Chemical Technology Process Package Design and Development Training Course
Unit One: Basic Concepts of Instruments
1. Measurement, Measurement Error, Direct Measurement and Indirect Measurement
1. What is measurement error? Answer: The difference between the measured value and the true value is called measurement error.
2. What are direct and indirect measurements? Answer: Direct measurement refers to comparing the measured parameter directly with a certain standard quantity. Indirect measurement involves substituting the directly measured data into certain formulas to calculate the required parameter value.
3. What is measurement? Answer: The process of converting and transmitting the measured parameter signal and comparing it with the corresponding measurement unit is called measurement.
2. Instrument Error, Variability, Sensitivity and Sensitivity Limits.
1. What is the allowable error of an instrument, instrument accuracy, and accuracy grade? Answer: Allowable error refers to the maximum percentage error allowed under specified normal conditions. Instrument accuracy refers to the value obtained by removing the percentage sign (%) from the allowable error. Accuracy grade is a series of standard percentage values defined uniformly by the state.
2. What is the variability of an instrument? Answer: It refers to the difference between the two measurements when using the same instrument to measure a certain parameter in a forward and reverse direction under unchanged external conditions.
3. What are the sensitivity and sensitivity limits of an instrument? Answer: Sensitivity expresses the degree to which the measuring instrument responds to changes in the measured parameter. Sensitivity is represented by the ratio of the change in the instrument’s measured value to the change in the measured parameter that caused this change. Sensitivity limit refers to the minimum change in the measured parameter that causes a visible change in the instrument’s indicated value.
4. What is the dynamic error of a measurement system? Answer: It refers to the difference between the instrument’s indicated value and the actual value of the measured parameter when the information of the measured parameter is in a fluctuating state.
3. Pressure, Engineering Atmospheric Pressure, Physical Atmospheric Pressure, Gauge Pressure, Absolute Pressure.
1. What is pressure? Answer: Pressure is the force acting vertically and uniformly on a unit area.
2. What is engineering atmospheric pressure? Answer: Engineering atmospheric pressure is a commonly used unit in industry, which is the pressure produced by 1 kg acting vertically and uniformly on an area of 1 square centimeter, expressed in kgf/cm².
3. What is physical atmospheric pressure? Answer: Physical atmospheric pressure equals the force produced by a mercury column of 760 mm height when the density of mercury is 13.5951 g/m³ and the gravitational acceleration is 980.665 cm/s².
4. Piezoelectric Effect, Magnetostrictive Effect, and Piezo-resistive Effect.
1. What is the piezoelectric effect? When certain crystals are mechanically deformed under pressure, anisotropic charges are generated on their opposing surfaces; this electrical phenomenon caused by deformation in the absence of an external electric field is called the piezoelectric effect.
2. What is the magnetostrictive effect? When ferromagnetic materials are subjected to pressure, not only does the mechanical stress within the material change with pressure, but the magnetic permeability of the material also changes; this phenomenon is known as the magnetostrictive effect.
3. What is the piezo-resistive effect? When semiconductor crystals are compressed, the symmetry of the crystal structure changes temporarily, altering the conductivity mechanism of the semiconductor, which manifests as a change in its resistivity; this effect is called the piezo-resistive effect.
5. Analog Display Instruments, Digital Display Instruments, Image Display Instruments.
1. Analog display instruments: These instruments simulate the continuous change of the measured parameter through the deflection angle or displacement of a pointer (recording pen). Their drawbacks include multiple transmission mechanisms, low accuracy, and slow response. Examples include moving coil display instruments, self-balancing display instruments, and automatic balancing display instruments.
2. Digital display instruments: These instruments directly display the measured parameter value in digital form, offering high accuracy and fast response.
3. Image display instruments: These instruments display information directly using graphics, characters, curves, and numbers on a screen.
Unit Two: Measurement Principles and Operation of Common Instruments
1. Measurement Principle and Operation of Thermal Resistance.
Principle: The temperature measurement principle utilizes the linear relationship between resistance value and temperature value within a certain temperature range of a thermistor.
Common thermal resistances include: platinum resistance, copper resistance, manganese resistance, and carbon resistance.
