What is an electric valve?
What is an electric valve?
Electric valve is simply to use electric actuator to control the valve, so as to realize the opening and closing of the valve. It can be divided into upper and lower parts, the upper part is an electric actuator, and the lower part is a valve. It can also be called air conditioning valve.
Electric valves are high-end products in automatic valves. They can not only switch on and off, but also regulate the valve position. The stroke of electric actuator can be divided into two types: 90 degree angular stroke and straight stroke. The special requirements can also meet 180 degree, 270 degree and 360 degree full stroke. The electric actuator with angular stroke cooperates with the valve with angular stroke to realize the internal rotation of the valve to control the pipeline fluid opening and closing, while the electric actuator with straight stroke cooperates with the valve with the valve with straight stroke to realize the control of the pipeline fluid opening and closing by the action of the valve plate.
Electric valves are usually connected by electric actuators and valves and become electric valves after installation and debugging. The electric valve uses electric energy as power to connect the electric actuator to drive the valve, so as to realize the valve’s switching and regulating action. So as to achieve the purpose of switching or regulating the pipeline medium.
Electric valves are usually driven by motors. It takes a certain amount of time to complete the opening or closing action, which can be adjusted. Compare withstand voltage shock. Solenoid valves are fast-opening and fast-closing, generally used in small flow and pressure, requiring high switching frequency; electric valves on the contrary. The opening of the electric valve can be controlled. The state of the valve is on, off and half off. The flow of the medium in the pipeline can be controlled, but the solenoid valve can not meet this requirement. Three-wire electric valve has three lines F/R/N, F represents forward action (or open action) control line, R represents reverse action (or close action) control line, N represents ground line. Solenoid valve is a kind of electric valve; it uses the magnetic field produced by electromagnetic coil to pull the valve core, thus changing the on-off of the valve body. When the coil is cut off, the valve core depends on the spring pressure to return.
The structure and composition of electric valve are introduced.
- Electric valves are mainly composed of valves and electric actuators. Electric actuators are used in electrical installation. The output shaft adopts the connecting form of stem nut and the size of the flange of the connecting disc, which drives the up and down motion of the valve stem. In order to open and close the valve, the electric ball valve can be driven by the stem to drive the gate to move up and down in a straight line.
- The valve is equipped with a manual operation mechanism. When there is no power supply, the valve can be opened and closed by manual operation. When the handwheel rotates clockwise, the valve can be closed. The pneumatic ball valve is opened when the valve rotates counter-clockwise.
- The flange in the valve adopts self-pressure sealing structure, and its sealing gasket adopts stainless steel metal gasket, which can withstand 4500 pounds of high-pressure sealing. The sealing performance of the ball valve is reliable.
- The gate adopts wedge single gate structure. The valve is small in size and compact in structure.
- The valve body adopts forging and welding structure, which fundamentally solves the quality problems caused by casting defects, eliminates the waste rate and improves the reliability of the product.
- Surfacing cobalt-based cemented carbide with gate and seat sealing materials.
- The valve stem and the valve cover are provided with a reverse sealing device. When necessary, the reversing sealing device can be used temporarily to isolate the medium from the packing box, but the reversing device can not be used to replace the packing.
In pipeline engineering, the correct selection of electrically operated valves is one of the guarantee conditions to meet the requirements of use. If the electric valve used is not properly selected, it will not only affect the use, but also bring about bad consequences or serious damage. Therefore, the electric valve should be correctly selected in the pipeline engineering design.
Working environment of electrically operated valves
In addition to the pipeline parameters, the environmental conditions of the electric valve operation should be well considered, because the electric actuator in the electrically operated valve is an electromechanical device, and its working condition is greatly affected by its working environment. Usually, the working environment of the electric valve is as follows:
- Indoor installation or protective measures for outdoor use;
- Outdoor open-air installation, wind, sand, rain, sun and other erosion;
- Flammable, explosive gas or dust environment;
- Wet tropical and dry tropical environment;
- The pipeline medium temperature is up to 480 ° C or more;
- The ambient temperature is below -20 °C;
- Environment vulnerable to flooding or immersion in water;
- Environment with radioactive materials (nuclear power stations and radioactive material testing devices);
- The environment on the ship or dock (with salt spray, mold, moisture);
- Where there is severe vibration;
- Where it is prone to fire.
