Electric actuator (electric actuator) is also called valve electric device or electric head. It is called in different industries. It is called valve electric device in industrial pipeline valve industry, and it is called electric actuator in electric instrument industry (electric actuator). mechanism).
Valves are very important equipment that is often used in industrial pipeline control. Electric valves are easy to obtain due to their power source and generally require no maintenance. They are more commonly used than pneumatic and hydraulic drive equipment. In industrial occasions, electric valves must have higher reliability and safety. The safety and reliability of electric valves usually depend on electric actuators (electric actuators), so the performance, control accuracy, function, The intelligent program is the comprehensive performance of the technical level of the electric valve machine. Therefore, in addition to some basic elements that must be considered when selecting an electric actuator (electric actuator), reasonable technical requirements should be put forward to maximize the value of the electric valve.
There are many types of electric actuators (electric actuators). Different types and functions of electric actuators (electric actuators) can be called electric valves after they are matched with valves. However,they often only pay attention to valves during the design and selection process. Ignore the parameters of the electric actuator (electric actuator), which not only makes the electric valve not play the best performance, but also brings unnecessary trouble during installation, commissioning and use, It even has serious consequences for production.
This article will explain the main considerations for the selection of electric actuators (electric actuators), as well as a brief introduction to the selection methods and techniques of electric actuators.Key points for selection of electric actuators (electric actuators)
1.Choose electric actuator(electric actuator)according to valve type
There are many types of valves,and the working principle is also different.Generally,the opening and closing control is realized by rotating the valve plate angle,lifting the valve plate(or spool),etc.When matching with the electric actuator(electric actuator),the first Choose electric actuator(electric actuator)according to the type of valve.1.Angular travel electric actuator(electric actuator)(rotation angle <360 degrees)
The rotation of the output shaft of the electric actuator(electric actuator)is less than one week,that is,less than 360 degrees(sometimes called the angle within 90 degrees and other strokes,greater than 90 degrees and less than 360 degrees is called partial rotation type),usually 90 degrees to achieve the valve Open and close process control.This type of electric actuator is divided into direct connection type and base crank type according to the different installation interface methods("Selection methods and skills of electric actuators"are explained in more detail).
a)Direct connection:refers to the form of direct connection installation of the output shaft of the electric actuator (electric actuator)and the valve stem.
b)Base crank type:refers to the form where the output shaft is connected to the valve stem through the crank.
This type of electric actuator is suitable for butterfly valves,ball valves,plug valves,etc.2.Multi-turn electric actuator(electric actuator)(rotation angle>360 degrees)
The rotation of the output shaft of the electric actuator is greater than one week,that is,greater than 360 degrees,and generally requires multiple turns to achieve the control of the valve opening and closing process.
This type of electric actuator is suitable for gate valves,globe valves,etc.
3.Straight travel (linear motion)
The movement of the output shaft of the electric actuator(electric actuator)is a linear movement type,not a rotating form.
This type of electric actuator is suitable for single-seat control valves and double-seat control valves.
2.Determine the control mode of the electric actuator (electric actuator)according to the production process control requirements PID adjustment function)
1.Switch type (open loop control)
Switch-type electric actuators (electric actuators)generally realize the opening or closing control of the valve.The valve is either in the fully open position or in the fully closed position.Such valves do not require precise control of the medium flow (incompletely stated here),There is also a switch type with valve position feedback form,the feedback signal is connected to DCS or other controllers to perform PID calculation,and then the actuator is controlled to open or close to achieve the purpose of controlling the opening degree).
