How to choose the right regulating valve?
The regulating valve is the terminal executive element in the industrial process control system. In the industrial process continuous production automatic control system, regulating valve is generally needed to control various process parameters in the process production to achieve the pressure on fluid, the adjustment of parameters such as temperature, flow and liquid level is usually called “hands and feet” in industrial process automation production “. Its application quality is directly reflected on the regulating quality of the system. As a terminal actuator in process control, people have a deeper understanding of its importance than in the past. The application of regulating valve is good or bad, in addition to the product quality and whether users are correctly installed, used and maintained, it is very important to correctly calculate the type selection. Due to the incorrect calculation and type selection, the system operation is unstable, and there are even many examples that cannot be put into operation. Therefore, users and system designers should fully realize the importance of regulating valves on site and pay enough attention to the selection of regulating valves.
The general principle of regulating valve selection is: under the premise of satisfying the use function, the selected regulating valve should have simple structure, reliable performance, low price, long life, convenient maintenance, etc. The following mainly introduces the selection of regulating valve type and the selection of accessories.
How to Choose a Regulating Valve?
There are many classification methods for regulating valves. At present, a classification method commonly used at home and abroad is divided into 9 categories according to structure, principle and function, namely straight-through single-seat regulating valve, straight-through double-seat regulating valve, sleeve regulating valve, angle regulating valve, three way regulating valve, diaphragm valve, butterfly valve, ball valve and eccentric rotary valve, these nine kinds of products are the most basic, the most common products, also known as standard products, others improved on this basis combined with practical application are called special products.
There are many types of valves available, each of which has its advantages and limitations. Basic requirements and options depend on their ability to perform specific functions, such:
- The ability to throttle or control flow.
- There is no turbulence or flow resistance when it is fully open because turbulence reduces head pressure.
- Quick opening and closing mechanism-multiple quick responses are required in case of emergency or for safety.
- Tightly closed to prevent high pressure leakage.
- The ability to allow only one-way flow-to prevent backflow.
- Open under preset pressure-program control to prevent equipment damage
- Ability to handle abrasive liquids-hardening materials to prevent rapid wear.
This paper discusses the basic types of valves, types of actuators, control actions and types of locators. According to the control action, they are linear motion and rotating motion.
Linear valve, also known as multi-turn valve, has a sliding stem design that pushes the closing element to the open or closed position. Common valves include stop valves and gate valves.
The rotary motion is also called a quarter turn valve. After the stem rotates 90 °, a quarter of the rotary valve will be fully on or off (0 °).
They operate much faster than linear motion valves. The common ones are ball valve and butterfly valve.
Characteristics and correct selection of standard regulating valve
Straight-through single seat regulating valve
The straight-through single seat regulating valve has only one valve core and one valve seat, which is easy to realize strict sealing. It can adopt metal-to-metal hard sealing, or metal-to-Teflon or other composite soft sealing, the standard leakage is 0.01%C(C is the coefficient of rated flow), and the allowable pressure difference is small and the flow capacity is small. For example, the allowable pressure difference of DN 100 single-seat regulating valve is only 120kPa, and the flow capacity is only 100. The flow path is complex and the structure is simple. It is suitable for clean Media with strict leakage requirements and small working pressure difference, but small specifications of regulating valves (DN1/2, 3/4, 20) it can also be used for occasions with large pressure difference. It is one of the most widely used regulating valves. After further design, it can be used as a cut-off valve. The shape of valve core determines the flow characteristics, and it loses its original characteristics after being flushed. Replacing the valve core can change the flow characteristics. However, the fluid medium has great thrust on the valve core, that is, the unbalanced force is large, and a larger thrust actuator is required. Therefore, such regulating valves are not suitable for high pressure difference and large diameter applications. When choosing this valve, special attention should be paid to the differential pressure checking to prevent being opened by the top.
