Selection of automatic control valves

I. Selection of control valves based on their functional capabilities 1. Control function 1) It is required that the control valve operate smoothly. 2) It has good adjustment performance when the opening is small. 3) Select the desired flow characteristic. 4) Meet the adjustable ratio requirement. 5) Low resistance and high flow ratio (the ratio of the valve’s rated flow parameter to its nominal size). 6) Fast regulation speed. 2. Leakage rate and shut-off pressure difference! These are two interrelated factors that are inseparable from one another. The leakage rate must meet the process requirements, and there must be protective measures to ensure the reliability of the sealing surfaces ； The cut-off pressure difference (the pressure difference when the control valve is closed) must be specified, so that the selected control valve has sufficient output force to overcome it; otherwise, it may lead to the selection of an actuator that is either too large or too small. 3. Clogging prevention! Even when the medium flowing through the control valve is clean, clogging can still occur. This is because contaminants such as oxide scales in the pipelines are carried into the control valve by the medium, leading to blockages. Therefore, it is necessary to consider the clogging prevention capabilities of the control valve. Generally, quarter-turn travel control valves have much better anti-clogging performance than linear travel control valves; therefore, in applications prone to clogging, it is recommended to use quarter-turn travel control valves. 4. Corrosion resistance! This includes resistance to erosion, cavitation, and corrosion; it relates primarily to the selection of materials for control valves and their service life, as well as economic considerations. The control valve should have good corrosion resistance and a reasonable price. 5. Pressure and temperature resistance! This relates to the selection of the nominal pressure and operating temperature of the control valve. Mainly, it involves properly selecting the main body material and internal component materials of the control valve. For example, the maximum operating pressure for carbon steel is 42 MPa, and the maximum operating temperature is 425 degrees Celsius ； The maximum operating pressure for austenitic steel is 32 MPa, and its operating temperature range is from -196 to 600 degrees Celsius ； Heat-resistant steels (molybdenum steels and chromomolybdenum steels with a molybdenum content of not less than 0.4%) have a maximum operating pressure of 42 MPa and a maximum operating temperature of 570 degrees Celsius. 6. Determination of the control valve type based on comprehensive economic considerations 1) High reliability. 2) Long service life. 3) Easy to maintain, with sufficient spare parts available. 4) The cost-performance ratio of the product is appropriate. 7. Sequence for selecting control valve types: Based on the performance advantages of various control valves and the principles mentioned above, the sequence for selecting control valves is as follows: V-port ball valve – butterfly valve – sleeve control valve – single-seat control valve – eccentric rotary valve – wedge ball valve – angle control valve – three-way valve – diaphragm valve. II. Selection of actuators: Among actuators, pneumatic actuators are the most commonly used, followed by electric actuators. Hydraulic actuators are used less frequently, while intelligent actuators are still in the stage of development. Therefore, when selecting an actuator, the focus lies on the comparison between pneumatic and electric actuators. Main considerations include issues related to reliability, safety, cost-effectiveness, sensitivity, etc. III. Valve Selection 1. Consideration of Process Conditions Before selecting a valve, it is necessary to conduct a thorough analysis of the control process, collect sufficient data, and understand the system’s requirements regarding control valves, including aspects such as operational performance, reliability, and safety. When selecting a control valve, choose one that is suitable, or one that is fairly suitable and has a lower cost. If the usage requirements are not strict and several types can be used, cost should be taken into consideration as the guiding factor ； If the usage requirements are high, there are not many types available to choose from ； In more extreme cases. For example, the medium is corrosive mud, and operation takes place under high pressure ； When the working medium contains highly abrasive particles and there is flashing, it is difficult to find a truly suitable control valve. The following discusses how to consider valve selection under various process conditions ； When faced with complex and comprehensive situations, how should they be resolved? (1) Flash evaporation and cavitation! Their conditions of occurrence and specific details have been explained earlier. Besides affecting the calculation of the flow coefficient, flashing and cavitation also cause vibration, noise, and damage to materials. Flash evaporation and cavitation occur only in liquid media. The first stage of cavitation is flashing. The outlet pressure of the valve is maintained below the saturated vapor pressure of the liquid, but this has caused erosion of the valve’s internal components. Since the flow velocity of the medium is highest near the contact line between the valve core and the valve seat seal, failure occurs here. The surface of the valve core after flash vaporization damage shows scratches. In the second stage of cavitation, the pressure behind the valve rises above the saturation vapor pressure. Due to