Next, I want to share a technical article with you. There are many practical formulas below. Regarding the multi-stage pressure-reducing regulator valve, let’s take a look at it together.Structural characteristics and types of multi-stage pressure-reducing regulating valve:
existControl valveThe root cause of the cavitation phenomenon in the valve is that the pressure difference between the front and back of the valve is too high.It is generally believed that whenΔp>2.5MPaWhen the fluid medium enters the throttling part inside the valve, the pressure drops suddenly, and the pressure drops to a lower level at the small flow cross-sectional area. When this pressure is lower than the saturated vapor pressure of the fluid at the current temperature, part of the liquid will appear. Vaporization forms a large number of tiny bubbles. When the fluid flows through the orifice and the pressure rises, these bubbles will burst and return to the liquid state, which will impact the valve body and valve core and cause noise, vibration and other hazards. In recent years, some control valve manufacturers at home and abroad have developed various types of anti-cavitation multi-stage pressure-reducing control valves specially used in harsh working conditions.CommonMulti-stage pressure-reducing regulating valveDivided into cascade regulating valve, multi-layer sleeve regulating valve,Labyrinth regulating valvealthough they are different in structure, they have a common working principle, which is to reduce the total pressure difference in stages by changing the structure, so that the pressure drop of each stage Δp1It is less than the critical pressure difference for cavitation, thus effectively avoiding the occurrence of cavitation and other hazards.
1cascade regulating valve
The cascaded multi-stage step-down structure is shown in the figure1As shown, this structure connects the original whole throttling area with multiple separate throttling areas in series, so that the larger pressure difference is converted into multiple smaller pressure differences, so that the pressure drop range of each time All are controlled above the saturated vapor pressure, so that cavitation no longer occurs.
Figure 1: Structural diagram of cascade control valve
The cascade regulating valve is mostly used in the occasion of working with liquid medium, and its characteristics are as follows:
1)The continuous pressure difference can be reduced during the opening and closing process, and the action of each level of throttle is lagging behind that of the previous level of throttle, which can reduce the continuous high pressure acting on the valve port during the opening and closing process, sharing the first Stage orifice pressure.
2)The flow resistance is small, and it can be used in occasions where the fluid cleanliness is not high, or even solid-liquid two-phase flow.
3)The cascaded spool is generally hardened by tungsten carbide spraying, which has good erosion and cavitation resistance.
4)Compared with other multi-stage pressure-reducing regulating valves, the manufacturing process is simpler, the processing is convenient, and the manufacturing cost is relatively low.
5)The cascade regulating valve generally has a limited number of step-down stages, mostly3~4level, and cannot be used in occasions where the differential pressure is too high.
2multi-layer sleeve control valve
The multi-layer sleeve type multi-stage step-down structure is shown in the figure2As shown, it is often used in power stations or chemical industries.
The typical structural feature of the multi-layer sleeve type control valve is that the throttling part of the valve core is composed of several layers of sleeves with small holes, and there is a certain gap between each layer of sleeves, so that the fluid can be buffered when flowing through the sleeves. , so as to control the fluid velocity within a certain range.
figure 2:Structural diagram of multi-layer sleeve type regulating valve
Its characteristics are:
1)The number of pressure-reducing stages of the multi-stage sleeve-type regulating valve can be designed to be larger, and the pressure-reducing capacity is stronger than that of the cascade type, and it can be used for the occasion of high pressure difference.
2)The multi-layer sleeve structure can not only meet the higher pressure drop requirements, but also ensure a larger flow rate during work.
3)The anti-cavitation performance is good. When it is used in liquid medium, the fluid flows from the outer sleeve to the inner side. The liquid medium is depressurized step by step in the sleeve to reduce the occurrence of cavitation and cavitation, and the fluid finally flows from the inner sleeve. The small hole on the sleeve is sprayed to the center valve cavity area, so that the bubbles will burst in the center of the sleeve, and will not directly damage the metal surface of the valve.
4)It has good anti-noise and vibration performance. When it is used for gas medium, it flows from the inner side of the sleeve to the outside. Compared with the inner side, the aperture and gap of the outer sleeve are enlarged, so that the gas medium continues to expand during the step-by-step depressurization process. It can effectively reduce the harm caused by noise and vibration.
5)The sleeve processing process is more complicated and the cost is higher. But it is easy to install and maintain, and easy to replace.
3Labyrinth regulating valve
The labyrinth disc multi-stage pressure-reducing structure is shown in the figure3As shown in the figure, the core throttling part is made up of multiple metal discs with labyrinth grooves. The fluid flows through the labyrinth flow channel through multiple collisions and turns, consuming energy, and the flow rate is also controlled during the step-by-step depressurization process.
image 3:Labyrinth regulating valvestructure diagram
Generally, it is mostly used in special occasions of high temperature and high pressure drop in nuclear energy, power station and other industries. The working medium is mostly superheated steam, and it can also be used in liquid medium. Its characteristics are as follows.
1)The turning series of the labyrinth flow channel is the step-down series of the labyrinth control valve, which can generally reach more than ten to twenty levels. Therefore, the labyrinth multi-stage step-down structure is the most common multi-stage step-down control valve. Strong, there are foreign products that can reach higher40MPa.