Straight-through two-seat regulating valve
The straight-through two-seat regulating valve has two valve core and two valve seats. Since the upward thrust of the upper valve core and the downward thrust of the lower valve core are basically balanced, the unbalanced force of the whole valve core is small and the allowable pressure difference is large, for example, the DN100 double seat regulating valve allows a pressure difference of 280kPa, which has a large circulation capacity. Compared with other regulating valves of the same diameter, the double seat regulating valve can flow more fluid, the circulation capacity of double seat regulating valves of the same caliber is 20% to 50% larger than that of single seat regulating valves. For example, the flow capacity of DN100 double seat regulating valve reaches 160. Therefore, in order to obtain the same flow capacity, the two-seat regulating valve can choose an actuator with smaller thrust. Double seat regulating valve adopts the bottom and top double guide, therefore, it is convenient to refit the normal body valve and the reverse body valve, that is, only need to install the valve core and valve seat reversely to change the normal body valve to reverse body valve, or change the reverse body valve into positive body valve, and don’t need to change the positive action or reaction type of the actuator. The upper and lower valve elements of the two-seat regulating valve cannot be closed at the same time, and the leakage is relatively large. The standard leakage is 0.1%C(C is the rated flow coefficient); The flow path is complex, not suitable for high-pressure difference applications, because in this kind of application, the valve is subjected to high-pressure fluid scouring is more serious, and it is easy to form flash and cavitation, which will increase the erosion to the valve body, similarly, it is not suitable for the control of fiber-containing media and high viscosity fluids.
Sleeve regulating valve
The sleeve regulating valve is also called cage valve. Its valve internals adopt valve core and Cage (sleeve). The sleeve can be a straight-through single seat regulating valve, or a two-seat regulating valve or angle regulating valve, there are two structures of double seal. The former is equivalent to a single-seat regulating valve, which is suitable for situations of a single-seat regulating valve; The latter is equivalent to a two-seat regulating valve, which is suitable for situations of a two-seat regulating valve. In addition, it also has the characteristics of good stability, convenient loading and unloading, convenient maintenance, noise reduction and cavitation effect reduction, but the price is 50% ~ 200% more expensive than single and double seat regulating valves, special winding sealing pads are also needed, which are widely used, second only to single and double seat regulating valves, but this valve should not be used for unclean media and easy media that are crystallized, stutter and scale.
Angle regulating valve
Angle regulating valve is a single-seat regulating valve with a special valve body structure, suitable for specific piping and fluid occasions, it is to change the direct valve body into a angle (equivalent to an elbow) valve body, its throttle, the force form is completely equivalent to single seat regulating valve. The characteristics of small leakage and small allowable pressure difference of single seat regulating valve are retained. In addition, due to its simple flow path with “self-cleaning” performance, it can be applied to unclean media, and can be further improved into anti-blocking angle valve, which is suitable for working conditions containing Suspended Particle Media, it is especially suitable for occasions where the installation space is limited.
Three-way regulating valve
The three-way regulating valve uses the valve core to guide itself, and the actuator must be replaced when replacing the gas on and off. What should be noted is that its gas on and off are different from other regulating valves, its gas opening and closing must be clear for which route, that is, horizontal position or vertical position. It has three channels, which can replace two straight-through single-seat regulating valves. It is used for shunt and combining two streams and the temperature difference is less than 150 ℃. When DN is less than 80mm, the closing valve can be used for shunt.
Diaphragm regulating valve consists of corrosion-resistant diaphragm and valve body lined with corrosion-resistant material. Its flow path is simple, and it is suitable for two cut-off occasions of unclean medium and Weakly corrosive medium. It is one of the earliest regulating valves. Because it has the flow characteristics similar to quick-opening, the control quality is poor, and it is also affected by the materials of diaphragm and lining, so it can not be used in high temperature and high pressure conditions, the general working pressure is less than or equal to 1.6MPa, the working temperature is less than or equal to 150 ℃. In addition, the diaphragm is easy to be damaged and has the disadvantages of short service life.
Butterfly Valve is equivalent to a section of pipeline to do the valve body, the center of the valve plate throttle, is used for control the most common rotary regulating valve. Applicable to low pressure, medium pressure or rare cases for high static pressure, large flow occasions, but the differential pressure is limited. It is of small volume and light weight, which is 4~10 times lighter than spherical type regulating valves of the same caliber. Its caliber and price are small, which is especially suitable for large caliber occasions, and the larger the caliber of the regulating valve, the more significant this feature is. Generally, when DN>300mm, it is usually completed by butterfly valve.
Ball Valve is a kind of mature old product, which is divided into “O”-shaped and “V”-shaped ball valves. The flow path is the simplest, the flow resistance is the smallest, the loss is the smallest, and the “self-cleaning” performance is the best. “O”-shaped ball valve is a kind of non-resistance regulating valve. Compared with the specification, the rated flow coefficient is the largest, and it is often used in occasions with large flow and unclean medium; “V”-shaped ball valve provides approximate logarithmic flow characteristics, and the ratio can be adjusted to large, “V”-shaped ball core has shear effect when it rotates with the valve seat, especially suitable for high viscosity, floating flow, pulp and other unclean, adjustment and cutting of fiber-containing medium. The price of ball valve is more expensive.
