What is a control valve

What is a control valve?

Control valve, also known as regulating valve, is the final control element in the field of industrial automation process control, which changes the medium flow, pressure, temperature, liquid level and other process parameters by receiving the control signal from the control unit and power operation. It is generally composed of actuator and valve. According to the stroke characteristics, the regulating valve can be divided into straight stroke and angular stroke; According to the power of the actuator, it can be divided into pneumatic control valve, electric control valve and hydraulic control valve; According to its function and characteristics, it can be divided into linear characteristics, equal percentage characteristics and parabolic characteristics. The control valve is suitable for air, water, steam, various corrosive media, mud, oil and other media.  Control valve commonly used classification: pneumatic control valve, electric control valve, hydraulic control valve, self regulating valve.

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Control valves play an important role in automatic control of modern chemical plants. The production of these plants depends on the correct distribution and control of the flowing medium. These controls, whether energy exchange, pressure reduction or simple container charging, need some final control elements to complete.
The regulating valve plays a role of variable resistance in the pipeline. It changes the turbulence of the process fluid or provides a pressure drop in the case of laminar flow. The pressure drop is caused by changing the valve resistance or “friction”. This process of pressure reduction is often called throttling. For gas, it is close to isothermal adiabatic state, and the deviation depends on the non ideal degree of gas (Joule Thomson effect). In the case of liquid, the pressure is consumed by turbulence or viscous friction, both of which convert the pressure into heat energy, resulting in a slight increase in temperature.
The common control loop includes three main parts. The first part is the sensitive element, which is usually a transmitter. It is a device that can be used to measure the adjusted process parameters, such as pressure, liquid level or temperature. The output of the transmitter is sent to the regulating instrument regulator, which determines and measures the deviation between the given value or expected value and the actual value of the process parameters, and sends the correction signal one by one to the final control element regulating valve. The valve changes the flow rate of fluid and makes the process parameters reach the expected value.
Control valve belongs to the control valve series, the main role is to adjust the medium pressure, flow, temperature and other parameters, is the final control element in the process loop.

Structure composition of control valve

Control valve is usually composed of electric actuator or pneumatic actuator and valve body. There are two types of straight stroke: straight through single seat and straight through double seat. The latter has the characteristics of large flow capacity, small unbalanced force and stable operation, so it is especially suitable for large flow, high pressure drop and large leakage. Angular stroke mainly includes: V-type electric control ball valve, electric butterfly valve, ventilation control valve, etc.

Working principle of control valve

The regulating valve is used to regulate the flow, pressure and liquid level of 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 consists of electric actuator or pneumatic actuator and control valve. Control valve is usually divided into 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 large flow, high pressure drop and less leakage.

Classification of control valve

Control valve is generally composed of actuator and valve. According to the power of the actuator, the control valve can be divided into three types: pneumatic control valve, electric control valve and hydraulic control valve. In addition, according to its function and characteristics, it can be divided into three types: linear characteristic, equal percentage characteristic and parabolic characteristic.
According to the stroke characteristics, the regulating valve can be divided into straight stroke and angular stroke. Straight stroke includes: single seat valve, double seat valve, sleeve valve, cage valve, angle valve, three-way valve and diaphragm valve; Angular stroke includes: butterfly valve, ball valve, eccentric rotary valve, full function ultra light control valve.

  • The control valve can be divided into manual control valve, pneumatic control valve, electric control valve and hydraulic control valve according to the driving mode, that is, the pneumatic control valve with compressed air as the power source, the electric control valve with electricity as the power source, and the hydraulic control valve with liquid medium (such as oil) pressure as the power source;
  • According to the regulation form, it can be divided into regulation type, cut-off type and regulation cut-off type;
  • According to the flow characteristics can be divided into: linear, logarithmic (percentage), parabolic, fast open.

Types of control valves

According to the use and function, main parameters, pressure, medium working temperature, special purpose (i.e. special, special valve), driving energy, structure and other ways, the most commonly used classification method is to divide the control valve into nine categories according to the structure, 6 kinds of straight stroke, 3 kinds of angular stroke.
Function by purpose

  • a. Two position valve: mainly used to close or connect the medium;
  • b. Regulating valve: mainly used for regulating system. When selecting the valve, it is necessary to determine the flow characteristics of the control valve;
  • c. Diverter valve: used to distribute or mix media;
  • d. Shut off valve: usually refers to the leakage rate is less than one hundred thousandth of the valve.

By main parameters
1. Classification by pressure

  • (1) Vacuum valve: the working pressure is lower than the standard atmospheric pressure;
  • (2) Low pressure valve: nominal pressure PN ≤ 1.6Mpa;
  • (3) Medium pressure valve: pn2.5 ~ 6.4Mpa;
  • (4) High pressure valve: pn10.0 ~ 80.0mpa, usually pn22, pn32;
  • (5) Ultra high pressure valve: PN ≥ 100MPa.

2. Classification according to working temperature of medium

  • (1) High temperature valve: T > 450 ℃;
  • (2) Medium temperature valve: 220 ℃≤ t ≤ 450 ℃;
  • (3) Normal temperature valve: – 40 ℃ ≤ t ≤ 220 ℃;
  • (4) Low temperature valve: – 200 ℃ ≤ t ≤ – 40 ℃.

Common classification
This classification method is divided according to principle, function and structure. It is the most commonly used classification method at home and abroad. It is generally divided into nine categories

Straight stroke pneumatic control valve

  • (1) Single seat control valve;
  • (2) Double seat control valve;
  • (3) Sleeve control valve;
  • (4) Angle control valve;
  • (5) Three way control valve;
  • (6) Diaphragm valve;
  • (7) Butterfly valve;
  • (8) Ball valve;
  • (9) Eccentric rotary valve. The first six types are straight stroke and the last three are angular stroke.

These nine products are also the most basic products, also known as ordinary products, basic products or standard products. All kinds of special products and special products are based on these nine types of products.
According to the purpose

  • (1) Soft seal shut-off valve;
  • (2) Hard seal shut-off valve;
  • (3) Wear resistant control valve;
  • (4) Corrosion resistant control valve;
  • (5) PTFE corrosion resistant control valve
  • (6) All corrosion resistant alloy control valve;
  • (7) Emergency shut-off or vent valve;
  • (8) Anti blocking control valve;
  • (9) Corrosion resistant and anti blocking shut-off valve;
  • (10) Heat preservation jacket valve;
  • (11) Large pressure drop shut-off valve;
  • (12) Small flow control valve;
  • (13) Large diameter control valve;
  • (14) Large adjustable ratio control valve;
  • (15) Low s energy saving control valve;
  • (16) Low noise valve;
  • (17) Fine and small control valve;
  • (18) Lining (rubber, PTFE, ceramic) control valve;
  • (19) Special ball valve for water treatment;
  • (20) Special valve for caustic soda;
  • (21) Special valve for ammonium phosphate;
  • (22) Chlorine control valve;
  • (23) Bellows seal valve.

By driving energy

  • (1) Pneumatic control valve;
  • (2) Electric control valve;
  • (3) Hydraulic control valve.

According to the structure
(1) Pneumatic control valve
① Classification according to the form of pneumatic actuator

  • (a) Membrane actuator. It is also divided into direct installation (positive action and reaction) and side installation (positive action and reaction).

  • (b) Piston actuator, also divided into proportional (positive action and reaction) and two position.

  • (c) Long stroke actuator.

  • (d) Rolling film actuator.

② According to the types of regulation:

  • (a) Regulation type;
  • (b) Cut off type;
  • (c) Adjustment cut-off type.