For electric valves in the above environments, the structure, materials and protective measures of the electric devices are different. Therefore, the corresponding valve electric device should be selected according to the above working environment.
According to the engineering control requirements, the control function of the electric valve is completed by the electric device for electric valves. The purpose of using an electric valve is to achieve non-manual electrical control or computer control of the valve opening, closing and adjustment linkage. The current use of electric devices is not just to save manpower. Due to the large differences in functions and quality of products from different manufacturers, the selection of electric devices and the selection ofvalves are equally important to the project.
As the requirements of the industrial automation level continue to increase, on the one hand, the use of electric valves is increasing, and on the other hand, the control requirements for electric valves are becoming higher and more complex. Therefore, the design of electric valves in electrical control is constantly being updated. With the advancement of science and technology and the popularization and application of computers, new and diverse electrical control methods will continue to emerge. For the overall control of electric valves, attention should be paid to the choice of the control method of the electric valve. For example, depending on the needs of the project, whether to use centralized control mode, single control mode, linkage with other devices, program control or computer program control, etc., the control principle is different. The valve electric actuator manufacturer only gives the standard electrical control principle, so the user department should make technical disclosure with the electric actuator manufacturer and clarify the technical requirements. In addition, when selecting an electric valve, consideration should be given to whether or not to purchase an electric valve controller. Because of the general situation, the controller needs to be purchased separately. In most cases, when using a single control, it is necessary to purchase the controller, because it is convenient and cheaper to purchase the controller than the user design and manufacture it. When the electrical control performance does not meet the engineering design requirements, the user should ask the manufacturer to propose modifications or redesigns.
Electric valves can be divided into switching electric valves and regulating electric valves according to valve position function; electric ball valves and electric butterfly valves according to valve position form; ordinary electric valves and micro electric valves according to valve body shape. Electric valves are usually switching type, but also adjustable type, such as: fan intake pipe to adjust water flow, etc. Switch-type electric valves are generally divided into two types: normal closed and normal open. Normal closed means that the valve is closed when the power is cut off. Normal open means that the valve is open when the power is cut off. In addition, according to the wiring, there are three-wire and two-wire systems. Most of the large-caliber electric valves are three-wire systems, while the small-caliber ones are two-wire and three-wire systems.
1. Valves for oil and natural gas wellhead installations
Valves for oil and natural gas wellhead installations are mainly single or double gate valves, forged steel parallel gate valves with or without diversion holes, mud valves, angle throttle valves, oil field parallel control valves, oil field straight-through check valves, water injection polymer parallel gate valves, clamp parallel gate valves, pilot safety valves and check valves, which meet API6A standards.
The nominal pressure level of electric valve for oil and natural gas wellhead installation electric valve is API 2000psi, 3000psi, 5000psi, 5000psi, 10000psi, 15000psi, 15000psi, 20000psi; nominal diameter DN46~228mm (113/16in~9in); nominal diameter DN46~228mm (113/16in 9in); temperature grade K (-60~182 C), L (-42-182 (-42-182 C), P (-29-182 C), R (room temperature), S (-18-166 (-18-166 C), T (-18-196 (-18-196 C), U (-18-18121 (-18-121 C), V (2-121), 2-A. FF, HH; material performance requirements according to 36K, 45K, 60K, 75K.
2. Valves for Long-distance Oil and Natural Gas Pipelines
Valves for petroleum and natural gas pipelines are mainly single gate or double gate, flat gate valves with or without diversion holes, forged or cast steel three-body, mounted or fully welded fixed ball valves, oil-sealed or pressure-balanced cock valves, swing or butterfly check valves, through ball check valves, pigging valves and so on, which meet the API6D standard of the United States.
The nominal pressure levels of these electric valves are CL150 (PN2.0MPa), CL300 (PN5.0MPa), CL400 (PN6.4MPa), CL600 (PN10.0MPa), CL900 (PN15.0MPa), CL1500 (PN25.0MPa), CL2500 (PN42.0MPa); nominal diameter is DN50-1500mm (2in-60in); fire resistance test technical requirements are ISO10497; pressure test of electric valves is 5208.