Operation: Thermal resistance employs a three-wire system when transmitting resistance signals to reduce measurement errors caused by the resistance of the wires. When checking its resistance value, pay attention to which two wires are common.
2. Measurement Principle and Operation of Thermocouples.
Principle: When two different metals are connected at one end, an electromotive force is generated between the other ends, and within a certain range, this electromotive force has a linear relationship with temperature.
Operation: A two-wire system is used, and the resistance value should be very small during checks.
3. Measurement Principle and Operation of Pressure Transmitters.
Principle: Thin film metal strain measuring elements.
Operation: When connecting wires, pay attention to the positive and negative terminals. When in use, open the hand valve slowly to avoid sudden impact on the diaphragm, and check for leaks at the instrument interface to prevent measurement errors. When disassembling, close the hand valve first, ensure it is closed, then slowly remove the instrument while shaking the transmitter to release any remaining gas.
4. Working Principle of Differential Pressure Level Gauges.
Principle: The diaphragm of a measuring box filled with silicone oil is connected through two coupling pipes, and the differential pressure creates a force that causes a twisting rod to deform. The twisting rod is placed in a thin film strain gauge arranged as a Wheatstone bridge to convert the pressure difference into a balanced voltage signal. The electronic amplifier converts the signal from the measuring chamber into a two-wire 4-20 mA DC output signal and can provide a Wheatstone bridge.
Operation: When connecting wires, pay attention to the positive and negative terminals. When in use, first open the balance valve of the transmitter, close the hand valves of the high and low-pressure chambers, open the hand valves on the pressure tap lines of the high and low-pressure chambers, and then slowly open the hand valves of the high and low-pressure chambers before closing the balance valve.
Unit Three: Basic Concepts of Automatic Control Systems
1. Regulated Object, Regulated Parameters, Regulating Parameters, Regulating Channel.
1. What is a regulated object? It refers to the equipment or machine being controlled during the production process.
2. What are regulated parameters? These are parameters within the regulated object that should be maintained within a predetermined amplitude and regulated.
3. What are regulating parameters? These are parameters that act on the regulated object and stabilize the regulated parameters.
4. What is disturbance? It refers to external factors that disrupt the system’s balance state and cause changes in the regulated parameters.
5. What is a disturbance channel? It refers to all links from the disturbance generation point to the regulated parameters.
2. Regulation Law, Transmitter, Regulator, Actuator.
1. What is regulation law?
It refers to the law governing the changes in the output signal of the regulator relative to the input signal over time.
When studying the regulation law of the regulator, the regulator is disconnected from the system to study the relationship between the output signal and the input signal independently. In analyzing the regulation law, a step signal is typically added at the input of the regulator; that is, when a certain deviation suddenly occurs, the output signal changes according to the step input signal.
The basic regulation law of the regulator is proportional (P), integral (I), derivative (D), and their combinations.
2. What is a transmitter?
The role of a transmitter in automatic detection and regulation systems is to convert various process parameters, such as pressure, differential pressure, temperature, flow, level, and composition, into corresponding standardized signals for transmission to indicating recorders, computers, and regulators for indication, recording, and regulation.
Transmitter types include differential pressure transmitters, pressure transmitters, temperature transmitters, flow transmitters, etc.
Composition: Typically consists of an input conversion part, amplifier, and feedback part. The input conversion part includes sensitive elements, which sense the measured parameters and convert them into a certain intermediate analog quantity. The intermediate quantity can be voltage, current, displacement, force, etc. The feedback part converts the output signal of the transmitter into a feedback signal. The amplifier amplifies the difference between the intermediate analog quantity and the feedback quantity and converts it into a standardized output signal.
3. What is a regulator?
A regulator typically performs PID calculations on the deviation between the input signal and the set signal and sends the result as a unified signal to the actuator for automatic regulation.
The regulator must have two key components for detecting deviation and performing PID calculations. The deviation detection circuit is generally referred to as the input circuit. The deviation signal is usually in voltage form, so both input and set signals are compared in voltage form within the input circuit. If the input signal is in current form, it must be converted into the corresponding voltage through a precision resistor. The input circuit must also include a switch for toggling between internal and external set circuits, a positive-negative action switch, and deviation indication parts. The PID calculation circuit is the key component that generates the regulation effect of the regulator, consisting of an amplifier and PID feedback circuit.