③ According to the type of movement:

  • (a) Straight stroke;
  • (b) Angular stroke.

④ According to the shape of spool:

  • (a) Flat spool;
  • (b) Plunger type valve core;
  • (c) Window valve core;
  • (d) Sleeve valve core;
  • (e) Multi stage valve core;
  • (f) Eccentric rotary valve core;
  • (g) Butterfly valve core;
  • (h) Ball valve core.

⑤ According to the flow characteristics:

  • (a) Straight line;
  • (b) Equal percentage;
  • (c) Parabola;
  • (d) Come on.

⑥ According to the type of upper valve cover:

  • (a) Ordinary type;
  • (b) Diffuse (endothermic) type;
  • (c) Long neck type;
  • (d) Bellows seal type.

(2) Electric control valve

① According to the types of electric mechanism:

  • (a) Angular stroke;
  • (b) Straight stroke;
  • (c) Multi rotary.

② According to the types of accessories:

  • (a) Servo amplifier;
  • (b) Limit switch.

③ According to the flow characteristics:

  • (a) Straight line;
  • (b) Equal percentage;
  • (c) Parabola;
  • (d) Fast ope.

④ According to the type of upper cover:

  • (a) Common type;
  • (b) Diffuse (endothermic) type;
  • (c) Long neck type;
  • (d) Bellows seal type.

(3) Manual control valve. According to the characteristics of valve core:

  • Conical; 
  • Plunger shape; 
  • Sleeve shape; 
  • Multistage shape; 
  • It is eccentric; 
  • Butterfly shaped; 
  • Spherical or hemispherical.

(4) (electric) hydraulic control valve.
(5) Intelligent regulating valve.

Single seat control valve

What is a single seat control valve?

There is a valve core and a valve seat in the single seat control valve body, which has the characteristics of small leakage. The valve has large unbalanced force, and its allowable pressure difference is smaller than that of double seat valve. When the pressure difference is high and the diameter is large, it is better to match the valve positioner. The valve with nominal diameter greater than or equal to 25 mm is of double guiding structure. The valve can be opened or closed by air as long as the connection position between the valve stem and the valve core is changed.

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Structural parameters of single seat control valve
There is only one spool and seat in the valve body of straight through single seat control valve. When DN ≥ 25, the spool is double oriented (now the fine and small single seat control valve has been changed to single oriented); When DN ≤ 25, the valve core is single guide. The valve was produced in 1940s abroad and designed jointly in 1960s. Its characteristics are as follows:

  • ① Because there is only one valve core, it is easy to ensure sealing and small leakage, but it can not be completely cut off. The standard leakage is 0.01% kV, so it is suitable for small leakage. After further design, it can be used as a shut-off valve.
  • ② Just because there is only one spool, the pressure difference has a large unbalanced thrust on the spool. Set the pressure in front of the valve as P1, the pressure behind the valve as P2, the stem diameter as DS and the seat diameter as DG.

Structure of single seat control valve
The straight through single seat regulating valve is composed of valve body, valve seat, valve core, guide sleeve, valve cover, valve stem, packing and other parts. The valve core and the valve stem are connected together. The connection method can be fixed by interference fit pin or screw connection pin, or the valve stem and valve core can be integrated. A guide sleeve is arranged between the valve cover and the valve body to guide the upward and downward movement of the valve core. The small hole on the guide sleeve connects the inner cavity of the valve body and the outlet end of the valve. The medium in the upper cavity of the guide sleeve is easy to flow into the outlet end of the valve through the small hole, which will not affect the movement of the valve core. There is only one spool and one seat in the valve body of straight through single seat regulating valve. It is characterized by small leakage and easy to ensure sealing. There are adjustment type and cut-off type in structure. They differ in the shape of the spool. The shape of regulating valve core is plunger; The cut-off valve core is flat or conical in shape. Another characteristic of the cut-off valve core is that the medium has a large force on the valve core, that is, the unbalanced force is large, especially in the case of high pressure difference and large diameter, so it is only suitable for the situation of low pressure difference. Otherwise, the actuator with high thrust or valve positioner should be selected properly.
There are two types of straight through single seat control valve spool: positive installation and reverse installation. When the valve core moves downward, the flow area between the valve core and the valve seat decreases, which is called forward installation; Otherwise, it is called reverse installation. The nominal size DN and seat diameter DN of the control valve indicate the size of the valve. For the single guide valve core with nominal size DN < 2.5mm, it can only be installed forward, not backward. Therefore, the reaction actuator must be used in the pneumatic opening. With the increase of the signal pressure, the flow area of the pneumatic control valve increases; On the contrary, the flow area decreases with the increase of signal pressure.
Application scope of single seat control valve
The single seat control valve is only suitable for the clean medium occasions with strict leakage requirements and small pressure difference. On the contrary, it can not be used when the medium is not clean and the pressure difference is large. But fine small single seat control valve can be used in these occasions.
Application of single seat control valve
The single seat control valve is an industrial field executive instrument composed of the control valve body and different electric actuators. It receives 4-20mA or pulse signal from the regulator to carry out closed-loop control and realize automatic control of flow, pressure, temperature, liquid level and other parameters. It can also select intelligent electric actuator to realize digital transmission with DCS, PLC, etc, To form a more intelligent control system. The power supply voltage of the electric control valve is 220 V and 380 v. the flow characteristics of the electric control valve are equal percentage, linear and fast. Compared with pneumatic control valve, electric control valve has the advantages of energy saving (only when working), environmental protection (no carbon emission), quick and convenient installation (no complicated pneumatic pipeline and air pump workstation), etc. it is widely used in the fields of electric power, chemical industry, metallurgy, environmental protection, water treatment, light industry, building materials and other industrial automation systems.
Single seat control valve is divided into single seat diaphragm control valve and single seat piston control valve. It uses compressed air as the power source, diaphragm or piston cylinder as the actuator, and uses electrical valve positioner, converter, solenoid valve, retaining valve and other accessories to drive the valve to realize the on-off or proportional regulation, Receive the control signal from the industrial automation control system to adjust the flow, pressure, temperature and other process parameters of the pipeline medium. Pneumatic control valve is characterized by simple control, fast response, and intrinsically safe, no need to take additional explosion-proof measures. The cost of pneumatic control valve is also lower than that of electric control valve. In chemical industry, petrochemical industry and other fields, pneumatic control valve has incomparable advantages over electric control valve.
The structure of single seat control valve is mainly divided into four types: single seat, sleeve type, double seat and cage type.
The main types of single seat control valve are: tee type, angle type, high temperature type, low temperature type, bellows type, etc.
Characteristics of single seat control valve
The main features and applications of single seat control valve are as follows:

  • The single seat control valve is mainly used in the working place where the allowable leakage is small and the pressure difference is small.
  • Single seat control valve adopts balanced valve core, with large unbalanced force, large allowable pressure difference and stable operation.
  • The single seat regulating valve is especially suitable for the working occasions where the allowable pressure difference is small and the pressure difference before and after the valve is small.
  • Single seat control valve is especially used in the occasions with large pressure difference and less strict requirements on leakage and clean medium.
  • Single seat control valve: combined type is mainly used to mix two kinds of fluid into a third kind of fluid; The split flow type is mainly used to divide a fluid into two streams.
  • Single seat control valve is especially suitable for high viscosity, containing suspended solids and granular medium fluid regulation, can avoid coking, sticking, blocking.
  • Single seat control valve is suitable for large flow, high temperature, leakage requirements are not strict occasions.
  • The single seat control valve is suitable for working below 0 ~ – 250 ℃.
  • The single seat tubular control valve is suitable for the regulation of extremely high temperature, inflammable, explosive, volatile and rare and precious metal media.

