How to choose a control valve?
The control valve is the terminal executive component in the industrial process control system. In the industrial process continuous production automatic control system, the control valve is generally used to control various process parameters in the process production, so as to achieve the adjustment of the parameters such as the pressure, temperature, flow and liquid level of the fluid. It is usually called “hand and foot” in the industrial process automation production. Its application quality is directly reflected in the regulation quality of the system. As a terminal executive component in process control, people have a deeper understanding of its importance than in the past. In addition to the quality of products and the correct installation, use and maintenance of users, it is very important to correctly calculate and select the type of control valve. Due to the error of calculation and selection, the system operation is not stable, some even can not be put into use. Therefore, users and system designers should fully realize the importance of the control valve in the field, and pay enough attention to the selection of the control valve.
- Working principle of control valve
- Classification of control valves
- Structure and composition of control valve
- Selection of control valve type
- Characteristics and correct selection of standard control valve
- Selection of accessories of control valve
- How to choose a control valve
The control valve is used to regulate the flow, pressure and liquid level of the medium. According to the signal of the regulating part, the opening of the valve is automatically controlled, so as to achieve the regulation of medium flow, pressure and liquid level. The control valve is divided into electric control valve, pneumatic control valve and hydraulic control valve.
The control valve is composed of electric actuator or pneumatic actuator and control valve. The control valve is usually divided into two types: straight through single seat control valve and straight through double seat control valve. The latter has the characteristics of large flow capacity, small imbalance and stable operation, so it is usually especially suitable for occasions with large flow, high pressure drop and less leakage.
1. Pneumatic control valve
- ① Classification according to the form of pneumatic actuator
(a) Membrane actuators. It also includes direct mounting (positive and negative) and side mounting (positive and negative).
(b) Piston actuator, which is divided into proportional type (positive action and reaction and two position type).
(c) Long stroke actuator.
(d) Rolling film actuator.
- ② According to the regulation types: (a) regulation type; (b) cut-off type; (c) Regulation cut-off type.
- ③ According to the type of movement: (a) straight stroke; (b) angular stroke.
- ④ According to the shape of valve core: (a) flat valve core; (b) plunger valve core; (c) Window valve core; (d) sleeve valve core; (E) multistage valve core; (f) partial rotation valve core; (g) butterfly valve core; (H) spherical valve core.
- ⑤ Classification according to flow characteristics: (a) straight line; (b) equal percentage; (c) parabola; (d) quick opening.
- ⑥ According to the type of upper valve cover: (a) common type; (b) scattered (suction) hot type; (c) long neck type; (d) bellows seal type.
2. Electric control valve
- ① Classification according to the form of electric Title Mechanism: (a) angle stroke; (b) straight stroke; (c) multi rotation type.
- ② According to the type of accessories: (a) servo amplifier; (b) limit switch.
- ③ Classification according to flow characteristics: (a) straight line; (b) equal percentage; (c) parabola; (d) fast opening.
- ④ According to the type of upper cover: (a) common type; (b) scattered (suction) heat type; (c) long neck type; (d) Bellows seal type.
3. Manual control valve. According to the characteristics of valve core, it can be classified as cone, plunger, sleeve, multistage, partial rotation, butterfly, spherical or hemispherical.
4. (Electric) hydraulic control valve.
5. Intelligent control valve.
control valve is usually composed of electric actuator or pneumatic actuator and valve body. There are two types of straight stroke: straight single seat type and straight double seat type. The latter is characterized by large flow capacity, small unbalanced force and stable operation, so it is usually especially suitable for occasions with large flow, high pressure drop and large leakage. Angle stroke mainly includes: V-type electric regulating ball valve, electric butterfly valve, ventilation control valve, eccentric butterfly valve, etc.
The general principle of control valve selection is: on the premise of meeting the use function, the selected control valve shall have simple structure, reliable performance, low price, long service life, convenient maintenance, etc. The following focuses on the selection of control valve type and accessories.
There are many classification methods for control valves. At present, a classification method commonly used at home and abroad is based on structure, principle and function, which is divided into nine categories, namely, straight single seat control valve, straight double seat control valve, sleeve control valve, angle control valve, three-way control valve, diaphragm valve, butterfly valve, ball valve and eccentric rotary valve. These nine categories of products are the most basic and common products. It is also known as standard product. Other products based on the improvement of practical application are called special products.
Through single seat control valve
The straight through single seat control valve has only one valve core and one valve seat, which is easy to achieve strict sealing. The hard sealing of metal and metal, or the soft sealing of metal and polytetrafluoroethylene or other composite materials can be used. The standard leakage is 0.01% C (C is the rated flow coefficient). The allowable pressure difference is small, and the flow capacity is small. For example, the allowable pressure difference of DN100 single seat control valve is only 120kPa, and the flow capacity is only It is 100. The flow path is complex and the structure is simple, which is suitable for the occasions of clean medium with strict leakage requirements and small working pressure difference. However, the small size control valve (DN1 / 2, 3 / 4, 20) can also be used for the occasions with large pressure difference, which is one of the most widely used control valves. When further designed, it can be used as a shut-off valve. The flow characteristics are determined by the shape of the valve core. After being washed out, the original characteristics will be lost. Changing the valve core can change the flow characteristics. However, the thrust of the fluid medium to the valve core is large, that is, the unbalanced force is large, and the actuator with large thrust is required. Therefore, this kind of control valve should not be used in the application of high pressure difference and large caliber. When selecting this valve, special attention shall be paid to the check of differential pressure to prevent it from being jacked open.
