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SAMSON Electric Valve Positioner Improves Control Valve Performance
Valve positioner along with the control valve has been used for decades, the emergence of valve positioner, the control valve control accuracy, anti-interference ability, response time, flow characteristics have been greatly improved, and now almost All control valves use a positioner, which has become an indispensable partner for control valves and plays an increasingly important role in controlled components. This article describes the working principle and function of the SAMSON valve positioner, by enumerating the analog and digital valve positioner, so that we can grasp the valve positioner more fully, the purpose is for us to better understand and apply the valve positioner. Although the valve positioner from the original gas / gas valve positioner, electrical / gas valve positioner to the current digital valve positioner, regional bus valve positioner, but their basic principles and main functions are not major changes. SAMSON valve positioner also follow the development of control technology, has gone through the pneumatic, electric, digital, regional bus valve positioner developed to the present. Among similar products in the world, SAMSON's valve positioner has won wide acclaim for its compact structure, low air consumption, reliable operation and many additional optional functions in the positioner. In order to facilitate the discussion, we first review the basic components in the locator. Introduction to the basic self-control components in the positioner - Electricity / gas converter principle With the development of instrument technology, the field of pneumatic instruments has been gradually occupied by the control of electric instruments and computers. Now only aerodynamic instruments are still used in some special occasions. The valve attachment "positioner" in the gauge is also being replaced by an electro-pneumatic (E / P) valve positioner from the original pneumatic valve (P / P) positioner. So in the electric / gas valve positioner into the electrical signal is how to convert it into gas signal? We SAMSON 6111 electric / gas converter as an example to introduce its working principle (see Figure 1): Figure 1 Function Diagram of 6111 Figure 1A Type 4763 pneumatic power amplifier (8) at the time of design; use the appropriate spring force ( 8.2) so that the output PA is kept at 100 mbar when the input signal is 0 mA, so that the output pressure will make a certain back pressure in the nozzle (7) through the constant orifice (8.4). When the input signal increases, the energized coil (2) cuts the lines of magnetic force of the permanent magnet (3) and generates an upward force → the back plate pressure (PK) is increased by the baffle (6) approaching the nozzle ↓ → Open spool (8.5) → Output PA ↑. (6) leaves the nozzle (7) → back pressure (PK) decreases → output pressure (PA) diaphragm (8.3) ↑ → spool (8.5) shut down → output pressure through the valve The core (8.5) is released. The output pressure remains the same when PA equilibrates with PK; the force generated by the electrical signal in coil (2) is also balanced with backpressure (PK). The electrical signals thus input are converted into gas signals. Positioner to SAMSON 4763 electric / gas valve positioner (Figure 1A) as an example, the main part of the locator shown in Figure 2. Figure 2 1. Feedback lever (1) 2. Feedback spring (6) 3. Feedback windbox (7) 4. Pneumatic power amplifier (7 lower part) 5. Electrical / pneumatic converter (21) Take SAMSON's 4763 locator as an example (see Figure 3). Figure 3 A) Valve position proportional to the input signal input signal ↑ → Pe point pressure ↑ → feedback bellows connecting rod (9) to the left → Compression spring (6), baffle (10.2) close to nozzle (10.1) → Output air pressure ↑ → Stem (for open valve) ↑ → Compression spring (6) → Feedback bellows link Action → Baffle (10.2) Leave the nozzle (10.1) → Output air pressure (Pst) ↓. When the force of the feedback spring is balanced with the force of the feedback bellows, the position of the valve remains the same as the input signal. B) Positioning When the input signal is not changed: Stem ↑ → Compaction actuator spring → Compression spring (6) → Connecting rod (9) in feedback air box rightward → Baffle (10.2) leaves due to changing process conditions Nozzle (10.1) → Output pressure ↓ → The spring force from the actuator moves the valve back to its original position. Due to changes in process conditions, the valve stem ↓ → release actuator spring → release spring (6) → move the connecting rod (9) in the feedback bellows to the left → baffle (10.2) close to the nozzle (10.1) → output air pressure ↑ → make the actuator The upward movement returns the valve position to its original position. 2. Digital Valve Positioner Digital valve positioner and analog valve positioner the main difference is: deviation calculation, feedback signal processing, input / output signal processing using digital integrated circuits and microprocessors for processing. Take SAMSON TYPE 3780 as an example, refer to Figure 4 and Figure 5: Figure 4 3780 Positinoer Figure 5 Function Diagram of 3780 Locator Input The analog signal is A / D converted to the microprocessor (2); the change signal of the valve passes the position The sensor (5) and A / D conversion also arrive at the microprocessor. Microprocessor based on the input signal and valve position signal deviation through the calculation and output control signal so that the gas supply solenoid valve 3 and exhaust solenoid valve 4 work so that the valve position and the input signal corresponding. The HART signal used for communication passes through the demodulator (1) to the microprocessor. 