Double seat control valve

What is a double seat control valve?

The double seat control valve is the executive unit of the instrument in the automatic control system. It is powered by AC220V power supply voltage and receives (4-20mA, 0-10mA or 1-5vdc) current signal or voltage signal from DCS, PLC system, regulating instrument, operator, etc. it can control the operation. It adopts the mechanical and electrical integration structure and has the functions of internal servo operation, position feedback of opening signal, position indication Manual operation and other functions, strong function, reliable performance, simple connection, high adjustment accuracy, change the valve opening displacement with straight stroke output thrust, achieve accurate adjustment and control of fluid medium process parameters.

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Structure of double seat control valve
The double seat control valve adopts double guide structure and is equipped with multi spring actuator. The utility model has the advantages of compact structure, light weight, sensitive action, large valve capacity, accurate flow characteristics, convenient disassembly, etc. It is widely used to accurately control the process parameters of gas, liquid, steam and other media, such as pressure, flow, temperature, liquid level to keep at a given value. It is especially suitable for the occasions where the pressure difference is large, the leakage is large and the medium is not very clean.
Double seat control valve is used to regulate the flow, pressure and liquid level of 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 consists of electric actuator or pneumatic actuator and control valve. The latter has the characteristics of large flow capacity, small imbalance and stable operation, so it is especially suitable for large flow, high pressure drop and less leakage. Valve is the control component of fluid transportation system, which has the functions of cut-off, regulation, diversion, prevention of countercurrent, pressure stabilization, shunt or overflow relief. The valves used in fluid control system, from the simplest stop valve to various valves used in extremely complex automatic control system, have many varieties and specifications. The valve can be used to control the flow of air, water, steam, various corrosive media, mud, oil, liquid metal and radioactive media.
There are standard type, bellows sealing type, jacket insulation type and other varieties. The nominal pressure rating of the product is dn25-400. The applicable fluid temperature ranges from – 250 ℃ to + 560 ℃. The leakage standard is grade III and grade IV. The flow characteristics are linear or percentage. A variety of varieties and specifications are available.
Characteristics of double seat control valve
Single seat control valve: it has the advantages of easy sealing and less leakage, but the imbalance force is large, so the working pressure difference should not be too high.
Double seat control valve: with the advantages of large rated flow coefficient and small unbalanced force, it is widely used.
Mode of double seat control valve
The electronic electric single seat and double seat regulating valve is composed of straight stroke electronic electric actuator and straight through double guide single seat and double seat valve. It has the advantages of compact structure, sensitive action, small pressure drop loss, large valve capacity and accurate flow characteristics. It can directly accept the control signals (4-20mA DC, 0-10mA DC or 1-5V DC) and single-phase power supply from the regulating instrument to control the operation. It can realize the automatic regulation and control of the fluid medium in the process pipeline. It is widely used to accurately control the process parameters of gas, liquid, steam and other media, such as pressure, pressure, etc Flow, temperature, liquid level and other parameters remain at the given value.
The double seat control valve is the executive unit of the instrument in the automatic control system. It is powered by AC220V power supply voltage and receives (4-20mA, 0-10mA or 1-5vdc) current signal or voltage signal from DCS, PLC system, regulating instrument, operator, etc. it can control the operation. It adopts the mechanical and electrical integration structure and has the functions of internal servo operation, position feedback of opening signal, position indication Manual operation and other functions, strong function, reliable performance, simple connection, high adjustment accuracy, change the valve opening displacement with straight stroke output thrust, achieve accurate adjustment and control of fluid medium process parameters.
2. The double seat control valve can be divided according to the action mode;

  • Positive action: electrically closed – normally open type (when the electrical signal increases, the valve position moves downward), type B.
  • Reaction: electric open normally closed type (when the electric signal increases, the valve position moves upward), type K.

3. Zdln electronic electric double seat control valve is a straight through double seat casting spherical valve with double seat plunger type valve core, which is suitable for the occasions with large pressure difference, large flow and not very clean medium and lax leakage requirements.
4. Zdln electric double seat control valve is a straight through low flow resistance double seat upper and lower double guide structure. The valve core is stable and reliable in the process of displacement. There are two valve core seats in the double seat valve body. The fluid medium flows out through the upper and lower valve core seats. It has large flow, large allowable pressure difference and small unbalanced force. It is a kind of balanced double seat valve.
5. By changing the shape of valve core; Different valve core shape will get different flow characteristics: equal percentage (logarithm) and linear characteristics.
This series of products are widely used in chemical industry, petroleum, metallurgy, power station, textile, papermaking, pharmaceutical and other industrial production process automation and remote control.
There are standard type, bellows sealing type, jacket insulation type and other varieties. The pressure grade of the product is pn1.64.06.4mpa; Nominal diameter DN20 ~ 300mm; The suitable fluid temperature is – 100 ℃ to + 500 ℃; It can be divided into normal temperature type, high temperature type and low temperature type.

Type of valve body

There are many kinds of control valve body, commonly used valve body types are straight single seat, straight double seat, angle, diaphragm, small flow, tee, eccentric rotation, butterfly, sleeve, spherical, etc.
In the specific selection, the following considerations can be considered:
(1) Valve core shape and structure
It is mainly considered according to the selected flow characteristics and unbalanced force.
(2) Wear resistance
When the fluid medium is a suspension containing high concentration of abrasive particles, the internal material of the valve should be hard.
(3) Corrosion resistance
As the medium is corrosive, the valve with simple structure shall be selected as far as possible.
(4) Temperature and pressure of medium
When the temperature and pressure of the medium are high and vary greatly, the valve core and seat materials should be selected. When the temperature and pressure vary little, the radiator should be added.
(5) Prevent flash and cavitation
Flash and cavitation only occur in liquid medium. In the actual production process, flashing and cavitation will form vibration and noise, shorten the service life of the valve. Therefore, flashing and cavitation should be prevented when selecting the valve.

Control valve actuator

In order to make the control valve work normally, the actuator should be able to produce enough output force to ensure the high sealing and valve opening.
For double acting pneumatic, hydraulic and electric actuators, there is generally no return spring. The magnitude of the force has nothing to do with its running direction. Therefore, the key to select the actuator is to find out the maximum output force and the rotation torque of the motor. For the single acting pneumatic actuator, the output force is related to the opening of the valve, and the force on the control valve will also affect the motion characteristics, so it is required to establish the force balance in the whole opening range of the control valve.
After the output force of the actuator is determined, the corresponding actuator is selected according to the requirements of the process environment. When there are explosion-proof requirements on site, pneumatic actuator should be selected. From the aspect of energy saving, electric actuator should be selected as far as possible. If the adjustment accuracy is high, the hydraulic actuator can be selected. Such as the speed regulation of transparent machine in power plant, the temperature regulation and control of catalytic reactor in refinery, etc.
The action mode of the control valve is only when the pneumatic actuator is selected, and its action mode is formed by the combination of the positive and negative action of the actuator and the positive and negative action of the valve. There are four types of combination, namely positive (air closing type), positive and negative (air opening type), negative (air opening type) and reverse (air closing type). The action modes of the control valve formed by these four combinations are air opening and air closing.
For the selection of the action mode of the control valve, it is mainly considered from three aspects

  • a) Process safety;
  • b) The characteristics of the medium;
  • c) Ensure product quality and minimize economic loss.