Eccentric rotary valve
Also known as Cam flexing valve, it combines the advantages of ball valve and butterfly valve, with simple flow path and good “self-cleaning” and adjusting performance, and is suitable for crystallization, stutter and unclean media; the valve body is small in volume and light in weight, and can be assembled flexibly according to the site installation position without changing any parts; The rated flow coefficient is big, which is 10% ~ 30% larger than the single-seat and double-seat regulating valve of the same diameter, the adjustable ratio is large, up to 100:1; The valve seat is reliably sealed. Due to the disturbing effect of the valve core support arm and the spherical eccentric rotation movement of the valve core, the required operating torque is reduced and some asymmetry is compensated, it can operate stably under flow-on, flow-off and high pressure difference; When proportional adjustment, a locator is required, and a straight line or equal percentage flow characteristic can be easily obtained by changing the position of the convex plate in the locator.
Selection of special regulating valves
For special application, on the basis of the regulating valve mentioned above, if the valve cover is lengthened and heat sink is added, it can be used in low temperature and high temperature situations; The use of multiple spring actuators can reduce the volume and weight of the entire regulating valve; in order to reduce low noise noise, a series of noise reduction measures can be adopted to design low noise noise regulating valve. In addition, there are Valve body separation regulating valves with valve body separation structure for convenient maintenance and cleaning; Quick cut-off regulating valves for quick requirements of interlocking actions; Small flow regulating valves designed for small flow control; in order to prevent leakage, it adopts bellows seal regulating valve, etc.
These special types of regulating valves are to meet the special process of production process or a specific use of the use of special valves, which are non-standard. They are characterized by complex working conditions, high requirements for use and small production batches. These regulating valves are usually evolved and improved according to the use requirements of standard products. Therefore, the basic shape should be determined first according to the non-particularity, and then the corresponding deformation shape and materials, etc. should be determined according to the particularity.
Selection of annexes
Accessories of regulating valve mainly include: valve positioner, valve position switch, pneumatic location valve, pneumatic relay, solenoid valve, air filter pressure reducer, hand wheel mechanism, valve position transmitter, converter and so on. Among them, valve locators include electrical valve locators and pneumatic valve locators, which are mainly used to improve the working characteristics of regulating valves, realize correct positioning and improve the linearity of regulating valve position, reduce the delay of regulating signal transmission, change the flow characteristics of the regulating valve, change the regulating valve’s response range to signal pressure, and realize split control and correct positioning. It is one of the most important accessories of regulating valve, and its quality will directly affect the performance and quality of regulating valve and regulating system. The following focuses on several main factors to be considered when selecting valve positioner:
- 1) Whether the valve positioner can realize the “split” function, that is, the valve positioner only responds to a certain range of input signals. If the valve positioner can realize this function, it can control two or more regulating valves with one input signal according to actual needs;
- 2) whether the adjustment of zero point and range is easy, whether the calibration is independent and how about the stability;
- 3) What is the precision of the valve positioner. Under the ideal working condition, corresponding to an input signal, the inner parts of the regulating valve (including the valve core, valve stem, valve seat, etc.) should be accurately located at the required position each time, regardless of the direction of the stroke or the load of the inner parts of the regulating valve;
- 4) What is the action speed of valve positioner and what are the frequency characteristics. Because the valve positioner can continuously compare the input signal and valve position, and adjust its own output according to the deviation between them. If the valve positioner responds quickly to this kind of deviation, the medium’s flow in unit time will be large, and the regulating system’s response to the set point and load change will be faster, that is to say, the error of the system will be smaller, the better the control quality is. Generally speaking, the higher the frequency characteristic is, that is, the higher the sensitivity to frequency response is, and the better the control performance is. It should be noted that the evaluation of frequency characteristics should adopt the method of combining experiments and theories instead of relying on the theory of one thought, and in the evaluation, the experimental method must be stable and scientific, at the same time, the valve positioner and the actuator should be considered together;
- 5) After the combination of valve positioner and regulating valve, how its positioning resolution changes. The positioning resolution has a very obvious effect on the control quality of the regulating system, because the higher the resolution, the closer the positioning of the regulating valve is to the ideal value, the fluctuation changes caused by the over-regulation of the regulating valve can be effectively suppressed, thus finally achieving the purpose of limiting the cyclic changes of the regulated quantity.
