Introduction

Level measurement methods, due to the complexity and diversity of the measured object and the measured medium, and there are different forms of level detection principles and methods. With the continuous development and progress of the load cell device mouth, from the original mechanical float or mercury column indication. As well as mica equipment to today's radar, ultrasonic, passive nuclei, speed motors, gravity sensors, inductive resistance, capacitive, etc. Various forms, but no matter which one, each has its own characteristics, due to changes in the measured object or medium can not be summarized simply. Such as industrial desulfurization and denitrification of the sewage pool water level, the purity of water is not stable, the pool is also part of the steam, if the capacitance type, the dielectric constant is constantly changing, the water level will not be a single function of capacitance. Another example is the use of ultrasonic level gauges, because water vapor condenses on the horn, which leads to measurement failure.

According to the measurement of solid materials, such as raw coal or other ore solid particles in thermal power and steel mills as well as in the cement industry, there is almost no reliable and simple means of measurement due to the uneven granularity of the material itself, which is often accompanied by moisture, dust and other factors. The field often use nuclear radiation ray or steel belt hammer principle is more, and occasionally there are ultrasonic or high-frequency radar as a means of continuous monitoring, with the output of 4mA ~ 20mA analog signal as the corresponding material level. However, no matter which means, it is difficult to operate stably for a long time in the process of on-site use, not to mention accurate calibration. Measuring means of nuclear radiation should belong to radioactive sources, installation, calibration, maintenance and waste storage, all involving staff protection measures and environmental requirements, generally not recommended. As we all know, the expensive heavy hammer steel belt type although the principle is simple available, but often years of operation, the hammer and the wire rope is easy to be wrapped by the material adhesion, wear and tear or buried hammer often occurs, resulting in wire rope breakage or drop hammer failure, affecting the stability of the measurement of the material level, to the operation and maintenance and maintenance of the inconvenience. In this harsh dust environment, the choice of high-frequency radar measurement principle is not a problem, but the raw coal hopper or ash silo material shape is irregular, coupled with a large concentration of dust, easy to block the radar source of the probe flare, as well as the installation location of the measurement error has a greater impact on the measurement of these factors can not satisfy the measurement of the device's long-term stable operation. Therefore, it seems very necessary to find a safe, economical, reliable and stable measurement means, which is also a problem that all professionals need to think about.

For this reason, this paper proposes a method of transforming the level measurement into a static weight detection of a silo by installing a weighing module on the basis of a silo in a dusty environment, expecting that the civil and mechanical professions can give cooperation when designing a raw coal silo or an ash silo, which is a kind of simple, reliable and accurate means of a kind of static weighing measurement without a transmission element.

Coal silo and ash silo level measurement object characteristics

1) Reliability: I hope that the measurement equipment is simple and reliable, accurate measurement, small maintenance, long-term online without failure.

2)Irregular material stacking cone: because of the irregularity of the shape of the material stacking, can not define the exact interface, so the level measurement, can not accurately measure the true level of the material, the location of the sampling port is particularly important to choose the relative.

3) poor environmental conditions: bin level measurement area dusty, harsh environment, to the installation and maintenance of the inconvenience.

4) Can not be calibrated and open: the operation process can not be calibrated to the real level of maintenance, maintenance and fault repair diagnosis is not convenient.

The use of weighing modules to measure the level of the method

In view of the above, according to the characteristics of the object to be measured, thus conceived a weighing scale weighing method, its principle is simple, easy to maintain, good reliability for the rules of the container can be measured by the weight to reflect the true level of material in the silo and the amount of material. Because the measurement of the level is also to know the amount of material inside the hopper, perhaps this is more direct and convenient. For the use of the weighing principle of this approach, in the design, breaking professional boundaries, civil engineering, process equipment, control of professional cooperation with each other to optimize the design, safety and reliability, to ensure reliable and stable operation. In order to facilitate operation and maintenance, only need to replace or calibrate the weighing elements in the overhaul cycle. Specific design of the load cell as the core of a material level measurement structure, which is an analog or switching according to the design of the measurement sketch. It is assumed that the object to be measured is an ash hopper and that the density of the ash medium is relatively constant. The "high level of the ash hopper" and the "low level of the ash hopper" are two load cells. Here a piezoelectric load cell is used.

When there is a certain weight of material falling onto the tray, until the tray is buried, the load cell will have a voltage output, the voltage signal to the DCS feeding system conversion can be made to make the material level of the screen state, but also to make the high and low material level state of the alarm status indication. When the level reaches a certain height, that is, the pressure of the pallet reaches a certain gravity, can be set to send out the material level alarm signal and trigger the alarm, the alarm range of insensitive intervals according to the characteristics of the medium and the site settings, in the field through the DCS system or by means of a relay and other ways of converting the required signal state. For high and low material level at the weighing module set up to measure the output can be switching signals, can also output analog signals, according to the needs of the process can be arbitrary choice. This way of installation position, according to the container material and the actual demand for the location of the open hole installation.

The shape and size of the pallet does not need to be too large, in order to prevent the accumulation of materials, in the design and production, it is best to make a hemisphere shape, or enterprise shape, there are some places as alarm and protection signals, in order to be reliable can be installed in the two state monitoring signals, to ensure the accuracy of the signal. According to this way, the strain-sensitive metering module is set at the high material level, and the measurement output can be a switching signal or an analog signal. The opening position is selected according to the material and characteristics of the container. In order to measure reliably, two strain sensing load cells are used, adopting the method of choosing one out of two, which ensures the reliability of the measurement. In the case of a stable medium density, the level and weight of a regular shape is a fixed proportional function of the relationship, so through the strain-sensing load cell in the measurement of the weight, it can also be converted to the corresponding level. The converse is also true, however, there are some objects where more importance is attached to the amount of material inside the container, such as grain stores. This way, more adapted to the density of a stable medium, pre-installation, the strain to measure the gravity of the location of the choice, the design of the time in the bottom of the container to install one or more load cells, measuring the weight of the entire container, remove the gross weight when the empty warehouse, which is the weight of the material.

The installation position of the weighing module, depending on the object to be measured, is installed and measured like a static rail scale, which is a more reliable and proven technology. It has been widely used in scale inspection. In order to be reliable, the actual site is generally more than the installation of several load cells, within the control system (such as DCS system) for sensor failure online diagnosis and calculation of the average signal or three to take two of the logical way, can make the signal is accurate and reliable, and there is a failure of the case, but also to ensure that the measurement of normal use. Weighing piezoelectric sensors installed in the specific location, according to different objects, the design of the design to choose the maintenance of convenient and easy to replace the place, so that the piezoelectric sensor output and weight corresponding to the level of the level voltage signal. The number of sensors and location selection, can be measuring the weight of the entire warehouse or library, but also by measuring a sub-force, and vice versa to calculate the total weight according to the process system to suit the local conditions. In short, as long as simple and reliable, easy to maintain and use. So that the interface between man (installation and maintenance personnel), machine (machine) and environment (environment) is optimized, without much operation and maintenance work.

Feasibility and significance of promotion

Through the weighing control system composed of load cells used to measure the level, is a new type of application technology for some of the harsh environment, the measurement medium special powder, coal, ash and ore, as well as sewage or grain and other media measurement, provides a viable method. Nowadays, measuring some solid particles also has a broad prospect, guiding peers to promote the use. Hope that the future engineering design, mechanical, electrical, instrumentation professionals work closely together to simplify the measurement of these more complex media, such as coal and ash silos, so that the level measurement of complex objects to be measured can be more simple, more reliable. More economical to meet the stable operation of the process system.