Thread Content
As the level of automation in petrochemical enterprises continues to increase, the selection and use of control valves become particularly important. At the production site, control valves directly regulate the process media. Some of these media have complex compositions, especially in cases involving high temperatures, high pressures, highly toxic substances, low temperatures, strong corrosivity, flammability, explosiveness, permeability, high viscosity, or tendency to crystallize. If the wrong valves are chosen, it can lead to difficulties in production control, resulting in a decline in quality and even serious production accidents. Therefore, great emphasis must be placed on the proper selection of control valves. Especially in recent times, some control valve manufacturers, in order to demonstrate their strong technical capabilities, often promise that providing only certain necessary parameters will be sufficient to meet the users’ requirements when selling their products. This indeed brings great convenience to users; however, their lack of knowledge about some details of the product also hinders subsequent repairs. Based on my own work experience and relevant information, I will discuss several aspects that should be taken into consideration during the selection of control valves. I hope this will be helpful to my colleagues in their work of selecting control valves, thereby ensuring the normal and stable operation of enterprises, as well as promoting technical exchanges among us. Select the appropriate structural form and material based on the process conditions. 1. How to choose the type of control valve? The pressure difference before and after the control valve is small, so a low leakage rate is required; generally, a single-seat valve can be used ； ? To regulate gases with low pressure differences and high flow rates, butterfly valves can be used ； ? To handle highly corrosive fluids, diaphragm valves can be used ； ? When both regulation and shutdown are required, an eccentric rotary valve can be used ； ? Sleeve valves can be used when there is high noise. 2. How to choose the material for control valves? The material is selected based on the working pressure, temperature, corrosivity of the medium, and the severity of cavitation erosion. ? Generally, cast steel should be chosen ； ? Cast iron can also be used when the requirements are not high (+120°C, below 1.6 MPa) ； ? 1Cr18Ni9Ti should be selected for high-temperature (450-600°C) or low-temperature (-60-250°C) applications ； ? For high-pressure applications (22–32 MPa), forged steel, 1Cr18Ni9Ti, and Cr18Ni12Mo2Ti should be used ； ? For highly corrosive media, 1Cr18Ni9Ti should be selected ； Based on the characteristics of the process medium, an appropriate flow characteristic is selected. The flow characteristic of a control valve represents the relationship between the relative flow rate of the medium passing through the valve and its relative displacement (the degree of opening of the control valve). Generally, by changing the cross-sectional area through which the valve element and seat allow flow, it is possible to control the flow rate. But in reality, due to various factors, such as changes in the cross-sectional area, there are also changes in the pressure difference before and after the valve, and these changes in pressure difference in turn cause changes in flow rate. When the pressure difference before and after the valve remains constant, the flow characteristic of the control valve is referred to as an ideal flow characteristic ； The structural characteristics of a control valve refer to the relationship between the displacement of the valve element and the flow area through the valve. This relationship is determined solely by the size and geometric shape of the valve element. In addition to the geometry of the valve, the influence of the flow coefficient under constant pressure differences is also taken into account. Therefore, the ideal flow characteristics of a control valve differ from its structural characteristics. The rational flow characteristics mainly include linear, equal percentage, parabolic, and fast-opening types. In actual production applications, the pressure difference before and after the control valve is always changing. The flow characteristics under such conditions are referred to as operating flow characteristics. Since control valves are often used in series or parallel with process equipment, the flow rate changes as a result of resistance losses. In practical use, the ideal flow characteristics are distorted into operating characteristics due to changes in the pressure difference before and after the valve. The ideal flow characteristics of control valves include linear, equal percentage, and quick-open types, which are commonly used in production. The parabolic flow characteristic lies between the linear and equal percentage types, and it can generally be replaced by the equal percentage type. The quick-open characteristic is mainly used for two-position control and program-controlled applications. Therefore, the selection of control valve characteristics refers to how to choose linear and equal percentage flow characteristics. Currently, empirical criteria are commonly used to select the flow characteristics of control valves, and considerations can be made from the following aspects: 1. Analysis of the quality of the control system. The figure below shows an automatic control system for a heat exchanger, which consists of the controlled process, transmitters, control instruments, and control valves. K1 is the amplification