Application of Coriolis force scale in measuring slag powder in cement plants

Current status of slag powder measurement

Industrial slag powders such as fly ash, slag powder, steel slag powder, carbide slag powder, etc. have been widely used in cement production as raw materials or mixing materials. The accurate measurement and stable control of slag powder materials are very important. However, for a long time, compared with For coal powder, raw meal, finished cement, etc., cement plants do not pay much attention to the measurement of slag powder materials.

At present, the equipment that can be used for slag powder measurement mainly includes: tape scale, plate flow meter, powder mass flow meter, rotor scale, ring balance scale, spiral weighing scale, weight loss scale, Coriolis force measuring scale, etc. The following problems are common in the selection and use of various types of slag powder weighing scales:

1) Some measuring scales with simple processes, such as tape scales, plate flow meters, spiral measuring scales, etc., may have problems such as low measurement accuracy, large flow fluctuations, difficult adjustment and control, material flushing, dust emission, and ash emission. However, some weighing scales are mostly limited by material characteristics and scale body structure, and are difficult to meet the requirements for continuous measurement of large flows.

(2) Cement plants pay more attention to the metering equipment itself and tend to ignore factors such as material characteristics, storage structure, flow aid and discharge, process design, equipment selection, etc. Especially for technical transformation projects, there are many on-site restrictions and slag powder measurement It is necessary to comprehensively consider multiple aspects such as optimizing process conditions, rationally supporting equipment, and strengthening on-site operation and maintenance management.

(3) Each cement plant has different sources and characteristics of slag powder, working conditions, workers’ operating habits, and equipment maintenance and management methods, and the effects of using weighing scales are also different. The measurement plan needs to be flexibly adjusted according to specific circumstances to form a “one factory, one discussion” customized plan.

(4) There are few disposable detection instruments configured in slag powder storage, unloading, transportation, measurement, control and other process links, and the level of intelligence needs to be further improved.

The Coriolis force measuring scale and its control system developed based on the principle of Coriolis force (referred to as “Coriolis force”) have simple structure, high measurement accuracy, large measuring range, small volume, and the zero point of measurement is affected by material adhesion and external force interference. Smaller, widely used in cement plants. This article combines the actual production and discusses the application of Coriolis force scale in the measurement of slag powder in cement plants and the optimization of equipment from the aspects of process and supporting equipment.

Working principle and equipment structure of Coriolis force scale measurement system

2.1 Working principle of Coriolis force scale

The basic working principle of the Coriolis force scale is shown in Figure 1. When the material flows through the Coriolis force scale, the material particles make relative radial motion in the uniform rotation reference system. They are affected by the inertial centrifugal force Fr, the centripetal friction force Fz and the Coriolis force. The force Fc acts, and the torque generated by the Coriolis force is proportional to the instantaneous mass flow rate of the material passing through, that is:

M=QωR² (1)
In the formula:
M——Coriolis force moment, N·m
Q——Material mass flow rate, kg/s
ω——angular velocity, 1/s
R——effective radius of measuring disk, m

From formula (1), it can be seen that the mass flow rate of the material to be measured can be obtained by measuring the torque generated when the material passes through the measuring plate that rotates at a constant speed on the Coriolis force scale.

The basic structure of the side-feed Coriolis force scale is shown in Figure 2. The structure of the Coriolis force scale measuring plate is shown in Figure 3. The measuring plate is equipped with radially arranged measuring blades. Driven by the motor, the transmission shaft drives The measuring plate rotates horizontally at a constant speed. After the material enters the Coriolis force scale, it leaves the measuring plate along the radial direction of the measuring blade. During this process, the measuring mechanism obtains the Coriolis force torque through the measuring blade to achieve dynamic measurement of the material mass flow.

2.2 Supporting stable flow feeding equipment

The Coriolis scale slag powder metering system has separate feeding and metering functions. The metering accuracy of the metering scale is related to the stability of the supporting feeding equipment. In order to achieve uniform and stable feeding of the system powder, a steady flow feeder is selected as the supporting feeding equipment of the Coriolis force scale. The photo of the small flow steady flow feeder is shown in Figure 4, and the photo of the large flow steady flow feeder is shown in Figure 5.

