Gas and Air Automation System

Contents

1. Introduction

1.1. Automatic Control Philosophy

1.2. Main tasks of the automatic boiler combustion control system

2. Main part

2.1. Automatic control of combustion process economy (supply

air)

2.2. Description of functional diagram of control object automation

2.3. Measurement of flow rate of liquid, gases or steam by flow meters with narrowing device

2.4 Selection of automation tools for measurement of pressure and flow rate of gas, steam and air

2.5.Selection of actuators

2.6 Selection of regulators

2.7 Interface Adapter Selection

2.8 Selection of secondary devices

2.9. Analyzing Dynamic Properties of an Object

2.10. Construction and description of generalized functional and structural diagrams

automation systems

2.11. Defining Object Parameters

2.12. Selecting Settings for the Adjuster

2.13. Calculation of automation system stability

3. Conclusion

4. List of used literature

1. Introduction

1.1. Automatic Control Philosophy

The construction of automatic control systems is based on the application or combination of two basic control principles.

The principle of automatic control for deviation of the controlled value from the set value has become the most widespread. The advantage of such systems is that the controlled amount is continuously under the control of the regulator. At the same time, the principle of deviation control has a significant disadvantage that the regulator is activated only after a mismatch occurs between the given and actual values ​ ​ of the controlled value, which makes it difficult to ensure high quality of the transition process. Such systems under certain conditions are capable of self-excitation, leading to the occurrence of fluctuations of a controlled value.

These disadvantages are devoid of automatic control systems based on the principle of compensation for disturbances. In such systems, the regulator receives information about disturbances acting on the object and compensates for their effect on the object through regulatory action. Perturbation compensation systems can achieve a higher quality of control than systems based on the deviation control principle, since the regulator comes into operation even before the deviation of the controlled value. Such systems are called open, since the regulator does not control the value of the adjustable value P. The disadvantage of these systems is that the adjustable value can deviate significantly from a given value. In addition, in real systems it is almost impossible to ensure the measurement and compensation of all disturbances acting on the object.

The most advanced are combined automatic control systems, using at the same time the principles of deviation of the regulated value and compensation of disturbances. At the same time, the advantages of both regulatory principles remain and their disadvantages are reduced.

Main part

2.1. Automatic control of the efficiency of the combustion process (air supply).

The control of the air supply to the boiler furnace shall ensure that the specified excess air ratio a is maintained, at which k. D. of the boiler has the highest value. The highest value of a can be constant or change with a change in load.

Air control by direct indicator. Since the air control process is characterized by the value of the excess air coefficient a, a direct indicator for it is the content of unused oxygen 02 in the combustion products or carbon dioxide C02 released during combustion.

Maximum value. d. The boiler is located at the boundary of disappearance in the gases of incomplete combustion products CO, H2, CH4, etc. At the highest value, the CO2 content changes with a slight slope, and the 02 content increases sharply. In addition, the optimum content of 02 in the gases depends little on the properties of the fuel, which has no place for CO2. These reasons led to the rejection of air control schemes for the CO2 content of gases. This is also prevented by a significant delay in the C02 signal to the autoregulation system. A large lag was observed during the operation of gas analyzers at O2, however, as a result of recent work, the effect of the lag was reduced so much that regulation of 02 became practically possible. In the automation of the combustion process, the signal on the content of 02 in the gases is most often used not as a main, but as a corrective, to optimize the process controlled by other devices.

Control of air flow by the specified load. The indicator of the specified boiler load is the signal from the main regulator by steam pressure in the steam line before the turbine. With manual remote control, the load signal is generated by the manual control setter. According to one of the common methods, the air supply is controlled by a signal from the main (correction) regulator or manual control setter ("assigned load - air" circuit).

Air control by fuel flow rate ("fuel-to-air" diagram). Maintenance of air flow is proportional to fuel flow according to the "fuel-to-air" scheme, it is used when the instantaneous fuel flow can be measured without noticeable delay, and the heat of fuel combustion remains constant for a long time. Natural gas from a particular field fully satisfies these conditions. Measurement of gaseous fuel consumption is not difficult, and quality indicators remain unchanged for a long time. In other cases, the fuel-to-air scheme does not meet the requirements of economy. Despite this, it is often used due to simplicity, with automation of industrial boilers of low productivity. In this case, instead of directly measuring fuel consumption, it is estimated by auxiliary indicators, for example, by the sum of the revolutions of the dust feeders serving the boiler, or by the position of the feeder capacity control. In the fuel-air control circuit, corrective action is often introduced on the content of 02 in the flue gases.

