valve is fully open, when the pressure difference between the front and rear of the valve is 1 kg / cm 2 and the medium weight is 1 g / cm 3, the medium mass passing through the valve per hour is m3 / h. For incompressible fluid, when Reynolds number is large enough, re > 10 5 for water and 5.5 × 10 4 for air
Where:
Δ P -- pressure difference before and after the valve (kg / cm2) y -- medium density (g / cm3)
Q - medium flow rate (m3 / h)
In the United States and other countries, the value of C is used to indicate the flow capacity of the valve. The value of AV is used to represent the flow capacity of valves in the internationally recognized I, e and C standards. The conversion relationship of the three is as follows:
Cv =1 ．17 C Cv =10 6 /24Av C=10 6 /28Av
The flow capacity of the valve depends only on the structure of the valve itself. When calculating the required valve flow capacity, it should be noted that the flow state in the valve will vary greatly with different media or flow conditions.
In the case of small flow rate, especially for viscous fluid and low pressure, the main constraint of fluid is laminar flow or a mixture of laminar flow and turbulent flow. In laminar flow, the medium flow through the valve has a linear relationship with the pressure difference before and after the valve. In the laminar and turbulent mixing States, with the increase of Reynolds number, the mass passing through the valve will increase even if the pressure difference remains unchanged. In the case of complete turbulence, the flow rate does not change with the change of Nuo number. However, the traditional method and calculation formula are still used to select the small flow control valve. However, the calculated value and the actual value deviate greatly. According to the data, when CV = 0.01, it is only used as a capacity index and has reference significance. The actual circulation capacity should be determined according to experience.
As the flow capacity decreases, the adjustable ratio of the valve will decrease. But at least 10:1 to 15:1 can be guaranteed. If the adjustable ratio is smaller, it is difficult to adjust the flow rate.
When the valve is used in series, with the change of opening, the pressure difference between the front and rear of the valve also changes, so the working characteristic curve of the valve deviates from the ideal characteristic. If the pipeline resistance is large, the straightness will become a quick opening characteristic and lose the regulating ability. The equal percentage property becomes a straight line property. In the case of small flow rate, because there is little pipeline resistance, the distortion of the above characteristics is not large, and the equivalent percentage characteristics are not necessary. From the manufacturing point of view, when CV = 0.05, it is impossible to produce the same percentage of side shape. Therefore, the main problem of small flow valve is how to control the flow in the required range.
From the point of view of economic effect, users hope that one valve can be used for both closure and regulation, which can be achieved now. But for the control valve, mainly to achieve the flow control, closing is secondary. It is wrong to think that the flow of small flow valve is very small, and it is easy to achieve closure when closing. The leakage of small flow control valve is also regulated in foreign countries. When CV value is 10, the leakage of the valve is specified as 3.5 kg / cm. Under air pressure, the leakage is less than 1% of the maximum flow rate.
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Self operated pressure regulating valve (hereinafter referred to as pressure valve) is a kind of energy-saving product which can automatically adjust the pressure by relying on the pressure change of regulating medium without external energy. It has the comprehensive functions of measurement, execution and control. It is widely used in petroleum, chemical, metallurgy, light industry and other industrial sectors and urban heating, belching system. This product can be used for pressure control device of non corrosive medium such as liquid, gas and steam.