Flow characteristic control analysis of the hottes

Control analysis of flow characteristics of regulating valve

1 flow characteristics of regulating valve

as we all know, regulating valve is an actuator directly in contact with fluid in automatic control. For thermal objects, the flow and pressure of the control fluid (often water) are related to the realization of the technical goals of automation of production process, air conditioning and so on. The correct selection of the structural form, flow characteristics and product specifications of the regulating valve plays a very important role in the stability and economic rationality of the automatic control system

there are two types of commonly used regulating valves: seat type and butterfly valve. With the development of production technology, there are more and more structural types of control valves. The selection of structural types of control valves is mainly based on the comprehensive consideration of process parameters (temperature, pressure, flow), medium properties (viscosity, corrosion, toxicity, impurity status) and the requirements of the control system (adjustable saving ratio, noise, leakage). In general, ordinary single and double seat valves and sleeve valves should be preferred. Because the structure of this kind of regulating valve is simple, the shape of valve core is easy to process, and it is more economical; Or according to the specific special requirements, select the corresponding structure of the regulating valve. After the structural type is determined, the specific specification of the regulating valve is related to whether the flow characteristics of the valve match the system characteristics, and whether the system is stable and economical. The flow characteristic of the regulating valve refers to the relationship between the relative flow of fluid flowing through the regulating valve and the relative opening of the regulating valve. It is easy to infer that the relative flow is positively correlated with the relative opening, that is, the smaller the valve channel is, the smaller the relative opening is, the smaller the relative flow is; The larger the valve channel is, the larger the relative opening is, and the larger the relative flow is. When the valve channel is zero, the flow is zero, that is, the valve is closed. From the hydrodynamics, the flow through the valve is positively correlated with the pressure difference before and after the valve, that is:

where: Q refers to the flow through the valve; P refers to the pressure difference formed before and after the valve; K is the coefficient

the differential pressure is often determined by the fluid channel formed by the valve opening (displacement l of the valve core). The smaller the opening is, the smaller the relative opening is, and the greater the differential pressure before and after the valve is; The larger the opening, the larger the relative opening, and the smaller the pressure difference before and after the valve. It can be said that the flow through the regulating valve is not only related to the opening of the valve, but also related to the pressure difference before and after the valve. When the opening of the regulating valve in operation changes, not only the flow changes, but also the pressure difference before and after the valve changes. For the convenience of discussion, first assume that the pressure difference between the front and back of the valve is certain, that is, first discuss the ideal flow characteristics, and then consider the actual situation of the regulating valve in the pipeline, that is, discuss the working flow characteristics

2 ideal flow characteristic

ideal flow characteristic is the flow characteristic obtained when the pressure difference between the front and back of the valve is fixed. It depends on the shape of the valve core. Therefore, the price increase of 10 yuan/ton in Shandong and Tianjin is also called structural characteristic. In an ideal situation, the flow only changes with the opening of the valve. From the perspective of control, it is a common method to observe the control index of the regulating valve and study the flow characteristics. In common regulating valves, there are four typical ideal flow characteristics, as shown in figure 1[1]

2.1 linear characteristic

the relative flow of the regulating valve has a linear relationship with the relative opening, as shown in the curve (1) in Figure 1. The slope of the curve remains unchanged, that is, its amplification coefficient remains unchanged. Taking the relative stroke equal to 10%, 50% and 80% as an example, when the stroke changes by 10%, the relative flow changes by 10%, and its relative change values (i.e. sensitivity) are 100%, 20% and 12.5% respectively

it can be inferred that under the condition of changing the same stroke, the relative opening of the valve is small, the relative flow change value is large, and the sensitivity is high; When the relative opening is large, the relative flow change value is small and the sensitivity is low. This often worsens the control performance of linear characteristic valves: when the opening is small, the amplification factor is relatively large, so that the collection accuracy can reach up to 1ms, and the process often oscillates; At large opening, the amplification factor is relatively small and the sensitivity is low, which is easy to slow down the action of the valve and prolong the adjustment time

2.2 logarithmic characteristic

the change of relative flow caused by the change of unit relative travel is in positive proportion to the relative flow at this point, as shown in curve (2) in Figure 1. Taking the same stroke l equal to 10%, 50% and 80% as an example, when the stroke changes by 10%, the flow changes by 1.9%, 7.4% and 20.5% respectively. It can be said that the amplification factor increases with the opening of the valve. Therefore, when the valve is opened at a small degree, the amplification factor is small and the operation is gentle and stable; When the opening is large, the amplification factor is large and the work is sensitive and effective. Similarly, the sensitivity of each point is 40%, which is equal everywhere (also known as equal percentage characteristic), which is convenient for control

2.3 fast opening characteristics and parabolic characteristics

fast opening characteristics are shown in the curve (3) in Figure 1. When the valve opening is small, the flow changes greatly. With the increase of the opening, the flow quickly reaches the maximum value, with large amplification coefficient and high sensitivity. When the valve opening is large, the flow changes little, the amplification coefficient is small, and the sensitivity is low. When used in occasions with low pressure and low regulation requirements, it is not easy to cause large pressure fluctuations when opening and closing. Parabolic characteristics are shown in curve (4) in Figure 1. The relative flow caused by the change of unit relative stroke of this valve is directly proportional to the square root of the relative flow value at this point. It is between curve (1) and curve (2), and its characteristics are close to those of logarithmic valve. However, due to the complex processing of its valve core, it is rarely used

3 working flow characteristics

when the regulating valve works in the process pipeline system, the resistance of the pipeline system changes or the pressure difference before and after the valve of the opening degree of the bypass valve changes, so that when the valve is opened at the same degree, the flow will no longer remain unchanged as the ideal flow characteristics, and the corresponding flow will change. We call the flow characteristic of the pressure difference change before and after the regulating valve as the working characteristic