Through double seat control valve
The straight through double seat control valve has two valve cores and two seats. Because the upward thrust of the upper valve core and the downward thrust of the lower valve core are basically balanced, the imbalance force of the whole valve core is small, and the allowable pressure difference is large. For example, the allowable pressure difference of the DN100 double seat control valve is 280kpa, and the flow capacity is large. Compared with other control valves of the same caliber, the double seat control valve can flow more fluid, with the same port. The flow capacity of double seat control valve is 20% – 50% larger than that of single seat control valve. For example, the flow capacity of DN100 double seat control valve is 160. Therefore, in order to obtain the same flow capacity, the double seat control valve can choose the actuator with smaller thrust. The double seat control valve adopts top and bottom double guide, so it is convenient to refit the body valve and the anti body valve, that is, the body valve can be changed into the anti body valve or the anti body valve can be changed into the body valve by reversing the installation of the valve core and the valve seat, without changing the positive or negative action type of the actuator. The upper and lower valve cores of the double seat control valve can not be closed at the same time, so the leakage is large, and the standard leakage is 0.1% C (C is the rated flow coefficient); the flow path is complex, so it is not suitable for the application of high pressure difference, because in this application, the valve is severely scoured by high pressure fluid, and it is easy to form flash and cavitation, which aggravates the scour on the valve body, so it is not applicable. In the control of fibrous media and high viscosity fluids.
Sleeve control valve
Sleeve control valve is also called cage valve. Its valve trim adopts valve core and cage (sleeve). The sleeve can be straight through single seat control valve, or double seat control valve or angle control valve. There are two structures: single seal and double seal. The former is equivalent to single seat control valve, which is suitable for single seat control valve; the latter is equivalent to double seat control valve, which is suitable for double seat control valve. In addition, it has the characteristics of good stability, convenient loading and unloading, convenient maintenance, noise reduction and cavitation reduction. However, the price is 50% – 200% higher than that of single seat and double seat control valves. It also needs special winding gasket, which is widely used, next to single seat and double seat control valves. However, this valve should not be used for unclean media and easy crystallization, stumbling and scaling media.
Angle control valve
Angle control valve is a single seat control valve with a special valve body structure, which is suitable for special piping and fluid occasions. It changes the straight valve body into an angle (equivalent to an elbow) valve body, and its throttling and stress forms are completely equivalent to the single seat control valve. The single seat control valve has the characteristics of small leakage and small differential pressure. In addition, because of its simple flow path with “self-cleaning” performance, it can be applied to unclean media, and can be further improved to anti blocking angle valve, which is suitable for the working conditions with suspended particles, especially in the situation where the installation space is limited.
Three way control valve
The three-way control valve is guided by the valve core itself. When replacing the air opening and air closing, the actuator must be replaced. It should be noted that the meaning of the air opening and air closing is different from other control valves. The air opening and air closing of the three-way control valve must be clear about which way to use, that is, the horizontal position or the vertical position. It has three channels, which can replace two through single seat control valves for the two groups of shunt and confluence and the occasions with temperature difference ≤ 150 ℃. When DN ≤ 80mm, confluence valve can be used for the occasions of shunt.
Diaphragm control valve
The diaphragm control valve is composed of a corrosion-resistant diaphragm and a valve body lined with a corrosion-resistant material. It has a simple flow path and is suitable for two-position cut-off occasions of unclean medium and weak corrosive medium. It is one of the earliest control valves. Due to its near quick opening flow characteristics, poor regulating quality and the influence of diaphragm and lining materials, it can not be used in high temperature and high pressure conditions. Generally, the working pressure is ≤ 1.6Mpa, and the working temperature is ≤ 150 ℃. In addition, the diaphragm is easy to be damaged and has short service life. Now, there are not many occasions for its use.
The butterfly valve is equivalent to a section of pipe to make the valve body, and the central valve plate is set for throttling. It is the most common rotary control valve used for control. It is suitable for low pressure, medium pressure or very few cases, but the pressure difference is limited. Its volume is small and its weight is light. It is 4-10 times lighter than the same caliber ball type control valve. Its caliber and price ratio is small. It is especially suitable for large caliber occasions. The larger the caliber of the control valve, the more remarkable this feature is. Generally, when DN > 300 mm, it is usually completed by butterfly valve.
Ball valve is a mature old product, which is divided into “O” and “V” 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” ball valve is a non resistance control valve. Compared with the specification, it has the largest rated flow coefficient, which is commonly used in the occasion of large flow and unclean media; “V” ball valve provides approximate logarithmic flow characteristics, and has large adjustable ratio. “V” ball core and valve seat produce shearing effect when rotating relative to each other, especially for the adjustment of unclean and fibrous media such as high viscosity, suspended flow, pulp, etc. Section and cut. Ball valves are more expensive.
