What is a pressure regulating valve?
What is a pressure regulating valve?
Pressure regulating valve, also known as self balancing valve, flow control valve, flow controller, dynamic balancing valve and flow balancing valve, is an intuitive and simple flow regulating control device. The application of flow regulating valve in pipe network can directly set the flow according to the design. Under the action of water, the valve can automatically eliminate the flow deviation caused by the residual pressure head and pressure fluctuation of the pipeline, regardless of the system The function of the valve makes the flow regulation of the pipe network complete at one time, which turns the network regulation into a simple flow distribution and effectively solves the hydraulic imbalance of the pipe network. Flow control valve is mainly used in: central heating (cooling) and other water systems, so that the pipe network flow distribution according to demand, eliminate the water system hydraulic imbalance, solve the problem of uneven heat and cold.
Working principle of pressure regulating valve
By receiving the signal from the industrial automation control system (such as 4 ~ 20mA), the valve is driven to change the cross-sectional area between the valve core and the valve seat, and control the flow, temperature, pressure and other process parameters of the pipeline medium. Realize the function of automatic regulation.
Figure 1 Schematic diagram and symbols of pressure regulating valve
As shown in the figure, the pressure reducing valve port 8 is in the normally open position. The opening of the valve port depends on the tightening degree of the handle 1. The tighter the handle is screwed down, the larger the opening of the valve port.
After the high pressure gas from the input port P1 passes through the pressure reducing valve port 8, the flow rate increases, but the pressure drops to P2, and then enters the actuator from the output port.
Most of the depressurized gas enters the actuator, and a small part of P2 gas enters the diaphragm chamber from the damping hole 7 in the figure, generating an upward thrust below the diaphragm 4. When the upward thrust is balanced with the downward spring force of the pressure regulating spring 2 (the balance at this time is a process of dynamic balance), the pressure regulating valve will have a stable pressure output.
If the input pressure P1 suddenly increases, the output pressure will obviously increase, and the pressure under the corresponding diaphragm 4 will also increase, and the diaphragm will move upward; and the spool 5 will also push the diaphragm upward under the action of the return spring 9, so that the pressure reducing valve port 8 tends to the position where the opening of the valve port decreases, and the throttling effect of the valve port will be enhanced until the output pressure P2 decreases, which makes the diaphragm move up and down The pressure at both ends is balanced again, and the output pressure at the pressure reducing valve port tends to be stable again.
On the contrary, if the input pressure P1 drops suddenly, the output pressure will obviously drop, the pressure under the corresponding diaphragm 4 will also decrease, and the diaphragm will move downward, so that the pressure reducing valve port 8 tends to the position where the opening of the valve port increases, and the throttling effect of the valve port will be reduced until the output pressure P2 increases, so that the pressure at the upper and lower ends of the diaphragm will reach the balance again, and the output pressure of the pressure reducing valve port will decrease the pressure is stabilizing again.
There are three main types of pressure regulators, namely direct acting, pneumatic diaphragm and self acting.
The self operated pressure regulator has the advantages of simple structure, convenient adjustment, operation and maintenance. It does not need external energy to drive, but uses the energy of natural gas in the gas pipeline to drive and adjust.
Working principle of self operated pressure regulating valve
Adjust the screw rod of the controller and give the pressure P2 (a certain value) behind the valve. At this time, the position of the nozzle and the baffle is in balance, the output pressure of the nozzle is fixed, the pressure difference of the diaphragm head of the regulating valve is constant (the pressure difference between the upper and lower parts of the diaphragm), the opening of the valve remains unchanged, the pressure behind the valve is P2, and the pressure in front of the valve is p1:1.
- a. If P2 increases, the pressure of the air chamber at the bottom of the director increases, so that the force of the diaphragm under the director is greater than the spring pressure, which makes the baffle move up close to the nozzle, the air quantity from the nozzle decreases, the pressure of the upper membrane cavity of the regulating valve increases, the pressure difference between the upper and lower membrane of the membrane head decreases, the regulating valve closes down, the gas flowing through the regulating valve decreases, and the output pressure behind the valve gradually decreases until the original given pressure value .