It is particularly worth mentioning that the switch-type electric actuator (electric actuator)can also be divided into a split structure and an integrated structure due to different structural forms.This must be explained when selecting the model,otherwise there will often be mismatches such as conflicts with the control system during field installation.
a)Split structure(commonly referred to as ordinary type):The control unit is separated from the electric actuator.The electric actuator cannot control the valve alone,and the control unit must be added to achieve the control.Generally,the controller or control cabinet is used for external control.Supporting
The disadvantage of this structure is that it is not convenient for the overall installation of the system,which increases the wiring and installation costs,and is prone to failure.When the failure occurs,it is not easy to diagnose and maintain,and the cost performance is not ideal(but the advantage is suitable for high temperature occasions,because the control circuit can be far away from the actuator In a high temperature environment).
b)Integrated structure (usually called integral type):The control unit and the electric actuator are packaged into one body,which can be operated on the spot without external control unit,and it can be operated by outputting related control information remotely.
The advantage of this structure is that it is convenient for the overall installation of the system,reducing wiring and installation costs,and easy to diagnose and troubleshoot.But the traditional integrated described later.
2.Adjustable type (closed-loop control)(ie servo integration)
The regulated electric actuator (electric actuator)not only has the function of a switch-type integrated structure,it can also accurately control the valve,thereby accurately regulating the medium flow. Due to space limitations,its working principle will not be described in detail here.The following is a brief description of the parameters that need to be noted when selecting an adjustable electric actuator
a )Control signal type (current,voltage)
The control signals of adjustable electric actuators(electric actuators)generally include current signals(4~20mA,0~10mA)or voltage signals(0~5V,1~5V).The type and parameters of control signals need to be clarified when selecting a model(Currently,4-20mA is the most commonly used).
b )Working form(electric open type,electric close type)(that is,positive action and reverse action)
The working mode of the regulated electric actuator(electric actuator)is generally the electric open type(taking 4-20mA control as an example,the electric open type means that the 4mA signal corresponds to the valve close,and 20mA corresponds to the valve open),another The type is electrically closed(taking 4-20mA control as an example,the electrically open type means that the 4mA signal corresponds to valve opening,and 20mA corresponds to valve closing).Under normal circumstances,the type selection requires a clear working form.Many products cannot be modified after leaving the factory (should be part of the product),and most intelligent electric actuators(such as CONAC series,NDL,NDQ series)can be modified at any time through on-site settings(For example,if parameters or jumpers are modified,the traditional analog control board must either change the jumpers or the motor forward and reverse wiring and sensor wiring.
c )Loss of signal protection
Loss of signal protection means that when the control signal is lost due to faults such as lines,the electric actuator (electric actuator)will open and close the control valve to the set protection value.The common protection values are full open,full close,and maintain the original position.In this case,and it is not easy to modify after leaving the factory (it should be difficult to modify the old analog control form),most of the intelligent electric actuators can be flexibly modified through on-site settings,such as CONAC series,NDL,NDQ series,etc.can be modified by menu parameters To achieve,where NDL and NDQ series can set the valve position to any set position when the signal is cut off.
3.Determine the output torque of the electric actuator (electric actuator)according to the torque required by the valve (thrust can be input)
The torque required for valve opening and closing determines how much output torque the electric actuator (electric actuator)chooses.Generally,it is proposed by the user or selected by the valve manufacturer.As an electric actuator(electric actuator),the manufacturer only applies the actuator It is responsible for the output torque of the valve.The torque required for the normal opening and closing of the valve is determined by the size of the valve,the working pressure and other factors.However,due to the difference in the processing accuracy and assembly process of the valve manufacturer,the torque required for the valves of the same specification produced by different manufacturers also has The difference is that even the torque of the same valve produced by the same valve manufacturer is one.Reasonable torque range.(The article"Method and Tips for the Selection of Electric Actuator"has a more detailed explanation)
Fourth,determine the electrical parameters
Because the electrical parameters of different electric actuators(electric actuators)manufacturers are different,the electrical parameters generally need to be determined when designing and selecting mainly including motor power,rated current,secondary control loop voltage,etc.,often in this regard As a result,the control system and electric actuator parameters did not match,resulting in malfunctions such as tripping,tripping of fuses,thermal overload relay protection and other faults.
