Information on process control valves: what is the use of each piece of data?
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When referring to the data on control valves, I was not entirely clear about some of the figures. I hope to discuss this with everyone. 1. Maximum shut-off pressure difference: This refers to the maximum pressure difference when the control valve is fully closed, and it influences the selection of the actuator for the control valve. The larger it is, the larger the cylinder of the control valve or the diaphragm disc on it must be, as well as the valve stem. (The pressure formula is p=F/S, where p represents pressure, F represents the vertical force (pressure), and S represents the area over which the force is applied.) The air supply for pneumatic control valves is generally 0.6–0.7 MPa; the greater the maximum closing pressure difference, the larger the disc can be. Generally, selecting the design pressure should work as well; however, this will result in a larger actuator and a thicker valve stem. 2. Vapor pressure, critical pressure: Used to determine choked flow in liquid states. The compressibility coefficient and isentropic exponent are used to calculate gases ； The superheat index is used to calculate superheated steam. 3. Vaporization volume: For example, if the condensate pipeline contains a gas-liquid mixture, treating it entirely as a liquid can lead to errors in the selection of control valves. 4. Solid content: This is rarely encountered, so it’s not very clear. Is it the same principle as the vaporization amount? 5. Pressure before the valve, pressure after the valve, pressure difference (key point): The pressure before the valve is the pipeline pressure; it is the pump outlet pressure behind the pump. The pressure difference across the control valve (the pressure difference at maximum flow) is generally increased by 0.5–1 bar to obtain the pressure downstream of the valve. However, the pressure behind the valve is generally calculated by reversing the back pressure; for example, in a reflux system, the pressure behind the control valve is determined by reversing the tower pressure. Verification: The pressure difference specified for control valves is generally that at the maximum flow rate. Instruments generally use this pressure difference to determine the maximum flow rate, normal flow rate, and minimum flow rate, which obviously leads to errors. The smaller the flow rate, the greater the pressure difference across the control valve. In fact, the pressure difference across the control valve at maximum flow can sometimes be much lower than that at minimum flow. Using the maximum flow differential will result in a valve that is too large, making it impractical to use; at normal flow rates, its opening degree is very small. This is the core value of the control valve; that’s how I understand it. But I still have many questions. Could someone more experienced explain it in more detail? 6. At the same time, some owners design their systems with very high capacity requirements, but they are actually unable to meet those demands; the operating load may be only half of what was designed. At this point, when selecting a control valve, it is necessary to consider how to appropriately increase the flow rate. Others have encountered this situation, but I haven’t. Has anyone encountered this? What should be done, and how can one determine its actual load? Or should it be ignored? 7. Factors such as temperature, density, and viscosity are things that need to be mentioned routinely; at the same time, attention should be paid to corrosivity, as well as to materials that are prone to polymerization. Materials with high viscosity may require heating, and should an easily cleanable control valve be chosen? The above are the author’s humble insights regarding data related to control valves; they are meant to serve as a starting point for further discussion. I hope everyone will share their views, as I aim to gain a thorough understanding of control valves.
Your question is very good. Regarding the significance of the data related to control valves, I will answer it based on your list of questions: 1. Maximum shut-off pressure difference: This is an important reference value for control valves; it tells us the maximum pressure that the valve can withstand when closed. Based on this value, an appropriate actuator can be selected. 2. Vapor pressure, critical pressure: These two parameters are primarily used for calculating the critical flow rates of liquid media, and they are related to the flow control performance of control valves. 3. Vaporization volume: The vaporization volume is very important for selecting control valves. If there is a two-phase flow of liquid and gas in the medium, failing to take the vaporization volume into account may lead to unstable operation of the control valve. 4. Solid content: This parameter may be related to the cleanliness of the medium; if there are many solids present in the medium, it may affect the lifespan of the valve seat and valve core of the control valve. 5. Pressure before the valve, pressure after the valve, and pressure difference: These three parameters are important indicators of the operating performance of control valves; they directly affect the flow rate of the valve as well as the stability of flow control. 6. Design capacity and actual operating volume: This is an issue that requires special attention. If the actual operating volume is much lower than the design capacity, it may lead to frequent opening and closing of the control valve, thereby affecting its lifespan; therefore, when selecting a valve, both the design capacity and the actual operating volume must be taken into account. 7. Temperature, density, viscosity, etc.: These are all important parameters in the selection of control valves; they affect the valve’s flow characteristics, corrosion resistance, and cleanliness, among other things. I hope the above answers are helpful to you. .