3. Nuclear Valves
The technical characteristics and requirements of electric valves for nuclear power plants are higher than those for conventional large-scale thermal power plants. Valves generally have gate valves, globe valves, check valves, butterfly valves, safety valves, main steam isolation valves, ball valves, diaphragm valves, pressure relief valves and control valves, etc. The highest technical parameters of representative electric valves are: maximum diameter DN1200mm (nuclear Level 3 Butterfly valves), DN800mm (nuclear Level 2 main steam isolation valves), DN350mm (nuclear level 1 main circuit gate valves); maximum pressure: about 1500 pounds; Temperature: about 350 C; Medium: Coolant (water boride), etc. Requirements for the production of nuclear grade electric valves: usually according to the nuclear industry standards EJ, ASME, IEEE and RCC-M, etc.
The specific types and parameters of the development of nuclear power valves are as follows:
(1) Gate valves without stuffing box:
A Hydraulic drive gate valve. The valve opens or closes the piston with the help of its own pressure water. The nominal diameter of the valve is DN350 and 400mm; the working pressure is PN17.5MPa; and the working temperature is 315 C. B Fully enclosed electric gate valve. The valve shall adopt a special shut-in motor to make the gate open and close through an inner planetary deceleration mechanism for immersion operation. The nominal diameter of the valve is DN100-800mm, the working pressure is PN2.5-45.0MPa, and the working temperature is 200-500 C.
Note: The advantages of the above two kinds of non-filler box gate valves are: no filler seal, avoiding leakage point, and reducing energy consumption. Disadvantage: Complex structure and high cost.
(2) Globe valves for nuclear power:
Globe valves for auxiliary pipelines. The valve usually has three kinds of structure, i. e. packing type globe valve, bellows type globe valve and metal diaphragm type globe valve. The valve medium is medium parameter (medium temperature, medium pressure) water and steam; nominal diameter: DN10 ~ 150mm.
(3) Butterfly valves for nuclear power:
Butterfly valves for cooling systems and systems for conveying air in containment. The valve usually has three kinds of structure, namely coaxial direct lining butterfly valve, eccentric metal seal butterfly valve and double-acting metal seal butterfly valve. The nominal diameter of the valve is DN < 2500mm, the working pressure is PN < 4.0MPa, and the working temperature is 100 ~ 150 C. In addition, the nominal diameter of the fast closing butterfly valve used in the duct system: DN400 ~ 1200mm is also listed as the direction of development.
(4) Pilot safety valve with detector for nuclear power:
Pilot safety valve with detector for nuclear island system. The pilot safety valve with detector can change the principle of position control release and filling medium according to the sensitive relationship between pressure and spring force balance, so as to avoid blocking problem in structure. The valve adopts a disc structure with positive spring pre-tightening and bellows sealing to ensure reliable sealing. The nominal diameter of the valve is DN600 mm, and the working pressure is PN1.265 MPa.
(5) Check Valve Isolation Valve for Nuclear Power:
Check valve isolation valve used in steam system has a structure similar to lift check valve. The nominal diameter of the valve is DN64~800mm (21/2 in~30 in); the working pressure is PN1.0~42.0 MPa (Class600~2500); and the working temperature is -29~1050 C.
(6) Main steam isolation valve for nuclear power:
Main steam isolation valve and main feed water electric valve for nuclear island and conventional island, nominal diameter: DN800mm; nominal pressure: 40.0 MPa; temperature 700 C;
In addition, the safety valve to meet the seismic requirements is also an urgent need to develop nuclear power electric valve.
Offshore Petroleum Valves
The oil production, blowout prevention, water injection, gas injection, underwater equipment and other systems of offshore oil production platforms need to develop some special electric valves which are resistant to sea water and salt spray erosion. These electric valves require strong corrosion resistance, storm resistance and other abnormal external forces, and require reliable sealing, flexible operation and convenient maintenance.
The specific types and parameters of offshore oil valves are as follows:
(1) Forged steel single gate or double gate with or without diversion orifice flat gate valve
The structure of the valve can refer to the U.S. Cameroon F-type gate valve, AF-type submarine electric valve (for 1000 meters under the sea), DF-type submarine electric valve (for 3000 meters under the sea), MCEVOY C-type and E-type gate valves.
