STUDY OF FACTORS AFFECTING THE EFFICIENCY OF RECIPROCATING COMPRESSORS

ABSTRACT

Today, 20-30% of all energy consumed in the mining industry is spent on the production of compressed air, compressor equipment, and the efficiency of compressor equipment depends on a number of technological factors.

This article presents a study of factors that negatively affect the efficiency of mining compressor equipment.

АННОТАЦИЯ

Сегодня 20-30% всей энергии, потребляемой в горнодобывающей промышленности, расходуется на производство сжатого воздуха, компрессорного оборудования, а эффективность работы компрессорного оборудования зависит от ряда технологических факторов.

В данной статье представлено исследование факторов, отрицательно влияющих на эффективность рудничных компрессоров.

Keywords: compressor, cooling system, heating system, air intake system, air cooler, temperature, performance, efficiency, energy saving.

Ключевые слова: компрессор, система охлаждения, система отопления, система воздухосбора, воздухоохладитель, температура, производительность, эффективность, энергосбережение.

Introduction

Along with electricity, pneumatic energy, i.e. compressed air energy, is widely used in the mining industry. In technological processes of mining, compressed air has a significant impact on the operation of mining machines and other pneumatic energy consumers, acting as an energy carrier. The main contribution to compressor efficiency depends on the operation of its cooling, lubrication and air traction systems, and improving their performance characteristics significantly reduces the cost of compressed air production.

Today, special attention is paid to the utilization of mining compressor equipment and the determination of factors affecting its efficiency.

Investigation of the influence of cooling system operation on compressor efficiency

The existing system of compressor equipment cooling has a number of significant disadvantages due to the peculiarities of their operation. These are, first of all, increased air pollution, lack of clean water sources, lengthening of main pipelines. All this together makes special requirements to the operation of compressor cooling systems. At the same time, the water used for cooling contains a large amount of salts and various impurities. In most cases the total water hardness reaches 20 °J mg-eq/l, which is almost 3 times higher than allowed by the norms of device and safe operation of stationary compressor equipment, air and gas pipelines. Solidification of cooling water leads to formation of soot on heat-exchange surfaces of compressor air coolers. For example, the experience of operation of compressors at Korakotan mine belonging to Navoi Mining and Metallurgical Combine showed that the rate of deposit formation is 8-10 mm and more per year. Due to the growth of enterprises, reduction of heat transfer rate leads to decrease in efficiency and safety of compressor equipment. In particular, at an air temperature of about 150 °C, spontaneous combustion of soot and oil (slag) layers may occur in air communications, causing detonation explosions. At the same time, analysis of compressor equipment operation at mining enterprises shows that after the second stage of the compressor the temperature of compressed air reaches 150-160 °С, and in the summer period - 170 °С and higher [3; p.68-71].

Analysis of the operation of compressor equipment of a number of mining enterprises in the world shows that the service life of air coolers at some compressor stations is reduced by 3-4 times compared to the specified periods, the main reason for this is frequent cleaning of air coolers [2; p.41-52].

Climate conditions are an important factor affecting the performance of the compressor cooling system. For example, when operating the water circulating cooling system in winter, freezing of the cooling radiators is observed, which also reduces the efficiency of water cooling. In summer, there is insufficient cooling of the circulating water due to high ambient temperature and unsatisfactory operation of the cooling radiator.

The need to transfer air heat during the compression process in a compressor station depends mainly on two aspects: ensuring safe operating conditions for the compressor station and increasing its efficiency. In multistage compressor equipment, the necessary conditions for its operation are achieved by efficient compression heat transfer in the stages and coolers [1; p.104-107].

In addition, to ensure optimal compressor operation, about 10 times more heat must be moved in the coolers than in the cylinders. The result of heat distribution to the elements of two-stage compressor equipment corresponds to the data of a number of authors. For example, according to the results of the scientific work of Y. N. Minyaev, the distribution of transferred heat to the elements of compressor equipment is as follows [4; p.55-62]:

• 8% of the heat is transferred from the walls of both cylinders;
• 40% is cooled in the intercooler;
• 46% is then cooled in the cooler;
• 6% is transferred to the compressed air line.

The effect of compressed air temperature varies in certain parts of the system. Therefore, the compressed air temperature in the intercoolers of reciprocating compressors must not exceed 60 °C. Reducing the compressed air temperature in the intercoolers by every 6 °C reduces the energy consumption by about 1% [2; p.41-52]. The difference between the temperature of compressed air and cooling water at the cooler outlet should not exceed 5-10 °C. An increase of up to 20 °C results in an energy overconsumption of up to 14% under the same conditions.

When the air temperature rises at the intercooler outlet, the power overrun is determined by the following expression [2; p.41-52]꞉

, kWh,                                 (1)

where t is the temperature of compressed air at the exit from the intercooler, °C;

 – actual power consumed by the electric motor, kW.