4. What is an actuator?
The role of an actuator in an automatic regulation system is to accept control signals from the regulating unit, causing changes in the opening of the regulating valve to achieve flow regulation.
3. Feedback, Positive Feedback, Negative Feedback.
1. Feedback: Refers to returning the system’s output signal back to the input in a certain way.
2. Positive feedback: Refers to feedback signals that increase the system’s input signal.
3. Negative feedback: Refers to feedback signals that decrease the system’s input signal.
4. Composition of Automatic Regulation Systems.
Automatic regulation systems consist of four parts: regulated object, regulating valve, measuring transmitter, and regulator.
5. Proportional, Integral, Derivative.
1. Proportional regulation law (P): Refers to a proportional relationship between the output signal and the deviation quantity. The advantage of proportional regulation is its fast response; the regulation effect is immediate when there is a deviation signal input, and the output varies proportionally with the deviation. The larger the input deviation signal, the stronger the output regulation effect; this is a significant feature of proportional regulators.
2. Integral regulation law (I): Its output signal is proportional to the integral of the deviation signal. When a deviation exists, the output signal of the integral regulator will continuously increase or decrease over time, and only when the input deviation equals zero will the output signal stop changing and stabilize at a certain value. The speed of change in the output signal of the regulator is proportional to the size of the input deviation and the integration speed, with the direction of the output change determined by the sign of the deviation.
3. Derivative regulation law (D): Refers to the output signal being proportional to the rate of change of the deviation signal. This type of regulator is used in systems where even a small deviation can trigger immediate regulation as long as a change trend appears, hence the term “lead” regulation. However, its output only reflects the rate of change of the deviation signal and cannot reflect the magnitude of the deviation, so it cannot be used alone and must be combined with proportional or integral regulation to form PD or PID regulators.
Unit Four: Principles, Classification, Characteristics, and Functions of Actuators
1. Principles of Actuators.
1. Working principle of electric actuators: The actuator consists of a servo motor, mechanical reducer, and position transmitter. The actuator receives output signals from the servo amplifier or electric operator, controlling the forward and reverse rotation of the servo motor, which, after the mechanical reducer, generates output torque to drive the regulating mechanism. Meanwhile, the position transmitter converts the angular displacement of the regulating mechanism into the corresponding 4-20 mADC signal to indicate the valve position and feedback to the input of the front magnetic amplifier as a position feedback signal to balance the input signal.
2. Working principle of pneumatic actuators: Pneumatic actuators receive the pressure signal output from pneumatic regulators or valve positioners and convert it into corresponding linear displacement of the push rod to drive the regulating mechanism.
2. Classification and Characteristics of Actuators.
Actuators can be classified into three main categories based on the form of energy used: pneumatic, electric, and hydraulic.
1. Actuators operated by pneumatic mechanisms are called pneumatic actuators or pneumatic control valves;
2. Actuators operated by electric mechanisms are called electric actuators or electric control valves;
3. Actuators operated by hydraulic mechanisms are called hydraulic actuators or hydraulic control valves;
Characteristics:
1. Pneumatic actuators have advantages such as simple structure, reliable and stable operation, large output force, convenient installation and maintenance, low cost, and fire and explosion resistance, making them widely used in petroleum, chemical, metallurgy, and electric power sectors. Their disadvantage is significant lag, making them unsuitable for long-distance transmission (limited to within 150 meters). To overcome this disadvantage, electric/pneumatic converters or electric/pneumatic valve positioners can be used to convert the transmitted signal to an electric signal while the field operation remains pneumatic.
2. Electric actuators have advantages such as fast action, especially suitable for long-distance signal transmission, and ease of integration with electronic computers. Generally, electric actuators are not suitable for fire and explosion-prone environments. However, if designed with explosion-proof structures, they can achieve fire and explosion resistance.
3. Selection Principles for Normally Open and Normally Closed Control Valves.
Selection Principle: When the pressure signal is interrupted, the safety of the equipment and operators must be ensured.
4. Functions of Electric/Pneumatic Valve Positioners.
Functions:
1. Improve the sensitivity and accuracy of pneumatic actuators, enhancing the static characteristics of pneumatic actuators.