The nominal diameter of this kind of electric valve is DN46-162mm (113/16in-63/8in); the nominal pressure level is API 2000psi, 3000psi, 5000psi, 10000psi, 15000psi, 20000psi; the working temperature is -59-343 C; the technical requirements are in accordance with API 6A.
(2) Double-disc butterfly check valve
Technical parameters of the valve: nominal diameter: DN53-280mm (21/16in-11in); temperature grade: K (-60-182 C), L (-42-182 C), P (-29-182 C), R (room temperature), S (-18-166 C), T (-18-182 C), U (-18-121 C), V (2-121 C).
(3) Angular throttle valve with pressure balancing stem
The valve is designed and manufactured according to API6A standard; its nominal pressure level is API3000psi, 5000psi, 10000psi; nominal diameter: DN50mm, DN80mm, DN100mm, DN150mm (2in, 3in, 4in, 6in); service temperature: -29-121 C. The throttle head of the valve can be designed as needle throttle head or sleeve throttle head.
Petrochemical and Electric Valves
Petrochemical and electric valve products, the focus of structural adjustment is to develop door-lacking products. As follows:
(1) High Temperature and High Pressure Regulating Valve
The structure design of high temperature and high pressure regulating valve for large thermal power units should be determined according to its flow characteristics and operation control requirements. It can be single seat, double seat or sleeve structure. The nominal diameter of the control valve is DN50-500mm, nominal pressure is less than 42.0MPa, and operating temperature is 510-570 C.
(2) Temperature and pressure relief valve
It is required by petrochemical industry to design a CVS-like valve and control the pressure and temperature in one valve. After decompression, the spray cooling control should be precise (within 4-7 C), close the valve tightly, with low noise and long service life.
(3) High temperature and high pressure large caliber safety valve
The design structure of high temperature, high pressure and large caliber safety valve for steam system can adopt spring type. The key problems to be solved in the design are: setting pressure, return pressure, sealing performance, repetitive action stability and spring fatigue. Its nominal diameter: DN200 ~ 400mm; nominal pressure: <5.0MPa; maximum operating temperature: 570 C.
(4) High pressure steam trap
High pressure steam traps for removing condensate water in high pressure steam system can be developed with thermal power HRW, HRF disc type, mechanical high pressure free floating ball type, thermostatic BK27, BK28, BK212 bimetal sheet type. The nominal diameter of this kind of electric valve is DN15 ~ 50mm, and the nominal pressure is 15.0MPa.
Environmental protection valve
The electric valves needed to be developed in this environmental protection system include ceramic seal rotary valves, PVC plastic valves, large sealing surface flat gate valves, stainless steel cock valves, etc.
Valves used in metallurgical systems require high wear resistance, temperature resistance and corrosion resistance of electric valves because the medium is usually powdery and suspended solids and the temperature is high.
The specific types and parameters of valve development for metallurgical system are as follows:
(1) Dome valve
Dome valves for pneumatic ash removal, feeding and discharging systems in thermal power plants. The valve is composed of valve body, driving shaft, driven shaft, sphere and so on. Its nominal diameter is DN50-300mm; working pressure is PN1.0-0.6MPa; and the maximum operating temperature is 1050 C. When designing the valve, the sealing surface should be sprayed with cobalt-based tungsten carbide alloy to make it harder and enhance wear resistance.
(2) Ash discharge flat gate valve
“Flat Sluice Valve for Ash Removal in Iron-making and Ash Removal System of Steel-making Plant”. The medium of the ash removal system, ash powder, is not only granular in shape (sometimes dispersed, sometimes mixed), but also viscous with carbon monoxide (CO). Because of the particles in the medium “ash powder”, the valve seat often suffers from different degrees of damage, abrasion and wear. Therefore, the main parts of the “ash discharge flat gate valve”, such as gate, valve seat and other parts, should have strong wear resistance. The operating pressure of the iron-making ash removal system is in the low pressure range of 0.25 MPa; the temperature is in the range of less than 250 C (generally normal temperature, sometimes the instantaneous temperature can reach 250 C); the nominal diameter is only from DN50mm to DN400mm (DN300mm is the most common diameter).
