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Discussion on the technical problems in the application of regulating valve
Abstract: This paper discusses the application of the valve need to pay attention to several technical aspects, and put forward reasonable suggestions based on the actual situation. 1 Introduction As technology advances, in the production process automation, used to control the fluid flow control valve has been in various industries. For heat and chemical process control system, as the final control of the quality of the process medium and safety indicators of the production process control valve, which in stable production, optimal control, maintenance and overhaul cost control plays a decisive role. As the control valve is to control the flow by changing the throttle mode, so it is an effective means of regulation, but also a part of the energy consumption will have a throttle. Take the chemical plant as an example. Along with the high-load operation of the device, problems such as corrosion, scouring, abrasion, vibration and internal leakage of the control valve occur continuously, resulting in shortening the service life of the control valve and lowering the reliability of the work, The production efficiency of the device is greatly reduced, which can lead to a full stop of the vehicle in serious cases. This is particularly important and urgent in the business management that now regards quality and benefit as life. In this regard, how to choose and install the control valve, the control valve at a high level of operation will be a key issue. The choice of control valve, the first to collect the physical properties of the process fluid and control valve operating parameters, the main fluid composition, temperature, density, viscosity, normal flow, maximum flow, minimum flow, maximum flow and minimum flow Import and export pressure, the maximum cut off the pressure and so on. In the specific selection of the control valve before the decision, but also must fully grasp and determine the structure of the valve itself, the form, material and other aspects of the characteristics of the main technical considerations flow characteristics, pressure drop, flash, cavitation, noise and other issues . 2 How to choose the flow characteristics The flow characteristics of the control valve refer to the relationship between the relative flow of the medium flowing through the valve and the relative displacement. [1] The mathematical expression is as follows: Q / Qmax = f (l / L) The relative flow rate is the ratio of the flow rate Q to the full-flow Qmax when the control valve is at a certain opening degree; l / L is the relative displacement, which is the ratio of the displacement l of the cutting core to the full opening displacement L when the control valve is at a certain opening degree. The general rule of thumb is that the flow characteristics of the regulating valve should be opposite to the characteristics of the regulating object and the characteristics of the regulator so that the overall characteristics of the regulating system approach to linearity. The selection is usually carried out under the requirements of the process system, but a lot of practical considerations have to be taken into account. (1) Linear flow control valve Linear flow characteristics refers to the relative flow control valve and the relative displacement of a linear relationship between the displacement of the unit caused by changes in the flow rate is constant. Linear flow characteristics of the valve used in the general election are: â‘ small pressure changes, almost constant; â‘¡ process system linear changes in the main parameters; â‘¢ most of the system pressure loss distribution in the control valve (change the opening, the valve differential pressure Change is relatively small); â‘£ small external interference, small changes in the given value, the adjustable range of small occasions. (2) Equal percentage control valve, etc. Percentage of flow characteristics, also known as logarithmic flow characteristics. It refers to the change of unit relative displacement caused by the relative change in flow and this point is proportional to the relative flow. The control valve amplification factor is changing, it increases with the relative increase in flow. Preferential selection of equal percentage of the valve occasion: â‘ actual adjustable range; â‘¡ opening changes, the valve differential pressure changes relatively large; â‘¢ piping system pressure loss; â‘£ process system load fluctuations; ⑤ regulating valve often Operate in a small opening. In addition to the above two commonly used flow characteristics, [2] there are other flow characteristics of the parabolic and quick opening characteristics of the valve, the ideal flow characteristics shown in Figure 1. In the sealing structure, if the flow characteristics of high precision, you can use high-precision flow characteristics of the metal seal, and soft-seal less accuracy. 