3. HART Locator The main differences between the HART locator and the analog locator are: 1) While the locator receives the 4 ~ 20mA analog signal, it can also receive a signal that is superimposed on the 4 ~ 20mA analog signal with a HART transmission protocol The exchange of signals. 2) Analog 4 ~ 20mA signal is used to control the valve position, the positioner can use analog signal generator valve position adjustment. 3) The HART signal is mainly used for the setting and configuration of the positioner as well as the fault diagnosis of the valve (depending on the positioner function). 4.PROFIBUS and Fieldbus locator main features are: 1) The signal transmission is through PROFIBUS or Fieldbus protocol area bus. 2) The positioner can not directly accept 4 ~ 20mA analog signal. 3) The locator contains input, output and PID control modules that conform to the relevant protocols. 4) Check and adjust the positioner; it must be done with equipment that can communicate with the positioner and contains the application software. The basic functions of the positioner 1. Proportional action and positioning action Proportional action: According to the input signal, the valve position of the valve corresponds to the input signal. Positioning role: When the input signal is fixed, the valve position from the process conditions change. 2. Power Amplifier For pneumatic input signal, the positioner can input the gas signal; through the positioner pneumatic power amplifier to amplify, so that the tiny signal can control the valve action. 3. To improve the control accuracy of the valve positioner is based on the input signal and the valve position deviation of the output signal to adjust, once the input signal and the valve position deviation, the positioner will automatically adjust the output signal to change the valve position until the valve position Corresponding to the input signal so far, so greatly improve the control accuracy of the valve. 4. To overcome the friction Due to the positioning of the positioner itself closed-loop control, when the friction changes (referring to the stem of the filler, the actuator seals and other parts of the friction); Locator can be caused by friction caused by the position deviation, automatic Increase or decrease the output pressure to the actuator to overcome the friction on the valve opening impact. 5. Change the way through the positioner we can change the role of the valve. According to the function of the valve, we can set the positive and negative reaction of the positioner according to Table 1. 6. Change the valve characteristics of the flow characteristics of the valve, in addition to valve manufacturing by changing the shape of the spool / seat to achieve; we can also be in the locator, by changing the feedback (analog positioner) and set the parameters ( Digital positioner) to change the flow characteristics of the valve. There are only digital positioners (3730-2 / 3, 3780, 3785, 3787) in the SAMSON positioner and 3767 analog positioner in the angular travel application; the flow characteristics of the valve can be changed by the positioner. Valve with the positioner to change the nature of the flow characteristics: By changing the relationship between the positioner input signal and valve position to change the valve opening, to achieve different valve flow characteristics. Let's take a look at Table 2 to see how the percentage-like characteristics of the shape of the spool and the percentage-like characteristics of the positioner can be achieved. From Table 2 we can find that when the flow characteristics of the control valve are changed by the shape of the valve plug, their opening degree (stroke sign) corresponds to the input signal regardless of the valve with linear and equal percentage characteristics. (But the actual valve internal opening is different) and through the positioner to achieve the same percentage of the valve opening; relative linearity, the input signal with the valve position is significantly different, and their relationship with the valve Structure and type as well as valve inherent flow characteristics. If the inherent flow characteristics of the valve are linear, we can easily use the positioner to change their characteristics, on the contrary we should exercise caution. 7. Signal Conversion / Splitter Control We can achieve the electrical / gas conversion through the positioner, you can also use the positioner to achieve the valve's split control. 8. Valve Fault Diagnosis (Digital Positioner) In the digital positioner, we can use the positioner to diagnose valve faults. 9. Integrate Other Functions (Reversing Switch, Solenoid Valve, Position Transmitter, etc.) With the positioner we can integrate the Reversing Switch, Solenoid Valve, Position Transmitter, etc. so that between the accessories of the valve The connection is more compact and reliable, but also improve the performance-to-price ratio of the positioner. Positioner Category There are many types and functions of SAMSON positioner. The main types we can summarize are as follows: 1. Pneumatic Positioner Pneumatic Positioners are: Type 3760,4765,3766,3761. 2. Electric / gas valve positioner analog positioners are: Type 3730-1,3760,3761,3766,4763; digital positioners are: Type 3730-2, 3730-3 (HART), 3780 (HART), 3785 ( PROFIBUS), 3787 (Fieldbus). 3. Electric valve positioner analog electric valve positioner: has been integrated in the 3274,5824 and other electric actuators. Conclusion Since the positioner plays an important role in the use of the valve, how to choose a good positioner in practical applications, the valve performance and cost, the stability of the process control has a great relationship, we only fully understand and understand the positioner, In order to give full play to its role.