The following factors that affect the sensitivity and accuracy of pneumatic actuators can be reduced:
a. Instability of the diaphragm and spring in the actuator part and the friction of all movable parts.
b. The unbalanced force caused by excessive pressure difference before and after the control valve.
c. The resistance against the movement of the valve stem caused by the high viscosity of the regulating medium or the presence of suspended solids, etc.
2. Accelerate the movement speed of the valve stem, reducing the system’s transmission lag.
Unit Five: Concepts and Measures of Explosion-Proof Instruments
1. Flameproof Instruments.
These are instruments whose casings can withstand the pressure of an explosion occurring inside them without causing an explosion outside; denoted by ‘d’.
2. Intrinsically Safe Instruments.
These are instruments whose electrical circuits do not generate sparks or heat effects that can ignite specified explosive mixtures under normal or fault conditions; denoted by ‘ia’.
3. Safety Barriers and Types of Safety Barriers.
Safety barriers are installed in the control room and serve as devices connecting control room instruments with field instruments. They transmit signals and control the energy flow into hazardous areas to ensure the system’s safe spark performance.
Types of Safety Barriers:
a. Resistor-type safety barriers: These use the limiting effect of resistors to keep the energy flowing into hazardous areas below a critical value to achieve explosion-proof purposes.
b. Zener safety barriers: These work based on the reverse breakdown performance of Zener diodes.
c. Relay amplifier-type safety barriers: These are developed from resistor-type safety barriers, utilizing high input impedance of amplifiers to increase the resistance in series in the input circuit to achieve safe spark prevention.
d. Isolating safety barriers: These limit the energy flowing into hazardous areas through isolation, pressure limitation, and current limitation measures to ensure safe spark performance. Main measures include insulation and energy limitation.
4. Precautions for Using Explosion-Proof Instruments.
1. Check whether the instrument casing has an EX mark and ensure that the explosion-proof mark corresponds with the regulations for hazardous substances on-site.
2. Intrinsically safe transmitters must be equipped with safety barriers to be used in hazardous locations.
3. The instrument casing must have good grounding.
4. In hazardous locations, power must be disconnected before opening the cover.
5. The specifications of the incoming cables for intrinsically safe transmitters are determined by the safety barriers certified in conjunction.
Unit Six: Basic Electrical Knowledge
1. Basic Electrical Knowledge
1. Ohm’s Law: The current I flowing through a resistor R is proportional to the voltage U across it. Its mathematical expression is: U=R*I
2. Total Circuit Ohm’s Law: The total circuit refers to a single loop. If the electromotive force of the power supply in the single loop is E, the internal resistance is r, and the load resistance is R, then the current is equal to the electromotive force divided by the total resistance of the loop. Its mathematical expression is: I=E/(R+r).
3. Kirchhoff’s Laws:
1. Current Law: At any moment, the sum of all currents entering a node equals the sum of all currents flowing out.
2. Voltage Law: The algebraic sum of the voltages around any closed loop equals zero.
4. Series and Parallel Resistance:
Series: When resistors are connected one after another, it is called series. The characteristic is that the current through each resistor is the same. The equivalent resistance R of series resistors is the sum of all resistors. R=R1+R2+R3+…+Rn
Parallel: When one end of each resistor is connected together and the other end is also connected together, it is called parallel. The characteristic is that each resistor experiences the same voltage. The reciprocal of the equivalent resistance of parallel resistors equals the sum of the reciprocals of all resistors. 1/R=1/R1+1/R2+1/R3+…+1/Rn
2. Working Principle of Variable Frequency Speed Regulation for Three-Phase Asynchronous Motors
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Operational Skills
1. Reading and Understanding Automatic Control Process Flow Diagrams
1. Symbols and Meanings
TCV–Temperature Control Valve TW–Temperature Measurement Test Head with Sleeve LI–Level Indicator
TI/T–Temperature Indicator TR–Temperature Recorder TS–Manual Multi-point Temperature Switching Indicator
TJI–Multi-point Temperature巡回 Indicator TIC–Temperature Indicating Regulating System with Manual Switching
PCV–Pressure Control Valve PI/P–Pressure or Vacuum Indicator PIS/PA–Pressure or Vacuum Alarm Indicator PISA–Pressure Interlock Alarm PIT/PRA–Centralized Pressure Indicator Recording Alarm
PFI–Compression Ratio Indicator TRC/PIC–Temperature Pressure Cascade Control System FO–Flow Limiting Orifice
FR–Flow Recorder FRQ–Flow Recorder Accumulator FI–Local Flow Indicator FIQ–Flow Indicator Accumulator TE/FRQ–Flow Recorder and Accumulator with Temperature Compensation LI/LX/TVR–Television Monitoring Level Gauge
2. Clarify the relationships between various points: for example, the adjustment amount of the LV level control valve is controlled by the measurement results of the LT level transmitter.