- b. If P2 decreases, the pressure of the air chamber at the bottom of the controller decreases, so that the force of the diaphragm at the bottom of the controller is less than the spring pressure, which makes the baffle move down from the nozzle, the air quantity from the nozzle increases, the pressure of the upper membrane cavity of the control valve decreases, the pressure difference between the upper and lower membrane of the membrane head increases, the opening of the control valve increases, the gas flowing through the control valve increases, and the output pressure behind the valve gradually increases until the original given pressure Value.
The adjustment accuracy of the self operated regulator is not high, but it can meet the requirements of gas transmission process. Therefore, a large number of self operated pressure regulators are used in gas transmission stations to regulate the gas transmission pressure. The gas pipeline adopts self operated pressure regulating valve with three functions of emergency shut-off, monitoring pressure regulation and main pressure regulation.
The self operated pressure regulating valve is mainly composed of command valve, pressure regulating valve, throttle needle valve, pressure pipe and emergency shut-off valve (as shown in the figure)
Fig. 2 structure diagram of self operated regulator
Types of pressure regulating valve
Because pneumatic control valve has the advantages of intrinsic explosion-proof and reliable performance, pneumatic control valve is still the main control valve at home and abroad.
In the past, the domestic production of small flow control valve. The maximum operating pressure can reach 100 kg / cm2, and the rated flow capacity C value can range from 0.05 to 0.0012. The valve seat aperture is 3 mm, the valve core is cylindrical, there are one or several V-shaped grooves on it, the valve stem stroke is 6 mm, the valve has no matching positioner, so the control accuracy is poor.
China has also introduced small flow control valve. The flow capacity is about 0.001, and the valve core is a cylinder with a notch. The working pressure is 300 kggcm2, stem stroke is 7 / 16 inch, and the valve core is conical. The valve is equipped with Moore’s top mounted positioner.
These valves are characterized by simple structure and light weight. The commonly used valve seat aperture is 1 / 8-1 / 4 inch (about 3.175a-6.35mm), and the stem stroke is 1 / 4-1 / 2 inch (about 6.35-12.7mm). The minimum flow capacity of this kind of valve can be 0.00006 or even smaller.
Generally speaking, the cylindrical slotted spool is better than the conical spool in characterization. It can obtain design characteristics by changing the groove depth, but the latter has good adjustability, because the fluid passing through the valve is distributed on the whole circumference of the spool section. This kind of valve is often used in the situation where the accuracy requirement is not very high. But the accuracy of capacity and reproducibility of characteristics are poor.
For a 1 / 16 inch orifice, the theoretical CV value is about 0.06, or just close to the upper limit of small flow. In order to further reduce the flow rate, it is necessary to reduce the stroke of the spool or restrict the opening of the flow hole.
Requirements for pressure regulating valve
For high pressure small flow control valve, a series of problems caused by high pressure and high pressure difference must also be considered. For example, the actuator must have enough output force to overcome the unbalanced force of medium, the strength of valve parts, high pressure sealing, and the most critical is the material and processing of valve core and seat.
The reasons for the damage of valve core and valve seat of high-pressure control valve are very complex, and the theories are not the same, but the erosion phenomenon (also known as velocity effect) caused by the movement of high-speed liquid (gas) flow relative to valve core and valve seat and the cavitation phenomenon of liquid medium under high-pressure difference are generally paid attention to. The former is the erosion trace related to the streamline, while the latter is the sponge hole.
In the case of cavitation, if the material of valve core and valve seat is not selected properly, the valve will be scrapped in a few days or months.
To solve the problem of cavitation, we should seek the methods to avoid cavitation and the materials to resist cavitation. There are several methods to avoid cavitation:
- 1. Improve the design of valve core and valve seat to make it have reasonable liquid flow velocity distribution and pressure distribution. Such as small flow control valve using narrow channel type spool, seat. The valve core and seat hole have very small taper, which is suitable for accurately controlling the flow under the condition of constant upstream pressure. It has been reported that it has been used at a pressure drop of 4200 kg / cm 2 due to its ability to absorb energy and reduce cavitation.
