As an important element in the flow control system, the control valve adjusts the medium flow rate of the system by changing its own resistance characteristics. Its regulating quality affects the efficiency and stability of the entire system. Design the control valve according to the regulation needs of the system. The key is to design the control valve structure and the flow area distribution of the valve core.
The existing researches on the flow characteristics of the control valve are mainly concentrated on the following aspects. First, the research on the flow regulation theory, including the establishment and improvement of mathematical models and the discussion of evaluation standards. The second is to analyze the flow characteristics and improvement methods based on specific valve types using tests or engineering experience; the third is to analyze the flow characteristics using CED technology for specific valve types. For the discussion of the relationship between the flow characteristics and the flow area distribution, the literature describes the calculation of the flow area distribution of a specific type of control valve with the help of experimental data and a line diagram between the fluid resistance coefficient and the flow area or using parameters such as the total flow coefficient The method of improving flow characteristics by adjusting the area-stroke distribution for the specific control valve was also proposed in , and compared with the experimental results. However, these studies are aimed at specific types of control valves and rely on empirical and experimental data. The significance lies in specific engineering applications, and there is no theoretical analysis of the relationship between flow characteristics and flow area distribution. For the flow characteristics of mesh sleeve valves, there is no relevant literature to analyze them.
In this paper, two simplified models with different structures are designed, and numerical simulation studies are performed to analyze the relationship between the flow characteristics and the distribution area of the circulation area and to find the rules from them. On this basis, a mesh sleeve valve was optimized, and the flow characteristics of the two types of control valves before and after the optimization were calculated and compared and analyzed.
Flow characteristics of the control valve
The flow characteristic of the control valve refers to the relationship between the relative flow rate and the relative opening degree of the fluid passing through the control valve, including the ideal flow characteristic and the working flow characteristic.
Ideal flow characteristics
The flow control system is composed of a control valve and other system components. The system flow depends on the total resistance of the system. Therefore, in different systems, the flow is not the same even with the same control valve and the same opening. The ideal flow characteristics of the control valve can reflect the regulating characteristics of the control valve itself: that is when the pressure difference between the front and rear of the control valve is constant, the relationship between the relative flow rate and the relative opening degree is the ideal flow characteristic of the control valve (also called the inherent nature of the control valve). Characteristics), the relationship curve is called the ideal flow characteristic curve, and is expressed by mathematical expression (l).。
Where: q / Q is the relative flow; // L is the relative opening.
In common control valves, there are four typical flow characteristic curves, namely linear type, logarithmic type (also called equal percentage type), quick-open type, and parabolic type. 3, 4. Linear flow control valve, the flow changes linearly with the stroke, the flow changes significantly at small openings (the ratio of the flow change to the flow is large), the adjustment sensitivity is high, and at large openings, the flow changes relatively slowly (flow The ratio of the amount of change to the flow rate is small), the adjustment sensitivity is low, so problems such as insufficient adjustment capacity under large openings, and so in some cases cannot meet the adjustment requirements. Control valve with logarithmic flow characteristics, the flow rate changes gradually, the flow rate changes slowly at small openings, the adjustment accuracy is high, the flow rate changes relatively fast at large openings, and it can maintain good adjustment capabilities. These characteristics make it suitable for engineering Has been widely used. The regulating characteristics of parabolic control valve are somewhere between linear and logarithmic. This type of control valve is also more common in engineering applications. The adjustment characteristics of the fast opening type control valve are opposite to those of the logarithmic type. The flow rate changes rapidly at a small opening degree to achieve the purpose of rapidly increasing the flow rate, while the flow rate changes slowly at a large opening degree. In addition to these four typical curves, there are hyperbolic, square root, and other flow characteristics, but they are not common.
In fact, when it comes to the actual control valve, its ideal flow characteristic curve is difficult to completely coincide with the four typical ideal flow characteristic curves, and it is often similar to some form. For example, the ideal flow characteristic of ordinary butterfly valves is usually between Between logarithmic curves. When selecting and designing a control valve, an appropriate flow characteristic curve must be selected according to specific requirements as a design reference.
For some types of control valves, according to the empirical formula or experimental parameters, the relationship between the ideal flow characteristic curve and the flow area distribution can be obtained, and the flow area distribution of the control valve can be calculated from the selected ideal flow characteristic curve. . Aiming at the control valves of several specific structural forms, the method of calculating the flow area distribution is introduced in the literature , but they all require experimental or empirical parameters as conditions, and are not suitable for control valves of other structures.
Working flow characteristics
The control valve installed in the regulating system, with the change of working conditions such as pipeline resistance, the pressure difference between the front and rear of the control valve is no longer constant. At this time, the relationship between the relative flow of the control valve and the relative stroke is called the control valve. Working flow characteristics. Expressed as a mathematical expression:
The meaning of the symbols in the Q L formula is the same as the formula (1).
The relationship between the ideal flow characteristic curve and the working flow characteristic curve can be established by formula (3) .
PR is the pressure drop ratio, which represents the ratio of the pressure difference between the front and back of the control valve to the total pressure drop of the regulating system.
The method of selecting the pressure drop ratio APR is described in the document . If the pressure drop ratio is selected, the corresponding ideal flow characteristic curve can be derived according to the working flow characteristic curve of the system requirement (3). The relationship between the curve and the flow area distribution is known, and the flow area distribution of the control valve can be derived.
Analysis of the flow characteristics of the simplified model
In this paper, two simplified models of the control valve are designed. According to the shape of the flow section, they are called fan model and ring model. The fan-shaped cross-section of the fan-shaped model is fan-shaped, and it is set to 0 and 360. The range changes uniformly with the stroke, and the relationship between the flow area and the stroke is linear. The circulation cross section of the ring model is circular, and its diameter is set to change uniformly with the stroke. It is easy to know that the relationship between the circulation area and the stroke is a square relationship. It should be noted that these two models are not directly applicable to engineering applications. This article is used for calculation and is intended for theoretical research.
The numerical simulation research in this paper is based on PROE software modeling, GAMBIT software meshing, and FLUENT software to solve. Establish the model according to the structure shown in Figure 2: pipeline diameter] 200 mm, pipeline length is taken 2,100 mm before the valve, 20 000 mm after the valve (the initial calculation takes 5 times the diameter to have a return flow at the outlet), and establish control valves respectively Models at different openings. Grid division: using unstructured grid division, the total number of fans in the fan-shaped model is about 130,000, and the ring model is about 100,000. Axial uniform distribution, similar to the ring model. Solution calculation: Set steady flow, select standard turbulence model, set fluid medium to 1 kg / L of water), and set inlet and outlet pressure difference to 105 Pa (total inlet pressure 105 Pa, outlet gauge pressure a). Flow, fit the model’s ideal flow characteristic curve, as shown in Figure 4.