Asphalt mixing plant (intermittent asphalt mixing plant, hot mix asphalt production equipment) is the core production equipment of highway municipal engineering. It is under the condition of high temperature of 150~180℃ for a long time, strong aggregate wear, dust diffusion and 24-hour continuous heavy load. It is easy to have a series of problems such as unstable production capacity, discharge temperature fluctuation, burner failure, measurement deviation, belt conveyor blocking, bag dust removal exceeding standard, PLC control system failure, abnormal noise wear of equipment, unqualified mixture products, etc.
According to the statistical data of road construction machinery industry, more than 75% of the shutdown faults of domestic asphalt mixing plantsare due to lack of daily inspection, failure to calibrate on time and inadequate lubrication and maintenance, which not only causes delay in construction period, waste of fuel oil and aggregate raw materials, environmental protection fines, but also produces unqualified asphalt mixture, directly induces early diseases such as rutting, loosening, cracking and water damage of pavement, and greatly increases the maintenance cost of roads in the later period.
Asphalt mixing plant as the core of road engineering hot mixing production equipment, long-term in high temperature, dust, strong wear, continuous heavy load extreme conditions, production process is prone to production fluctuations, temperature out of control, equipment failure, environmental protection standards, mixture quality unqualified and other problems. Most of the equipment abnormalities are not caused by sudden failures, but chain production hidden dangers formed by the superposition and accumulation of five core factors: equipment wear, raw material fluctuation, artificial operation deviation, environmental seasonal change and lack of maintenance management. All kinds of problems have strong conductivity. If single detail control is not in place, it will gradually evolve into systematic production failure. The specific causes are as follows:
Continuous wear and aging of equipment machinery: the core parts of asphalt mixing plant face the continuous friction and erosion of high hardness angular aggregates for a long time, and the wearing parts such as mixing blades, drying cylinder lifting plates, conveyor belts, weighing hoppers, dust filter bags, etc. wear quickly. At the same time, the equipment operates in a high temperature environment above 150℃ for a long time, and the bearings, seals, oil pipelines and transmission structures are accelerated to age and fail. If they are not repaired and replaced in time, it is easy to cause sudden failures such as stagnation, leakage and shutdown.
Fluctuation of raw material quality: uneven quality of raw materials is one of the core causes of production failures. The excessive moisture content of aggregate, deviation of particle gradation from design standard, excessive mud content, aging of asphalt resin, excessive fuel impurities and insufficient calorific value will greatly increase the operating load of drying system, combustion system, metering system and dust removal system, which will directly lead to a series of secondary quality and equipment problems such as incomplete drying, insufficient combustion, imbalance of oil-stone ratio and excessive dust.
Non-standard manual operation on site: most production hidden dangers stem from human operation deviation. The operator changes the production parameters at will, the cold bin blanking ratio is uneven, and the stirring time is unreasonable, which will directly disrupt the balance of the production process; some personnel short-circuit the safety interlock device for convenience and ignore the abnormal operation of the equipment, which is the main man-made reason for the temperature out of control, the ratio disorder, the burner flameout and the equipment safety accident.
Environmental and seasonal conditions interference: outdoor open-air operation characteristics, so that mixing plant production by the season, weather and environment greatly affected. High temperature in summer is easy to cause aging of asphalt storage tank oil and condensation of dust collection bag; low temperature in winter will lead to high viscosity of heavy oil, solidification of pipeline, rapid heat dissipation of equipment, and significant decrease in drying efficiency; in rainy weather, the moisture content of aggregate rises sharply, greatly increasing the drying load of equipment, easily causing problems such as insufficient production capacity and temperature fluctuation.
Preventive maintenance system is seriously lacking: daily inspection, cleaning, lubrication, calibration and replacement of wearing parts are the basis for ensuring stable operation of equipment, but most mixing stations have problems of heavy production and light maintenance. Long-term skipping of zero calibration, dust cleaning, bearing lubrication, seal maintenance and other basic work, resulting in sensor drift, filter bag clogging, belt deviation, bearing oil shortage and other minor problems continue to accumulate, and finally upgraded to the whole line shutdown, quality scrap, environmental protection standards and other major failures.
