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Analysis on the control level of magnetic column level gauge when operating conditions change

Time: August 26, 2019 15:44:04 source: Author:
If it is required to list a series of factors that can improve the heat rate of thermal power plant, the accurate measurement of drum liquid level will be close to the top due to its influence on the performance of magnetic turnover column level gauge. This is especially true when electricity markets and demand patterns change.
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Many traditional thermal power plants are designed for basic load operation, and the load variation is small for several days and weeks at a time. Now, factories often have to increase and decrease every day to balance the intermittent production of renewable energy. This has an impact on the equipment and requires more complex control to maintain efficiency and ideal heating rate over a wider operating range. Let's take a look at the level control in the magnetic column level gauge and see how it affects efficiency, operation and maintenance.
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Drum water level of magnetic column level gauge
Although of different designs and sizes, most subcritical magnetic tumbler level gauges have a drum at a higher point, where steam is collected and the tubes are placed together. It is important to maintain the correct water level in the drum. If the water level is too high, it can be taken into the steam pipe, and if it reaches the main turbine, it may damage the blades. If the water level is too low, the part of the magnetic column level gauge may become dry and overheated. Both conditions can lead to long and expensive shutdowns, so simply replacing one inch or two inch levels can result in travel.
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Maintaining this critical level is challenging because the magnetic tumbler level gauge drum is a very turbulent and chaotic place with high temperature and high pressure. Ideally, feed water should be added at the same rate and the steam extracted. When steam consumption is very stable, this can be controlled, but when the load rises and falls, the level can change rapidly. The increased load will absorb the steam, causing the pressure in the drum to decrease and allowing more steam to bubble in the tube. This will raise the level and cause a trip if there is a sufficiently large change in a short period of time. Reducing steam consumption may have the opposite effect.
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In the American Society of Mechanical Engineers (ASME) magnetic column level gauge and pressure vessel code pg-60.1.1, the above 400 psi operation is required to have two direct reading meters or sight glasses to indicate the level drum any magnetic column level gauge. It's a bit old-fashioned, and it dates back to the days when firemen controlled it manually. The specification allows one of the direct reading instruments to be replaced by two indirect level measurement methods, including differential pressure (DP), displacer, conductivity or radar technology.
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Generally, all types of magnetic level gauge drum level gauge are installed in external chamber (Fig. 1). This mounting device has several terms, including reins and stationary wells, but always includes two connected to the drum - above and below the liquid level - so it will be at the same level as the drum itself. Both connections have valves so that the chamber can be isolated during operation so that the instrument can be replaced or repaired without shutting down the magnetic column level gauge. Chambers also helps to reduce turbulence, which is a feature of the magnetic tumbler drum.
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Level measurement method
Let's take a look at how these four great measurement techniques work in this situation, each with its own advantages and disadvantages.
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DP。 DP is a traditional hot spot in many applications, but it has some serious disadvantages for the drum of magnetic column level gauge. First, it depends on knowing the density of the liquid Magnetic level meter The density of the liquid may vary greatly. The variation of water density between 0 psi and 1000 psi may result in 26% error in DP level reading. It is clear that the automation system controlling the magnetic tumbler level gauge will use a density closer to the operating conditions, but the density can still vary rapidly based on the amount of bubbling generated during the change in internal pressure, as described earlier. Therefore, when there is no level sensor, DP sensor can record the change of level.
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Another serious drawback is the use of pulse lines to reach the transmitter. These are often maintenance intensive, require regular blowdown and leak repair, and they are affected by other sources of blockage.
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Replacement. Like DP, displacers also rely on knowing the density of the liquid, so they are subject to the same accuracy limit. Displacers are mechanical in nature and have moving parts that must be operated freely in difficult environments, which usually require frequent calibration and maintenance.
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Conductivity. The conductivity sensor is arranged on a line extending along the side of the chamber to report the level according to the number of sensors immersed. This method is very reliable, but the resolution is low because of the small distance between sensors. These instruments are very useful for the level control system, but they do not provide the required accuracy.