the sudden collapse of the bubbles, all the energy is concentrated at the point of collapse, generating an extremely high impact force that can reach several thousand newtons. This forces cause severe damage to the valve core, valve seat, and valve body; this type of damage is known as cavitation. This effect is similar to sand being sprayed onto the surface of the valve stem, tearing apart the solid surface layer and creating a rough, pitted outer surface. The destructive effects caused by cavitation are very severe. Under the conditions of cavitation caused by high pressure differences, even valve cores and seat sealing surfaces with very high hardness can only be used for a short period of time. In such cases, appropriate methods and measures should be adopted to select the control valve. (2) Wear: The valve core, valve seat, and the medium are in direct contact; due to continuous throttling and flow interruption, wear is severe when the flow velocity of the medium is high and it contains solid particles. The wear caused by solid particle impact is related to the kinetic energy of the particles, and the magnitude of this kinetic energy depends on the flow velocity of the medium and the size of the particles. According to physical theories, the degree of wear and damage is proportional to the mass of the particles, as well as to the square of the flow velocity of the medium, which shows how significant the effect of velocity is. When the medium is a suspension containing a high concentration of abrasive particles, the valve core and valve seat mating surfaces experience severe friction every time they close. The sealing pair wears out due to the repeated compression of the particles. Over time, poor sealing will occur. When selecting valves, attention should be paid to wear, and appropriate measures and solutions should be taken. 3. Corrosion: Control valves that operate in corrosive media should have as simple a structure as possible, as this facilitates the installation of linings, especially those that are expensive and special. The selection of valve type should be suitable for the corrosive medium being used. Single-seat and double-seat control valves made of austenitic stainless steel, duplex stainless steel, or special alloys (Hastelloy, Inconel, Monel, etc.) can be selected; alternatively, diaphragm valves, clamp valves, lined butterfly valves, ball valves, and other types may also be chosen. Butterfly valves can be made from cast alloys or coated with electroplating. Ball valves and angle valves can be manufactured from bars or forgings, and the sealing rings can be made of reinforced polytetrafluoroethylene. Angle valves can be lined with tantalum or other corrosion-resistant materials. If the coating is too thin, it will not provide corrosion resistance. If the medium is a very strong organic or inorganic acid, a all-titanium control valve can be used. (4) High pressure difference: First, the materials of the valve core and valve seat surfaces of the control valve need to be considered. These materials must be able to withstand the effects of high speed and high pressure from the medium. The guidance structure must be good to ensure the stability of the flow plane; in addition to the effects of dynamic forces, factors that cause instability in the actuator must also be eliminated. The most widely used valve is the throttle valve. Angle-type throttle valves or 2-way throttle valves with balanced trim can reduce the requirements placed on the actuator. In the absence of a balanced valve trim, a flow-open type can be used as it is the most stable. If the discharge medium is corrosive, it is best to use a flow-sealed type. Because when using the flow-open type, the medium can damage the main components. Under the effect of a high pressure difference, it is easy for the liquid to experience flashing and cavitation; therefore, anti-cavitation control valves should be used. 6. High temperature: When selecting control valves, it is essential to consider using shell and internal component materials with high-temperature strength, such as WC6, WC9, or austenitic stainless steel, duplex stainless steel, Inconel, etc. The materials used must not bond, plasticize, or creep under the influence of high temperatures. The gap cannot be too small. 7\Low temperature + When the temperature is below )(’, it is necessary to use shell materials and internal components that are resistant to low temperatures. In the low-temperature range of -29 to -196 °F, the shell material and the internal components must possess sufficient impact toughness. Special measures must be taken to maintain the heat capacity of the valve, protecting it from cooling loads, while also keeping the temperature in the packing box above 0 degrees.

2nd floor: Reasonable; consider high temperature, pressure difference, and pressure balance equation 25*15

I’d like to ask the experts: although angle valves have better resistance to clogging compared to straight-stroke valves, their cost is also much higher. How should one consider this?

The denitration projects carried out by environmental protection companies are, in terms of design quality, not worth praising.

The valve core is made of 316, and the valve body is made of 304.

Pneumatic diaphragm actuator, single-seat valve; valve body material: 304; valve core and seat: 316 or 316L. Equipped with a pressure reducing valve and a valve positioner; cost being irrelevant, an intelligent type of positioner should be chosen whenever possible.

Single-seat valve, large capacity, equal percentage

15063629978 is the same WeChat ID; you can add me and I can help you choose the right option.