Compressed air cooling has a major impact on the key performance indicators of compressor equipment. It is known that an increase in the temperature of the compressed air in the intercooler leads to an increase in the work required in the next stage. Therefore, on the one hand, it is necessary to cool the air compressed in the intercooler as much as possible, and on the other hand, as a result of the increase in heat exchange surfaces, the temperature of the air leaving the intercooler is further reduced. For example, to cool compressed air to a level that differs from the temperature of the water entering the cooler by 5 °C, an 18-20 % larger heat exchanger surface is required compared to a difference of 8 °C, which means that the hydraulic resistance also increases [4; p.55-62].

Thus, the efficiency of the cooling system depends on the energy costs of the compressor equipment. In addition, small and frequent heat can be cooled in the post-processing and compressor operation process, as well as the technical and economic performance of the compressor station and the main influence on the efficiency of intermediate and internal cooling.

Stationary compressors, existing pneumatic devices in accordance with the rules of device and safe operation of air and gas pipelines, if the compressor capacity is more than 10 m³/min, with air collectors, smoothing pressure drops, aftercoolers and oil- Between the cooler and air collectors must be installed water separators. The purpose of the aftercooler is to cool the compressed air leaving the last stage of the compressor and to remove water and oil vapors. Water-oil separators are designed to clean the air lines of water and oil. The air collector also partially fulfills the function of an oil-water separator.

Investigation of the influence of compressor air intake contamination on its efficiency

Contamination of compressed air has a negative physical and chemical impact on pneumatic devices and reduces their durability by 3-7 times. Up to 80% of pneumatic systems failures are related to unsatisfactory air quality. Pollution of compressed air occurs through: the atmosphere, the compressor itself and pipes, on average, each 1 m³ of urban air contains about 140 million particles of dust, 80% of which are particles smaller than 2 microns, not captured by filters when hit the compressor. In the atmosphere, in addition to particulate matter, hydrocarbons (up to 0.05-0.5 mg/m³), vapors of unburned fuel and oils up to 0.5 mg/m³, microorganisms up to 3850 units/m³, bacteria and fungi up to 10 mg/m³, moisture contains up to 10 -11 mg/m³.

In addition, particles and oil are added as a result of wear and tear on the friction mechanisms of the compressor itself. 5-50 mg/m³ of oil particles in the form of aerosols and vapors are added to the compressed air, due to the high temperature in reciprocating compressors, the oil partially decomposes, oxidizes and forms oil bodies in the friction mechanisms and valves. This, in turn, adversely affects compressor efficiency.

During transportation air in the pipeline additionally rusts up to 3-4 mg/m³, layering of corrosion and fittings, presence of capillary moisture increase air consumption by 30-40%.

The amount of moisture generated in compressor equipment reaches a significant amount, and on average, more than 150-200 liters of condensate is generated during an 8-hour shift.

The presence of water in the form of vapor in the air does not cause any problems in compressor operation, but the formation of condensed moisture droplets in compressed air causes very serious operational problems such as flushing of protective oil in the pneumatic system. tools and machinery, corrosion and rusting of metals in ducts, wear and tear of pneumatic tools and increased maintenance costs, interruptions in the use of pneumatic tools and increased maintenance costs, freezing in cold climates, freezing and clogging of fittings [4; p.55-62].

Dust particles from the air enter the cylinders of reciprocating compressors, cause premature wear of the friction surfaces and mix with the lubricant to form oil deposits. Oil buildup leads to reduced compressor performance and explosion due to reduced valve closure and piston ring wear. Particles of mechanical impurities enter the compressor with air and accelerate the erosion of surface parts, which also causes a decrease in compressor performance and efficiency [5; p.44-47].

Outside the compressor station, air is sucked into compressors at a height of at least 3 meters from the side illuminated by the least amount of dust and sunlight.

Filters are designed to clean the air sucked into the compressor to the maximum permissible amount of dust, i.e. to 0.5 mg/m³, the air velocity should be equal over the entire surface of the filter and should not exceed 3 m/s.

Excessive contamination of air sucked into the compressor, i.e. increase in the permissible amount of abrasive dust particles in the sucked air, leads to mechanical wear of compressor parts.

For example, compressors 2VM10-63/10 compressor station "Karakotan" mining department "Navoikon Metallurgical Plant" JSC "GMZ №1" as a result of increased pollution of the intake air, the efficiency of the compressor decreased, the compressor overheated, the piston, piston rings, cylinder failed, such malfunctions as erosion, erosion and contamination of the pusher and drive valves, oil contamination, clogging of the air filter on the inlet tract was observed. The percentage of shutdowns caused by these failures is shown in Figure 1.

Figure 1. Causes and their proportion of downtime caused by contamination of the compressor air intake

The time required to eliminate the above downtime will increase compressor downtime (idling) for more than a year, which in turn will reduce compressor efficiency.

