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Application of Pressure Transmitter

time£º2018-10-18 16:49    source£ºshelok pressure transmitter

 Application of Pressure Transmitter


¡¾Abstract¡¿: Briefly introduce the principle of compensating anti-blocking blower and the importance of material switch on electric dust ash hopper, and use pressure transmitter and anti-blocking purger to detect the ash position alarm of power plant ash, pointing out its The problems of the original capacitive level gauge and the advantages after the technical transformation, maintenance methods and precautions.




    The company has three UG-260/9.8-M pulverized coal boilers produced by Wuxi Boiler Factory. The dust removal method is electric dust removal. In order to ensure the safe and stable operation of the dust collector, it is installed on the electric dust ash bucket of each furnace. There is a top and bottom level gauge to detect the level of the ash bucket. The original installed Siemens Pointek cls200 capacitive switch to detect the material level, because the ash powder furnace has large amount of ash, fine dust, a certain amount of static electricity, large medium fluctuation, high temperature, relatively high installation position, etc., often affects the accuracy of the material level. Determination, no material false positives, no material reported times, seriously affecting the boiler efficiency and system production capacity stability, so timely elimination of material level failure, to ensure reliable operation of the boiler is particularly critical. Through multiple program comparisons, our company has chosen a set of economically viable technical solutions. After running the practice, the program is safe, reliable and effective. The following describes the application of our company to the technical transformation plan of the ash bucket level gauge.


[2, the necessity of the gray bucket level switch installation]


1 The upper level is actually the upper limit of the material level. The process requirement must always control the material level below the upper limit. Otherwise, it will cause serious consequences. The first case is because the upper level meter has gray alarm and no Acting as an alarm, the gray level rises to the electric field, causing the electric field to be short-circuited, and the power supply and the rapping and cleaning system are damaged or invalid. This leads to serious exhaustion of smoke and dust, resulting in environmental accidents. The second case is that the ash position continues to rise, and the ash accumulation weight in the electric dust removal exceeds the design load, causing the ash bucket or the electric dust to collapse;


2 The lower material level is actually the lower limit value of the material level. As far as possible, the gray level is kept at a position not lower than the lower material level. The static pressure difference generated by this gray level assists the electric locker to lock the gas and reduce the gas. The air leakage rate is reduced by two flights. In order to reduce the occurrence of the above events, the hopper level switch must be accurate.


[3. Reasons for technical change]

 3.1 Working principle of capacitive material meter


A capacitive level gauge requires two electrodes: a reference electrode and a measuring electrode of a variable capacitor. In capacitive level measurement, the surrounding environment (usually the vessel wall) acts as a reference electrode and the probe acts as a measuring electrode. The medium is the material in the container, and if the measuring electrode is insulated, it is an insulating layer. The capacitance depends on the surface area of ​​the electrode, the distance between the electrodes, and the dielectric constant of the medium in the container. The dielectric constant represents the storage capacity of a material. The relative dielectric constant of air (vacuum) is 1; the dielectric constant of all other materials is greater than 1. When the dielectric constant is greater than 1, it is proved that there is material inside.


3.2 The cause of the problem with the capacitive level gauge in the company

    Because the coal type burned by the pulverized coal furnace often changes, the conductive medium constant of the produced ash powder also changes accordingly, and because the frictional force of the ash powder is large, it is easy to stay on the measuring rod and form a false level; there is another reason That is, the design temperature of the material meter is up to 200 ° C, and our flue gas temperature is 110 ° C. When the measuring rod of the level gauge is used for a long time under the design high temperature work, the top of the measuring rod is crushed by gravity and coal ash. After time, the measuring rod inserted into the ash bucket is deformed and hits the hopper shell to form a false level.


[4. Introduction of technical reform program]

 4.1 Structure of the technical transformation plan


    Our technical retrofit solution is to install an H B C environmentally-friendly compensating anti-blocking purging device at the location where the level gauge is to be installed. The device has the advantages of reasonable structure, convenient installation, never clogging, accurate measurement and the like. The compensating anti-blocking purging device has two parts: a constant air flow control box and a sample purging device (built-in compensator), and its structure is shown in Fig. 1.

HBC compensation type anti-blocking purging device installation diagram


4.2 Main features of compensating anti-blocking blower

    Using the dynamic pressure compensation method of fluid mechanics, the pressure of the measuring point is truly reflected online. The purge pressure is greater than the measured pressure and completely solves the blockage of the dust pressure measurement line.

    The voltage regulator is used to regulate the gas supply provided by the power plant, ensuring stable flow output of the flow controller and achieving long-term reliable operation.

    No manual purge is required, which greatly reduces maintenance during operation. The control box is IP 56, easy to install, easy to maintain and easy to adjust.


