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Discussion on the Technology of Level Measurement Instruments such as Differential Pressure Transmitter in Pressure Vessel Measurement of Thermal Power Plant
1 overview of this article
With the development of the times and the improvement of technology level, the automation level of thermal power plant production is also increasing year by year. As an important measurement work of thermal power plant, the accuracy and stability of its measurement requirements are particularly important because it is not only related to The stable operation of the entire unit also plays a vital role in achieving the production benefits.
The measurement points of the pressure level of the thermal power plant pressure vessel mainly include: steam drum, deaerator, drain box, height increase and so on. Among them, the measurement of the drum level is the most important and the most critical measurement point. If the full working condition cannot be measured and the water level protection cannot be put into operation during startup and commissioning, it will increase the probability of accidents and expand the danger of accidents. If there is a problem during heating, the boiler will not work properly and the continuous and stable operation of the unit cannot be guaranteed.
The power plant's level measurement (analog output) of pressure vessels mainly includes: differential pressure transmitter , guided wave radar, capacitance probe type, etc. Among them, the differential pressure transmitter is a more traditional measurement method. In recent years, with the development of level gauge technology, especially the capacity of capacitive probes, guided wave radar and other media to improve the ability to withstand the media process conditions, the improvement of the measurement cylinder technology It is increasingly used in high pressure and high temperature applications. This article introduces several methods of liquid level measurement in thermal power plants, and analyzes their respective problems, which provides a certain reference for the optimization of liquid level measurement methods and equipment selection in future power plants.
2Drum level measurement
2.1 Differential pressure transmitter measurement method
2.1.1 The liquid level measurement of boiler drum adopts differential pressure transmitter method, and its sampling is single room and double room.
2.1.2 There are major problems
(1) Due to the absolute temperature of the water side, the temperature and pressure consistency of the measuring cylinder and the boiler body cannot be guaranteed, resulting in a large negative difference between the sampling water level and the boiler body water level.
(2) The sampling volume is large, the measurement delay is large, and the change of the water level cannot be immediately reflected, resulting in poor quality of the measurement signal adjustment.
(3) The number of sampling pipes is large, and the thermal insulation in the winter and outdoor heating are complicated, the construction volume is large, and the installation cycle is long.
(4) It consists of multiple parts such as differential pressure, pressure, temperature, flow, system calculation, etc., with low integration reliability and large maintenance.
(5) Affected by the temperature and pressure change of the drum and the ambient temperature, it cannot perfectly solve the problem of density compensation of the system measurement error caused by the absolute temperature on the water side.
(6) The direct use cost (condensation tube heat dissipation) and maintenance cost (annual calibration and construction) of the balanced drum drum level meter are high, and its comprehensive use cost is far from the value of surface purchase cost response.
(7) When the boiler is started, the reference water column (P +) sampling tube is empty, and it takes time to wait for the condensate to be filled. During this time, the balance container level gauge cannot work normally (normal differential pressure conditions cannot be established), and the boiler starts The previous water level cannot be monitored centrally, and protection cannot be invested.
2.2 Guided Wave Radar Measurement
Guided wave radar sensors are capable of transmitting very short frequency radar pulses with a duration of about 6 nanoseconds to about 26 nanoseconds. The radar pulses reflected by the medium are received by the antenna as echoes. The running time of the radar pulse from transmission to reception is directly proportional to the distance from the antenna to the surface of the measured medium. The calculated level is converted into an output signal.
A 50MW thermal power plant boiler drum level in Shaanxi uses guided wave radar measurement method. The specific connection method is to cancel the balance container and use a connecting pipe to directly connect the measuring cylinder with the drum (isolation valve is installed in the middle). The guided wave radar is installed at The top of the measuring cylinder, because the engineering boiler is tightly closed outdoors, is located in the high and cold area of northern Shaanxi. In order to avoid the external low temperature influence, the measuring cylinder uses electric mixing heat. After a period of trial operation, the following problems still exist:
(1) The influence of liquid level fluctuations cannot be eliminated, especially when the liquid level changes frequently during startup.
(2) Pressure and temperature changes interfere with the actual measurement value, and the accuracy is not guaranteed.
(3) There is still a deviation in the matching between the measuring cylinder and the level gauge, which cannot be used as a protection value point, and it cannot achieve accurate measurement under all working conditions.
Cause analysis (as shown in Figure 3).