For example, to eliminate downtime caused by piston and piston ring wear, the cylinder piston group is completely opened and reassembled after repair work, which takes an average of one shift for the repair crew. In addition to the complete start-up and restart of the compressor, downsizing increases energy consumption.

Also, for example, contamination and erosion of the valves, which makes it difficult to open and close the valves at the demand level, the presence of abrasive particles of pollutants in the air sucked into the compressor leads to erosion of the valve plates, as a consequence When the oil is contaminated, soot forms on the surface of the plate and spring. As a result, the efficiency and pressure of the compressor is reduced.

This type of malfunction can only be eliminated by replacing the valves, which in turn leads not only to time-consuming repairs, but also to increased operating costs.

Cellular and self-cleaning filters are mainly used in compressor stations. Cellular and self-cleaning filters can be individual for each compressor or common for the entire compressor station.

Generally, compressors are less likely to shut down due to cylinder wear and fouling because the cylinder material is more durable than the piston ring material and the rings are more likely to wear than the cylinder surfaces. However, in some cases, there is wear of cylinder walls due to ring wear and shrinkage, abrasive particles in the air-oil mixture inside the cylinder. When this situation occurs, it is eliminated by grinding or sanding the cylinder surface. In some cases, a complete cylinder replacement is required.

As a result of clogged air filters in the suction path, the volume of air drawn into the compressor decreases and the energy input to the suction piston increases. Compressor equipment currently used in industrial plants mainly uses two types of air filters: cellular and self-cleaning.

Although mesh filters are structurally simple, they are difficult to clean and this is their disadvantage, self-cleaning filters are characterised by relative ease of cleaning, but they are structurally more complex and cannot operate in the pulsating air flows that occur when piston compressors are operating [7].

The use of self-cleaning and cellular filters does not allow to completely clean the air supplied to the compressor equipment, and cleaning of cellular filters requires considerable labour and time. All this causes a decrease in the efficiency of compressor equipment. Based on the above, it is determined that it is possible to increase the efficiency of compressor equipment by effective filtration of the air sucked into it.

Investigation of compressor lubrication system malfunctions caused by contamination 

The lubrication system of reciprocating compressors is known to consist of two parts: the cylinder and spool lubrication system and the drive train lubrication system.

Lubrication of wear parts of the cylinder, such as compressor valves, piston rings, retaining rings and seals, should be performed in accordance with the recommendations specified in the technical documentation by the compressor equipment manufacturer. The service life of these components is directly related to the type of oil, oil purity, oil size and lubrication frequency [1; p.104-107].

Lubrication of reciprocating compressor cylinders and seals is mainly carried out by the following methods [1; p.104-107]:

• Spray oil from the crankcase. Oil from the crankcase is captured by special nozzles and distributed over the cylinder surface as the piston moves. At subsequent shaft revolutions the piston transfers the incoming oil to other working surfaces of the cylinder. The main disadvantage of this method is the impossibility to regulate the flow of lubricants. In addition, the contact of a large volume of oil with hot air causes the formation of oil bodies inside the cylinder, and the oil may be distributed unevenly throughout all parts of the cylinder;
• The compressor is a compressor, but it is also a compressor that is not a compressor. The compressor should not be used as a compressor for any other purpose. It is important to be aware of the fact that my bilan cylinder is not just a cylinder, but also a cylinder that is a part of the cylinder.
• Lubrication of cylinders and seals under pressure, i.e. forced cylinder lubrication. The oil is supplied by an oil plunger pump. In horizontal type compressors, the oil supply is at the top point or at two points if the cylinder diameter is more than 500 mm. Vertical cylinder compressors also have one or more oil inlet points, the number of which also depends on the cylinder diameter.

Oil from the pump is fed into the cylinder through tubes. This method is considered to be the most efficient and allows full lubrication of all working surfaces of the cylinder and its parts.

Lubrication of motion mechanisms is carried out in two ways [2; 41-52].

1. Spraying oil on the working surfaces of driving mechanisms is used in small-sized compressors and is intended for short-term operation. Oil is poured into the crankcase bath, turns into a mist when the shaft rotates and only then passes through the bearings to the rubbing surfaces.

This method does not provide efficient heat transfer, so the atomisation system is often used in domestic or semi-professional compressor models. The main disadvantages of this method are insufficient control of the oil level in the crankcase, rapid oil contamination, clogging of filters and mixing of oil with compressed air.

Forced lubrication of the motion mechanisms, i.e. the circulation method, is carried out by supplying oil in a closed cycle through the compressor crankcase. The oil enters the pump through a filter. The forced lubrication system is equipped with a pressure gauge and relief valves. Today, most reciprocating compressors used in our country's mining enterprises are equipped with a forced lubrication system.