3.1 working flow characteristics when connecting pipelines in series

in the project, the regulating valve is installed on the pipeline system with resistance, as shown in Figure 2. When the total pressure difference at both ends of the system is constant, the pressure difference on the regulating valve will decrease with the increase of flow [2]. As the valve is opened larger, the pressure difference before and after the valve decreases. Therefore, when the relative opening of the valve is the same, the flow at this time is smaller than that under the ideal flow characteristics. When the valve opening is large, the pressure difference before and after the regulating valve decreases and the flow is large

in Figure 2, P is the total differential pressure of the pipeline system, P1 is the differential pressure of the regulating valve, and P2 is the differential pressure on the series pipeline and equipment. Let s= (p1m/(P), where s is the weight coefficient of the valve, and p1m is the differential pressure at both ends of the regulating valve when the valve is fully open. When the valve is changed to different pipe resistance, the s value is different. With the increase of pipeline resistance, s value decreases. Under different s values, for the control valve whose ideal characteristics are linear and equal percentage flow characteristics, the working characteristics are shown in figure 3[3]

it can be seen from Figure 3 that when s=1, that is, the total pressure of the system acts on the regulating valve and remains constant, it is an ideal characteristic. With the decrease of s value, the flow of the fully open regulating valve decreases, but the relative flow Q at a certain relative opening increases with the decrease of s value (q=q/q100, Q100 represents the flow of the fully open regulating valve when the pipeline has resistance). Therefore, compared with the ideal flow characteristics, the working characteristics are distorted and become a group of upward arching curve clusters. In this way, when the opening is small, the amplification factor is larger and the sensitivity is higher; At large opening, the amplification factor is smaller and the sensitivity is lower. At the same time, if we call the flow when the relative opening is zero as the minimum flow, and the reciprocal of the ratio of the minimum flow to the maximum flow Q100 as the adjustable ratio, then with the decrease of s value, the adjustable ratio of the valve becomes smaller due to the influence of the resistance of the series pipeline. It can be inferred that the relationship between the adjustable ratio R and the weight of the valve is:

where R is the adjustable ratio when the test material is the ideal flow characteristic, which is called the ideal adjustable ratio; RS is the adjustable ratio of working flow characteristics, which is called the actual adjustable ratio

the smaller the adjustable ratio, the lower the regulating capacity of the regulating valve; The larger the adjustable ratio is, the stronger the regulating ability of the regulating valve is. However, the actual adjustable ratio cannot be too large compared with the ideal adjustable ratio, because the energy consumption of the system should be considered. Generally, s is between 0.3 and 0.5 [4], and the actual adjustable ratio is controlled between 0.55 and 0.70 of the ideal adjustable ratio

3.2 working flow characteristics when parallel pipes

Figure 4 shows the parallel connection of regulating valves. Although the pressure at both ends of the regulating valve is constant, the opening degree of the parallel bypass valve will also affect the flow characteristics of the regulating valve. If Q100 represents the flow through the regulating valve when the regulating valve is fully open, Qmax represents the maximum flow of the main pipe, and X represents the degree of bypass, then. Under different x values, the working flow characteristics are shown in figure 5[5]. It can be seen from the figure that when x is equal to 1, the bypass valve is closed, and the working flow characteristic of the regulating valve is the ideal flow characteristic. With the gradual opening of the bypass valve, the flow of the bypass valve increases, the x value decreases, the flow characteristics do not change, but the adjustable ratio decreases greatly. The relationship between the actual adjustable ratio and the bypass degree x is:

in practical application, there is always the influence of series pipes, which makes the adjustable flow of the regulating valve become very small, and even the regulating valve has little regulating effect. Generally, the minimum value of X is expected to be no less than 0.8[6], so that the maximum flow of the regulating valve is 80% of the total flow, and the working characteristic curve is closer to the ideal characteristic, and the adjustable ratio r will not be reduced too much. For the linear valve, the sensitivity is reduced when the opening is small, which can avoid the occurrence of oscillation. For logarithmic valves, the amplification factor is smaller when the opening is small, and the sensitivity change of the whole stroke tends to be constant, which is approximately equal to the percentage characteristic, and still can maintain a high adjustment quality. For quick opening characteristic valve and parabolic characteristic valve, the working characteristic curve has the same change trend, which should also be paid attention to in use. It should also be pointed out that when working in parallel, the flow of (1-x) Qmax cannot be adjusted, because this part of the flow flows out through the bypass valve. From the perspective of control, when the relative opening of the regulating valve is small, the relative flow is small. Compared with the ideal characteristics, the regulation of the regulating valve is slow, the regulation time is extended, and the regulation capacity is reduced

4 stability analysis of regulating valve

the regulating valve acts on the system in practical application. It is not enough to only discuss the regulating valve itself or simply discuss the relationship between the valve and the system. An overall analysis should be carried out. Generally speaking, the system as a whole can be divided into two parts: the regulating system and the regulated object. The former includes three parts: the measuring sensor, the regulator and the actuator (the actuator also includes the regulating mechanism, the regulating valve and the heater). Taking temperature as an example, the signal connection between various components is shown in Figure 6. Generally speaking, the task of controlling the regulated quantity in the regulated object is completed only after the regulated signal passes through the cycle of the regulated quantity of the regulated object of the regulated comparator, the regulator, the regulating valve of the regulating mechanism, the heating (cooling) equipment and the regulated object

from the perspective of the regulated object, the response curve of most thermal objects under the action of step signal conforms to the exponential attenuation law, as shown in figure 7[7]. In the transition process, the amplification coefficient KC of the modulated object is not a constant relative to its input signal, and often changes from small to large. And from regulation