If the structure of the valve is designed with a diversion hole and “enlarging the sealing surface”, it can not only improve its wear resistance, meet its sealing performance, but also avoid ash entering the bottom chamber of the valve body. In the design, we can make full use of the existing flat gate valve products with diversion holes, and change the seat to the sealing state of “enlarging the sealing surface”. For wear resistance, the metal-to-metal hard seals have the longest life, but the metal-to-metal hard seals with “enlarged sealing surface” allow leakage in one standard (for example, the design of zero leakage will be very expensive and difficult to process); and the second is that the seals are inferior to non-metallic materials. Its sealing is inferior to that of non-metallic materials because a layer of oxide is produced when the metal surface is abrasive. Although this layer of oxide covers the corroded parts and can slow down the further corrosion of the metal, if sliding occurs, the oxide layer will be removed, which will further corrode the exposed metal surface, thus accelerating wear and weakening its sealing ability. Yes. The use of non-metallic materials can not only save costs, reduce the difficulty of processing, but also ensure its sealing performance.
Because the working temperature of the iron-making ash removal system in steel works is generally normal temperature, sometimes the highest temperature in an instant is only 250 C. Therefore, the use of non-metallic materials (the proposed use of para-polyphenyl) as a sealing surface can not only meet the working conditions of temperature, wear resistance, but also meet its sealing performance. Non-metallic material, para-polyphenyl, is suitable for use at temperatures below 300 C and harder than rigid PTFE. It can meet the working conditions of ash removal system in ironmaking plant and is an ideal sealing surface material. Seat material is recommended to use 1Cr13.
(3) Stainless steel valves resistant to strong acid corrosion
Stainless steel valves resistant to strong acid corrosion are used in metallurgical systems. The nominal diameter of the valve is DN50-2400mm, nominal pressure is 0.05-1.6MPa, and maximum service temperature is 1050 C.
Valves for alumina industry
In the production process of alumina industry, the medium is mostly alkaline slurry and granular material, and the scarring phenomenon is very serious, and the erosion phenomenon of the medium on the body cavity and internal parts of the electric valve is also very serious. Therefore, alumina industrial valves require scar resistance, erosion resistance and erosion resistance.
Electric valves, which are used to regulate the analog flow of liquid, gas and air duct media, are AI feedback. Electric valves can also be used as two-switch control in the control of large valves and wind systems. There can be AI feedback signal, which can be controlled by DO or AO, compared with large pipes and wind valves.
- It can meet the requirements of most working conditions and the working conditions that ordinary valves can not be used.
- It has both switching and adjusting functions, with valve position indication and output.
- It is suitable for almost all media, with a maximum viscosity of 600 mm2/s (lis).
- High temperature resistance, chemical corrosion resistance, wear resistance and durability, water hammer impact resistance;
- With manual function, it can be equipped with handwheel type, explosion-proof environment actuator can be equipped with explosion-proof type.
- Two-way circulation, simple operation, stable control and long service life.
Electric valves are usually composed of electric actuators and valves. The electric valve uses electric energy as power to drive the valve through the electric actuator to realize the valve’s switching action. Thus, the purpose of switching on and off the pipeline medium is achieved. The action distance of electric valve is larger than that of ordinary valve. The action speed of electric valve can be adjusted. The structure is simple and easy to maintain. It can be used to control the flow of various fluids, such as air, water, steam, various corrosive media, mud, oil, liquid metal and radioactive media.
Pneumatic valves are valves driven by compressed air. Compressed air is used to drive the movement of multiple groups of combined pneumatic pistons in the actuator, transfer force to the cross beam and internal curve track, drive the hollow spindle to rotate, compressed air disc is fed to each cylinder, change the position of inlet and outlet air to change the rotation direction of the spindle, according to the requirement of rotating torque required by the load (valve), the number of cylinder combinations can be adjusted to drive the load (valve) to work. Pneumatic valves can be used to control the flow of air, water, steam, various corrosive media, mud, oil, liquid metal and radioactive media.
The advantages of electric valve and pneumatic valve are as follows:
- Pneumatic valve has good effect on gas medium and small diameter liquid, low cost and convenient maintenance. Disadvantage: Affected by fluctuation of air pressure, air pressure and water content are easily affected in northern winter, resulting in freezing and inactivity of transmission part. Generally, pneumatic power is faster than electric power. Electric power is dual-purpose. And the price of pneumatic master and gas dual-use is relatively high.