Development history of control valve

The development of control valves has a history of 80 years since the beginning of the 20th century. There are ten categories of control valve products, self operated valves and positioners:

  • 1920s: the original control valve for stabilizing pressure came out.
  • 1930s: represented by double seat valve and single seat valve with “V” notch, V-type regulating ball valve came out.
  • 1940s: positioner appeared, new varieties of control valve were further produced, such as diaphragm valve, angle valve, butterfly valve, ball valve, etc.
  • 1950s: ball valve was widely used, three-way valve instead of two single seat valves was put into the system.
  • 1960s: after the series improvement design, standardization and normalization of the above products in China, there was a complete series of products in China. We are still using a large number of single seat valve, double seat valve, angle valve, three-way valve, diaphragm valve, butterfly valve, ball valve seven products are still the level of products in the 1960s. At this time, foreign countries began to introduce the eighth structure control valve – sleeve valve.
  • 1970s: another new structure product – eccentric rotary valve came out (the ninth largest type of control valve). In this period, sleeve valve was widely used abroad. At the end of 1970s, China jointly designed sleeve valve, which made China have its own series of sleeve valve products.
  • 1980s: during the period of reform and opening up, China successfully introduced petrochemical equipment and control valve technology, which promoted the use of sleeve valve and eccentric rotary valve, especially sleeve valve, which has the potential to replace single seat valve and double seat valve, and its use is more and more widely. At the end of the 1980s, another major development of the control valve was the CV3000 and the fine and small control valve in Japan. In terms of structure, they changed the single spring pneumatic diaphragm actuator to the multi spring diaphragm actuator. The structure of the valve was only improved, not changed. Its outstanding feature is that the weight and height of the control valve are reduced by 30%, and the flow coefficient is increased by 30%.
  • 90’s: 90’s control valve focus on reliability, special difficult product research, improvement and improvement. By the end of 1990s, Hualin company launched the tenth structure product – full function ultra light valve. Its outstanding feature is a breakthrough in reliability, function and weight. Breakthrough in function – the only product with full function, therefore, many products with incomplete functions can be replaced by one product to simplify the selection, use and variety; Breakthrough in weight: 70-80% lighter than single seat valve, double seat valve and sleeve valve, and 40-50% lighter than fine small valve; The breakthrough of reliability solves all kinds of unreliable factors of traditional control valve, such as the reliability of sealing, positioning, action, etc. With the advent of this product, China’s control valve technology and application level have reached the advanced level at the end of 1990s; It is a great breakthrough to the control valve; In particular, the electronic full-function ultra light valve will become the mainstream of the control valve in the next century.

Maintenance of control valve

After the regulating valve is in normal operation, maintenance and maintenance shall be carried out. As part of the automatic control system, the maintenance of the regulating valve shall be carried out simultaneously with the automatic instruments and other equipment.
The maintenance of regulating valve is similar to that of general instrument, which can be divided into passive maintenance, preventive maintenance and predictive maintenance. Passive maintenance is a maintenance method when the equipment such as regulating valve fails. Because the equipment is not maintained until it fails, it often causes the production process to stop, even the equipment is damaged or casualties occur in serious cases. Passive maintenance is not expected in the production process. Preventive maintenance is a maintenance method based on the past operation experience and time. For example, the common regular maintenance is preventive maintenance. It establishes corresponding maintenance schedule according to the operation of different equipment, and maintains it when the equipment has not failed. Because the failure does not occur, maintenance is carried out, so the probability of failure occurrence can be greatly reduced. But this maintenance method does not analyze the actual situation of the valve currently used, and it often disassembles and checks the regulating valve which can also use a certain time, which wastes time and resources. Predictive maintenance starts from the analysis of the current used regulating valve data, foresees the status of the regulating valve, so that the regulating valve can be used to the maximum extent.

Daily maintenance work content of regulating valve

The daily maintenance work of regulating valve is divided into two parts: patrol inspection and regular maintenance. The patrol inspection work is as follows.

  • 1. Know the operation of the regulating valve to the process operators on duty.
  • 2. Check the supply energy (air source, hydraulic oil or power supply) of the regulating valve and related accessories
  • 3. Check the operation of the hydraulic oil system.
  • 4. Check whether there is leakage at the static and dynamic sealing points of the regulating valve.
  • 5. Check the connecting lines and joints of the regulating valve for looseness or corrosion.
  • 6. Check the regulating valve for abnormal sound and large vibration, and check the supply condition.
  • 7. Check whether the action of the regulating valve is flexible and whether it changes in time when the control signal changes
  • 8. Listen for abnormal vibration or noise of valve core and valve seat.
  • 9. Contact and handle problems in time.
  • 10. Record the patrol inspection and file it.

The regular maintenance work is as follows:

  • 1. Clean the outside of the regulating valve regularly.
  • 2. Adjust the packing box and other sealing parts of the regulating valve regularly. If necessary, replace the sealing parts to keep the sealing of the static and dynamic sealing points.
  • 3. Add lubricating oil to parts to be lubricated regularly.
  • 4. Regularly discharge and clean the air source or hydraulic filtration system.
  • 5. Check the connection and corrosion of each connection point regularly, and replace the connection if necessary.

Regular calibration of regulating valve

Units that have not yet been carried out in predictive maintenance of regulating valves shall conduct regular verification of regulating valves. Regular verification is preventive maintenance.
According to different process production process, the regular calibration of regulating valve should have different calibration periods. The periodic calibration period of each regulating valve can be determined according to the data provided by the manufacturer. It can be carried out at the same time as the overhaul is carried out during the process production. When some regulating valves are used in high pressure, high pressure drop or corrosive occasions, the inspection period should be shortened.
The main content of the inspection is the static performance test of regulating valve, and the corresponding test items can be added if necessary, such as the flow characteristics test of regulating valve. Regular verification requires testing equipment and instruments, and replacement parts. Therefore, manufacturers can usually be entrusted to complete the verification.

Maintenance of regulating valve

The maintenance of regulating valve includes emergency maintenance, regular maintenance and predictive maintenance. Emergency maintenance is the maintenance of regulating valve which fails to meet the requirements of process operation. Regular maintenance usually includes routine maintenance and maintenance at the same time as process shutdown overhaul. Predictive maintenance is based on the analysis results of predictive maintenance, and it is aimed at the maintenance of the relevant regulating valve components. Emergency maintenance is the maintenance after the failure of the regulating valve. Regular maintenance and predictive maintenance are the maintenance before the failure of the regulating valve. In general, the daily maintenance of the regulating valve is carried out by the instrument maintenance personnel, and the regular maintenance at the same time as the overhaul is carried out by the manufacturing technicians.
1) The daily inspection and maintenance of regulating valve includes the following contents:

  • 1. Stress relief. Various stresses due to improper installation or combination. For example, the high temperature medium produces thermal stress, tight installation, and stress caused by unbalanced solid force. The unbalanced stress acts on the regulating valve, which makes the valve stem and guide parts deformed, and it can not leak correctly with the valve seat, and the variation is increased. Therefore, the stress relief maintenance should be carried out in daily maintenance.
  • 2. Remove rust and dirt. Check the connecting pipe of regulating valve frequently for rust, welding slag, dirt, etc. and remove them in time after finding out. Because these dirt will cause the valve core and seat wear, affect the normal operation of the regulating valve. Generally, filter devices such as filter screen can be installed in front of regulating valve and cleaned regularly.
  • 3. Check the support of the regulating valve. The support of the regulating valve makes the parts of the regulating valve in a position free from the influence of gravity. If the support is not correct, the valve stem and the valve seat cannot be centered, which will increase the difference and reduce the sealing performance. Therefore, check whether the support of the regulating valve is suitable.
  • 4. Remove the dirt from the air source, hydraulic oil and other energy supply. The air source and hydraulic source are the energy sources of regulating valve operation. Impurities in compressed air and hydraulic oil can block orifice and pipe, causing failure. Therefore, it is very important to check the air source and hydraulic oil regularly and discharge the filter regularly.
  • 5. Inspection of gear drive. Gear transmission of the hand wheel mechanism, electric actuator and hydraulic actuator shall be checked regularly to add lubricant to prevent the occurrence of undercut. Check whether the brake and limit devices are flexible and easy to use.
  • 6. Packing box inspection. The wear and compression force of the packing shall be checked, and the packing box shall be replaced regularly to ensure that the packing can be sealed and the shadow direction of friction can be reduced. No lubricating oil shall be added to the packing box without oil lubrication.
  • 7. Inspection of safe operation. For the regulating valve and related accessories used in explosive dangerous places, check the safety operation, such as whether the sealing cover is tight, the operation of safety barrier, power supply, etc., so as to ensure the safe operation of the regulating valve and related accessories.
  • 8. Transportation and custody. During transportation and storage, the regulating valve shall be fixed with special support to prevent looseness; The relevant accessories installed on the regulating valve, such as valve positioner and handwheel mechanism, shall be firm, and the feedback rod connected with the regulating valve shall be prevented from external force damage; Plastic film sealing shall be applied to each connection interface to prevent the invasion of external objects; The connection port of the regulating valve can be sealed by matching flange and blind flange, or sealed with adhesive paper to prevent the invasion of external objects. When transporting, a firm wooden box shall be installed, and the impact of wind sand, rain water and dust prevention and other adverse transportation environment conditions shall be adopted. The environmental conditions for transportation and storage shall meet the requirements of product specification.

2) The main contents of daily maintenance of regulating valve and accessories are as follows:

  • 1. Replacement of diaphragm of pneumatic actuator. The diaphragm of pneumatic film actuator is retracted during operation, so it is easy to fatigue damage. Rubber diaphragm of the same specification shall be used for replacement. When tightening, the diaphragm shall be stressed evenly to prevent leakage and damage of the diaphragm.
  • 2. Grind. The valve core and the valve seat will leak after running for a certain time, and the internal leakage between the piston and the cylinder block will also occur. At this time, grinding shall be carried out. It can be used for manual grinding, mechanical grinding, coating treatment and inlay. The abrasive size of diamond used for grinding shall be appropriate, and the grinding force shall be uniform and appropriate. After grinding, it shall be polished, and meet the requirements of required smoothness and precision, meet the requirements of valve core and valve seat alignment, etc. and the sealing test shall be conducted after the final assembly.
  • 3. Packing box replacement. The same type of packing box shall be used when replacing the packing box. When replacing, the packing should be carefully checked out, and the packing should be removed correctly to prevent damage to the valve stem. The installation of new packing box shall be in accordance with the requirements of the instruction, and the cut shall be staggered to prevent the screw thread of the stem from scratching the packing. The packing compression force shall be uniform and appropriate to prevent stress and increase friction.
  • 4. Replacement of drive components. If some parts of the transmission parts in the regulating valve and accessories are worn, parts can be replaced, repaired, etc. After replacement and repair, the transmission shall be flexible and the transmission clearance shall be as small as possible.
  • 5. Cleaning of pneumatic amplifier. When the orifice of pneumatic amplifier is blocked due to the dirt in the compressed air of instrument, the orifice shall be cleaned, and the appropriate wire can be used for dredging and cleaning. When reassembling, the amplifier diaphragm shall be evenly stressed to prevent blockage or leakage. The amplifier gain can be adjusted by adjusting the pressing force of the steel ball to prevent resonance.

Characteristics of control valve

First generation products
Since the beginning of the 20th century, the regulating valve is still at the level of the first generation of products. The regulating valve is characterized by:

  • ① The leading products are single seat valve, double seat valve and sleeve valve;
  • ② The function of this generation of products is not complete, so we have to rely on expanding product varieties and variants to adapt to various occasions, resulting in a wide range of varieties and specifications, especially high requirements for the use, calculation, selection, adjustment, maintenance and spare parts of control valves;
  • ③ The reliability is poor and there are many problems in use;
  • ④ It’s very heavy.

Nowadays, compared with the traditional old products, the more popular CV3000 and fine small valves have three more 30%: the weight is reduced by 30%; The height decreased by 30%; The discharge coefficient is increased by 30%. But they didn’t bring a qualitative breakthrough, they just improved the traditional products.
Second generation products
The second generation products should have a qualitative breakthrough in reliability, function and weight. The regulating valve is characterized by:

  • ① The full-function ultra light control valve has replaced many products with poor reliability, incomplete function and heavy weight. It has replaced the first generation of leading products, such as single seat valve, double seat valve and sleeve valve, and has become the second generation of leading products;
  • ② The electronic electric full-function ultra light control valve has gradually replaced the traditional combination of “pneumatic valve + electric valve positioner + air source” due to the poor reliability of the original actuator.

From the appearance, the second generation products should have the characteristics of light, miniaturization and instrumentation.
Third generation products
Its outstanding feature is intelligent and meets the requirements of fieldbus

  • ① Interface with computer;
  • ② Self diagnosis improves reliability and reduces failure rate;
  • ③ Improve the characteristic curve of the valve, so as to change the regulating quality of the valve;
  • ④ The varieties of the control valve and the requirements for the use of the control valve are further simplified;
  • ⑤ In terms of indicators, it can meet the requirements of modern industry and reach a new level of performance. For example, the new processes and systems introduced by some industrial departments put forward some harsh requirements for regulating valves.

Flow characteristics of control valve

The flow characteristic of the control valve is the relationship between the relative flow of the medium flowing through the control valve and its opening under the condition that the pressure difference between the two ends of the valve remains constant. The flow characteristics of the control valve are linear, equal percentage and parabolic. The significance of the three flux characteristics is as follows:
20210714034610 46364 - What is a control valve

Equal percentage characteristic

There is no linear relationship between the relative stroke and the relative flow rate of the equal percentage characteristic. At each point of the stroke, the change of the flow rate caused by the change of the unit stroke is proportional to the flow rate at this point, and the percentage of the flow rate change is equal. So its advantage is that the flow is small, the flow change is small, when the flow is large, the flow change is large, that is, in different opening, it has the same adjustment accuracy.

Linear characteristic

The relative stroke of linear characteristic is linear with the relative flow rate. The change of flow caused by the change of unit stroke is constant. When the flow rate is large, the change of the relative value of the flow rate is small. When the flow rate is small, the change of the relative value of the flow rate is large.

Parabola characteristic

The flow rate is proportional to the two sides of the stroke, which has the intermediate characteristics of linear and equal percentage characteristics.
From the analysis of the above three characteristics, it can be seen that in terms of its regulation performance, the equal percentage characteristic is the best, with stable regulation and good regulation performance. The parabola characteristic is better than the linear characteristic in regulating performance. Any one of the flow characteristics can be selected according to the requirements of different application occasions.