3 control valve pressure drop system to consider Regulating valve as a process control system in the terminal components, is the most commonly used actuator. According to the requirements of the process control system, the control valve should have the working characteristics of low energy consumption and can fully match the system. However, these two requirements can not be met or even contradictory in the use of control valves. In order to get the same flow Qmax circumstances, the choice of a smaller caliber regulator, although the other resistance unchanged and the total resistance must be relatively large, resulting in a large total system pressure drop. If the logistics of the driving force is generated by the pump, it means that you must choose the larger power of the pump and motor, which inevitably bring about large energy consumption. When the flow rate of the medium in the pipe system increases, a series of dynamic changes occur in the pressure drop of the fluid generated when the fluid passes through the various mounting components on the pipe. The pressure drop caused by the regulating valve, which is the main component of the pipe fluid control, is A very important and easily overlooked factor, we analyze the system related to the control valve, not only to take into account the problem of the control valve itself, but also to take into account the control valve pressure drop on the system dynamic balance. Figure 2 is a simple process flow diagram with a regulator installed, where D1 and D2 are process tanks, G1 is the pump, V1 is the regulator and E1 is the heat exchanger. Here, it is possible to decompose the pressure change in the pipe fluid into several parts, namely: ΔPp (pump boost), ΔPv (pressure drop across the regulator), ΔPa (pressure drop across the heat exchanger), ΔPt Pressure drop), ΔPg (fluid potential energy conversion pressure drop). Figure 3 is a flow-pressure graph of the fluid system showing the pressure balance of the line at different flow rates. The pressure characteristic equation for the corresponding pump in this system is: ΔPp = ΔPfo - (1 / Ï) * (F / Cp) 2 where: ΔPfo is the pressure at the outlet of the pump at zero flow; p is the mass of the liquid medium Density; F is the mass flow of the liquid medium; Cp is a constant. The pressure drop characteristic equation of the fluid on the pipeline is: ΔPt = (1 / Ï) * (F / Ct) 2 The pressure drop characteristic equation of the fluid on the heat exchanger is: ΔPa = (1 / Ï) * ) 2 The pressure drop characteristic of the fluid on the regulator can be expressed similarly: ΔPv = (1 / Ï) * (F / Cv *) 2 where Cv * is a dynamic flow constant that depends on the valve Rod position changes and changes. Below the maximum pressure drop in the system control valve and the minimum pressure drop for further discussion. Figure 4 is a simple and typical control valve flow control system. In this system, the relationship between the flow rate (including the maximum flow rate and the minimum flow rate) and the pump outlet pressure and the tank inlet pressure is shown in (1) and (2) in FIG. 5. From Figure 6, we can see the maximum pressure drop and the minimum pressure drop allowed by the control valve in the distribution of pressure variations throughout the system and the associated factors. Relatively speaking, when the flow rate is maximum, the pressure drop of the regulating valve is the smallest; when the flow rate is minimum, the pressure drop of the regulating valve is the maximum. In addition, it must be noted: [2] In order to make the control valve to work in a normal state, remember not to make the control valve minimum pressure is zero! [3] On the control valve of the pressure difference, here for several common situations further elaborated: â‘ In a pipeline system driven by a pump, the corresponding pressure drop at the rated flow control valve should be the system dynamic pressure drop 30% or 15PSI; â‘¡ In the centrifugal compressor suction or discharge port if the valve is installed, the control valve in normal operating conditions, pressure drop should be aspiration absolute pressure of 5% or for the system dynamics The pressure drop of about 50%; â‘¢ For the use of static pressure tank between the process to drive the flow of media pipeline system, the pressure drop under normal operating conditions should be low-end tank pressure of 10% About 50% of the dynamic pressure drop of the system; â‘£ The pressure drop of the regulating valve connected to the horizontal steam line under normal working conditions should be 10% of the absolute pressure of the steam system design or 5 PSI. 4 control valve flashes and cavitation The flow of liquid in the control valve often flash and cavitation two phenomena. Their occurrence not only affects the selection and calculation of caliber, but also causes serious noise, vibration and material damage. In this case, the working life of the control valve will be greatly reduced, which is necessary to elaborate. Under normal circumstances, as a liquid state medium, inflow, flow through, out of the control valve remain liquid. The pressure curve shown in Figure 7 (1) below. Flashing is a liquid state medium that flows into the regulator valve in a liquid state. The pressure of the fluid is lower than the vapor pressure Pvapor as it flows through the converging valve in the converging valve. The liquid medium becomes a gaseous medium and its pressure is no longer Return to the gasification pressure above, out of the control valve media has remained gaseous. The pressure curve shown in Figure 7 (2). Flashing is like a sandblasting phenomenon that acts on the valve body and