- Electric valve has good effect on liquid medium and large diameter gas, and is not affected by climate. Not affected by the pressure of air pressure. Disadvantage: High cost, bad in humid environment.
- There are not many brands of slow-acting electric valves that can be explosion-proof; pneumatic valves move quickly, and the price of explosion-proof is relatively higher than that of electric bottoms (what accessories are the key pneumatic valves equipped with, big brand accessories will be more expensive than electric valves).
- Electric valves are used in some places with large diameters, because pneumatic valves are difficult to achieve, but the stability of electric valves is not as slow as pneumatic switches, and the actuator will be jammed for a long time. Pneumatic valves have fast and accurate switching speed, but need a stable air source.
Electric valves and solenoid valves are important actuators in industrial pipeline control. They are widely used in various industries. Their main differences are as follows:
I. Working Principle
- Solenoid valve: Solenoid valve is composed of solenoid coil (solenoid) and valve core and valve body. When the solenoid coil is electrified or power off, it drives the valve core to move, so that the fluid passes through the valve body or is cut off.
- Electric valves: Electric valves drive the valve stem through the motor and drive the valve core to move, so as to achieve the flow through or cut off;
II. Valve type:
1. Solenoid valves:
- Two-way solenoid valve: the valve body has an inlet and an outlet connected with the pipeline. There are two kinds of control modes: normally open (when the solenoid valve is turned on, the fluid is cut off), and normally closed (when the solenoid valve is turned on, the fluid passes through the valve);
- Three-way solenoid valve: three ports of valve body are connected with pipeline and two holes are used for three control modes: normal closed type (pressure opening closes when power is off, exhaust opening connects to cylinder opening, pressure opening connects to cylinder opening and exhaust opening closes when power is on), normal open type (pressure opening connects to cylinder opening when valve is off, exhaust opening closes, pressure opening closes when valve is on, cylinder opening connects to exhaust Port, general type (allow valve to be connected to one of the normally closed or open positions);
- Four-way solenoid valve: generally used with the operation of double-acting cylinder, there are four or five pipeline connections, a pressure port, two cylinder ports and one or two exhaust (oil) ports.
2. Electric valves:
- Electric valve matched with electric angular stroke actuator: electric actuator rotates valve plate angle to control and regulate valve closure and control pipeline flow. The rotation of output shaft of electric actuator is less than one week, i.e. less than 360 degrees, usually 90 degrees, to realize valve opening and closing process control. Such electric valves are mainly butterfly valves, ball valves, cock valves, etc.
- Valve body and multi-turn electric actuator (rotation angle > 360 degrees) matched with the electric valve, multi-turn electric actuator output shaft swing more than a week, that is, more than 360 degrees, usually need more than one turn to achieve valve opening and closing process control. The main electric valve gates are gate valves, globe valves, etc.
- Electric valve matched with straight-stroke electric actuator (linear motion): the movement of the output shaft of the driving part of the electric actuator is linear motion, not rotational form, and the opening and closing control is realized by lifting the valve plate. This kind of electric valve door mainly consists of regulating valve, single seat regulating valve, double seat regulating valve, etc.
III. Control Mode
- The solenoid valve is driven by coil and can only play the role of opening or closing.
- Electric valves are driven by motors. The operation of opening or closing takes longer than that of solenoid valves. The flow can be regulated by the input current and voltage signals of actuators. The general control signals are current signals (4-20mA, 0-10mA) or voltage signals (0-5V, 1-5V).
IV. Scope of application:
- The solenoid valve is driven by solenoid coil, which has small driving force and is easy to be damaged by voltage shock. It is generally suitable for pipelines with diameters less than DN50 or below. It has short switching time and special high frequency solenoid valve, especially for pipelines with high operating frequency or short switching time.
- Electric valves are generally driven by motors, which are more resistant to voltage shocks and have a large driving force. They can control large-caliber valves. Moreover, electric valves can regulate the flow of pipelines, especially for large-caliber pipelines or places where the flow of pipelines is required to be regulated.