Application of control valve

Control valves play a very important role in automatic control of modern chemical plants. The production of these plants depends on the correct distribution and control of liquid and gas flowing. These controls, whether energy exchange, pressure reduction or simple container charging, need some final control elements to complete. The final control element can be regarded as the “physical strength” of automatic control. Between the low-energy level of the regulator and the high-energy level of the flow fluid control, the final control element completes the necessary power amplification.
Control valve is the most widely used type of final control element. Other final control elements include metering pumps, regulating baffles and louver baffles (a variant of butterfly valves), variable pitch fan blades, current regulating devices, and motor positioning devices different from valves.
Although the control valve is widely used, other units in the control system probably do not have as little maintenance work as it does. In many systems, operating conditions such as temperature, pressure, corrosion and contamination are more serious than those of other components. However, when it controls the flow of process fluid, it must be operated satisfactorily and with minimal maintenance.
In the pneumatic control system, the pneumatic signal output by the regulator can directly drive a spring type thin film actuator or a piston actuator to make the valve move. In this case, the energy required to determine the valve position is provided by compressed air, which should be dried in outdoor equipment to prevent freezing, and purified and filtered.
When a pneumatic control valve and an electric regulator are used together, an electric pneumatic valve positioner or an electric pneumatic converter can be used. The air supply system of compressed air can be considered as the same as that of all pneumatic control system.
In terms of adjustment theory, the regulating valve has both static and dynamic characteristics, which affects the success or failure of the whole control circuit. The static characteristic or gain term is the flow characteristic of the valve, which depends on the size of the valve, the combination structure of the spool and seat, the type of actuator, the valve positioner, the pressure before and after the valve, and the nature of the fluid.
The dynamic characteristics are determined by the combination of actuator or valve positioner actuator. For slower production processes, such as temperature control or liquid level control, the dynamic characteristics of valves are generally not a limiting factor in terms of controllability. For faster systems, such as liquid flow control, the control valve may have obvious lag, so the controllability of the loop must be considered. In general, only the experts of control system need to care about the dynamic stability of the control valve. The formal consideration about the application of valve positioner, as discussed in Chapter 9, will meet the needs of most control valve devices.
The history of automatic control valve can be traced back to the self operated pressure regulating valve, which includes a spherical valve with a heavy rod. The heavy object is used to balance the valve core force, so as to get a certain degree of regulation. Another early form of self operated pressure regulating valve is the pressure balanced pressure regulating valve. The pressure of the process is connected to the diaphragm chamber of the spring diaphragm pressure regulating valve by pipeline. No matter the pressure reducing valve, pressure regulating valve behind the valve or differential pressure regulating valve, they are all made from the change of this basic valve.
The appearance of pneumatic transmitter and regulator will inevitably lead to the application of pneumatic throttle valve. In essence, they are pressure reducing valve or pressure regulating valve behind the valve, and the instrument compressed air is used to replace the process fluid. Many companies producing pressure reducing valves have developed into control valve manufacturers. The application of control valve continues to develop in terms of quantity and complexity. The improvement of valve body and accessories of many valves can be used to solve various problems. The purpose of this manual is to familiarize engineers with the problems and factors of control valve, and help Instrument Engineers select the best valve body, actuator and accessories in application.
According to the stroke characteristics, the regulating valve can be divided into straight stroke and angular stroke. Straight stroke includes: single seat valve, double seat valve, sleeve valve, angle valve, three-way valve and diaphragm valve; Angular stroke includes: butterfly valve, ball valve, eccentric rotary valve, full function ultra light control valve. The control valve can be divided into pneumatic control valve, electric control valve and hydraulic control valve; According to the regulation form, it can be divided into regulation type, cut-off type and regulation cut-off type; According to the flow characteristics can be divided into: linear, equal percentage, parabolic, fast open. The control valve is suitable for air, water, steam, various corrosive media, mud, oil and other media.

Treatment of control valve

Cleaning method
The welding slag, rust and slag in the pipeline cause blockage or jamming in the orifice, guide part and balance hole of the lower valve cover, resulting in pulling and scratch on the surface of the valve core and guide surface, and indentation on the sealing surface. This often occurs in the early stage of new system operation and after overhaul. This is the most common fault. In this case, it must be removed for cleaning and slag removal. If the sealing surface is damaged, it should also be ground; At the same time, open the bottom plug to flush the slag falling into the lower valve cover from the balance hole, and flush the pipeline. Before putting into operation, the control valve shall be fully opened and the medium shall flow for a period of time before entering into normal operation.
External scour method
When ordinary valve is used to regulate the medium which is easy to precipitate and contains solid particles, it is often blocked at the orifice and guide. Flushing gas and steam can be connected at the bottom plug of the lower valve cover. When the valve is blocked or stuck, open the external gas or steam valve to complete the flushing work without moving the control valve, so that the valve can operate normally.
Install the air filter pressure reducing valve
For small caliber control valve, especially ultra small flow control valve, its throttle clearance is very small, and there should be no slag in the medium. In case of blockage, it is better to install a filter on the pipeline in front of the valve to ensure the smooth passage of medium. For the control valve with positioner, the positioner doesn’t work normally, and the most common fault is the blockage of its throttle port. Therefore, when working with the positioner, the air source must be handled well. The usual method is to install the air filter pressure reducing valve on the air source pipeline in front of the positioner.
Increase throttle clearance
For example, the solid particles in the medium or the welding slag and rust washed out in the pipeline can not pass through the throttling orifice and cause blockage, jamming and other faults. The throttling parts with large throttling clearance, such as valve core and sleeve with windowed and open throttling area, can be used instead. Because the throttling area is concentrated rather than circumferentially distributed, the fault can be easily eliminated. If it is a single seat valve or a double seat valve, the plunger shaped valve core can be changed into a V-shaped valve core or a sleeve valve. For example, a double seat valve in a chemical plant is often stuck, so it is recommended to use sleeve valve, and the problem is solved immediately.
Medium scour method
By using the scouring energy of the medium itself, it can scour and take away the things that are easy to precipitate and block, so as to improve the anti blocking function of the valve. The common methods are as follows: 1; ② Streamline valve body is adopted; ③ When the orifice is placed at the place where the erosion is most severe, the erosion resistance of the material of the orifice should be improved.
Straight through to angle
The straight through flow is inverted s flow, the flow path is complex, and there are many dead zones in the upper and lower cavity, which provides a place for the medium precipitation. Angular connection, the medium as if flowing through 90 elbow, good scouring performance, small dead zone, easy to design into streamline. Therefore, when the straight through control valve is slightly blocked, it can be changed into an angle valve.
Increase the gap
If the clearance between valve core and sleeve is too small, or different materials are used, the sleeve control valve used in steam pipeline may be blocked by heat due to different expansion coefficient. The gap can be appropriately increased during machining (on the premise of meeting the flow control requirements) to prevent the occurrence of similar situations.
Resolvent
1) Grinding method
Fine grinding, eliminate traces, reduce or eliminate the seal clearance, improve the surface finish of the seal, in order to improve the sealing performance.
2) Using unbalanced force to increase specific pressure of seal
The sealing pressure produced by the actuator to the valve core is certain. When the unbalanced force has the tendency of opening the valve core, the sealing force of the valve core is the subtraction of the two forces. On the contrary, the sealing force of the valve core has the tendency of closing the valve core. The sealing force of the valve core is the addition of the two forces, which greatly increases the sealing specific pressure, and the sealing effect can be increased by more than 5-10 times, It is usually the flow open type. If the sealing effect is not satisfied, it will be changed to the flow closed type, and the sealing performance will be doubled. In particular, the two position shut-off valve should be generally used as the flow closed type.
3) Method of improving sealing force of actuator
Improving the sealing force of the actuator to the valve core is also a common method to ensure the valve closing, increase the sealing pressure and improve the sealing performance. The common methods are as follows:

  • ① Working range of moving spring;
  • ② The spring with small stiffness is used;
  • ③ Add accessories, such as with positioner;
  • ④ Increase the pressure of gas source;
  • ⑤ The actuator with higher thrust is used instead.