Figure 2 shows the compressor forced lubrication system.

1-collector of the compressor, 2-motor, 3-oil pump, 4-oil pump, 5-oil filters, 6-oil radiator, 7-filter of fine oil purification (fine filter), 8-filter collection, 9-10-11 -pipes, stages I and II-compressor

Figure 2. Compressor forced lubrication system

The compressor forced lubrication system shown in figure 2 works as follows: oil is fed through the pump (3) to the filter (5), oil from the filter is fed to the cooler (6) and then through the tube (9) to the compressor manifold (1). In the compressor manifold it lubricates the shaft, crosshead and moving parts and at the same time acts as a cooling agent. At the outlet it passes through a collecting filter (8), the collecting filter serves to settle the dirt particles in the oil. The oil from the collecting filter is fed back to the pump (3) via a pipe (10). When there is an increase in contaminant particles in the oil, the oil is directed to the manifold (3) through the fine filter (7).

Oil is fed from the lubricator (4) into the cylinder section of the compressor through tubes (11) where the oil is atomised into the cylinders rather than recirculated. The oil from the cylinder is mixed with compressed air and exits through valves, the oil is then separated and disposed of in a separator.

With a compressor forced lubrication system, the oil stays inside the compressor, meaning it is in constant circulation. Since there is no loss of oil volume, this method is considered the most efficient.

The following requirements are imposed on the oils in the lubrication system: the oil temperature should be within +220...+240 ℃ with a flash point and an ignition temperature of about +400 ℃. The oils should have properties that do not change during the compressor operation (do not stratify, do not mix with gases, do not form deposits and soot, etc.) and do not contain water and mechanical impurities, the average service life of oils should be 2500 motor hours. Also, their acidity values must be normal and their content of water-soluble acids and alkalis must be within the specified limits.

Lack of oil, as well as excessive lubrication of moving parts of compressors and cylinders, have the same negative impact on equipment performance.

Excessive lubrication can cause pistons and other compressor components to become unstable, valves to jam, valves to slow or delay opening and damage, and compressor performance to suffer.

Lack of oil results in premature wear of moving parts, sounds, equipment vibration and pulsation, and cylinder deposits.

The reliability of the lubricant distribution system for lubricating cylinder components, as well as the use of compatible lubricants and their correct consumption, are important for the overall reliability of reciprocating compressors.

Special attention should be paid to oil parameters and cleanliness when lubricating the compressor's sliding mechanisms, as changes in the initial oil parameters and oil contamination cause wear of the sliding and friction mechanisms, resulting in an increased number of emergency stops.

Reduced oil viscosity and oil contamination due to heating of the compressor shaft bearings will cause the compressor to stop suddenly and will also cause abrasive wear of the bearings and their linings. This will also result in bearing misalignment on the shaft journal.

Oil contamination leads to corrosion of the shaft housing, the part associated with the balls, bending of the shaft and premature failure.

The cleanliness and viscosity of the oil also determine the performance of the crosshead, rod and bearings that transmit shaft motion to the piston. The formation of abrasive particles in the oil results in erosion of the crosshead, its sliding surfaces and the crosshead axis, reducing its size and shortening its service life.

Figures 3a and b show malfunctions caused by changes in initial parameters and contamination of oil lubricating moving mechanisms and their photos.

Figure 3a. Malfunctions caused by contamination of the oil lubricating the mechanism mechanisms

a – bearing depreciation, b – depreciation of the moving surface of the kreutzkopf, v – depreciation of the shaft surface

Figure 3b. View of erosion of compressor drive mechanisms

All of the above factors are caused by oil contamination in the lubrication system and changes in initial parameters. Filters installed in the lubrication system of compressor equipment cannot always ensure cleanliness of circulating oil, i.e. excessive wear and tear of compressors operating in industrial plants and poor quality of spare parts cause dirt and contaminating particles to separate from them during the operation. In addition, the filters used in the lubrication system are not up to the required level. Based on the above, it can be concluded that by improving the filters in the compressor lubrication system and improving their operation, it is possible to achieve unforeseen stoppages of compressor equipment and reduce operating costs.

References:

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2. Khatamova D.N., Djuraev R.U. Energy saving during operation of mining compressor units based on modernization of their cooling systems // International Journal of Future Generation Communication and Networking. 2021. Vol. 14. №2. P. 41-52.
3. Khatamova D.N., Urunova Kh. Improvement of the cooling system of compressor units // Universum: texnicheskiye nauki. 2021. №5 (86). P. 68-71.
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5. Xatamova D.N., Djurayev R.U. Issledovaniye vliyaniya temperaturi vsasivayemogo vozduxa na effektivnost porshnevogo kompressora [Study of the influence of intake air temperature on the efficiency of a reciprocating compressor] // Universum: texnicheskiye nauki. 2021. №6 (87). P.44-47.