Is the pressure relief valve a regulating valve

　　The pressure reducing valve regulates the inlet pressure to a certain outlet pressure and relies on the energy of the medium itself to keep the outlet pressure stable. The regulating valve is installed on the fluid pipe, and the valve core can move in the valve body. It receives the control signal output from the regulating control unit, and controls the flow of the controlled medium with the help of the power to change the flow area of the fluid, so as to achieve the purpose of adjusting the technological variables. The process variables mentioned here include pressure, temperature, flow, liquid level, composition and so on. From this, it can be seen that the pressure relief valve only regulates the variable of a pressure, so it is one of the regulating valves. The name of the pressure relief valve should be called the self - Powered post - valve regulator. There are many kinds of regulating valves.

　　First, let's talk about the principle of the differential pressure regulating valve of the air compressor. First of all, we need to summarize the control valve. Then we need to pay attention to the problems that should be paid attention to when designing the twin screw air compressor.

　　1. Characteristics of small flow regulating valve

　　The so-called small flow regulation valve, as the name implies, is a small circulation capacity of the regulating valve.

　　The flow capacity of the valve is an indicator of the capacity of the valve under a unified condition. Our country is represented by the C value. It is defined as: when the valve is fully open, when the pressure difference between the front and rear valves is 1 kg / cm 2, the medium weight is 1 g / cm 3, the mass of the valve flowing through the valve is 3 meters per hour. For an incompressible fluid, in a state of sufficient turbulence (when the Reynolds number is large enough, for water Re>10 5; to air Re>5.5 x 104)

　　In the form:

　　P - pressure valve before and after (2 kg / cm) gamma - medium severe (g / cm 3)

　　Q 1 medium flow (m 3 / M)

　　The United States and other countries use the C value to indicate the flow capacity of the valve. It is recognized internationally that the I, E and C standards of electricity are used to express the flow capacity of the valves with the value of Av. The conversion relationship between 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 flow capacity of the valve, we should pay attention to different medium or flow conditions, and the flow state in the valve will vary greatly.

　　In the case of small flow, especially when the viscous fluid and low pressure work, the main constraint of the fluid is often the mixed state of laminar flow or laminar flow and turbulence. When laminar flow, the medium flow through the valve and the pressure difference between the front and back of the valve have a linear relationship. In the mixed state of laminar flow and turbulence, with the increase of Reynolds number, the meso mass flow through the valve will increase even if the pressure difference is constant. In full turbulence, the flow rate does not change with the Reynolds number. In spite of this, the selection of small flow regulating valves is still carried out in traditional methods and formulas. But the calculation value deviates from the actual value very much, according to the data introduced under Cv =O.01, it is only a capacity index, which is of 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 the 10:l to 15:1 can also be guaranteed. If the adjustable ratio is small, it is difficult to regulate the flow.

　　When the valve is used in series, with the change of the opening, the pressure difference between the front and back of the valve is also changed, so the working characteristic curve of the valve is deviated from the ideal characteristic. If the pipe resistance is large, the straightness will turn into a fast opening characteristic, and the adjustment ability is lost. The equal percentage feature will become a linear feature. In the case of small flow, due to little pipe resistance, the above characteristic distortion is not much, and the equivalent percentage characteristic is not necessary in fact. From the manufacturing point of view, it is not possible to produce an equal percentage of the side shape when the Cv =O.05 is below. Therefore, the main problem for the small flow valve is how to control the flow within the required range.

　　From the economic effect, users hope that a valve can be used at the same time to interception and adjustment, and now it can be done. But for the regulating valve, it is mainly to realize the control of the flow, and the closure is secondary. It is considered that the flow of small flow valve itself is very small, and it is very easy to cut off the flow when it is closed, and it is wrong. In general, the leakage of small flow regulating valves is also stipulated in foreign countries. When the Cv value is 10 one, the leakage of the valve is specified as: at 3.5 kg / cm. In the air pressure, the leakage is less than 1% of the maximum flow.

　　Two. Type of small flow regulating valve

　　The pneumatic regulating valve has the advantages of essential explosion-proof, reliable performance and so on. At present, the domestic and foreign regulating valves are still mainly pneumatic.

　　In the past, small flow regulating valves were formally produced in China. The maximum use pressure is up to 100 kg / cm 2, and the rated circulation capacity C can be from 0.05 to O 0012. The valve seat diameter is 3 millimeters, the spool is cylindrical, with one or several V grooves on it, and the valve stem runs 6 millimeters, and the valve has no matching locator, so the control accuracy is poor.

　　In recent years, China has also introduced small flow regulating valves. The circulation capacity is about 0. 001, and the valve core is a cylinder with a gap. The working pressure is 300 kg / cm 2 stem stroke 7 / 16 inches, the valve core is conical, and the valve is equipped with the top mounted positioner of Moore company.

　　The characteristics of these valves are simple structure and light weight. The commonly used seat aperture is 1 / 8~1 / 4 inches (about 3 175A-6.35 mm) and the stem stroke is 1 / 4 to L / 2 inches (6.35 to 12.7 mm). The minimum flow capacity of this type of valve can be made to O.00006 and even smaller.

　　Generally speaking, the cylinder slotted spool is better than the conical valve in the aspect of characterization. It can get the design characteristics by changing the groove depth, but the latter can adjust well, because the valve is through the valve.