FIELD OF INVENTION
This invention relates to a system for determination of steam water ratio in water wall tubes of large coal based steam generating units using capacitive probe.
BACKGROUND OF THE INVENTION
Coal fired boilers often experiences internal deposit formation in riser tubes and deposits layer continuously increases due to operation of boiler during the period of condenser tube leakage/seepage. Gradually the internal deposit layer starts building up and develops localized under deposit corrosion and increase in metal temperature due to reduction in heat transfer. In case the metal temperature increases more than design temperature, metal degradation starts and tube failure occurs due to overheating. Presently the practice is to measure the oxide scale thickness/deposit quantity measurement in water wall tubes during the overhauls wherein the chemical cleaning is recommended when the deposit exceeds the recommend levels. Moreover, when the heat transfer is not uniform in all the sides of furnace due to changed position of fire ball, some part of the wall gets high heat flux and chances of overheating of tubes is more and also steam temperature rises to the desired level and operator controls the temperature by attemperation without knowing exactly the cause. The boilers are high temperature and pressure boilers having 100-500 MW coal fired power plants with pressure of 110-190 kg/cm2 and around 540°C steam outlet temperature.

Thermocouple based thermal monitoring system is known to have been provided in the art that calls for embedding thermocouples on or in the thickness part of the hot side to boiler tube to determine the heat flux for intelligent soot blowing operation, identifying material degradation and also monitoring the internal deposits. None of the technology is available to monitor the combustion process and heat transfer based on void fraction (steam as voids in water) measurement principle for such a high pressure and temperature application. A lot of work has been successfully done to determine void fraction ratio of two phase flow by capacitive (intrusive type), capacitive sensor (non-intrusive) or combination of both but no technology on similar principle exists for high temperature and pressure applications to meet the objective for coal fired steam generating units where density difference between steam and water is also variable and matters at high pressures. In view of this, need was felt to develop high temperature and pressure resistant capacitive type intrusive, sensor, signal condition and processing, data acquisition system and software along with density compensation for monitoring steam and water ratio in fraction variations with respect to time (trending) rather than precise absolute values.
OBJECTS OF THE INVENTION
The primary object of the present invention is to propose a system for determination of steam water ratio in water wall tubes of large coal based steam generating units using capacitive probe which is efficient and effective.

Another object of the present invention is to propose a system for determination of steam water ratio in water wall tubes of large coal based steam generating units using capacitive probe so as to access the heat transfer to boiler tubes.
Further object of the present invention is to propose a system for determination of steam water ratio in water wall tubes of large coal based steam generating units using capacitive probe so as to detect the cause of large heat transfer deviation.
STATEMENT OF INVENTION
According to this invention, there is provided a system for determination of steam water ratio in water wall tubes comprising of an instrumented water wall tube connected to signal conditioning and data acquisition unit and electronic circuit wherein said water wall tube housing a temperature and pressure resistive capacitance sensor.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Further objects and advantages of this invention will be more apparent from the ensuing description when read in conjunction with the accompanying drawings and wherein:

Fig. 1 shows: Natural circulation Test Rig. Fig. 2 shows: System of the present invention.
DETAIL DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS
This invention discloses a system for determination of steam water ratio in water wall tubes of large coal based steam generating units using capacitive probe comprising of an instrumented water wall tube connected to signal conditioning and data acquisition unit and electronic circuit wherein the instrumented water wall tube comprising of ceramic coated high temperature and high pressure resistive capacitive sensor.
The ratio is determined at operating pressure of 1 atmosphere and at temperature of for example 100°C. The steam water ratio is monitored on real time basis by operator to find out cause of large heat transfer deviation so as to take corrective action accordingly. This reduces boiler tube failures and results in best combustion performance.
Steam water ratio in water wall tubes of large capacity coal fired steam generating units is determining by sensing steam voids with capacitive method using high temperature ceramic capacitive probe. The capacitive probe comprising an electrode connected to body of the tube itself and

another electrode connected to testing insulated one having high pressure sealing. Due to change of dielectric property of steam and water, capacitance varies and obtained signal is processed to obtain frequency signal and fluctuates depending upon volume of voids.
For testing system of the present invention, natural circulation testing is used.
Reference may be made to fig. 2. The water wall tube housing a capacitance sensor, which is connected to an oscillator circuit. The circuit is provided to detect capacitance of the sensor and in connection with a display through frequency conversion unit, data acquisition unit and signal processing and software unit. The frequency conversion unit converts the capacitance into required frequency, which is transmitted to the data acquisition unit. This unit converts frequency into voltage/current, which is processed through said signal processing and software unit to obtain steam water ratio. Finally, this ratio is displayed on said display.
The sensor comprising of a metal cap covered with an electrode covering and connected to terminal of said oscillator circuit by means of insulating tube such a ceramic tube passing through a metal cap. Said metal cap is provided with a metal washer.

Natural circulation test rig is provided as shown in Fig. 1 comprising of atleast two heaters of 1.5 KW for heating the water and creating naturally produced steam bubbles. The pressure of the circulation system is unpressureised and temperature is 100°C. Capacitive sensor for experimentation purpose is for ex. PTFE coated hydra step as one electrode and another electrode is connected with the body as shown. Presence of steam in water affects the dielectric of the fluid and hence capacitance varies. Oscillator circuit is connected with the capacitance to generate output as frequency, which varies due to change in capacitance. The change in frequency is in the range of 0-3 KHz for voids of 0 to 50% against the base frequency of 42 KHz. To validate the void ratio, flow meter is put in air line so as to determine the volume of air being injected in 10 sees time. Signal obtained in 10 sees is evaluated to determine void ratio that fairly matches with the measurements.
In order to assess the heat transfer to boiler tubes, quantification is done for the amount of steam in riser tubes as a ratio to water by volume in all the side walls at different elevations. This ratio is monitored on real time basis by operator to find out what exactly is the cause of large heat transfer deviation and takes corrective actions in real time or makes plan for permanent solution in off line so as to reduce boiler tube failures and best combustion performance.
It is to be noted that the present invention is susceptible to modifications, adaptations and changes by those skilled in the art. Such variant embodiments employing the concepts and features of this invention are intended to be within the scope of the present invention, which is further set forth under the following claims:-

WE CLAIM:
1. A system for determination of steam water ratio in water wall tubes comprising of an instrumented water wall tube connected to signal conditioning and data acquisition unit and electronic circuit wherein said water wall tube housing a temperature and pressure resistive capacitance sensor.
2. A system as claimed in claim 1 wherein said sensor is connected to an oscillator circuit, which detects capacitance of the sensor.
3. A system as claimed in claim 1 or 2 wherein said circuit is connected with a display to indicate the steam water ratio through frequency conversion unit, data acquisition unit and signal processing and software unit.
4. A system as claimed in any of the preceding claims wherein said sensor is coated with an insulating material such as ceramic.
5. A system as claimed in claim 4, wherein said sensor comprising of an electrode connected to body of the tube and another electrode is connected to insulated one having high pressure sealing such as herein described.
6. A system for determination of steam water ratio in water wall tubes substantially as herein described with reference to the accompanying drawings.