Fifth,choose the shell protection level and explosion-proof level according to the occasion of use
1.Enclosure rating
The enclosure protection level refers to the protection against foreign objects and water resistance of the electric actuator casing.It is represented by the letter IP followed by two digits.The first digit is from 1 to 6 indicating the level of foreign substance(see Table 1),and the second digit is 1 to 8 indicate the waterproof rating (see Table 2).
Table 1. The level of anti-foreign objects represented by the first digit
Table 2. Waterproof level represented by the second digit
2. Explosion-proof grade In places where explosive gas, steam, liquid, flammable dust, etc. may cause fire or explosion hazards, explosion-proof requirements must be put forward for electric actuators, and the explosion-proof form and category should be selected according to different application areas. The explosion-proof grade can be expressed by the explosion-proof mark EX and explosion-proof content (refer to "Explosion-proof Electrical Equipment for Explosive Environments" GB3836-2000).
The contents of the explosion-proof mark include: explosion-proof type + equipment category + (gas group) + temperature group
a) Explosion-proof type: According to the explosion-proof measures taken, it can be divided into intrinsically safe type, flameproof type, increased safety type, positive pressure type, pouring type, sand filling type, etc. Their logos are shown in Table 3 below.
Table 3: Explosion-proof forms and signs
b) Equipment category:
Table 4. Equipment categories
Class II flameproof "Exd" and intrinsically safe "Exi" electrical equipment are further divided into categories IIA, IIB, and IIC according to their maximum test safety clearance or minimum ignition current ratio applicable to explosive gas mixtures. The relationship between them is shown in Table 5 below:
Table 5. Gas Group
c) Temperature category:
Electrical equipment is divided into T1~T6 groups according to its maximum surface temperature, so that the maximum surface temperature of the corresponding T1~T6 group of electrical equipment cannot exceed the allowable value of the corresponding temperature group. The relationship between temperature group, equipment surface temperature and ignition temperature of flammable gas or steam is shown in Table 6 below:
Table 6. Temperature Group
Explosion-proof electric actuators sold by manufacturers of electric actuators (electric actuators) must obtain an explosion-proof certificate. However, some users do not follow the rules during installation, which will greatly reduce the explosion-proof performance. For example, the fine cable does not meet the requirements of the electric actuator (electric actuator). If the cable is too thin, it will leave a gap at the cable entrance and not reach To explosion-proof performance.
Selection methods and skills of electric actuators
The selection of electric actuators (electric actuators) is mainly carried out from the following aspects:
1. Structure type of electric actuator (electric actuator)
According to the movement mode of the controlled object, electric actuators are divided into the following structural types: angular stroke, straight stroke, partial rotation type, and multi-rotation type. According to the connection and installation methods (generally for angular stroke electric actuators (electric actuators), it is divided into base type and direct connection type.
Let's talk about what kind of structure type electric actuator (electric actuator) is selected for various valves and dampers, or what kind of controlled object is suitable for electric actuators (electric actuators) of various structures.
1. Angular travel electric actuator (electric actuator)
The motion mode of the output shaft of the electric actuator with angular stroke (electric actuator) rotates by angle, and the general rotation range is 0~90 degrees. Such actuators are generally suitable for dampers, butterfly valves, ball valves, V-shaped valves, etc.
Angular travel electric actuators (electric actuators) are divided into direct connection type and base type according to the connection mode. When the direct connection type is connected, the output shaft of the electric actuator is directly connected to the valve stem. The base type connection method is to connect the actuator output shaft and the valve stem by means of a ball hinge + connecting rod.
2. Straight travel electric actuator (electric actuator)
Straight travel electric actuator (electric actuator) output shaft movement is linear movement, so this type of actuator is suitable for valves with linear movement (except for shut-off valves and gate valves, which will be discussed later). Seat valve, double seat valve, sleeve valve, angle valve, three-way valve, diaphragm valve, etc.