downstream of the pipeline, causing severe erosion of the regulator and the inner surface of the pipe, while also reducing the flow capacity of the regulator. Cavitation, as a liquid state medium, flows into the regulator valve when it is in a liquid state. When it flows through the converging point in the regulating valve, the pressure of the fluid is lower than the gasification pressure. The liquid medium becomes a gaseous medium, and its pressure returns to the gas The pressure Pvapor above, and finally out of the control valve before the medium has become liquid. The pressure curve shown in Figure 7 (3) below. [4] It is possible to initially determine the existence of cavitation based on a few phenomena, which give rise to a hissing sound when the cavitation starts, and when the cavitation progresses completely stable, the regulating valve makes a rattle sound The sound of gravel as it passes through the regulator. Cavitation damage to the regulating valve and the inner parts is also great, but it also reduces the flow control valve, as the same flash. Therefore, [4] we must take effective measures to prevent or minimize the occurrence of flashover or cavitation: â‘ try to install the control valve in the lowest position of the system, which can relatively increase the control valve inlet P1 and outlet P2 pressure shown in Figure 8; â‘¡ upstream or downstream of the control valve to install a stop valve or orifice to change the original installation of the control valve pressure drop characteristics (this method is generally more effective for small flow conditions) , As shown in Figure 9; â‘¢ use of specialized anti-cavitation inner parts can also effectively prevent flash or cavitation, it can change the fluid flow in the control valve changes, thereby increasing the internal pressure; â‘£ try to use more material Hard control valve, because in the event of cavitation, for such control valve, it has a certain erosion resistance and wear resistance, under certain conditions can make cavitation exist, and will not damage the control valve Pieces. On the contrary, for the soft material control valve, due to its poor erosion resistance and wear resistance, when the cavitation occurs, the internal components of the control valve will soon be worn, which can not be in the case of cavitation Working normally. In a word, there is not any engineering material that can adapt to the cavitation under severe conditions and can only be analyzed objectively to select a relatively reasonable solution. Noise Analysis of Control Valves Cavitation and noise are two major hazards of regulating valves in controlling high differential pressure fluids. Regulating valve noise is the main source of pollution in the petrochemical industry. In addition to the use of low-noise structure in the selection of the control valve, the valve operating conditions change is to eliminate or reduce cavitation and noise, the fundamental method. Regulating valve in the work, should pay attention to its noise situation, analysis of the noise generation mechanism can better monitor the working status of the valve and effectively handle the problems that occur, the following is an example. â‘ Mechanical Vibration - When the spool moves horizontally in the sleeve, the clearance between the spool and the sleeve should be as small as possible or a sleeve with a hard surface can be used. Natural Frequency Vibrations - such as spools or other components that have an inherent vibrational frequency, can be changed by special casting or forging to change the characteristics of the spool and, if necessary, to change other types of valves core. â‘¢ spool instability - as the valve core vibration caused by fluid pressure fluctuations caused by fluctuations in the noise, which is generally due to the adjustment of the loop actuator and other damping factors, which can be readjusted damping coefficient or In the direction of spool displacement plus damping facilities. â‘£ Mechanical fluidity of media - The media also emits noise when it flows in pipes or regulating valves. For this case, it is not explained in detail here (cavitation can also cause noise). Of course, some noises can not be eliminated, and only noise protection measures such as wearing earplugs should be taken as far as possible. 6 Conclusion The selection and application of control valve is a professional and technical work involved in a wide range of systems work, to do this work, not only in theory to fully understand its various characteristics, but also with the actual use of experience to Comprehensive analysis and judgment, so that the combination of theory and practice scientifically. References [1] Mingchi East editor of "Selection Guide for Control Valves" Chengdu: Sichuan Science and Technology Press, 1996.172 ~ 174 [2] Mingze East editor "control valve application" Chengdu: Sichuan Science and Technology Press, 1989.110 ~ 119 [3] Mingchi East editor "control valve calculation, selection, use" Chengdu: Chengdu University of Science and Technology Press, 1999.120 ~ 131 [4] Mingci East, left the National Day editor of "Automation Instrumentation Failure to deal with" Beijing: Chemical Industry Publishing Society, 2002.212 ~ 224 Author Institution: Instrument Factory of Quzhou Juhua Group Engineering Co., Ltd. Zip Code: 324004 E-mail: