As a bulk material, pneumatic conveying has a history of more than 100 years. Compared with conventional mechanical transportation and vehicle transportation, it has unique advantages: fully enclosed pipeline transportation, no secondary pollution; transportation route layout can be flexibly changed according to the site ; Using air as the medium, no other medium is required; various parameters of the entire conveying process can be monitored by computer-aided technology. It is widely used in electric power, building materials, coal, chemical and food industries.
Pneumatic conveying system is divided into pressure feeding type, suction feeding type and hybrid type. Either way, the pneumatic conveying system is composed of several basic structures such as blower, feeding device, conveying pipeline, separator, and dust collector. The principle is that the material enters the silo through the feeding hopper. When the height is certain, the feed valve is closed. At this time, the compressed air enters the silo pump through the air inlet valve, so that the material and gas are mixed to form the material and air flow, and then the discharge valve is opened, and the material and air flow are opened. It can be sent to the storage warehouse through the pipeline.
From the emergence to the widespread application of pneumatic conveying, it has experienced the research transition from dilute phase to dense phase, which has promoted the continuous development of pneumatic conveying.
When the particle volume ratio (material/gas) in the airflow does not exceed 0.05, and the porosity of the solid-gas mixing system is greater than 0.95, it is called phase-phobic transport; when the particle volume ratio (material/gas) in the airflow exceeds 0.2, solid-gas mixing When the porosity of the system ε < 0.8, it is called dense phase transport. Dense phase conveying usually takes the silo pump as the sending tank, the conveying pressure is 100~600kPa, the gas velocity is 2~8m/s, the solid-gas ratio is greater than 15, the gas velocity is low, the wear is small, and the conveying efficiency is high. Dense phase conveying is intermittent conveying, with stable conveying and long conveying distance, small diameter of conveying pipe and convenient installation. Dense phase conveying systems are widely used in the loading and unloading of bulk materials such as plastic particles, milk powder, pharmaceuticals, cement, molding sand, coal ash, etc. in chemical, food, pharmaceutical, building materials, mining, metallurgy and electric power industries. In this paper, the common problems in the dense phase pneumatic conveying system are analyzed, and the causes and influencing factors are analyzed.
Frequently Asked Questions about Dense Phase Pneumatic Conveying Systems
(1) Blockage Pipe blockage is one of the most common and difficult-to-handle faults in dense-phase pneumatic conveying systems, accounting for more than 40% of the failure rate of pneumatic conveying systems. Especially at the connection between the horizontal section and the vertical section of the conveying pipeline, the blockage is more serious. Due to the long and overhead transmission pipeline, maintenance is very difficult.
(2) Component wear Common component wear, such as more than 40% of the outlet pipe failure rate of the silo pump. Especially in the pipeline valve wear and damage is serious, and the maintenance workload is large. The rotary feed valve under the buffer silo wears and damages quickly and leaks powder seriously.
(3) Unsmooth blanking The dense phase pneumatic conveying system takes a long time to discharge, and powder blocking occurs during the operation of the powder storage silo, resulting in difficult blanking.
(4) Insufficient output The output of the dense phase pneumatic conveying system is too small or insufficient, and it is difficult to adapt to the pneumatic ash removal requirements of large-scale installations, such as thermal power plants.
Causes and Influencing Factors of Common Problems in Dense Phase Pneumatic Conveying System
1. System design and selection
Dense-phase pneumatic conveying is a complex motion process of gas-solid two-phase flow. At present, the determination of the parameters of the dense phase silo pump pneumatic conveying system is still based on the calculation method of the suspended dilute phase positive pressure pneumatic conveying, and the important parameters are still selected by experience. The key factors affecting dense phase pneumatic conveying are energy consumption and stability, which need to be considered during 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 pipe, inclined pipe, pipe fittings, valve fittings, etc.) and separator pressure loss, etc. . Usually an empirical formula is used, based on the pressure loss of the conveyed gas, and the pressure loss factor of the conveyed material is used to consider the pressure loss of the conveyed material. In the design calculation, more consideration should be given to the places that are likely to cause pressure drop or large flow resistance, and reasonable materials and design schemes should be selected.
1.2 Solid gas ratio
For materials of a certain particle size, the solid-gas ratio has an optimal value, which can not only ensure the transportation of materials, but also ensure the minimum gas consumption. If the solid-gas ratio is higher than the optimum value, although the material is transported out, the gas consumption is large, the flow velocity of the material in the conveying pipeline is high, and the wear of the material on the pipeline becomes larger.
1.3 Setting the lower/upper limit of the pressure of the silo pump
The setting of the lower limit of the pressure of the silo pump is more important. If the value is set too large, the material in the pump or the pipeline will not be transported completely, which will affect the second transport. If the value is set too small, the materials in the pump or pipe have already been conveyed, which will cause waste of compressed air, prolong the conveying time and reduce the conveying efficiency. Generally, the lower limit of silo pump pressure is set as silo pump delivery pressure plus 0.01～0.03MPa. The upper limit of the pressure of the silo pump is set as the pressure in the actual delivery process of the silo pump plus 0.02～0.04MPa.
1.4 System selection and layout
Different types of silo pumps have different parameters such as system output, delivery pressure and solid-gas ratio. Improper selection will cause problems such as insufficient output or excessive energy consumption. The poor pneumatic ash removal in most power plants is manifested as insufficient conveying capacity, mainly due to small design selection and changes in coal types. The dense phase pneumatic conveying system currently mainly includes positive pressure single-silo pump system, positive pressure multi-silo pump system and low positive pressure system. The application range of its system parameters is shown in the table below.
Parameter application range of dense phase pneumatic conveying system
2. Conveying medium
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 required for pneumatic conveying is 0.55MPa. In the process of conveying materials by the silo pump, if the pressure is lower than 0.55MPa, the conveying pipeline will be easily blocked, the silo pump will not be able to deliver the material, or the conveying time will become longer. The purification degree of the compressed air source has a great influence on the pneumatic conveying. If the compressed air source is not completely purified, there will be a lot of water and oil in the compressed air source. When the oil and water come into contact with the powder material, they will stick to the gasification plate of the silo pump, which will affect the flow pattern and material inside the silo pump. Fluidization reduces the delivery efficiency of the silo pump, which in turn affects the smooth delivery of the system.
2.2 Material quality
(1) Material particle size The smaller the material particle size, the stronger its fluidity.
(2) The material contains water Some materials, such as fly ash, have many pores and cracks on the surface, and the maximum porosity is 60% to 70%. This structure has strong fluidization properties in the dry state, but also has a strong adsorption effect on water. If the equipment is not well sealed and causes internal water seepage, the ash will agglomerate in contact with water, causing the system to block.
(3) There are many sundries in the material. If the interior of the conveying system is not cleaned up, the sundries will enter the hopper, which will cause the silo pump and the conveying pipeline to be blocked.
3. Intermittent operation
The material conveyed by the silo pump is operated intermittently. During the alternate conveying process, the material will gradually deposit at the bottom of the conveying pipe or at the bend, causing blockage. At the moment when the discharge valve is opened to start conveying, the wear on the valve core of the discharge valve is relatively large, and the elbow of the pipeline is impacted and worn. In the process of loading materials into the silo, the materials at the bottom of the silo will be pressed together for a long time and bonded together, which is not easy to disperse and fluidize, and cause blockage in the horizontal pipeline at the conveying outlet.
4. Component failure
4.1 Air leakage of pneumatic valve
The feed valve, exhaust valve, discharge valve, delivery valve and blow-off valve used in pneumatic conveying system are all pneumatic valves. Wear the rubber sealing ring, and then wear the door panel and valve body.
4.2 Level meter failure
The level gauge used in the pneumatic conveying system has high accuracy, and the adjustment of the level gauge is more important. If the sensitivity is adjusted too sensitively, it will cause the silo pump to feed too little. If the sensitivity is not adjusted enough, the silo pump will be fed too much, the fluidization space in the silo pump will be reduced, the material concentration will be larger, and the pipe will be easily blocked.
4.3 Pressure transmitter failure
The internal pressure of the silo pump plays a very critical 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, causing the set pressure to be unable to be reached during the fluidization process, and the silo pump quits operation due to the overtime of pressurization. ②The lower limit of the pressure of the silo pump is not reached, and the conveying process ends, resulting in the accumulation of dust in the pipeline, which affects the transportation of other silo pumps in the same conveying pipeline. ③The contact voltage is 24v, and the contact is easily affected by the environment and causes interference. ④Contact arc carbonization, poor contact. ⑤ The pressure pipe is blocked, so that the pressure value of the transmitter remains unchanged or changes very little, causing the pipe to be blocked by mistake.