From top to bottom, the steady-flow feeder can be divided into horizontal rotary structures such as a pressure reducing chamber, a pressure equalizing chamber, and a steady-flow chamber. The decompression chamber and pressure equalizing chamber are equipped with material distribution and stirring devices, which can fully activate the material and ensure uniform material pressure and bulk density. There is a horizontal rotating dividing wheel in the stable flow chamber, and a frequency converter is used to adjust the motor speed to ensure that the materials enter the weighing scale evenly, stably and in a controllable manner. The rotors of each layer of the steady flow chamber are made of elastic sealing materials, with zero gap and full sealing. They have good wear resistance, temperature resistance and aging resistance, and are easy to replace and maintain.
Compared with feeding equipment such as flow valves, screw conveyors, and vertical rotary dividing wheels, the horizontal rotary steady-flow feeder has the advantages of good sealing, sensitive adjustment, and low maintenance costs, and can effectively solve the problem of high-fluidity materials. The problem of material running and punching is eliminated, and the influence of material pressure in the warehouse on the feeding process is eliminated. At present, the company has developed multiple models of steady-flow feeders, with a maximum diameter of ~2m and a feeding capacity of >600m³/h, which meet the current increasing demand for slag powder measurement.

2.3 Optimize air-tight, dust-proof and shock-proof structure

In order to adapt to the high dust and easy vibration conditions at the bottom of the slag powder warehouse, the company has optimized the air sealing structure of the Coriolis force scale bearing, the dustproof sealing structure of the measuring mechanism, the conductive spring of the measuring mechanism and the shockproof legs. At the same time, The installation size of the large-flow Coriolis force scale has been optimized. Based on the original “standard side-feed type” Coriolis force scale, various models of Coriolis force scales have been developed to meet different on-site working conditions.

03 Coriolis force scale is used for equipment optimization in slag powder measurement

3.1 “Low Space, Small Height” Measurement System Solution Development

The slag powder metering system is mostly installed at the bottom of the concrete warehouse, and the installation space is limited. At the same time, the installation of the mixer and chute also requires space at the bottom of the warehouse, and the existing bottom foundation generally does not allow secondary excavation of the pit. Therefore, although the range of the slag and powder metering system has been continuously increased, its installation height has been repeatedly compressed.
In response to this situation, we proposed a “non-standard side-feeding” metering plan (Figure 7) based on the “standard side-feeding” metering plan (Figure 6). This solution retains the basic structure of the guide cone and measuring plate at the core of the weighing scale, changes the shape of the top cover of the weighing scale and its feeding method, and reduces the installation height of the non-standard connector between the steady-flow feeder and the weighing scale. . Adopting a “non-standard side-feeding” metering scheme, a canvas clamp and an inner sleeve structure are installed at the non-standard connection point between the flow stabilizer and the metering scale to achieve equipment shock absorption; a flow-assisted inflatable box is added to the slope , ensuring smooth discharge. In addition, we changed the lower cone of the weighing scale to an inflated fluidized layer, and improved the integrated design of the weighing scale outlet and the rear-stage conveying chute. This “non-standard side-feeding weighing scale + outlet chute integration” measurement solution further reduces the installation height, as shown in Figure 8.

6 Standard side feed metering scheme

For a large-flow slag powder metering system >300t/h, due to its increased volume, even according to the aforementioned structural combinations, it is still difficult to meet the on-site installation space requirements. In order to solve this problem, we developed a non-standard large-scale single-layer steady-flow feeder with a certain material sealing function. It adopts an upper-mounted drive form to avoid interference with the installation of subsequent weighing scales; instead, a central feeder is used This type of weighing scale separates the driving shaft and working shaft of the weighing scale, and the measuring plate synchronously transmits the Coriolis force torque to an independent measuring mechanism through belt transmission. This “large flow steady flow feeder + center feeding weighing scale” measurement solution greatly reduces the overall installation height, as shown in Figure 9.

9 Measurement scheme of “high flow stable flow feeder+central feeding type weighing

Practical applications have proven that various combinations of slag powder metering systems can fully meet the needs of precise control of feed flow and slag powder metering.