- 2. When the condition is sufficient, the gas is inflated in the liquid flow to partially or completely eliminate the low pressure area.
- 3. The valves are used in series to reduce the pressure drop of each valve.
- 4. Make the pressure difference between the front and back of the valve lower than the maximum allowable pressure difference of cavitation phenomenon of the medium at the inlet temperature of the control valve.
- 5. When the medium works in the “flow open” state, the allowable pressure difference is more than three times larger than that in the “flow closed” state.
Structure of pressure regulating valve
The self operated pressure regulating valve does not need other external energy sources, such as power supply and gas source, but only depends on the energy of the medium itself. It is energy-saving, environmental friendly and easy to use. After the installation, it can be put into automatic operation after setting the pressure value. Therefore, it is widely used in the occasions where the control accuracy is not high and the power supply and gas source are lacking. But in the process of use, we must pay attention to the particularity of the selection, otherwise it is easy to cause accidents. In the process of using, we should pay attention to the selection and installation environment. Therefore, it is very important to understand the working principle and structure of the self operated pressure regulating valve in detail.
Characteristics of pressure regulating valve
The so-called small flow control valve, as the name suggests, is a very small flow capacity control valve.
The flow capacity of the valve is the valve capacity index under the unified conditions. In China, it is expressed by C value. It is defined as: when the valve is fully open, when the pressure difference between the front and back of the valve is 1kg / cm2 and the medium weight is 1g / cm3, the medium mass (3m / h) flowing through the valve every hour. For incompressible fluid, the flow capacity of the valve only depends on the structure of the valve itself under the condition of sufficient turbulence. When calculating the required valve flow capacity, it should be noted that when the media or flow conditions are different, the flow state in the valve will be very different.
In the case of small flow rate, especially in the case of viscous fluid and low pressure, the main constraint of fluid is laminar flow or the mixed state of laminar flow and turbulence. In laminar flow, there is a linear relationship between the medium flow through the valve and the pressure difference before and after the valve. In the laminar and turbulent mixed state, with the increase of Reynolds number, even if the pressure difference does not change, the medium mass through the valve will increase. In the case of complete turbulence, the flow rate does not change with Reynolds number. However, the traditional method and calculation formula are still used to select the small flow control valve. However, the calculated value deviates greatly from the actual value. According to the data, when CV is below 0.01, it is only used as a capacity index and has reference significance. The actual circulation capacity should be determined by 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.
When the valve is used in series, with the change of opening, the pressure difference before and after 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 regulation ability. The equal percentage property becomes a straight line property. Under the condition of small flow rate, because there is little pipeline resistance, the distortion of the above characteristics is small, and the equal percentage characteristics are not necessary. From the manufacturing point of view, when CV = 0.05 or less, 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 within the required range.
From the perspective of economic effect, users hope that a valve can be used for both closure and regulation, which can also be achieved. But for the control valve, the main is to achieve the control of flow, closing is secondary. It is wrong to think that the flow of small flow valve itself is very small and it is easy to achieve closure when it is closed. In foreign countries, the leakage of small flow control valve is also regulated. When CV is 10, the leakage of the valve is 3.5kg/cm. Under air pressure, the leakage is less than 1% of the maximum flow.
Application of pressure regulating valve
Self operated pressure regulating valve (hereinafter referred to as pressure valve) is a kind of energy-saving product, which can automatically adjust the pressure only depending on the pressure change of regulating medium without external energy. It has the comprehensive functions of measurement, execution and control. Widely used in petroleum, chemical, metallurgy, light industry and other industrial sectors and urban heating, heating system. This product can be used for pressure control device of non corrosive liquid, gas and steam medium.
Source: Network Arrangement – China Valve Manufacturer – Yaang Pipe Industry Co., Limited (www.metallicsteel.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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