Eccentric rotary valve
It is also called cam flexure valve. It combines the advantages of ball valve and butterfly valve. It has simple flow path, good self-cleaning performance and adjustment performance, and is suitable for crystallization, stutter and unclean medium occasions. The valve body is small in volume and light in weight, and can be flexibly assembled according to the site installation position without replacing any parts. The rated flow coefficient is large, which is 10% – 30% larger than the single seat and double seat control valves of the same caliber. The adjustable ratio is large, up to 100:1; the valve seat is sealed reliably. Due to the disturbing effect of the support arm of the valve core and the eccentric rotation movement of the spherical surface of the valve core, the required operating torque is reduced, and some asymmetry is compensated, and the valve can operate stably under the conditions of flow opening, flow closing and high pressure difference; when the proportion is adjusted, a positioner is required, which can easily obtain a straight line or equal by changing the position of the convex plate in the positioner. Percentage flow characteristics.
Selection of special control valve
For special application, on the basis of the above-mentioned control valve, if the upper valve cover is lengthened and heat sink is added, it can be used in low and high temperature occasions; the volume and weight of the whole control valve can be reduced by using the actuator with multiple springs; and a series of noise reduction measures can be used to reduce the noise to design a low-noise control valve. In addition, there are valve body separation control valve with valve body separation structure for maintenance and cleaning; fast cut-off control valve for fast interlocking action; small flow control valve designed for small flow control requirements; bellows seal control valve used to prevent leakage, etc.
These special types of control valves are designed to meet the requirements of special process production process or a specific use of special valves, which are non-standard. They have the characteristics of complex working conditions, high use requirements and small production batch. These control valves are usually evolved and improved from standard products according to the use requirements. Therefore, the basic type should be determined first according to the non particularity, and then the corresponding deformation type and material should be determined according to the particularity.
The accessories of the control valve mainly include: valve positioner, valve position switch, pneumatic holding valve, pneumatic relay, solenoid valve, air filter pressure reducer, hand wheel mechanism, valve position transmitter and converter, etc. Among them, the valve positioner includes electric valve positioner and pneumatic valve positioner, which are mainly used to improve the working characteristics of the control valve, realize correct positioning, improve the linearity of the position of the control valve, reduce the transmission lag of the regulating signal, change the flow characteristics of the control valve, change the response range of the control valve to the signal pressure, and realize the split control and correct positioning. It is one of the most important accessories of the control valve. Its quality will directly affect the performance and quality of the control valve and the 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 range” function, that is, the valve positioner only responds to a certain range of input signal. If the valve positioner can realize this function, two or more control valves can be controlled by one input signal according to the actual needs.
- 2) Whether it is easy to adjust the zero point and range, whether the calibration is independent, and how stable it is;
- 3) What is the accuracy of the valve positioner? Under the ideal working condition, the internal parts of the control valve (including the valve core, valve rod, valve seat, etc.) should be accurately positioned at the required position each time, regardless of the direction of travel or the load borne by the internal parts of the control valve.
- 4) What is the action speed and frequency characteristic of the valve positioner? Because the valve positioner can constantly compare the input signal and valve position, and adjust its own output according to the deviation between them. If the valve positioner responds to the deviation quickly, the flow of medium in unit time will be large, and the response of the regulating system to the change of set point and load will be faster, that is to say, the smaller the system error is, the better the control quality will be. Generally speaking, the higher the frequency characteristic, that is, the higher the sensitivity to the frequency response, the better the control performance. It should be noted that the evaluation of frequency characteristics should be based on the combination of experiment and theory, rather than on theory. In the evaluation, the experimental method must be stable and scientific, and the valve positioner and actuator should be considered together.
- 5) After the valve positioner is combined with the control valve, how does the positioning resolution change? The positioning resolution has a very obvious effect on the control quality of the regulating system, because the higher the resolution is, the closer the positioning of the control valve is to the ideal value, and the fluctuation caused by the over adjustment of the control valve can be effectively suppressed, so as to ultimately limit the periodic change of the regulated quantity.
- 6) Whether the maximum rated air supply pressure of the valve positioner matches the rated operating pressure of the actuator, whether the installation and connection are convenient, and how much maintenance and degree are, etc.
In addition to the valve positioner, other types of accessories are relatively simple and do not need to be repeated here. All accessories play the role of supplementary functions and ensuring the correct operation of the control valve. The principle to be grasped in the selection is necessary increase and unnecessary abandonment, otherwise it will only improve the operation cost of the control system and reduce the reliability.
The correct selection of the control valve is the first and the most critical step in the application of the control valve. The selection directly affects the use effect of the control valve, and then affects the regulating quality of the system. Of course, the selection work is more complex, but also a knowledge, which needs to be explored and summarized in practical application. Therefore, it is necessary for us to master certain methods and skills in the selection of control valve, on the premise of being familiar with relevant professional knowledge. Only in this way can we truly play the role of “hands and feet” in industrial process automation control.
Source: China Control Valve Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.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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