- 6) Whether the maximum rated gas supply pressure of the valve locator matches the rated operating pressure of the actuator, whether the installation and connection are convenient, how much maintenance is and how much maintenance is, etc.
In addition to the valve positioner, other categories of accessories are relatively simple and do not need to be repeated here. All attachments play the role of supplementary function and ensuring the correct operation of the regulating valve. When selecting, the principle to be grasped is necessary increase and unnecessary abandoning, otherwise, it will only improve the operating cost of the control system and reduce reliability.
The correct selection of the regulating valve is the first step to apply the regulating valve, and it is also the most critical step. The quality of the selection directly affects the use effect of the regulating valve, thus affecting the regulation quality of the system. Of course, the selection of the model is more complicated, and it is also a knowledge that requires continuous exploration and summary in practical application. Therefore, it is necessary for us to master certain methods and skills in the selection of regulating valves and on the premise of being familiar with relevant professional knowledge. Only in this way, only in this way can it really play its role of “hands and feet” in the automatic control of industrial process.
Distinction between regulating valve and control valve
At present, there is basically no clear distinction between control valve and regulating valve in China. Control valve and regulating valve are often called together in general, and valves with actuator are more called regulating valve, however, the concept of control valve has not been deeply rooted in the hearts of the people. Look at those websites that do control valve. They all call control valve a regulating valve. I think there are some concepts here that need to be clarified. Superficial opinions are only valuable.
The definition of control valve should be: the most commonly used terminal control element in process control industry is control valve. The control valve regulates the flow of fluid, such as gas, steam, water or chemical mixture, to compensate for load disturbance and make the controlled process variables as close as possible to the required set point. The sensor feeds back the signals of pressure, temperature, flow and so on to the regulator, and the regulator compares the measurement signals with the set value, then outputs the control signal to the control valve according to the result of the calculation, the valve opening is adjusted by the control valve, thus realizing the automatic adjustment of temperature, pressure, flow and other parameters. According to this definition, the valve must be combined with sensors and controllers (PLC, DCS or pure regulators) to form a control loop (whether single or multi-loop) to control process variables, can be called a control valve. Therefore, all pneumatic, electric and hydraulic valves equipped with actuators should be called control valves, including solenoid valves, because they can form a control loop with the control system to realize automatic control. Whether they receive switching signal or proportional signal, the control principle is the same. All manual valves are not control valves. Some valve manufacturers also have the name of self-control valve, the definition of which should be the same as control valve.
The regulating valve is actually a throttle valve. For throttle valve, its intention is to produce a pressure drop and reduce the flow, so it can have a valve seat with a diameter significantly smaller than the valve diameter. The design of throttle valve is to generate pressure drop to reduce linear pressure, flow rate and temperature, while the structure of switch valve allows linear flow through fluid without obvious pressure drop. Such as O-type ball valve, butterfly valve, tube valve, etc., their flow resistance is very small, the pressure drop generated when the medium passes through is very small, so it is suitable for switch control, not suitable for throttling control. The throttle valve needs to design suitable flow characteristics, valve core and valve seat diameter according to the requirements of process technology, while the switch valve is not required. The throttle valve is equipped with an actuator, which is an electric, gas or hydraulic regulating valve. Therefore, in a narrow sense, the regulating valve is limited to a limited number of throttle valves such as single-seat valve, double-seat valve, sleeve valve, angle valve, three-way valve, v-type ball valve, etc. (so there are also manual regulating valves in the industry).
It can be said that all control valves are control valves, but control valves are not necessarily control valves. As for self-operated control valve, it should not be classified as control valve, because whether it is the self-operated pressure regulating valve, or the temperature and flow regulating valve, it depends on the power of the medium itself to drive the valve, control variables also have to be set manually, because they cannot form a control loop with the control system. They should be included in the class of automatic valve such as safety valve, drain valve, pressure reducing valve and hydraulic control valve.
In my opinion, if you want to simplify the classification of valves, it is better to divide them into three categories: switch valve, throttle valve and automatic valve.
Difference between regulating valve and reducing valve
The regulating valve can adjust the flow, adjust the pressure, adjust the temperature, adjust the liquid level, adjust the weight and other functions. It is a professional, comprehensive and multi-functional regulating valve. The decompression valve can only depress, and the pressure difference before and after the valve and the adjustable range are small, while the regulating valve can freely set the target value. The pressure reducing valve can be said to be a small classification of the regulating valve, but the regulating valve can not be said to be a kind of the reducing valve. The following is the introduction of the model knowledge, advantages and disadvantages of the regulating valve and reducing valve under the valve of Tai Chen. Which one is better? It depends on the use requirements of the site, and technical parameters can also be provided, so that our technicians can choose the model for you.