4.3 Working principle of compensating anti-blocking blower

   The measuring principle of the device is to prevent the plugging point by using the method of continuous ventilation in the measuring point, and to use the hydrodynamic dynamic pressure compensation method to eliminate the differential pressure generated by the reverse purging airflow to ensure accurate measurement value. Between the sampling point A of the intelligent pressure transmitter and the purge port B, the pressure difference ΔP in the tube due to the gas flow, the gravity of the flowing gas is γ, the air flow rate of the section is V, and the air resistance parameter of the section is ξ, set the pressure point of the pressure transmitter to the PA point, and the pressure at the pressure point of the pressure tube to PB.



It can be seen from the formula that the change in gas flow causes a change in gas flow rate and a change in Δ P . In the conventional purging method, in order to ensure the measurement accuracy, only a small flow rate can be selected, which is easy to cause blockage. For example, if the flow rate is increased, the transmitter must generate zero drift, and then the transmitter and the display instrument must be adjusted. The zero method causes the display system to be confusing, and it is difficult to correct the problems during operation. The faults and deviations of the pressure regulating valve and the flow controller will directly affect the accuracy of the measurement, and the daily maintenance workload is quite large, in order to overcome the above drawbacks. To improve the accuracy of the measurement, a pressure compensation device is added to the system. The compensator uses the aerodynamic principle to eliminate the differential pressure Δ P . The pressure change of the backflush air does not affect the measured value, because the actual signal is sampled from the pressure compensating device, which solves the anti-blocking and sampling, and truly measures the actual pressure. value.


[5. Technical changes and results]

 5.1 Precautions


   When selecting the transmitter, the intelligent differential pressure transmitter with a range of -2000pa to +2000pa and a withstand voltage of 3MPa should be selected. The air pressure of the anti-blocking purging device is 0.1M pa, and the flow rate is 1m3/h.


 5.2 Technical transformation results

    When there is no material level, the pressure measured on the transmitter is the negative pressure of the flue gas (about -1700pa); when the material level rises to the measuring port, the pressure of the loosening device rises slowly, and the measuring pressure is positive. The change between the negative pressures is because the ash that starts to accumulate is blown away by the compressed air. When the material level continues to rise and the compressed air cannot blow away the ash of the measuring port in time, the compressed air of the blower can be blown to the transmission. Inside the device, the pressure shows positive pressure. According to the increase of ash, the pressure gradually increases, possibly reaching several thousand Pa. In this way, the operator can judge whether there is material in the ash bucket according to the change of the pressure, and the result of measuring the material level is reached.


[6 Installation and Maintenance Precautions]

6.1 Installation Precautions


1) The purge sampling head is generally inclined at an angle greater than 30 degrees from the horizontal plane. The outer side of the ash bucket is higher than the inner side of the ash bucket, and the front end of the purging sampling head is retracted to the surface


2) The constant air flow control box is installed in the wall and should be installed perpendicular to the horizontal plane to ensure the normal indication of the flow controller.

3) The pipe connection should be Φ 14X 2 seamless steel pipe, stainless steel pipe or metal hose. Before connecting (welding), the garbage and dust in the pipe must be cleaned with compressed air to ensure the pipeline is clean; Where there is welding, it must be welded without leakage. The connecting part is sealed with gasket to ensure that there is no air leakage at each connection point.

4) Before the first use, it is necessary to clean the garbage and welding slag impurities in the pipeline to ensure a clean gas source. Turn off the voltage regulator before the initial operation, measure the wind pressure value of the ash bucket, and observe the fluctuation of the ash bucket. When there is no purge, the accurate measurement will not be affected. After the production system is stable, slowly open the voltage regulator to 0.1M P a, adjust the purge flow rate to 1m3 / h, the value measured should be the same as when no purge, if the deviation slightly adjust the purge flow.


6.2 Maintenance Precautions

    1. The air source treatment component (pressure regulating filter) must be installed on the front pipe of the constant air flow control box, and has an automatic drainage function to block the garbage and water vapor in the pipeline from entering the voltage regulator and the rotameter and compensator. To ensure the long-term normal operation of the equipment. In order to prevent the clogging of the pressure regulating filter of the air source treatment component, the garbage of the filter in the pressure regulating filter must be cleaned regularly;

    2. Open the large rotameter flowmeter during the inspection every day to allow short-term entry of multi-point compressed air to clean the dust of the pipeline and ensure the accuracy of the measurement;


[7 Conclusion]

    After this transformation and put into operation, the operation has been relatively stable, and the material level can be accurately measured, and the failure efficiency is greatly reduced. In addition to the daily maintenance, the site has no extra work intensity, and greatly reduces the maintenance personnel's work, and does not often go to the sky. Checking the material level detecting device, improving the maintenance safety factor, and accumulating experience for the transformation of other auxiliary equipment control systems in our factory.

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