It is worth noting that the five causes do not exist independently, but are interrelated and transmitted layer by layer, forming a vicious circle. For example, if the moisture content of aggregate is too high, it will force the burner to work overload, cause temperature fluctuation, sensor drift, and finally lead to imbalance of oil-stone ratio and batch scrapping of mixture; blockage of dust removal bag will cause equipment pressure difference to rise, poor exhaust, incomplete combustion, and excessive flue gas dust, which will not only affect equipment production capacity, but also trigger environmental protection violations. Therefore, all kinds of problems of asphalt mixing plant are essentially comprehensive manifestations of imbalance in all-round controlof working conditions, materials, operation and maintenance.
The output fluctuates between high and low, and the rated capacity of the equipment cannot be reached continuously, which directly causes the material interruption of the paving working face and the increase of overtime cost of the team; the raw material drying is insufficient, and the proportion imbalance produces a large amount of waste materials, and the loss of fuel oil and sand is increased by 20%~40%; the long-term low load operation will disrupt the thermal balance of the whole equipment, induce the temperature and dust removal chain failure, and the economic loss of the small and medium-sized mixing station in a single day can reach several thousand yuan.
(1) The mixture ratio setting does not match the on-site production: the laboratory target mixture ratio is directly applied to the production system, and it is not dynamically corrected according to the sand moisture content and aggregate cleanliness. The discharge flow rate of the cold bin is unbalanced, and the material flow of the drying cylinder and the mixing tank is blocked.
(2) Unqualified aggregate grading: the diameter of crushed stone, stone powder and fine sand exceeds the designed screening interval, the vibrating screen frequently blocks, the material with excessive particle size continues to return, and the effective material passing amount decreases greatly; the material stacking in the material yard is separated by layers, the aggregates are mixed, and the continuity of blanking is broken.
(3) Sand moisture content exceeds the standard: the specification requires that the average moisture content of aggregate is ≤5%, and the moisture content of sand stacked in the open air in rainy season can reach 8%~12%. The drying cylinder needs to consume more fuel evaporation moisture, and the drying speed cannot keep up with the feeding speed, forming a bottleneck in the whole line.
(4) insufficient burner and fuel performance: diesel, heavy oil impurities, low calorific value, atomization combustion efficiency decreased, drying cylinder heating speed is insufficient; flame adjustment imbalance, ignition failure, damper ratio imbalance, drying efficiency continued to decline.
(5) unreasonable operation parameters: dry mixing, wet mixing time is too long, cold material belt conveying speed, drying drum drum speed parameters do not match the raw material characteristics; material shortage in the bin, frequent no-load and full-load switching of equipment.
(6) Bottleneck of conveying and screening machinery: belt deviation and slip, idler jam, screen blockage, belt scale calibration failure, intermittent cold material supply, stable production cannot be guaranteed.
The dynamic optimization mixture ratio systemarranges technicians to sample and detect the moisture content of aggregates every 2 hours, corrects the production mixture ratio in real time in the PLC control system, and starts the automatic compensation program of moisture content; avoids fixed parameters, and fine-tunes the blanking ratio of each bin according to the ambient temperature in the morning, middle and evening, and the dry and wet degree of sand, so as to ensure that the materials enter the drying cylinder at a constant speed.
Strictly control the aggregate quality of the material yard. Sand andgravel piles are fully covered with rainproof tarpaulin and stacked in different warehouses and layers to avoid mixing and segregation of large and small aggregates. Screening test shall be conducted for each batch of incoming sand and gravel, and the materials blocked by the vibrating screen shall be cleaned regularly. Fine sand shall be stored separately to reduce the fluctuation of overall moisture content.
Optimize the drying efficiency of combustion system,select fuel with up to standard calorific value and less impurities, preheat heavy oil in advance to ensure fluidity; clean burner nozzle and fuel oil filter before each shift starts, adjust the ratio of damper and fuel oil to ensure stable flame and full combustion; adjust combustion power according to dry and wet degree of aggregate, moderately increase firepower and reduce belt feeding speed in wet material stage.
Calibrate the equipment operation process parameters andconduct trial production commissioning in combination with the mixing station model (intermittent/continuous, mobile/fixed station) to shorten redundant mixing time; stabilize the material level of cold bin and prohibit empty bin operation; reduce the cold material conveying speed under high moisture content conditions in rainy season to give sufficient drying time for drying drum.
Normal maintenance of conveyor and screening equipmentWeekly calibration of belt scales, frequency conversion feeder zero point; daily cleaning of screen adhesion powder, adjustment of belt tension, correction device, replacement of worn rollers, cleaners, eliminate blanking jam, material scattering and blocking problems
Long-term treatment of high-moisture aggregate and large-volume projects can be upgraded to large-volume drying cylinders and high-power burners; small-scale projects can avoid overload forced production and reduce equipment loss and capacity fluctuation from the source.
Arrange operators to record real-time output, aggregate moisture content and burner load curve every hour, shut down immediately for 3 consecutive hours when the production capacity is lower than 80% of the rated value, give priority to checking the moisture content of raw materials and combustion conditions, and then check the conveying metering equipment, which can quickly locate the fault point.
The standard temperature range of asphalt mixture discharge is 150\180℃ (modified asphalt needs to be raised to 170\185℃). Temperature fluctuation is the primary cause of early pavement damage:
Burner is the core component of asphalt mixing plant heating. Once it fails, it will directly cause insufficient capacity, temperature out of control and flue gas exceeding standard, which belongs to the core equipment failure with the highest shutdown rate.
Ignition delay, multiple ignition failures; flame fluctuation, black smoke, flame licking the inner wall of drying cylinder; abnormal increase in fuel consumption; frequent triggering of automatic shutdown by equipment safety interlock; continuous low aggregate drying temperature.
Graded investigation and rectification scheme
(1) Quick on-site inspection for each shift (required before commencement)
Check fuel storage, fuel supply pressure, clean fuel filter; wipe flame detector lens to confirm no carbon deposition; start fan to observe whether air door opens and closes smoothly, check whether flame has tempering and black smoke; check whether fuel pipeline has leakage and freezing blockage.
(2) Weekly deep maintenance
Disassemble and clean the burner nozzle and electrode; use combustion analyzer to detect flue gas components, accurately adjust the fuel and air ratio; clean dust at the fan inlet, test the action sensitivity of the damper actuator; check the ignition circuit insulation layer, tighten and loosen the terminal.
(3) Quarterly/annual overhaul maintenance
Disassemble the complete burner assembly, clean the carbon deposit and oil stain inside; replace the aging nozzle, ignition electrode and sealing gasket; repair the damaged refractory layer; match the heavy oil preheating device in winter to ensure the fuel atomization quality; reserve the vulnerable spare parts such as nozzle, sensor and filter, and replace them quickly when they fail.
It is strictly prohibited to artificially short-circuit flame monitoring, over-temperature protection, pressure interlock and other safety devices. In history, many explosion accidents in mixing stations are caused by unauthorized removal of safety protection; locking and tagging and pressure relief procedures must be implemented for shutdown maintenance to prevent disassembly and assembly of fuel pipelines under pressure.
The measurement accuracy of asphalt, aggregate and mineral powder directly determines the pavement performance of mixture. If the weighing error exceeds ±0.3%, it will cause unqualified laboratory test of mixture, which is the core pain point of quality control of mixing station.
Too much asphalt content: mixture flooding, road rutting, anti-skid performance decline; too little asphalt content: weak aggregate bonding, loose pavement peeling, serious water damage; mineral powder, aggregate measurement deviation: mixture void ratio imbalance, Marshall stability can not meet the specification standards, engineering acceptance can not pass.
Zero drift is not calibrated in time: long-term vibration of equipment, alternating cold and hot ambient temperature, continuous zero drift of load cell, daily no peeling and clearing.
Weighing bucket accumulated material pollution: asphalt residual agglomeration, stone powder dust accumulated on the hopper, sensor stress surface, increase extra load, readings continue to be high; discharge door sealing is not tight, ore powder, fine aggregate continuous leakage.
Load cell hardware damage: high temperature, dust, rain erosion caused by sensor circuit moisture, insulation damage; long-term overload, vibration caused by sensor elastomer aging, measurement accuracy permanently reduced.
Mechanical structure defects: discharge door wear closed not tight, material residue; equipment base fixed loose, continuous vibration interference weighing data during production.
Control system adaptation problem: PLC program does not set moisture compensation, only according to the dry material ratio measurement wet aggregate, the actual effective mix ratio deviation from the design value.