3. Multi-turn electric actuator (electric actuator)
The multi-turn electric actuator (electric actuator) output shaft movement mode is rotary, and the full stroke exceeds 360 degrees. It is suitable for controlled objects such as gate valves and globe valves. Multi-turn electric actuators are special and can be reconfigured. The first-stage reducer is converted into an angular-stroke electric actuator (electric actuator) or a straight-stroke electric actuator (electric actuator), or even a multi-turn type. The output force (torque) increases after the first-stage deceleration, and the movement speed Slow down.
2. The output force (torque) of the electric actuator (electric actuator)
The most important parameter of electric actuators (electric actuators) is the output force. For straight-stroke electric actuators (electric actuators), the unit is N or Kg, 10N = 1Kg, which is called thrust. For angular electric actuators For the mechanism (electric actuator), the unit is Nm or Kg.m, 10Nm=1Kgm. The size of the output force should be determined according to the maximum driving force of the controlled object. Generally, the actuator needs to retain 30% of the margin to be insured. Since the calculation of the driving force of the valve is very complicated, the following gives some common valve empirical data:
Commonly used valves require driving force for application actuators (30% allowance has been set aside)
Gate valve (caliber 25 to 1800mm, pressure 1 to 320Kg/cm2 or 1 to 32MPa, torque unit Kg.m)
Globe valve (caliber 15 to 350mm, pressure 6 to 320Kg/cm2, torque unit Kg.m)
Ball valve (caliber 50 to 1000mm, pressure 6 to 100Kg/cm2, torque unit Kg.m)
Butterfly valve (caliber 50 to 2000mm, pressure 2.5 to 25Kg/cm2, torque unit Kg.m)
3. Stroke of electric actuator (electric actuator)
This parameter is also a more important parameter. For the angular stroke electric actuator (electric actuator), the stroke is fixed at 0~90 degrees. For the straight stroke, the stroke unit is mm, which needs to be adjusted according to the valve spool stroke. It is determined that the straight stroke electric actuator (electric actuator) conventional stroke has the following gears: 10mm, 16mm, 25mm, 40mm, 60mm, 100mm. For multi-turn electric actuators (electric actuators), in general, the number of turns of such actuators can meet the needs.
Four, electrical interface
Determine what kind of signal the electric actuator (electric actuator) needs to feed back to determine the control method of the electric actuator (electric actuator).
way to control:
1. Analog quantity control: The general analog quantity control is 4~20mA, and there are also 0~10mA, 1~5V, etc. The latter two are not used much now. Generally adjustable (integrated) electric actuators can be controlled by analog.
2. Switching control: the general switching control signal is "on" and "off" command signals, and there may also be a "stop" command signal. The switching quantity mainly includes the following types: passive, 24V DC, 220V AC. Generally, if a servo amplifier is equipped, the switching quantity is 220V AC. If the servo amplifier is not used to control the actuator and the 220V AC switch-type electric actuator (electric actuator) is selected, it is necessary to pay attention that the 220V switching power provided must be greater than the actuator motor power.
Feedback signal:
1. Analog feedback: Analog feedback signals are all the valve position feedback signals corresponding to the current valve position output by the electric actuator (electric actuator). There are also 4~20mA, 0~10mA and 1~5V. Both are now rarely used. Generally, those equipped with servo amplifiers require analog valve position feedback signals. If the DCS or display instrument needs to display the current valve position, the electric actuator must also require an analog valve position feedback signal
2. Switch feedback signal: According to the process requirements, it may be necessary for the electric actuator to output a fully closed and fully open switch signal, and even other meaningful signals, such as an actuator failure alarm signal. Generally, the switching value is passive, but it is also active.
In addition to the several important parameters mentioned above, some parameters are not so important, but may also be very important according to process requirements, such as installation size, weight, control accuracy, full travel time, protection level, etc.