Advantages and disadvantages of regulating valve and reducing valve:
The advantage of the regulating valve is that it has many adjustable parameters, a large range and high precision. The adjustable current, voltage regulation, temperature regulation, voltage stabilization and pressure relief can be used, but it has complex structure and high cost, the disadvantages of requiring external force and signal source. The pressure relief valve has the advantages of simple structure and low cost, while its disadvantage is that it can only work in the situation where pressure fluctuation is not big and can only function of pressure relief. In some occasions where the pressure fluctuation is not big and the pressure is relatively stable, the reducing valve can be used to replace the regulating valve, such as N2 steam outside the battery limit. The pressure transmitter needs to be added to the adjustment valve, which has good adjustment performance, but considering saving investment, a pressure reducing valve can stabilize the pressure behind the valve.
Picture of regulating valve:
The difference between the function and use of regulating valve and pressure reducing valve:
The work of the regulating valve needs external gas/power supply and 4-20 mA signal output by the instrument to work, while the decompression valve depends on its own energy and the function of the spring, the set value must be set by tightening the screw on the valve on the spot, and the control valve can set the set value of the regulator on DCS at will, which can have a wider object to be adjusted, such as temperature, flow, pressure, liquid level, etc. Compared with the control valve, the pressure reducing valve has a much smaller setting range. The previous Taichen Valve introduced the whole series of regulating valves, so in practical application, the self-operated regulating valve and the reducing valve can be compared, both of which do not need external assistance, the following Tai Chen briefly introduces the selection knowledge of the two.
Knowledge of self-acting pressure regulating valve and pressure reducing valve selection?
The self-operated regulating valve works by relying on the pressure and temperature of the medium itself flowing through the valve, introducing the medium into the actuator through the signal pressure guide tube, and acting as the driving energy of the regulating valve, automatically adjusts the parameters of the medium with the acting force of the spring, do not need external power supply and secondary instrument control valve. Taichen self-regulating valves are mainly divided into self-operated temperature regulating valves, self-operated differential pressure regulating valves, self-operated pressure regulating valves, self-operated flow regulating valves, etc., which can work in various medium parameters.
Pressure reducing valve is depending on the energy of the medium itself, the inlet pressure is reduced to a required value of the outlet pressure, so that the outlet pressure automatically remains stable. In the view of fluid mechanics, the decompression valve is a throttle element whose local resistance can be changed, that is, it can change the flow velocity and kinetic energy of fluid by changing the throttle area, thus causing different pressure loss, so as to achieve the purpose of decompression. Then rely on the adjustment of the control and regulating system to balance the pressure fluctuation behind the valve with the spring force, so that the pressure behind the valve remains constant within a certain error range.
Picture of pressure reducing valve:
Which one is better, the pneumatic pressure regulating valve or the pressure reducing valve?
- 1. The purpose of the two is different. The self-operated regulating valve focuses on adjustment. The decompression valve is a simple decompression;
- 2. The pressure reducing valve can be adjusted subjectively. If the pressure fluctuation before the valve is large, the pressure behind the valve also changes, and the pressure difference needs to be adjusted manually. While self-operated control valve is based on a set when the fluctuation before the valve is relatively large, depending on its own pressure as power, it can achieve automatic back pressure stability or the pressure before the valve is stable;
- 3. The pressure reducing valve needs to manually adjust the pressure difference, if the pressure before the valve changes, the pressure behind the valve is also changed, can not be automatically adjusted to a fixed pressure. The self-operated regulating valve can automatically achieve stable back pressure or the pressure before the valve is stable;
- 4. The adjustment precision of the pressure reducing valve is higher, and the general error is 0.5%, while the adjustment precision of the self-operated regulating valve is 5%;
- 5. Self-operated regulating valve can control pressure, differential pressure, temperature, flow and so on, while decompression valve function only plays the role of decompression;
- 6. Self-operated regulating valve can stabilize the pressure before and after the regulating valve, while the pressure relief valve can only adjust the pressure behind the regulating valve to play the role of decompression.
Source: Network Arrangement – China Valves Manufacturer – Yaang Pipe Industry Co., Limited (www.ugsteelmill.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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