3.2 Design of anti-pressure material system for steady flow feeder

Coriolis force scale measurement systems are often installed at the bottom of large concrete warehouses with thousands of tons, and can withstand large material pressures in the warehouse. During cold start, if the material in the warehouse has not been inflated and fluidized, the steady-flow feeder will need to bear a larger starting load; or when the low-frequency rotation torque is insufficient, the steady-flow feeder may “press material”. “Jump stop situation.
In order to solve this kind of “material pressing” problem, the following process layout and equipment system optimization can be carried out.

(1) For large flat-bottomed concrete warehouses that do not have concrete pressure reducing cones, install simple steel pressure reducing cones; rationally arrange inflatable boxes at the bottom of the concrete warehouse, and use low-pressure air sources such as flow-assisted Roots blowers to achieve “high pressure” “Air pressure, small flow”; reasonably control the material level in the warehouse, standardize the material entry and exit system, etc.

(2) Change the manual spiral gate used for maintenance in the front stage of the steady-flow feeder to a pneumatic gate valve with controllable stroke. According to the real-time working conditions and feeding flow rate, the gate opening can be adjusted in time to adjust the steady-flow feeding. The amount of material pressure at the top of the machine.

(3) The preferred inverter for the steady-flow feeder is a vector-type inverter, which ensures 10% to 150% torque output when the steady-flow feeder speed is <5Hz, and at the same time increases the power configuration of the steady-flow feeder.

(4) In order to avoid over-adjustment of the feed amount in situations such as material interruption or material flushing, the steady-flow feeder control system adopts a segmented PID control strategy to adaptively adjust different PID limits based on the deviation between the feedback flow and the set flow. Amplitude parameters to solve the problem of “oscillation adjustment”.

3.3 Development of metal composite wear-resistant measuring disc

Fly ash or slag powder with large particle size is highly grindable, and the supporting measuring equipment, especially the measuring plate of the measuring scale, must have high wear resistance. In the early days of our company’s application of the Coriolis force scale, when the material flow was coarse fly ash, the blades of the measuring plate were worn out within less than half a year of operation, resulting in complete measurement failure.
To this end, the company has carried out research on wear-resistant materials and related technologies for weighing scales. The surface of the measuring disk has been subjected to vacuum sintering composite treatment of nickel-based materials, so that the surface of the measuring disk can obtain a high wear-resistant material with a Rockwell hardness HRC>68 and excellent toughness. layer and grinding it into a mirror-like state, which solves the problem of rapid abrasion of the measuring disc and the adhesion of fine powder to the measuring disc. The vacuum sintering furnace independently developed by our company is shown in Figure 10, and the wear-resistant measuring plate after vacuum sintered metal composite is shown in Figure 11.

Application case of Coriolis force scale used for measuring slag powder

4.1 Metering system process optimization plan in renovation project

Before the transformation, most of the cement companies slag powder storage warehouses used CP-type concrete warehouses. The slag powder gas was sent into the warehouse by tank trucks, and a rough metering scheme of double-pipe/single-pipe screw conveyors was used at the bottom of the warehouse. There were feeding fluctuations, Inaccurate measurement, material flushing, etc. During the technical transformation, we replaced the original screw conveyor with a steady-flow feeder and a Coriolis force measuring scale. The typical process flow and on-site photos before and after the technical transformation are shown in Figure 12 and Figure 13.

(1) For large flat-bottomed concrete warehouses that do not have concrete pressure reducing cones, install simple steel pressure reducing cones; rationally arrange inflatable boxes at the bottom of the concrete warehouse, and use low-pressure air sources such as flow-assisted Roots blowers to achieve “high pressure” “Air pressure, small flow”; reasonably control the material level in the warehouse, standardize the material entry and exit system, etc.

(2) Change the manual spiral gate used for maintenance in the front stage of the steady-flow feeder to a pneumatic gate valve with controllable stroke. According to the real-time working conditions and feeding flow rate, the gate opening can be adjusted in time to adjust the steady-flow feeding. The amount of material pressure at the top of the machine.

(3) The preferred inverter for the steady-flow feeder is a vector-type inverter, which ensures 10% to 150% torque output when the steady-flow feeder speed is <5Hz, and at the same time increases the power configuration of the steady-flow feeder.

(4) In order to avoid over-adjustment of the feed amount in situations such as material interruption or material flushing, the steady-flow feeder control system adopts a segmented PID control strategy to adaptively adjust different PID limits based on the deviation between the feedback flow and the set flow. Amplitude parameters to solve the problem of “oscillation adjustment”.