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Radar. This leaves behind the radar, in which case the radar is usually a guided wave radar (GWR), named for its use of a probe as a waveguide. Due to the waveguide, GWR is suitable for operation in narrow cavities and has many advantages.
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First, GWR is independent of liquid density. It detects the point at which the dielectric constant (DK) changes on the surface of the liquid. Secondly, it can withstand the temperature and pressure encountered in the magnetic column level gauge environment. Third, it's very accurate and can update readings multiple times per second, so it can record changes very quickly.
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The disadvantages of GWR are important but can be mitigated. First, the emitter head must be mounted at a high level, possibly at least with reference to the length of the reflector. There needs to be a clear space through which the pulse can pass before it reaches the surface of the liquid. As a result, most of the existing chambers cannot be used as is. Some chambers can have extensions at the top to increase the length, or new chambers may have to be added.
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Secondly, just as DP and displacer are affected by the change of liquid density, GWR is affected by the dielectric properties of steam. According to the vapor density, the radar pulse slows down when working in a magnetic column level gauge - higher pressure means slower pulse. Because the pulse takes longer to pass through the steam and rebounds as it passes through the steam, if the GWR transmitter is calibrated in air, it will provide a reading indicating that the level is below the actual level. The transmitter of the instrument can overcome this problem by dynamic steam compensation.
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The diameter step size of the probe is always at a fixed distance from the transmitter. Each time a pulse is transmitted, this step acts as a reference reflector, sending its own echo back together with the echo from the liquid surface (Fig. 2). The transmitter signal processor looks for the deviation of the reference reflection time to determine whether the signal has slowed down and how much has been reduced. Then the correction factor is calculated and applied to the echo of liquid surface. This operation is performed on each pulse, so the correction is dynamic and reduces the measurement error to less than 2%.
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The disturbance of magnetic column level gauge is serious, whether it is to suck water into steam pipe or allow part of magnetic column level gauge to dry. Therefore, it is very important to adopt appropriate instruments and control strategies to prevent these situations. At the same time, the wrong stroke that triggers the shutdown of the magnetic column level gauge will interrupt the production and need to restart the process, both of which are very expensive. As a result, operators often use three-level instruments arranged in a two-thirds voting scheme to start the shutdown. This may sound like a complex and expensive solution, but it would be more than cost justified to lose only one production during peak output due to instrument failure.
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Remember that ASME pg.60.1.1 requires at least two indirect tools, so only one is required for the actual voting plan. Normal installation practices include installing three chambers around the drum. If a GWR transmitter is used, the erector should be careful to ensure that the chamber is positioned so that the top is the distance required above the drum.
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Ideally, all three chambers should be at exactly the same level, and the drum itself should be perfectly level, but this is not usually the case. If the GWR does not match the three levels, the transmitter will not agree. This is because the reading is based on the distance from the flange to the liquid level, so if the flange position is different, the reading will be different. Laser measuring equipment can help to quantify the differences and therefore can be corrected in GWR transmitters.
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If this measurement correction is not practical, installers can use a technique called "floating chamber.". The three chambers with GWR transmitter are closed to the magnetic column level gauge drum at the top and bottom. They're connected to each other by means of a drain at the top of each other, through a pipe to the bottom. The water is forced through the pipe so that all three chambers reach the same natural level. This can be a tricky process, and if it is not properly implemented, it can be subject to major errors, so it is often chosen as a later means.
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The readings from the three GWR transmitters can be matched in their configuration to correct for any height differences that are occurring during installation. If there is a difference during operation, it may be a sign of flow or expansion, which may lead to uneven distribution in the drum. If this proves to be a long-term problem, it may be better to reposition the chamber close to the middle of the drum.
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The need for baseline load power plants to adjust their output according to the intermittent output of renewable energy and other generators makes the operation more complicated. Therefore, the control of magnetic column level gauge is more important than ever to maintain plant stability. Proper operation depends on effective level instruments, and GWR transmitters can fill the bill to maintain stability, efficiency and profitability.