4) Single seal and soft seal are adopted
For the control valve with double seal, single seal can be used, which can improve the sealing effect by more than 10 times. If the unbalanced force is large, corresponding measures should be taken. For the valve with hard seal, soft seal can be used, which can improve the sealing effect by more than 10 times.
5) Switch to the valve with good sealing performance
If we have to, we can consider to use the valve with better sealing performance. For example, we can use elliptic butterfly valve instead of ordinary butterfly valve, and then we can use cut-off butterfly valve, eccentric rotary valve, ball valve and specially designed cut-off valve.

Leakage solution

1) Method of adding sealing grease
For the valve without sealing grease, the sealing grease can be added to improve the sealing performance of the valve stem.
2) Adding filler method
In order to improve the sealing performance of packing to valve stem, the method of adding packing can be adopted. It is usually in the form of double-layer and multi-layer mixed fillers. If the number of fillers is increased from 3 to 5, the effect is not obvious.
3) Replacing graphite packing method
When the temperature is between – 20 ~ + 200, the sealing performance of PTFE packing is obviously decreased, aging is fast and the service life is short. Flexible graphite packing can overcome these shortcomings and has a long service life. As a result, some factories will replace the tetrafluoro packing with graphite packing, and even the newly purchased control valve will replace the tetrafluoro packing with graphite packing. However, when graphite packing is used, the backlash is large, and some creeping phenomenon occurs at the beginning, which must be considered.
4) Change the flow direction and place P2 at the end of the valve stem
When △ P is large and P1 is large, it is obviously more difficult to seal P1 than P2. Therefore, the method of changing the flow direction can be adopted to change P1 at the stem end to P2 at the stem end, which is more effective for valves with high pressure and large pressure difference. For example, bellows valves should usually consider sealing P2.
5) Using lens gasket sealing method
For the sealing of the upper and lower covers, the sealing of the valve seat and the upper and lower valve bodies, if it is plane sealing, under high temperature and high pressure, the sealing performance is poor, causing leakage, the lens gasket sealing can be used, and the satisfactory effect can be obtained.
6) Replace the sealing gasket
Most of the sealing gasket is still made of asbestos board, which has poor sealing performance and short service life at high temperature, causing leakage. In this case, spiral wound gasket and “O” ring can be used instead.

Vibration solutions

1) Increasing stiffness method
For oscillation and slight vibration, the stiffness can be increased to eliminate or weaken. For example, it is feasible to select spring with large stiffness and use piston actuator.
2) Increasing damping method
Increasing the damping means increasing the friction to the vibration. For example, the plug of sleeve valve can be sealed with “O” ring, and graphite packing with larger friction can be used, which has a certain effect on eliminating or weakening the slight vibration.
3) Method of increasing guide dimension and reducing fit clearance
Generally, the guide dimension of the shaft plug valve is small, and the fit clearance of all valves is generally large, which is 0.4 ~ LMM, which is helpful to produce mechanical vibration. Therefore, when slight mechanical vibration occurs, the vibration can be weakened by increasing the guide dimension and reducing the fit clearance.
4) Changing the shape of throttle and eliminating resonance
Because the so-called vibration source of the control valve occurs at the orifice with high-speed flow and rapid pressure change, changing the shape of the throttle can change the frequency of the vibration source, which is easier to solve when the resonance is not strong. The specific method is to turn the valve core surface within the vibration opening range of 0.5 ~ 1.0 mm. For example, a self operated pressure regulating valve was installed near the family area of a factory, and the whistling caused by resonance affected the rest of workers. After we dropped the surface of valve core by 0.5mm, the resonance whistling disappeared.

Installation points of control valve

  • 1) The installation position, height, inlet and outlet direction must meet the design requirements, and the connection should be firm and tight.
  • 2) The handle of all kinds of manual valves installed on the insulation pipeline shall not be downward.
  • 3) The valve must be visually inspected before installation, and the valve name plate shall comply with the provisions of the current national standard “general valve marks” GB 12220. For the valve with working pressure greater than 1.0 MPa and cut-off function on the main pipe, the strength and tightness test shall be conducted before installation, and the valve can only be used after it is qualified. During the strength test, the test pressure shall be 1.5 times of the nominal pressure, the duration shall not be less than 5min, and the valve shell and packing shall be qualified if there is no leakage. In the tightness test, the test pressure is 1.1 times of the nominal pressure; The duration of the test pressure shall meet the requirements of GB 50243, and the valve disc sealing surface shall be qualified if there is no leakage.

Note:

  • 1. The valve should be stored in a dry room, and both ends of the passage must be blocked, and it is not allowed to be stacked.
  • 2. The long-term storage control valve should be checked regularly to remove the dirt, and the moving parts and processing surface should be coated with anti rust oil to prevent rust.
  • 3. The valve should be installed on the horizontal pipeline, and should be installed vertically. 
  • 4. Install according to the medium indicated by the arrow in the figure.

Factors affecting normal operation of control valve and Countermeasures

In the chemical control system with high degree of automation, the control valve, as the terminal executive device of the automatic control system, receives the control signal to realize the regulation of the chemical process. Its action sensitivity is directly related to the quality of the regulating system. According to the actual statistics, about 70% of the faults are caused by the control valve. Therefore, in the daily maintenance, summarize and analyze the factors affecting the safe operation of the control valve and its countermeasures.

Blocking

The frequent problem of regulating valve is blocking, which occurs in the initial stage of new system and overhaul. Because of the blockage of welding slag and rust in the pipe at the throttle and guide position, the medium circulation is not smooth, or the packing is too tight in the maintenance of regulating valve, resulting in the increase of friction, resulting in the phenomenon of small signal not acting and large signal acting too much.
Troubleshooting: quickly open and close the auxiliary line or control valve, so that dirt can be washed away by the medium from the auxiliary line or control valve. Another way is to clamp the valve stem with a pipe clamp. Under the condition of additional signal pressure, turn the valve stem with positive and negative force to make the valve core flash over the card. If not, the problem can be solved by increasing the air source pressure, increasing the driving power and moving up and down repeatedly for several times. If it still doesn’t work, it needs to be disassembled.

Leakage

Internal leakage of valve and improper length of valve stem.

When the valve is opened by air, the valve stem is too long and the upward (or downward) distance of the valve stem is not enough, resulting in a gap between the valve core and the valve seat and insufficient contact, resulting in internal leakage due to lax closing. Similarly, the valve stem is too short, resulting in a gap between the valve core and the valve seat, can not fully contact, resulting in tight closure and internal leakage.
Solution: should shorten (or extend) the stem of the control valve, so that the length of the control valve is appropriate, so that it does not leak.

Packing leakage.

After the packing is put into the stuffing box, the axial pressure is exerted on it through the gland. Because of the plasticity of the packing, it produces radial force and contacts with the valve stem closely, but the contact is not very uniform. Some parts are loose in contact, some parts are tight in contact, and even some parts are not in contact. Control valve in the process of use, there is a relative movement between the stem and packing, this movement is called axial movement. In the process of use, with the influence of high temperature, high pressure and strong permeability of the fluid medium, the stuffing box of the control valve is also the place where more leakage occurs. The main cause of packing leakage is interface leakage, and there will be leakage for textile packing (the pressure medium leaks along the tiny gap between the packing fibers). The interface leakage between the valve stem and the packing is caused by the gradual attenuation of the packing contact pressure and the aging of the packing itself. At this time, the pressure medium will leak outward along the contact gap between the packing and the valve stem.
Solution: in order to make it convenient for packing, chamfering at the top of the stuffing box, and placing a metal protection ring with small clearance (the contact surface with the stuffing can not be inclined) at the bottom of the stuffing box to prevent the stuffing from being pushed out by the medium pressure. The metal surface of each part of the stuffing box contacting with the stuffing should be finished to improve the surface finish and reduce the stuffing wear. The packing is made of flexible graphite, because of its good air tightness, small friction, small change after long-term use, small burning loss of wear, easy maintenance, no change in friction after re tightening the gland bolt, good pressure resistance and heat resistance, no erosion of internal medium, and no pitting or corrosion of metal in contact with the valve stem and stuffing box. In this way, the sealing of the valve stem packing box is effectively protected, and the reliability and long-term performance of the packing seal are ensured.