factor of the transmitter, K2 adjusts the amplification factor of the instrument, K3 is the amplification factor of the actuator, K4 is the amplification factor of the control valve, and K5 is the amplification factor of the controlled object. Obviously, the overall amplification factor K of the system is given by: K=K1*K2*K3*K4*K5. K1, K2, K3, K4, and K5 represent the amplification factors of the transmitter, control instrument, actuator, control valve, and control object respectively. In the event of changes in load, this value ensures that the control system can still maintain the desired quality standards ； It is then desired that the overall amplification factor remain constant throughout the entire operating range of the control system. Generally, the amplification factors of the transmitter, the regulator (once tuned), and the actuator are constant, but the amplification factor of the process variable always changes with operating conditions; therefore, the characteristics of the process variable are often nonlinear. Therefore, the characteristics of the control valve should be chosen appropriately so that changes in the valve’s amplification factor can compensate for changes in the amplification factor of the process variable, thereby keeping the overall amplification factor of the system constant or nearly constant and improving the quality of the control system. Therefore, the selection of the flow characteristic for the control valve should satisfy: K4*K5 = constant. To counteract the phenomenon where the amplification factor decreases as the load increases, if a control valve with an equal percentage characteristic is used, whose amplification factor increases as the load increases, these effects can cancel each other out. As a result, the overall amplification factor remains constant, resulting in a nearly linear behavior. When the amplification factor of the control object is linear, a linear flow characteristic should be adopted to keep the total amplification factor constant. 2. Considering the process piping layout, control valves are always used in conjunction with pipes, equipment, etc. Due to differences in the system piping layout, the presence of piping resistance causes variations in the pressure drop across the control valve; as a result, the actual flow characteristics of the valve differ from its ideal flow characteristics. The desired working characteristics must be selected based on the system’s features, and then the appropriate ideal characteristics should be chosen considering the piping conditions. The pressure difference across the control valves, all process equipment, and piping systems is referred to as the total system pressure difference. When the control valve is fully open, the ratio of the pressure difference ΔPmin across the valve to the total system pressure difference ΔP is called the S value; that is, S = ΔPmin/ΔP. Taking into account the conditions of the process piping, the appropriate ideal characteristic can be selected by referring to the table below. Table 1 Table considering process piping conditions Piping condition S = 1–0.6 S = 0.6–0.3 S < 0.3 Operating characteristic of the valve Linear Equal percentage Linear Equal percentage Not controllable Ideal characteristic of the valve Linear Equal percentage Equal percentage Equal percentage Not controllable From the table above, it can be seen that when S is between 1–0.6, the selected ideal characteristic is consistent with the operating characteristic. When S=0.6–0.3, equal percentage should be chosen if a linear operating characteristic is required; this is because valves with an equal percentage characteristic exhibit an ideal behavior, and when S=0.6–0.3, the distorted operating characteristic is already close to linear ； When the required operating characteristic is an equal percentage, its ideal curve should be more concave; in this case, it can be compensated by using the cam profile of the valve positioner or by employing a hyperbolic characteristic. In cases where S10MPa or the inlet pressure P1 > 10MPa. ? Situations where the proportional control system and control valves need to switch between air-open and air-close modes during operation. ? Situations where it is necessary to change the flow characteristics. ? The regulator has a wide proportional band, yet it is required that the valve respond to small signals. ? Control valves without spring mechanisms are used, such as proportional pneumatic piston actuators. Conclusion: In addition, the issue of leakage from control valves must be considered, with attention paid to the selection of gaskets. Tetrafluorine filler, as the operating temperature ranges from -40 to 250°C. When the upper and lower temperature limits change significantly, its sealing performance deteriorates markedly, it ages quickly, and its lifespan is short. Flexible graphite packing can overcome these drawbacks, offering a longer service life. However, the backlash of graphite fillers is large, and crawling phenomena occur during initial use; this aspect must be taken into consideration. Asbestos sheets are used as sealing gaskets, but their sealing performance is poor at high temperatures; their lifespan is short as well, and they tend to cause leaks ； Wound gaskets and “O”-rings provide good sealing performance under high temperature and high pressure. In summary, there are many considerations involved in selecting control valves. It is hoped that the information presented in this article will be helpful to colleagues in the field of control valve application, and any errors or shortcomings are welcome to be pointed out.