For this type of CP concrete warehouse, we started from the on-site material characteristics and process conditions, comprehensively considered the related process links such as feeding, storage, flow assistance, unloading, measurement, and control, and summarized a set of measurement technology suitable for Coriolis measurement system process optimization plan for the modified project:

(1) Focus on and check the range range of the Coriolis force scale and the actual measured bulk density of the material to ensure that the equipment selection covers the actual use range as much as possible.

(2) In the design stage of the technical transformation plan, try to reserve a calibration discharge bypass at the downstream stage of the weighing scale outlet to facilitate on-site physical calibration.

(3) If there is a central chamber pressure reducing cone structure in the original concrete warehouse, it is necessary to add an exhaust pipe (the pipe is connected to the chute or dust collector at the bottom of the warehouse); if there is no pressure reducing cone in the original concrete warehouse, it is recommended that the cement plant install a pressure reducing cone at the bottom of the warehouse. Install steel pressure reducing cone and exhaust pipe. The photo of the steel pressure reducing cone installed in the slag powder warehouse is shown in Figure 14.

(4) Before installing the Coriolis force scale, the concrete warehouse must be thoroughly cleaned to ensure that it is clean and free of debris and agglomerations, and repair blocked or damaged inflatable boxes and pipes.

(5) Reasonably arrange the flow-assisted inflatable boxes in the warehouse and at the outlet at the bottom of the warehouse, and strictly control the air volume of the Roots blower. If the air volume of the original Roots blower is too large, it needs to be replaced with a fan with a small air volume or supporting frequency conversion control equipment.

(6) If there are a lot of debris in the materials, it is necessary to add a quick-open isolation net at the feed inlet at the top of the warehouse (silo) and clean it regularly; add a buffer channel at the entrance of the warehouse; if space permits, it can be installed at the bottom of the warehouse Install a crushing valve.

(7) Ensure that there is an accurate and reliable material level indicator instrument in the warehouse, maintain a certain material level in the warehouse, and try to avoid low material level work.

(8) In order to ensure dust collection and negative pressure in the pressure reducing cone exhaust pipe, steady flow feeder, weighing scale, and discharge chute, install a small-height stand-alone dust collector at the bottom of the warehouse or nearby outside the warehouse, and pay attention to dust collection. The pipe installation angle ensures that the feeding and metering equipment operates under slightly negative pressure.

(9) Set up a reasonable maintenance steel platform on site to ensure that the equipment is installed firmly and avoid vibration during equipment operation. (10) Ensure that the air volume of the centrifugal fan supporting the discharge chute and the number and location of the flow-assisted air ducts are reasonably designed.

(10) Ensure that the air volume of the centrifugal fan supporting the discharge chute and the number and location of the flow-assisted air ducts are reasonably designed.

Through the standardized layout of the on-site process in the early stage and the remote continuous tracking and monitoring in the later stage, most of the slag powder metering systems currently in operation have achieved good results. Compared with before the technical reform, the slag powder feeding is more stable, the measurement accuracy and control reliability are greatly improved, the established technical reform goals have been achieved, and good social and economic benefits have been achieved.

4.2 Application example analysis

The selected two sets of Coriolis force scale measurement systems for a project, one (model KXT (H)-280) for measuring 280t/h mixed slag powder, and one (model KXT (S)-320) ) is used to measure clinker powder at 320t/h. The separately measured mixture and clinker powder enter the mixer to mix and formulate finished cement.

In order to solve the caking problem in the warehouse, both sets of metering systems adopt the metering solution of “large flow steady flow feeder + center feeding Coriolis force scale”, as shown in Figure 15, both are equipped with self-developed by Special double-shaft mixing type crushing valve. At the same time, in order to ensure the stability of feeding, a section of overflow non-standard parts is added between the crushing valve and the steady-flow feeder, and is connected to the negative pressure dust collection pipe to ensure that the system operates in a slightly negative pressure state. The actual scene of the clinker powder metering system and mixed powder metering system is shown in Figure 16.

Since the project was put into operation, the two measuring systems have been evenly and stably fed, with precise and controllable measurement, stable and reliable operation, effectively ensuring the stable quality of finished cement.

by Weixiantang, Leijin