- The solenoid valve can be reset when power is cut off, and the reset device of the electric valve needs to be added.
- Solenoid valves are suitable for a wide range of media, such as acid, alkali, water, gas, oil, steam and other media can be used, but for the medium viscosity, cleanliness has a high requirement. Before installing solenoid valves, filters should be installed to ensure the normal operation of solenoid valves, low leakage rate and high safety.
- Electric valves are not only suitable for controlling general fluid media, but also suitable for controlling pulp, sewage, media containing fibers and micro-solid particles. They do not require high cleanliness of media and have higher leakage rate than solenoid valves.
Whether solenoid valve or electric valve, scale will not only cause valve leakage, even affect the normal work of the valve seriously, so how to eliminate the impact of scale has become a common concern of the industry.
Poor performance of control valve
The process of solenoid valve involves a wide range, but the poor performance of control valve can be summarized because of different design actuators and filling materials.
The existence of dead zone in the process will make the process variables deviate from the original set point. Therefore, the output of the controller must be increased enough to overcome the dead zone, and only this corrective action will occur.
The main factors affecting the dead zone are as follows:
- (1) Friction, drift, shaft torsion, dead zone of amplifier. Various control valves are different in their sensitivity to friction. For example, rotary valves are very sensitive to the friction caused by high seat load, so this should be noticed when using them. However, for some types of seals, high seat load is necessary to obtain closure grade. In this way, the design of the valve is very poor, and it is easy to cause a large dead zone. The effect on the deviation of the process is obvious, and it is absolutely decisive.
- (2) Valve wear is inevitable in normal use, but the wear of the lubrication layer is the most severe. According to the experiment, the lubrication rotary valve only passes through hundreds of cycles, and the lubrication layer can almost be used with a rigid brush. In addition, the load caused by pressure can also lead to the wear of the sealing layer, which are the main factors leading to the increase of friction. The result is that the performance of the control valve is devastating.
- (3) Packing friction is the main source of friction of control valves. The friction caused by different fillers is very different.
- (4) Different types of actuators also have a fundamental impact on friction. Generally speaking, spring-film actuators are better than piston actuators.
The application fields of electric valves are canning/bottle system, brewing and beverage technology, chemical industry, fluid mixing device, food industry concrete and cement industry, vacuum technology, water treatment device, pneumatic device, laundry, medical equipment, boiler water supply, process control and other fields.
- Electric valves should be stored in dry and ventilated rooms, and both ends of the passage should be blocked.
- Electric valves stored for a long time should be inspected regularly to remove dirt and coated with rust-proof oil on the processing surface.
- After installation, regular inspections should be carried out. The main inspections are as follows:
- (1) Wear condition of sealing surface.
- (2) Trapezoidal thread wear of stem and stem nut.
- (3) Whether the filler is out of date or not, and if it is damaged, it should be replaced in time.
- (4) After the electric valve is repaired and assembled, the sealing performance test should be carried out.
Electric valves in operation, all kinds of valves should be complete and intact. Bolts on flange threads and brackets are indispensable. Threads should be intact and intact. No loosening is allowed. The fastening nuts on the handwheel should be tightened in time if loosening is found, so as not to wear the connection or lose the handwheel and nameplate. If the handwheel is lost, it is not allowed to be replaced by a live wrench. It should be matched in time. Packing gland is not allowed to be skewed or without pre-tightening clearance. For electric valves in environments susceptible to rain, snow, dust, sand and other pollutants, the valve stem should be equipped with protective cover. The ruler on the electric valve should be complete, accurate and clear. Lead seals, caps and pneumatic accessories of electric valves should be in good condition. Thermal insulation jacket should be free from depression and crack.
It is not allowed to knock, stand or support heavy objects on electric valves in operation, especially non-metallic electric valves and cast iron electric valves.
The electric device of the electric valve is one of the devices used to operate the valve and connect the valve. The device is driven by electricity and its motion process can be controlled by stroke, torque or axial thrust. Because the working characteristics and utilization of valve electric device depend on the type of valve, the working specifications of the device and the position of the valve on the pipeline or equipment. Therefore, it is very important to know the correct choice of valve electric device and to consider how to prevent overload (working torque is higher than control torque).