Figure 3 Actual distribution of medium in the measuring cylinder
The temperature difference will cause the condensation rain area to form on the steam side. Once the condensation rain area reaches a certain density, the guided wave radar will make incorrect measurements. To eliminate the condensation rain area, the diameter of the measuring cylinder needs to be increased, and the temperature difference must be eliminated. However, these two points still have some difficulties in practical engineering applications. On the one hand, manufacturers can't provide accurate values at present. In addition, the oversized diameter measuring cylinders increase the implementation difficulty for installation and mixing.
The guided wave radar level measurement technology has an instantaneous power consumption of several hundred milliamps at normal temperature and pressure. Because steam and condensed water can cause significant attenuation of microwaves, the power consumption of guided wave radar level gauges under pressure vessels will double. The meter is a continuous application for many years. Excessive power consumption will also reduce the service life of the product. The interference caused by the condensation rain zone to the guided wave radar level gauge cannot be completely eliminated by increasing the power consumption. If a balance container is added between the measuring cylinder and the steam communication pipe of the drum to eliminate the influence of the steam side, Added detection links and failure points.
2.3 Intelligent capacitance measurement method
Working principle: Utilize the relationship between the liquid level change and the capacitance change caused by the measuring probe, and use the special mode system software to compensate the detected capacitance change and output an analog signal proportional to the level change.
At present, domestic manufacturers rely on their own research and patents to develop a three-probe capacitive level gauge measuring cylinder with automatic compensation for level measurement. This product is a coaxial sleeve measuring probe with the high end (above the level change of the measured medium *** high point) and the low end (below the level change of the measured medium *** low point) ) Connect a coaxial sleeve measuring probe in parallel in the form of a communication tube, that is, a three-probe measuring cylinder composed of three coaxial sleeve measuring probes # 1, # 2, and # 3, where # 1 is the main measurement, The probe measurement height is generally based on the effective measurement range that meets the site requirements. The production height of # 2 and # 3 probes is generally one sixth to one third of the height of # 1. At the same time, the physical indexes such as the inner diameter of the metal tube of the three probes, the outer diameter of the metal rod, the thickness of the insulation layer, and the material are the same. It is used to ensure that the three probes can measure the same medium at the same time and that the probes are the same. The measured capacitance change caused by the change in the level of the measured medium at the same unit height under the environment is completely the same. The # 2 probe is placed in an empty cylinder, that is, the high-end state during actual measurement, and is used to # 1 and # 3 the measurement cylinders. The capacitance calibration value is modified, which mainly eliminates the influence of changes in the dielectric constant of the gas or vapor medium and the change in the adhesion rate of the measured medium to the probe on the measurement. # 3 Probe is placed in the medium full during the actual measurement. The tube is the low-end state. It is used to modify the calibration value of the # 1 measurement tube full capacitance, which mainly eliminates the influence of the dielectric constant change of the measured medium on the measurement. The three measurement tubes are supplemented by three electrical measurement indicators that are consistent and have good accuracy. The capacitance measurement probe and related transmission display unit constitute a set of liquid level gauges with automatic compensation function for liquid level measurement. According to the actual application of multiple projects, the function of accurate and continuous measurement and control of the liquid level under all working conditions (boiler start, stop, sewage, accident conditions, etc.) is basically realized. There is no “false water level” measurement, and the contact signal output is provided. After that, it fully meets the monitoring and protection needs.
2.4 High level liquid level measurement
The characteristics of high-pressure heater and deaerator are similar to those of the drum. The traditional differential pressure transmitter and the balance container are used to measure the liquid level and the application of the drum has similar problems. Guided wave radars have been successfully applied in deaerators, high and low water heaters, and hot water well water levels. Especially low water heaters and hot water wells have good water levels. The deaerator and high temperature are mainly because the steam side temperature and pressure are not high. The measurement accuracy is lower than that of the drum. Although there are certain fluctuations, it is more stable and less maintenance than the traditional differential pressure transmitter. In order to reduce the occurrence of condensed rain areas, it is also important to select a measuring cylinder for mixing and heat insulation.
3. Conclusion
If the investment permits, the drum level measurement should be a three-probe capacitive product from the perspective of achieving full operating condition monitoring and protection, eliminating hidden safety hazards, and promoting the continuous and stable operation of the thermal power plant. Guided wave radar and capacitance measurement methods for liquid level monitoring applications such as high and low additions, deaerators, and hot water wells are also future trends.
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