2)Excellent anti-cavitation erosion and noise reduction and vibration reduction performance, multi-stage turning labyrinth flow channel can effectively control the fluid flow rate, avoid cavitation, noise and vibration and other adverse phenomena.
3)By combining different forms of labyrinth discs, the labyrinth control valve can achieve different flow characteristic adjustment curves.
4)Labyrinth discs require high manufacturing precision, and are generally surfacing with Stellite alloy, which has a long service life;Installation and maintenance are relatively simple, and the disk is easy to replace.
5)The labyrinth flow channel has high requirements on the cleanliness of the fluid medium, otherwise the labyrinth flow channel is prone to blockage.
Multi-stage pressure-reducing regulating valvecvCalculation of value
Flow Coefficient(CV)It is generally used to indicate the flow capacity of the valve. In order to select a suitable regulating valve, the necessary flow rate must be calculated according to the conditions of use.cvvalue, and then select the appropriate regulating valve model according to the rated flow coefficient. In the compressible condition, the pressure of the fluid decreases during the throttling process, the volume expands, and the density decreases. The flow situation in the valve is much more complicated than that of the incompressible.Therefore, for multi-stage pressure-reducing regulating valves that are generally used in compressible working conditions, the calculation method of the flow coefficient is also relatively special.cvThere are two commonly used methods for calculating the value, the compression coefficient method and the expansion coefficient method.
1Compression factor method
Compression factor method in20century50It was proposed by the Soviet Union and was one of the early formulas for calculating the discharge coefficient under compressible conditions. The compressibility factor method takes into account the compressibility of gas, and adds a gas compressibility factor ε to the general liquid calculation formula to correct the liquid calculation formula. This method greatly simplifies the calculation model, simplifies the different forms of regulating valves into the same flow nozzle, and then considers that the flow process of the gas medium in the nozzle is an adiabatic process, and then uses the energy balance equation to derive the calculation formula, which is:
In the formula gammaN——Gas gravity under standard conditions, unit iskgf/m3(1kgf=9.8N);
Q——The volume flow rate under the standard condition, the unit ism3/h;
T– gas temperature, in units ofK;
p1——The pressure before the valve, the unit iskgf/m2(1kgf=9.8N);
∆p——Pressure difference before and after the valve, unit iskgf/m2.
compression factorε can be determined by experiment, generally for air experiment:
In addition to the compression coefficient method, there were also methods such as the pre-valve density method, the post-valve density method, and the average density method in the early days. Early formulas can only be applied to occasions where the degree of pressure recovery is not high, and better calculation accuracy can be guaranteed in the non-critical flow range.However, due to the simplification of the calculation model by the formula, with∆p/p1When it is increased to the critical pressure difference ratio, a large error will occur, and the requirements cannot be met in the transition zone and critical zone.
2Expansion coefficient method
For the early calculation formulas, the influence of the pressure recovery characteristics of the valve on the calculation was not considered.20century70In the 1990s, some foreign manufacturers proposed a series of late formulas represented by the expansion coefficient method, polynomial method and sine method, and improved the early formulas, which can better meet the calculation accuracy from the non-critical region to the critical region. Compared with the early formula, the calculation result of the later formula represented by the expansion coefficient method is more economical and can reduce unnecessary waste.Among them, the expansion coefficient method is favored for its simplicity of calculation.IECRecommended as standard formula.The expansion coefficient method introduces the expansion coefficient from the calculation formula used in the case of liquidYCorrected to obtain, whenY=1the expansion coefficient method is also applicable to incompressible liquid conditions.
In the formulaρN——gas density under standard condition, unit iskg/m3;
Q——The volume flow rate under the standard condition, the unit ism3/h;
T1——gas inlet temperature, in units ofK;
p1——The pressure before the valve, the unit iskPa;
x– differential pressure ratio,X=∆p/p1;
Z– Compression factor.
Coefficient of expansionYIt refers to the ratio of the flow coefficient of the compressible medium to the flow coefficient of the incompressible medium at the same Reynolds number. It represents the density change of the fluid when it flows from the valve inlet to the smaller flow area downstream of the orifice, and the change of the contraction area when the pressure difference changes.
In the formulaFK– specific heat ratio coefficient,FK=K/1.4.
Since the calculation formula itself does not contain the actual density of the fluid at the upstream condition, the expansion coefficient method introduces the compressibility factorZTo compensate for the deviation between the actual gas and the ideal gas under certain conditions.Coefficient of expansionYUsed to correct for changes in gas density from the valve inlet to the throat,Yand the ratio of the area at the throat to the inlet area, the shape of the channel, and the pressure difference ratioxReynolds number and specific heat ratio coefficientFKand other factors.The expansion coefficient method has comprehensively considered many factors affecting the flow of compressible fluid, so it can ensure high calculation accuracy in the entire flow range, and is suitable for various types ofvalveis widely used.
After reading the above article, do you now have a deep understanding of the structural characteristics and types of multi-stage step-down regulating valves? And the above formula, get it up~