2.2. Description of functional diagram of control object automation

Controlled parameters:

A) input parameters:

PE1 - steam pressure in steam line;

PE42 - gas pressure in the gas pipeline ;

PE5 - air pressure in the air duct

QE3 - gas flow rate;

B) output parameters:

QE3 - gas flow rate;

QE2 - air flow rate;

Functional diagram of the system-stabilization of gas and air supply to the boiler furnace, made in accordance with GOST 21.40485. The flow rate of gas and air is stabilized by a QRC controller which maintains a predetermined value thereof as follows.

The signal of the sensor 6, proportional to the current value of the gas pressure and flow rate in the gas pipeline, is supplied through the normalizing converter and the recording device QR2 to the comparison unit of the QRC regulator. The second signal entering this unit is the sensor signal 7. At the same time, the controller comparator receives a signal from the setter H. If the algebraic sum of these signals is equal to zero, the controller output is absent.

Otherwise, the QRC controller comparator generates a mismatch signal, which in the electronic regulator units is amplified to a predetermined value and converted in accordance with a predetermined control law.

From the output of the QRC controller, the signal is transmitted to the power amplifier ns through a switch designed to select the manual-automatic mode.

Amplifier ns amplifies signal to preset power and supplies it to control winding of electric actuator M consisting of motor and reduction gear box arranged in one housing.

The actuator M is designed to move the rotary flap 7A mounted on the air duct. Movement of the damper reduces or increases the flow section of the pipeline due to which the air flow rate changes.

In parallel, the gas supply is controlled by changing the pressure in the steam line of the steam line .

Input signal of pressure sensor is supplied through recording device PR to PRC regulator, processed by computer, which compares current value of measured parameter with preset value on setter H. Output signal from regulator is supplied through switch to power amplifier NS, from where output signal is transmitted to executing mechanism.

The change in gas and air flow occurs until the controller output is zero, i.e. until the controlled gas and air flow into the boiler furnace reaches a predetermined value.

If the current values of the measured values do not correspond to the specified values, light and sound alarms are triggered on the operator's board.

2.8 Selection of secondary devices

Depending on the type of output value of the sensor, the following secondary devices are selected:

- automatic bridges at sensor output parameter - active resistance value;

automatic potentiometers if the sensor signal is DC voltage;

instruments with differential transformer circuit (DCC) at output value of the sensor - AC voltage;

- milliampermeters (FBW), if the sensor output is a unified DC signal of 0-4 mA and 420 mA.

The measuring range of the secondary instrument is determined by the operating range of the sensor. The readings and recording errors of the secondary instrument shall not exceed the sensor error.

The inertia of sensors and secondary devices should be, as a rule, 1-2 orders of magnitude less than the inertia of control objects.

Automatic recording devices of KSU2 type (bridges, potentiometers and devices with a differential transformer circuit) work with electronic amplifiers of the UPD type (UPD1, UPD-2, UPDZ.

In single-channel instruments of KSU2, KSU-4 types, the measured value is recorded continuously on the diagram tape when the carriage moves along the scale. recording device of single-channel device consists of writing unit fixed on carriage.

In multichannel devices, the measured value is recorded by cyclically applying color dots on the diagram tape with indication of the serial number of the channel at the moment of carriage stop. The digit appearing in the carriage window indicates the channel number, the signal of which will be recorded in the subsequent printing cycle.

recording device of multi-channel device consists of directly printing drum with dots with corresponding numerals applied on its surface. Depending on the types of recording devices themselves, corresponding printing devices are installed on 4, 6 and 12 measuring points. For convenience of control and decryption of controlled parameters, feeding device has holder of felt sectors impregnated with paint.

Select secondary device of FCS 4 type with output signal 420 mA with accuracy class 0.5.

Conclusion

In this work, the automation system for the supply of gas and air to the boiler current according to the proportional-integral law was considered, consisting of two parts:

1) control circuit of gas supply to boiler furnace by steam pressure in steam line;

2) circuit of air supply to boiler furnace by gas flow rate;

Automation tools are selected for each loop. Further, based on the equation of motion, the time constant of the control object and the lag time, dynamic time and frequency characteristics were constructed, their analysis of which it follows that the control object is high-frequency, that is, it responds to sufficiently fast changes in the controlled value, it is characterized by low inertia, the absence of lag time.

Testing of the closed-circuit automation system for stability according to the Nyquist criterion gave positive results. Thus, the automation system of the plant boiler is suitable for use.

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