Standardized calibration process: before starting each shift, the weighing hopper shall be peeled and cleared; the aggregate scale, asphalt scale and mineral powder scale shall be calibrated with standard weights every week; after overhaul, replacement of sensors and relocation of equipment, a complete set of measurement calibration shall be carried out, and calibration records shall be kept. The asphalt weighing hopper must be completely emptied to avoid residual asphalt adhesion affecting the next weighing.
Regularly clean and maintain the seal. Clean the asphalt caking and accumulated mineral powder on the inner wall of the weighing hopper every day. Check the sealing strip of the discharge door and replace it immediately if it is worn to prevent powder leakage. Install dust and heat insulation protective cover outside the sensor to avoid direct corrosion by high temperature dust.
Sensor long-term protection Regularly check the waterproof and grounding devices of the terminal, and make the line moisture-proof in rainy season; avoid overload feeding, and install shock absorbers at the vibration parts of the equipment; replace the aging sensor in batches in advance to prevent sudden failures.
PLC system for production data linkage control enables aggregate moisture content compensation function. Laboratory samples are taken every 2 hours to detect oil-stone ratio and screening index, and automatic metering data of mixing station are compared. If there is deviation, stop calibration immediately; operators are strictly prohibited to modify metering parameters without permission.
Cold material belt, elevator and aggregate belt are material conveying channels. Conveying failure will directly cause aggregate supply interruption, grading segregation, and chain multiple problems such as output, temperature and proportion.
The belt is deviated and the grinding edge is torn, and the belt slips and idles; material is scattered on both sides of the material, and material stuck on the belt surface is scattered on the return journey; the cold silo arches and blocks the material, and the discharging is intermittent; the rollers and bearings make abnormal noises and get stuck, and the conveying load suddenly increases.
Bag dust collector (bag filter) is the core equipment of environmental protection in mixing station. The increase of pressure difference and the damage of filter bag will cause dust emission without organization, trigger the shutdown of environmental protection department for rectification and fine, and at the same time, the recovery of mineral powder will be interrupted, and the mixture gradation will be unbalanced.
The pressure difference between the inlet and outlet of the dust collector continues to rise, and the exhaust resistance of the fan is large; white/black dust and smoke can be seen in the chimney; the filter bag is frequently damaged and leaking ash; the pulse ash cleaning effect is poor, the dust on the surface of the bag is agglomerated and the bag is pasted; the ash hopper is full of accumulated material. Overflow the drying cylinder.
Intelligent asphalt mixing station relies on PLC programmable controller and temperature/weighing/pressure sensor to realize full automatic production. The fault concealment of electrical control system is strong, which is easy to cause shutdown without warning.
The equipment stops safely without any reason, and the central control screen reports errors; the temperature, weighing and pressure data are not displayed or the values jump; the HMI operation interface is stuck, and the parameters cannot be modified; the communication of each equipment is interrupted, and the independent mechanism cannot be started and stopped.
Abnormal noise and abnormal vibration of equipment are pre-warning signals of mechanical damage. Most major shutdown faults have continuous abnormal noise in the early stage. Failure to handle them in time will cause irreversible damage to bearings, gears, cylinders and stirring cylinders, which will greatly increase maintenance costs.
Mixture quality defects are the comprehensive results of the previous eight system failures, all production, measurement, temperature control, transportation, dust removal failures will eventually be reflected in the finished material, directly affecting the project acceptance and road service life.
The mixture is mottled in color and incomplete in aggregate wrapping; the ratio of oil stone fluctuates; the coarse aggregate and fine aggregate of the mixture are separated; the porosity and stability of indoor Marshall test are not up to standard; the mixture temperature is stratified, local overheating aging, local low temperature white material; the mixture remains moisture, and the pavement is foamed and peeled off after paving.
Proportioning segregation problem: calibrate weighing system, stabilize cold material blanking speed, stack aggregates in stock yard, avoid mixing of large and small materials, mix time to meet specification requirements, and ensure full mixing of aggregates and asphalt.
Temperature defects: stabilize burner conditions, calibrate temperature sensors, control aggregate moisture content, eliminate heat loss of drying cylinders, and eliminate local uneven cooling and heating of mixtures.
Aggregate cleanliness is insufficient: the bag dust removal system operates normally, the recovered mineral powder is supplied stably, the incoming sand is washed with soil impurities in advance, and the ultrafine soil powder is reduced to affect the bonding performance.
