As a bulk material transportation method, pneumatic conveying has a history of more than 100 years. Compared with traditional mechanical transportation and vehicle transportation, it has unique advantages: the pipeline transportation method is fully enclosed without secondary pollution; the layout of transportation routes can be flexibly changed according to the site; air is used as the medium, and no other medium is required; the entire transportation process The various parameters can be achieved by monitoring computer-aided techniques. Widely used in electric power, building materials, coal, chemical, food and other industries.
Pneumatic conveying system is divided into three types: pressure conveying, suction conveying and mixed conveying. No matter what kind of pneumatic conveying system, it is composed of basic structures such as blower, feeding device, conveying pipeline, separator, dust collector, etc. The principle is that the material enters the silo through the hopper. When the height is fixed, close the feed valve. At this time, the compressed air enters the silo pump through the intake valve, so that the material and the air are mixed to form the flow of the material and the air. Then the discharge valve is opened, and the material and air flow are conveyed to the silo through the conveying pipeline.
当粒子体积比(材料/气体)在空气流中不超过0.05和孔隙比ε> 0.95固体气体混合系统,它被称为稀疏相运输;当粒子体积比(材料/气体)在空气流中超过0.2,孔隙比ε< 0.8固体气体混合系统,它被称为密相输送。在密相输送中，通常采用料仓泵作为输送罐。输送压力100-600kpa，气速2-8m / s，固气比大于15，气速低，磨损小，输送效率高。密相输送属于间歇输送，输送稳定，输送距离长，输送管径小，安装方便。密相输送系统广泛应用于化工、食品、制药、建材、矿山、冶金、电力等行业的塑料颗粒、奶粉、化工、水泥、型砂、粉煤灰等大宗物料的搬运输送。本文针对密相气力输送系统存在的常见问题，分析了产生问题的原因及影响因素。
Causes and Influencing Factors of Common Problems in Dense Phase Pneumatic Conveying System
System Design and Selection
Dense-phase pneumatic conveying is a complex gas-solid two-phase flow process. At present, the parameters of the silo pump pneumatic conveying system for dense phase conveying are still based on the calculation method of dilute phase positive pressure pneumatic conveying. The key factors affecting dense phase pneumatic conveying are energy consumption and stability, which should be considered in parameter design and equipment selection.
1.1 System pressure loss
The pressure loss of the system includes the pressure loss of the feeding device, the pressure loss of material acceleration and lifting, the pressure loss along the pipeline (horizontal straight pipe, vertical straight pipe, elbow, inclined pipe, pipe fittings, valves, etc.), the pressure loss of the separator Wait. Generally, an empirical formula is used, and according to the pressure loss of the conveyed gas, the pressure loss coefficient is used to consider the pressure loss of the conveyed material. In the design and calculation, more consideration should be given to the places prone to pressure drop or large flow resistance, and reasonable materials and design schemes should be selected.
1.2 Solid gas ratio
For materials with a certain particle diameter, there is an optimal solid-gas ratio, which can not only ensure the transportation of materials, but also ensure the minimum gas consumption. When the solid-gas ratio is higher than the optimal value, although the material is transported out, the air consumption is large, the flow velocity of the material in the conveying pipeline is large, and the material wears the pipeline greatly.
1.3 Setting of the upper and lower limits of the pressure of the silo pump
It is important to set the lower limit of the silo pump. If the value is set too high, the material in the pump or pipeline will not be fully conveyed, which will affect the second conveyance. If the set value is too small, it means that the materials in the pump or pipeline have been transported, which will cause waste of compressed air, prolong the conveying time and reduce the conveying efficiency. The lower limit of the pressure of the silo pump is generally set as the output pressure of the silo pump plus 0.01-0.03mpa. The upper limit of the pressure of the silo pump is the pressure of the actual conveying process of the silo pump plus 0.02-0.04mpa.
1.4 System selection and layout
Due to the different types of silo pumps, the parameters of system output, delivery pressure and solid-gas ratio are also different. The main reasons for the poor performance of pneumatic dust removal in power plants are insufficient conveying capacity, mainly due to small design selection and changes in coal types. At present, the dense phase pneumatic conveying system mainly includes positive pressure single-silo pump system, positive pressure multi-silo pump system and low positive pressure system. The scope of application of the system parameters is shown in the table below.
Application Scope of Dense Phase Pneumatic Conveying System Parameters
2.1 Quality of compressed air source
The quality of the compressed air source refers to the pressure and purification degree of the compressed air source. The minimum pressure of compressed air pneumatic conveying is 0.55mpa. When the pressure is lower than 0.55mpa, the conveying pipeline is easily blocked, the material cannot be pumped out through the silo pump, or the conveying time becomes longer. The degree of purification of the compressed air source has a great influence on pneumatic conveying. If the compressed air source is not thoroughly purified, there will be a lot of water and oil in the compressed air source. When oil and water contact powder materials, they will adhere to the gasification plate of the silo pump, which will affect the flow pattern and material fluidization in the silo pump, reduce the efficiency of silo pump delivery, and affect the smooth delivery of the system.
(1) The smaller the particle size, the stronger the fluidity.
(2) The material contains water. Some materials with high water absorption, such as fly ash, have many pores and cracks on the surface, and the maximum porosity is 60% ~ 70%. This structure has strong fluidity in the dry state and a strong adsorption capacity for water. If the equipment is not well sealed, it will cause internal water seepage, and the ash will agglomerate with water, causing the system to block.
(3) There are many sundries in the material. If the inside of the conveying system is not cleaned, the sundries will enter the hopper and block the silo pump and conveying pipeline.
The silo pump conveying is an intermittent operation. During the alternate conveying process, the material will gradually deposit at the bottom or corner of the pipeline, causing blockage. When the unloading valve is opened for transportation, the spool of the unloading valve is greatly worn, and the elbow of the pipeline is impacted and worn. In the process of loading the material into the silo, the material at the bottom of the silo will be squeezed and bonded together for a long time, which is not easy to disperse and fluidize, resulting in blockage of the horizontal pipe at the conveying outlet.
4.1 Air leakage of pneumatic valve
The feed valve, exhaust valve, discharge valve, discharge valve and plug valve used in the pneumatic conveying system are all pneumatic valves. When the rubber sealing ring in the valve body is aging, deformed, and the sealing is not tight, the dry particle airflow will first wear the rubber sealing ring, and then wear the door plate and valve body.
4.2 Level gauge failure
The level gauge of the pneumatic conveying system has high precision, and the adjustment of the level gauge is very important. If the sensitivity adjustment is too sensitive, the silo pump feed volume is too small. If the sensitivity adjustment is not enough, it will lead to excessive feeding of the silo pump, reduce the fluidization space in the silo pump, increase the material concentration, and easily cause pipeline blockage.
4.3 Pressure transmitter failure
The internal pressure of the silo pump plays a key role in the entire operation control process, which directly affects the operation of the system and the judgment of faults. Common faults of the pressure transmitter: ① The pressure pipe leaks, the delivery cannot reach the set pressure, and the silo pump stops when it is pressurized over time. ②The lower limit of the pressure of the silo pump has not been reached. After the conveying process is completed, the accumulation of dust in the pipeline will affect the transportation of other silo pumps in the pipeline. ③The contact voltage is 24 V, and the contact is easily disturbed by the environment. ④ Contact arcing and carbonization are poor. ⑤ The pressure suction pipe is blocked, so that the pressure value of the transmitter is always the same or has a small change, resulting in the pipe being blocked by mistake.