The Correct Selection of Electric Devices Should Be Based on
- Operating Torque: Operating Torque is the most important parameter for selecting valve electric device. The output torque of the electric device shall be 1.2-1.5 times of the maximum operating torque of the valve.
- Operating thrust: There are two main structures of the valve electric device, one is the direct output moment without the thrust disc, the other is the one with the thrust disc, at which time the output moment is converted to the output thrust through the valve stem nut in the thrust disc.
- Number of rotating rings of output shaft: The number of rotating rings of output shaft of valve electric device is related to the nominal diameter of valve, the pitch of valve stem and the number of thread heads, and is calculated according to M=H/ZS (in formula: M is the total number of rotating rings that should be satisfied by electric device; H is the opening height of valve, mm; S is the pitch of valve stem drive thread, mm; Z is the number of valve stem thread heads).
- Stem diameter: For open-rod valves of multi-turn type, if the maximum diameter of the stem allowed by the electric device can not pass through the valve stem of the matched valve, the electric valve can not be assembled. Therefore, the inner diameter of the hollow output shaft of the electric device must be larger than the outer diameter of the stem of the open stem valve. For some rotary valves and CONCEALED-ROD valves in multi-rotary valves, although the passage of stem diameter is not considered, the diameter of stem and the size of key groove should also be fully considered when selecting and matching, so that the valve can work normally after assembly.
- Output speed: valve opening and closing speed is fast, easy to produce water hammer phenomenon. Therefore, we should choose the appropriate opening and closing speed according to different conditions of use.
- Installation and connection modes: the installation modes of electric devices are vertical installation, horizontal installation and landing installation; connection modes are: thrust disc; valve rod passing through (open rod multi-rotary valve); dark rod multi-rotation; no thrust disc; valve rod not passing through; part of rotary electric devices are widely used, which are indispensable equipment to realize valve programmable control, automatic control and remote control, and they are mainly used in closure. On the road valve. However, the special requirements of valve electric devices should not be neglected – they must be able to limit the torque or axial force. Usually, the valve electric device adopts the coupling which restricts the torque.
When the specifications of electric devices are determined, the control torque is also determined. When it runs in a predetermined time, the motor usually does not overload. However, if the following conditions occur, it can be overloaded:
- Low power supply voltage, not get the required torque, so that the motor stop turning.
- Torque restriction mechanism was set incorrectly to make it larger than the stop torque, and consequently excessive torque was generated continuously, which made the motor stop turning.
- The heat generated by intermittent use such as point motion accumulates, exceeding the allowable temperature rise of the motor.
- For some reason, the circuit of the torque restriction mechanism fails, which makes the torque too large.
- The excessive ambient temperature reduces the heat capacity of the motor.
The above are some reasons for overload. The motor overheating caused by these reasons should be considered in advance, and measures should be taken to prevent overheating.
In the past, fuses, over-current relays, thermal relays and thermostats were used to protect motors. However, these methods have their own advantages and disadvantages. There is no absolutely reliable protection method for such load-changing equipment as electric devices. Therefore, it is necessary to adopt a combination of various methods. However, due to the different load conditions of each electric device, it is difficult to put forward a unified method. But in most cases, we can also find common ground.
Overload protection methods can be summarized into two types
- Judging the increase or decrease of motor input current;
- Judge the heat of the motor itself.
In either case, the time margin given by the thermal capacity of the motor should be taken into account. It is difficult to make it consistent with the thermal capacity characteristics of the motor in a single way. Therefore, we should choose a reliable way of action according to the cause of overload – combined mode, in order to achieve comprehensive overload protection.
The motor of the Rotock electric device, because it has a thermostat embedded in the winding which is the same as the insulation level of the motor, when the rated temperature is reached, the motor control circuit will be cut off. The thermostat itself has a small heat capacity, and its time-limited characteristics are determined by the heat capacity characteristics of the motor, so this is a reliable method.
Basic protection methods for overload
- The thermostat is used for the overload protection of motor running continuously or starting operation.
- The thermal relay is used to protect the motor from blocking.
- Fuses or over-current relays are used for short-circuit accidents.
The correct selection of valve electric device and the prevention of overload are closely related, which should be paid attention to.
Source: China Electric Valves 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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