Insufficient mechanical mixing efficiency: replace worn mixing blades regularly, clean asphalt caking on the inner wall of mixing cylinder, and ensure sufficient mixing space and uniform mixing.
Whole process quality control: sampling and temperature measurement of each vehicle mixture, hourly laboratory screening, oil stone ratio detection, data synchronization central control system, immediate shutdown in case of quality fluctuation, troubleshooting of upstream equipment, and elimination of unqualified materials from the source.
Preventive maintenance is the core means to reduce downtime rate, control operation and maintenance cost, ensure mixture quality and meet environmental protection requirements. It is divided into five levels of standardized maintenance list per shift, weekly, monthly, quarterly and annual, taking into account condition monitoring and regular maintenance, supporting account records, spare parts reserve and personnel training.
Conveyor system: belt rectification, cleaning roller accumulation, inspection cleaner, calibration frequency conversion feeder;
Metering system: full-range calibration aggregate scale, asphalt scale, mineral powder scale, cleaning weighing bucket accumulated material;
Combustion system: clean nozzles and fuel filters, adjust air-fuel ratio, and test flame detectors;
Dust removal system: check whether the filter bag is damaged, discharge dust from the ash hopper, test the pulse ash cleaning solenoid valve;
Electrical system: check PLC fault log, clean dust from control cabinet, tighten terminal blocks.
Drying cylinder and mixing cylinder: Check the wear degree of lifting plates and mixing blades, and repair the insulation layer and seals of the cylinder body;
Mechanical transmission: the whole machine gearbox, bearing grease filling, vibration testing, replacement of slightly worn rollers, seals;
Temperature control system: fully calibrate all temperature measuring thermocouples, overhaul asphalt tank heating circulation system;
Raw material control: sand screening, moisture content detection, optimization of yard rain prevention, anti-segregation stacking scheme.
Complete set of equipment disassembly cleaning: drying drum, mixing cylinder, dust collector internal depth descaling;
Replace wearing parts in large quantities: conveyor belt, filter bag, bearing, nozzle, gasket, sensor;
Equipment structure maintenance: frame weld, anchor bolt, cylinder wear repair, foundation settlement detection;
Electrical system upgrade: line aging replacement, PLC program backup, lightning UPS system detection;
Complete calibration of metering, temperature control and dust removal system of the whole machine, and issue maintenance inspection report.
Spare parts warehouse reserve: standing burner nozzle, filter bag, weighing sensor, belt, bearing, sweeper and other high-loss parts, fault quick replacement;
Digital ledger: CMMS equipment management system is used to record the time of each maintenance, calibration and replacement of components, and fault trend analysis is formed;
Personnel training: regular training for operators and maintenance technicians to master fault pre-identification, basic calibration and safety maintenance specifications;
Predictive operation and maintenance tools: equipped with infrared camera, vibration detector, smoke analyzer, early detection of hidden wear, insufficient combustion problems.
After the implementation of the whole preventive maintenance system, the sudden shutdown failure of equipment can be reduced by more than 70%; the loss of fuel oil and aggregate waste can be reduced by 15%~30%; the replacement cycle of wearing parts such as bags and belts can be extended; the qualified asphalt mixture can be stably produced to avoid the loss of pavement rework; the stable discharge standard can avoid the environmental protection shutdown penalty, and the annual operation cost of the mixing station can be comprehensively reduced.
All kinds of faults in asphalt mixing plant have strong correlation, such as unstable output, temperature fluctuation, burner fault, metering deviation, conveying blocking, dust removal exceeding standard, electrical failure, mechanical wear and mixture quality defect. If a single equipment fault is not dealt with in time, it will cause chain problems in the whole line.
Compared with passive repair after fault shutdown, establishing graded standardized preventive maintenance system, from raw material control, equipment calibration, daily cleaning and lubrication, regular maintenance and control of the whole process, is the most efficient and economical solution to all kinds of problems in mixing station. The mixing station management personnel and equipment operation and maintenance personnel shall establish the operation and maintenance thinking of "early warning, source control and closed-loop rectification", regularly carry out equipment inspection and calibration, and continuously optimize raw material control and production process parameters, which can not only ensure stable production capacity and high-quality mixture output, but also realize multiple objectives of energy saving and consumption reduction, environmental protection and safe production.