2. Handling Common DCS Faults
1. Alarm Fault: Some points in the DCS control system are equipped with alarm signals. When handling alarm signals, it is essential to distinguish the alarm categories, such as: if there is no liquid level in the tank, causing a low-level alarm, analyze the reasons for the low alarm, such as the inlet valve not operating, poor regulation relationship settings, inaccurate measurements by the level transmitter, or too little liquid quantity, and handle based on the inspection results.
2. Operating Station Freeze: Simply activate the operating station.
3. Loose Terminals: Regularly tighten screws.
Unit Three: Usage of Digital Multimeters and Signal Generators
Usage of Digital Multimeters
1. Determining the Digit Count of Digital Multimeters: Common digital multimeters have 31/2 digits, 32/3 digits, 33/4 digits, 41/2 digits, etc. Generally, 31/2 to 41/2 digit digital multimeters are portable, while those with 51/2 digits and above are desktop models. The 31/2 digit displays values from 0000 to 1999, 32/3 digit displays from 0000 to 2999, and 33/4 digit displays from 0000 to 3999.
2. Resolution of Digital Multimeters: The resolution of a digital multimeter is the physical quantity value represented by the last digit; resolution increases proportionally with the range. For example, the resolution of the DT-930-F DC voltage at the 200mV range is 10μV, while at the 2V range, it is 100μV.
3. Characteristics of Automatic Range Switching: Some mid-to-high-end digital multimeters can automatically switch ranges during measurement, reducing measurement errors and making usage very convenient. However, each measurement starts from the maximum range and gradually decreases to the appropriate range, resulting in longer wait times for readings when measuring small values.
4. Comparison of Digital Multimeters with Analog Multimeters: Higher accuracy; easier reading; higher sensitivity; convenient to use; wide measurement range; overload protection; minimal impact on circuit state when measuring voltage or current.
5. How to Measure Voltage:
1. Insert the black probe into the “COM” hole and the red probe into the “V/Ω” hole.
2. Set the range switch to the appropriate range, and connect the probes in parallel with the power source or load for measurement;
6. How to Measure Current:
1. Insert the black probe into the “COM” hole and the red probe into the “A” hole (for currents below 200mA) or into the “10A” hole (for currents between 200mA and 10A).
2. Set the range switch to the appropriate range, and connect the probes in series with the load for measurement;
7. How to Measure Resistance:
1. Insert the black probe into the “COM” hole and the red probe into the “V/Ω” hole.
2. Set the range switch to the appropriate range, and connect the probes in parallel with the load for measurement;
3. Ensure the power supply to the circuit is turned off when measuring resistance.
Usage of Low-Frequency Signal Generators
Using the XD-2 type signal generator as an example:
1. Turn the “output fine adjustment” knob on the instrument panel counterclockwise to its lowest setting, power on the device, and preheat for 3-5 minutes;
2. Frequency Selection: Select the desired frequency by setting the “frequency range” switch on the panel to the corresponding gear, then use the three “frequency adjustment” knobs “*1”, “*0.1”, “*0.01” to adjust to the desired frequency according to decimal principles.
3. Voltage Output: The sine voltage is output from the “output” terminal on the panel. Adjust the “output fine adjustment” knob to reflect a certain value on the voltmeter while setting the “output attenuation” switch to a certain position (changing the “output attenuation” switch does not affect the voltmeter reading); at this point, the output voltage amplitude is the voltmeter reading reduced by the corresponding multiple of the selected “output attenuation” dB value.
Source: Network
Editor: Chemical Engineer Club
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