Deformation and leakage of valve core and valve seat.

The main reason for the leakage of the core and seat is that the casting or forging defects in the production process of the control valve can lead to the strengthening of corrosion. The leakage of the control valve can also be caused by the passage of corrosive medium and the erosion of fluid medium. Corrosion mainly exists in the form of erosion or cavitation. When the corrosive medium passes through the control valve, it will produce erosion and impact on the valve core and seat materials, making the valve core and seat oval or other shapes. With the passage of time, the valve core and seat do not match, there is a gap, and leakage occurs if the valve is not closed tightly.
Solution: the key is to control the material selection and quality of valve core and valve seat. Select corrosion-resistant materials, and resolutely eliminate the products with pitting, trachoma and other defects. If the deformation of valve core and valve seat is not too serious, it can be ground by fine sandpaper to eliminate traces and improve the sealing finish, so as to improve the sealing performance. In case of serious damage, replace the valve with a new one.

Oscillation

The spring stiffness of the control valve is insufficient, and the output signal of the control valve is unstable and changes rapidly, which is easy to cause the oscillation of the control valve. It is also said that the frequency of the valve selection is the same as that of the system, or the pipe and base vibrate violently, which makes the control valve vibrate accordingly. When the valve is not properly selected, there are sharp changes in flow resistance, flow velocity and pressure in the small opening of the control valve. When the valve stiffness is exceeded, the stability becomes poor and oscillation occurs in serious cases.
Solutions: because the causes of oscillation are various, the specific problems are analyzed in detail. For slight vibration, the stiffness can be increased to eliminate it. If the spring with large stiffness is selected, the piston actuator structure should be used instead. The pipe and base vibrate violently, and the vibration interference is eliminated by adding support; If the frequency of valve selection is the same as the system frequency, replace the valve with different structure; The oscillation caused by working at small opening is due to improper selection of type and larger value of flow capacity C. It is necessary to select the type with smaller value of flow capacity C again or adopt split range control or sub master valve to overcome the problem of working at small opening of control valve.

Valve positioner failure

The common positioner works on the principle of mechanical force balance, i.e. nozzle baffle technology, which mainly has the following fault types:

  • 1) Because of the mechanical force balance principle, there are many movable parts, which are easy to be affected by temperature and vibration, resulting in the fluctuation of the control valve;
  • 2) Using nozzle baffle technology, because the nozzle hole is very small, it is easy to be blocked by dust or unclean air source, so the positioner can not work normally;
  • 3) Based on the principle of force balance, the elastic coefficient of the spring changes in bad conditions, which causes the nonlinearity of the control valve and the decline of the control quality.

Intelligent positioner is composed of microprocessor (CPU), A/D, D/A converter and other components, its working principle is different from ordinary positioner. The comparison between the given value and the actual value is purely electric signal, not force balance. Therefore, it can overcome the disadvantage of force balance of conventional positioner. But when it is used for emergency stop, such as emergency shut-off valve, emergency vent valve, etc. These valves are required to be stationary in a certain position, and only in case of emergency can they be operated reliably. Staying in a certain position for a long time is easy to make the electrical converter out of control, resulting in the dangerous situation of small signal not acting. In addition, because the position sensing potentiometer used for valve works on site, the resistance value is easy to change, resulting in the dangerous situation of small signal not acting and large signal fully open. Therefore, in order to ensure the reliability and availability of intelligent positioners, they must be tested frequently.

Calculation formula of control valve

The flow coefficient kV of the control valve is an important parameter of the control valve, which reflects the flow capacity of the control valve, that is, the capacity of the control valve. According to the calculation of the flow coefficient kV of the control valve, the diameter of the control valve can be determined. In order to select the diameter of the control valve correctly, it is necessary to calculate the Rated flow coefficient Kv value. The definition of the rated flow coefficient kV of the control valve is: under the specified conditions, that is, the pressure difference between the two ends of the valve is 10Pa, the density of the fluid is lg/cm, and the flow rate passing through the regulating valve at m/h or t/h during the rated stroke.
1. Calculation of Kv value of general liquid
a. Nonblocking flow
  • Discriminant: △ p
  • Calculation formula: Kv=10QL

In the formula:

  • FL – pressure recovery coefficient, see attached table
  • FF – Critical pressure ratio coefficient of fluid, FF = 0.96-0.2820210714040756 47795 - What is a control valve
  • PV – saturated vapor pressure (absolute pressure) of medium at valve inlet temperature, kPa
  • PC – thermodynamic critical pressure (absolute pressure), kPa
  • QL – liquid flow m/h
  • ρ- Liquid density g/cm
  • P1 – front pressure (absolute pressure) kPa
  • P2 – pressure behind valve (absolute pressure) kPa
b. Blocking flow
  • Discriminant: △P≥FL(P1-FFPV)
  • Calculation formula: Kv=10QL
In the formula: the meaning and unit of each character are the same as before
2. Calculation of Kv value of gas
a. General gas
  • When P2 > 0.5p1
  • When P2 ≤ 0.5p1
In the formula: QG – gas flow under standard state nm / h
PM – (P1 + P2) / 2 (P1 and P2 are absolute pressure) kPa
  • △P=P1-P2
G – specific gravity of gas (air g = 1)
T – gas temperature (℃)
b. High pressure gas (PN > 10MPa)
  • When P2 > 0.5p1
  • When P2 ≤ 0.5p1
In the formula: Z- Gas compression coefficient Refer to GB / T 2624-81 design, installation and use of throttling devices for flow measurement
3. Low Reynolds number correction (high) viscosity Calculation of liquid Kv value)
When the liquid viscosity is too high or the flow rate is too low, the flow state of the fluid flowing through the control valve is changed due to the decrease of Reynolds number. When Rev < 2300, the fluid is in low-speed laminar flow. Therefore, the Kv value calculated according to the original formula has a large error and must be corrected. In this case, the calculation formula should be as follows:

In the formula:

  • Φ― The viscosity correction coefficient was obtained from the fr-rev curve of Rev; QL – liquid flow M/h.
  • For single seat valve, sleeve valve, angle valve and so on, there is only one flow path.
  • For double seat valves butterfly valve A valve with two parallel flow paths.

In the formula:

  • kV ‘- the flow system calculated without considering viscosity correction
  • ν ― fluid Kinematic viscosity mm/s
  • Fr- REV curve
  • Fr- REV diagram
4. Calculation of Kv value of water vapor
a. Saturated steam
  • When P2 > 0.5p1
  • When P2 ≤ 0.5p1
In the formula: G — steam flow rate kg / h, meaning and unit of P1 and P2 are the same as before, k-steam correction coefficient, K value of some steam is as follows: Steam: k = 19.4; Ammonia steam: k = 25; Freon 11: k = 68.5; Methane and ethylene vapor: k = 37; Propane and propylene steam: k = 41.5; Butane and isobutane vapor: k = 43.5.
b. Superheated steam
  • When P2 > 0.5p1
  • When P2 ≤ 0.5p1
In the formula: △t – superheat degree of steam ℃, GS, P1 and P2 have the same meanings and units as before.

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.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@steeljrv.com

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