Control System of Power Plant
● Boiler instrument are used for safety and control the boiler. It balance the mass and energy in and out of the boiler are achived. Fuel input, combustion air, automization of air or stream and feed water The fuel is the major energy input and the outputs are steam, flue gas, blow-down, radiation and soot blowing
● A device, like photoelectric cell, that receive and respond to a signal. A device which detects a variable quantity measures and converts the measurement into a singals to be recorded. A sensor is a device that measures a physical quantity and convert it into signal which can be read by an instrument
Types of Controls and Control Systems :
There are basically three types of industrial instrumentation systems for power plant control : analog, microprocessor, and computer.
1. Analog:
Analog control is the representation of numerical quantities by means of physical variables such as current, air pressure, voltage, rotation, resistance, electromagnetic field (EMF), etc. Analog control over the last 30 years has consisted primarily of two types:
a) Pneumatic - the use of air pressure (or other gases occasionally) as the power source to represent numerical values,
b) Electronic - the use of current, voltage, resistance, EMF etc. as the power source to represent numerical values.
2. Microprocessor-based control stations :
These are a digital standalone single controller type, or a split-architecture control system offering powerful, configurable control capability on a modular basis. These units can accept standard analog electronic inputs plus digital inputs and give analog outputs plus digital outputs.
1. Supervisory Control (SC):
This control can perform all of the control functions, operator displays and graphics, reports and calculations for efficiency and controller tuning, or a computer can be used as a supervisory control for analog control system, microprocessor based control units, or as a data logger with graphic displays. Choice of analog versus microprocessor based control units or computer (DDC) (SC) should be based on relative cost and future requirements. Consideration should be given to the ability to readily interface to or add to a utilities energy management system.
2. Pneumatic control systems:
Pneumatic control systems should only be considered when adding to an existing power plant already equipped with pneumatic control instruments.
The following are the instrument used:
1.Temperature gauge : A thermocouple is a junction of two dissimilar metal that produce voltage related to temperature difference. It can be used to convert heat into electrical control.
(a) Feed water temperature
(b) Oil temperature
(c) Flue gas temperature
(d) Water temperature
2. Fluid flow measurement gauge : Rotameter
3. Steam flow measurement
4. Combustion air flow measurement (oxygen analyzer): provide an indication of excess of air in the flue gas, so it gives alarm and trim, so instrument control combustion air. Eg. Pitot tube
5. Pressure measurement
(a) Steam Pressure
(b) Feed water Pressure
(c) Gas Pressure
(d) Oil Pressure
(e) draft Pressure
6. Drum water level control : (Shrink and swell)
● When rapid increase in load, a severe increase in level occur, Shrink and swell is a result of pressure changes in the drum changing water density. The water in the drum contains steam bubbles as water is boiled hence increase in volume of the bubbles resulting increases in volume so drop in steam pressure from the load increase and the increase in steam generation from the greater firing rate to match the load increase.
● If the level in the drum is too high at this time, it may result in water carryover into the super-heater or the turbine. The firing rate cycle can result in drum pressure cycles. The drum pressure cycles will cause a change in drum level. The firing rate change has an effect on drum level. The significant cause of shrink and swell is rapid changes in drum pressure expanding or shrinking the steam bubbles due to load changes.
Combustion Control:
Combustion control comprises a series of devices on aboiler developed to satisfy steam demands automatically and economically by controlling furnace combustion rates through adjustments of the burning components while maintaining a constant set point such as a fixed but adjustable pressure or temperature and an optimum (adjustable) ratio of fuel to air.
a) For the metering type (proportioning plus reset plus rate action).
b) Combustion safe guard.
Turbine flow meters :
● Turbine flow meters are widely used for liquid and gas applications, but they have very limited use for steam flow measurement. One major issue has to do with the effect of condensation on measurement accuracy. When condensation occurs in a flowing stream, the fluid becomes a mixture of steam and water, which creates two-phase flow. The condensate can harm the blades, and interfere with measurement accuracy.
● Turbine tlow meters do have an advantage since they can perform better at low flow rates than vortex flow meters. They also have a good turn down ratio .The most favorable condition for steam flow measurement by turbine meters is measurement of dry steam. However, because steam is so readily influenced by changes in temperature and pressure, steam flow conditions can change very quickly. Steam is most stable at the boiler, but cold spots can occur in the line, bringing about condensation.
Pressure Gauges :
● Pressure gauges are usually direct-connected and field mounted. Size and ranges are specified by user. Local mounted gauges give a "backup" reading and also help operators in determining if equipment or pressure systems are working satisfactorily. Accuracy is normally 0.5 to 1 percent of span. For test and calibration purposes, use 0.25 to 0.5 percent gauges.
Temperature Sensors:
The basic types of temperature sensors are thermometers, thermocouples, and resistance temperature detectors.
1. Thermometers :
Thermometers are located on equipment and piping to provide local temperature indication.c.g mercury thermometer convert measured temperature into expansion and contraction of a liquid which can be read on calibrated glass tube. A thermocouple convert temperature to an output voltage which can read by voltmeter.
2. Thermocouples :
Thermocouples provide a reliable and accurate temperature measurement for most remote temperature sensing applications. Thermocouples can be used with pneumatic and electronic transmitters or they can be direct connected to some instruments. Thermocouples are non-linear
● These are common passive sensing element thit respond to temperature in a measurable way. They are self-powered, requiring no excitation, and can operate over a wide temperature range (upto 2000). They can respond quickly, with almost no significant delay in system operation
● The voltage measurement depends only on the temperature difference (T1 - T2). As T1 changes, the voltage output of the thermocouple changes proportionally to the change in temperature, but not linearly. The voltage output ranges from about 10 to 77 mV (depending on the thermocouple type and measurement temperature) The correlation of temperature versus voltage establishes a relationship that is unique to the various thermocouple types.
3. Resistance temperature detector:
Resistance temperature detectors offer a temperature range about the same as a copper-constantan thermocouple with detection of temperature changes of 0.03 degrees F (0.02 C). The resistance detector dons not have a reference junction, as a thermocouple, since it operates on the measured change in the resistance of metal or semiconductor (thermistor) with temperature Platinum, because of its inherent stability and linearity, is becoming the standard of the industry with some copper and nickel still being used. Copper is quite linear but nickel is quite nonlinear. Resistance detectors can be used with pneumaticand electronic transmitters or can also be direct connected to some instruments.
Characteristics :
1. high temperature resistibility up to 500 °C
2. cheap
3. high shock and vibration resistibility
4. suitable for industrial temperature measurement
5. more accurate class 1/3b is available
6. excellent response speed due to the small size