The last edit to this post was made by bfdlwolf on 2023-10-20 at 10:55. Article 6.1.5 of the \"Guidelines for Selecting Pipe Sizes in Petrochemical Process Units\" SH/T 3035-2018 states: \"The pressure drop of control valves shall be determined based on the process operating conditions and the characteristics of those valves.\" Under normal flow conditions, the pressure drop across the control valve should not be less than 30% of the pressure drop in the pipeline system; in pipeline systems with relatively stable flow, it can be set at not less than 20% of the pipeline system’s pressure drop. The provisions explanation provides a detailed overview of this article; it is recommended to take a look. Furthermore, the pressure drop in the piping system here refers to the pressure drop associated with flow rate, including the resistance losses incurred while fluid flows through pipes, heat exchangers, mixers, nozzles, valves, and piping fittings; it does not include back pressure or head differences. Therefore, before setting the conditions, the pressure drop of the piping system under normal flow conditions should be calculated, and the pressure difference for the control valve should be determined based on this value.
This post was last edited by The Man in Black with a Knife on 2023-10-20 at 11:15. However, when specifying the conditions for the control valve, such as when calculating the head of the return pump, it is necessary to first provide the pressure drop across the control valve; generally, a value of 0.5–1 bar is used. At this point, the pipeline layout has not yet been finalized, so the pressure drops associated with the pipelines and equipment are all estimates only. I know what the standards say, but when doing the actual calculations, it doesn’t seem to be used that way. Friend, could you explain it in more detail? No, give an example.
The PID diagram has identified the equipment involved in the pipeline system of the control valve (such as towers, vessels, heat exchangers, mixers, etc.), the instruments (such as flow meters), the types and quantities of valves, as well as the pipe diameters. The equipment layout diagram determines the locations of the devices involved in the pipeline system where the control valves are situated; it can be used to draw pipeline layout plans, and it essentially helps to determine the length of the pipelines as well as the number of pipe fittings. By combining the above conditions, the pressure drop of the piping system can be calculated. The pump head also needs to take into account the pressure drop in the piping system, and this is before considering the conditions of the control valve. Additionally, some engineering companies stipulate that under normal conditions, the pressure drop (ΔP) across the control valve should generally not be less than 0.07 MPa. If the calculated pressure drop ΔP exceeds 1.38 MPa, it is necessary to discuss the matter with engineers from related fields such as instrumentation before making a decision
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This table is quite similar to the condition table used in our institute. 1. Maximum closing pressure difference: Generally, the design pressure is used. 3. Vaporization volume: The valves and calculations for two-phase flow must be different, right? 4. Solid content: This is generally determined by the type of valve designed; for high solid content levels, issues such as valve jamming need to be taken into consideration. 5. Pressure before the valve, pressure after the valve, and pressure difference (key point): Follow the process requirements if any exist. If there are no specific requirements, just choose a normal valve pressure drop, such as 50 kPa. As for the specific methods for selection and calculation, there are software tools available in the field of automation control. 6. At the same time, some owners design their systems with very high capacity requirements, but they are actually unable to meet those demands; the operating load may be only half of what was designed. In such cases, I usually communicate with the owner to obtain an accurate figure, and if possible, include it when selecting the appropriate options; otherwise, any issues that arise on site later will be quite troublesome.
Give it a try and see how many you can get right. Start with the designation: FV10201. FV stands for flow; F refers to flow rate; V denotes valve. 201 is the project section number, 01 is the sequence number, while the pipeline number indicates the identifier and order of the pipeline. The pipeline grade, pressure rating, material, and the corresponding medium type are also important factors. Pipeline specifications include outer diameter and wall thickness. The material used is 20# steel. The name of the medium is coal tar. The medium state is liquid phase, with 1 phase in total. Flow rate is expressed in terms of minimum and maximum values, as well as the normal flow rate. Pressures before and after the valve, including positive and back pressures, are also relevant parameters. Operating temperature, density, maximum shut-off pressure difference, and the pressure that the valve disc can withstand are other important factors. The design pressure is 1.1 times the pressure before the valve. Design temperature is operating temperature plus 20 degrees. Dynamic viscosity, superheating index, whether there is superheating, saturated vapor pressure (which should be positive), solid content, wear considerations, amount of vaporization in the liquid phase, and fault shutdown mode (FC) are also important aspects to consider