I Introduction
The 260 unit of the first rolling mill of Henan Anyang Iron and Steel Company was put into operation in the 1980s, with a designed production capacity of 200000 t/a for bar production. With the continuous upgrading of technological transformation, the production capacity of the unit has exceeded 900000 tons per year. Since 1998, multiple renovations have been carried out on the finishing frame, cold bed section, rolling mill, etc., replacing the original mechanical transmission with hydraulic transmission.
There are generally three maintenance methods for hydraulic systems, namely offline overhaul, diagnostic maintenance, and proactive maintenance (proactive preventive maintenance). Proactive preventive maintenance is a type of condition based maintenance, which is a new equipment maintenance method based on the theory and laws of equipment failure, and is carried out before equipment failure occurs.
II The generation and harm of pollution
The production environment in the metallurgical industry is often high temperature, dusty, water soaked, heavy load, and harsh working conditions. Hydraulic systems working in such environments are prone to malfunctions that can affect production. 70% of hydraulic system failures are caused by contamination of hydraulic oil.
Common pollutants in hydraulic systems include various solid particles, moisture, gases, chemical solvents, etc. The possibility of solid particles invading the system is the highest among these pollutants, and the harm to the system oil is also the greatest. Pollution can cause the following hazards to hydraulic systems:
2.1 Sudden malfunction of hydraulic components
This type of failure is usually caused by sudden damage or malfunction of components. The pollutants that cause this failure are mainly solid particles. Most solid particles are abrasive in nature. The fitting clearance of hydraulic component moving pairs is generally 5-15 μ m. For valve components, when pollutants enter between the moving parts, they will scratch the surface and cut out new abrasive particles, exacerbating wear, expanding the fitting clearance, or blocking the throttle hole and damping hole, causing performance changes and vibration and action failure; For pump components, pollutant particles can cause increased wear and heat on the sliding parts (plunger and cylinder bore, cylinder body, and distribution plate) of the plunger pump, leading to a decrease in pump efficiency; For hydraulic cylinders, pollution particles can accelerate the wear of seals, increase leakage, and reduce power. Polluting particles may also cause damage to the cylinder or piston rod, resulting in scrapping.
2.2 Performance degradation of hydraulic components
This type of failure is mainly caused by wear and tear, usually occurring gradually and predictable. The pollutants in the system can accelerate the wear of sliding parts of hydraulic control valves, leading to internal leakage or oil leakage inside the valve, increasing the friction between the valve core and sleeve, reducing the response speed of the valve, accelerating the wear of hydraulic pumps, hydraulic motors, and hydraulic cylinders, damaging seals, and causing internal and external leakage.
The pollutants that cause failure are mainly moisture and air. Moisture entering the hydraulic system causes the oil to emulsify and deteriorate, promoting bacterial growth, thereby reducing the lubrication and anti-corrosion effects of hydraulic oil, and corroding the surface of components. Under the influence of temperature, pressure, and chemical reactions, moisture can deteriorate the physical and chemical properties of oil, leading to its scrapping. When air is mixed into hydraulic oil, it exists in the form of bubbles and occupies a certain space, just like particulate pollutants. Bubbles collapse or shrink rapidly as oil enters the high-pressure zone from the low-pressure zone, causing the oil to lose its incompressibility. At the same time, the surrounding oil particles rush towards the center of the bubble at high speed, generating impact and converting kinetic energy into pressure energy and heat energy, causing temperature rise, noise, cavitation, and vibration, resulting in reduced efficiency, deterioration of lubrication conditions, insufficient pressure in the driving system, and soft operation. Under high-pressure and high-temperature environmental conditions, air can easily cause hydraulic oil to oxidize and deteriorate, generating harmful acidic substances or gel like precipitates, and corroding metal parts. All of these issues, although not causing the hydraulic system to immediately stop running, can make the system work unstable, slow down and cause malfunctions, reduce operational accuracy, and decrease system reliability.
2.3 Shortening the lifespan of hydraulic components
This form of failure is caused by multiple factors. The invasion of pollutants exacerbates the wear between components, the invasion of pollutants (water and air) exacerbates the corrosion of components, and the deterioration of the working environment of components due to the deterioration of oil, all of which can shorten the lifespan of components.
III Target cleanliness of hydraulic system
Strictly controlling the degree of oil contamination can significantly reduce system failures, and target cleanliness control technology for hydraulic systems is one of the important means of active maintenance of hydraulic systems.
The purpose of target cleanliness control in hydraulic systems is to maintain the cleanliness of the oil within the target value during the operating life that the hydraulic system should be able to achieve, so that oil contamination cannot become a factor causing component and system failures (including indirect failures and fading failures between sudden failures). Before oil contamination causes system failures, changes in oil cleanliness should be regularly detected, and necessary measures such as proactive maintenance should be taken to prevent problems before they occur.
The evaluation standards for the target cleanliness of hydraulic systems are NAS1638 and ISO 4406 standards. The standard uses automatic particle counter detection method for quantitative detection of oil cleanliness control.
4 Maintenance methods for hydraulic equipment
4.1 Control the inherent pollution sources inside the hydraulic system
(1) Flushing work before system installation. The cleaning process must strictly control the formula of acid washing solution, passivation solution, and neutralization solution. Secondly, a larger flow flushing device should be selected to make the liquid flow in the pipeline turbulent, manipulate the actions of various components, and flush out pollutants. After cleaning, drain the rinsing solution in a hot state. Finally, sampling and testing.
(2) Management of hydraulic components and hydraulic oil. ① Hydraulic components should be selected from legitimate manufacturers, especially filters and accessories (filter cartridges), and pollution control requirements should be proposed to the manufacturer. During the transportation and management of accessories, all oil ports should be sealed to prevent contamination from entering. ② Strengthen the pollution management of hydraulic oil. Oil procurement must be sampled and inspected, and oil with qualified indicators must be filtered before being injected into the storage tank. During the storage process, it is necessary to pay attention to sealing and store it in a dedicated container indoors to prevent oxidation and deterioration. Regular sampling and spot checks should be conducted, and the storage period should not exceed the limit. Expired oil cannot be used without passing inspection. ③ When adding hydraulic oil to the system, a refueling cart should be used for filtration to ensure that the cleanliness of the hydraulic oil is below NAS10 level.
4.2 Control the invasion of external pollutants
(1) Reliable dust prevention devices should be installed at the ventilation holes and piston rods of the fuel tank, and hydraulic oil that leaks should not directly flow back to the tank.
(2) When the system malfunctions and needs to be disassembled, it is necessary to prevent environmental pollution of the system. Components that cannot be immediately reinstalled should be protected.
(3) When disassembling and repairing hydraulic pipelines, it is necessary to clean the dirt at the joints to avoid introducing dirt into the system during disassembly and installation. Newly replaced steel pipes should be sandblasted or pickled to thoroughly remove the dirt inside the pipes. The newly assembled hose needs to undergo a pressure test and flushing. After flushing, the hose joint should be covered with a plastic bag to prevent dirt from entering.
(4) Do not easily disassemble hydraulic components. When disassembly is necessary, the parts should be cleaned, dried, and placed in a clean place. Before dismantling, remove external dust and oil sludge.
When cleaning, diesel or kerosene should be used, and parts should be cleaned with a brush or silk cloth. It is strictly prohibited to use cotton yarn for cleaning to prevent the oil passage from being blocked by cotton yarn. Rubber seals cannot be cleaned with gasoline to prevent corrosion, aging, and deterioration of the rubber components, which can lead to a decrease in sealing performance. Keep clean during assembly.
(5) Regularly remove oil and dust from the surface of the hydraulic system, clean and replace the oil filter element. Check if the seals and oil pipelines are damaged, and if the accumulator is leaking air. Timely discharge and regularly clean the oil tank to prevent hydraulic oil from deteriorating.
(6) Prevent the infiltration of coolers or other water sources into the system.
4.3 Pollution control of the system
(1) Regular sampling and testing. Each hydraulic system oil should be sampled and tested every two months to promptly understand the impact of particulate pollutants (including moisture) on the system. Trend analysis should be conducted on the results detected each time to grasp the pollution situation of the system.
(2) Regularly replace the filter element for filtration. The pressure difference before and after is an important parameter for understanding the degree of system pollution. In general, when the pressure difference AP30.35MPa occurs, the filter will automatically sound an alarm, and the filter element must be replaced.
(3) Bypass circulation cooling filtration. Typically, larger hydraulic systems are equipped with an independent external circulation cooling and filtration system. Its main function is to continuously circulate and filter solid particles that settle at the bottom of the fuel tank and are suspended in the oil, ensuring that the cleanliness of the oil meets the standards, while controlling the temperature of the system at 30-50 ℃.
(4) Attention should be paid to refueling and filtering the car. ① New oil is not necessarily clean oil. ② Never replace oil change with oil replenishment. ③ The filtering car should be cleaned regularly and the filter element should be replaced.
5. Conclusion
After years of practice, the maintenance and management of hydraulic equipment in the metallurgical industry should focus on the cleanliness of the control system oil. The basic work of active maintenance should be integrated into the operation, maintenance, and overhaul of conventional hydraulic equipment. Active maintenance of hydraulic systems should begin during normal operation of the equipment, using various means to take preventive measures, monitor the cleanliness of the system oil quality, eliminate hidden dangers that may cause hydraulic system failure at any time, move the accident defense line forward, and ensure the normal operation of hydraulic equipment in the metallurgical industry.
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