Basic knowledge of screw compressor lubricants and troubleshooting

Basic knowledge of screw compressor lubricants and troubleshooting

There are many reasons for the high temperature of the screw compressor.

This article only solves the failure of the lubrication system, provides basic knowledge of lubricating oil and cleaning methods, in order to extend the service life of the screw compressor.

The high temperature phenomenon of air compressor operation is one of the common problems in the after-sales service. The exhaust temperature of the air compressor host is an important factor affecting the safe, reliable and stable operation of the air compressor.

Over-temperature operation of air compressors is a common phenomenon in summer, so it often fails to attract the attention of air compressor managers. The occurrence of high temperature not only affects production, but also aggravates the loss of equipment and shortens its normal service life. In addition, high temperature will accelerate the deterioration of air compressor oil, causing lower lubrication performance, lower flash point, etc.

Sometimes high temperature can also cause spontaneous combustion of oil under the combined action of internal and external factors, resulting in air compressor burning, and even explosion accidents.

Therefore, great attention should be paid to the high temperature problems that occur in the operation of the equipment, and hidden dangers should be eliminated in time to ensure the gas requirements for production.



The normal exhaust temperature of air compressor equipment is between 75°C and 95°C. When the exhaust temperature is lower than the pressure dew point, condensation will occur, causing more water in the system, causing lubricating oil to emulsify and affecting the lubrication effect. In severe cases, the amount of condensed water may cause all the lubricating oil in the oil and gas barrel to be discharged into the air pipe network due to the specific gravity. This not only increases the difficulty of post-processing, but also causes the main engine bearing to not be lubricated in time and burn out, which will eventually cause the main engine to malfunction. The high discharge temperature increases the gap reserved by the compressor to consider expansion, and high leakage reduces the efficiency of the compressor, resulting in the same power consumption, but a decrease in gas production.

Too high temperature will also cause the lubricating oil to undergo thermal decomposition under the catalysis of metal, producing harmful free carbon and acidic substances, resulting in carbon deposits and siltation, blocking the oil circuit, and in severe cases, the main engine will be coked and stuck.

Although the screw air compressor is designed with high temperature protection (110℃ automatic shutdown), the long-term high temperature problem of the air compressor must be treated with caution.

Reasons for high temperature of screw air compressor

The air compressor panel shows that the exhaust temperature is high, and the temperature needs to be confirmed first

Whether the sensor is displayed incorrectly can be confirmed with a portable infrared thermometer or thermal imaging. Other reasons can be roughly divided into the cooling system, lubrication system, equipment host and other system design parts to confirm separately.

1. Cooling system

2. The factors that can be considered in the cooling system include environmental factors, indoor ventilation, blockage of the cooler, insufficient cooling water and cooling air volume, etc.

1. Environmental factors and indoor ventilation

Because a large amount of heat is generated during equipment operation, if this heat is not exported to the outside in time, it may cause closed circulation of air inside the room, that is, the heat discharged from the heat exchange of the air compressor will be sucked in again by the air compressor inlet, such a vicious circle , The equipment will continue to be hot. If the ambient temperature exceeds 　40℃ based on the ventilation status of the installation site, forced ventilation is necessary. Precautions for indoor ventilation:

●Position of inlet and outlet

The air inlet should be located as low as possible in the house where it is not easy to inhale environmental dust and harmful gases. In harsh environments such as dusty environments, air inlet filtering devices should be properly equipped. The exhaust vent is generally set on the top of the roof or wall, with additional protective facilities

●Exhaust channel

The circulation area of the exhaust duct should be large enough;

　The exhaust passage should be as short as possible to reduce the flow resistance. According to the compressor's exhaust volume, calculate the resistance of the exhaust passage. If the pressure loss exceeds 20Pa, an auxiliary exhaust fan should be installed;

In order to facilitate maintenance, a detachable structure of the exhaust channel is used, and a windshield such as canvas or tinplate can be used for connection and balance to avoid excessive drawing and vibration transmission. Flexible adjustable flaps can be installed, and the passage can be closed to avoid low temperature in winter.

2. Cooler blockage. Coolers include air-cooled plate-fin heat exchangers and water-cooled shell-and-tube heat exchangers. With the passage of time, due to the environment or the quality of the cooling water, the cooler will be blocked. This can be passed The temperature difference between the inlet and outlet of the cooler can be confirmed. The plate-fin heat exchanger is a heat exchange tube bundle composed of rectangular plates and heat exchange tubes. The blockage of the air-cooled heat exchanger can be repeatedly purged with compressed air above 0.2MPa, and the whole can be disassembled and cleaned in severe cases. For the water-cooled tubular heat exchanger, it is mainly composed of an external shell and an internal cooler. Because the equipment of different manufacturers may have different media flowing through the tube side and the shell side, and the quality of the cooling water used on site, the blockage of the tube heat exchanger is required Cycle cleaning by different chemicals.

3. Insufficient cooling water and cooling air

4. Insufficient cooling air volume

5. View the control panel

6. Whether the start and stop temperature of the fan is set reasonably, and whether the cooling fan is activated or damaged in time; the wind-proof net of the suction port should be cleaned regularly to prevent clogging; in addition, the design and installation of the exhaust channel are unreasonable, the exhaust volume is too small or the channel resistance If it is large, it will affect the air volume of the equipment and cause the exhaust temperature of the air compressor to rise.

7. Insufficient cooling water

8. Check the cooling water temperature. The high water temperature reduces the temperature between the cooling water and the lubricating oil and reduces the heat exchange effect; in addition, the amount of cooling water is insufficient, and the heat of the high-temperature lubricating oil cannot be cooled by the circulating cooling water in time, resulting in increasing temperature high. It is usually required that the cooling water backwater pressure is not less than 0.15MPa.

Three, lubrication system

Oil-injected screw air compressors do not require special oil pumps like oil-free ones. The circulation of lubricating oil in screw compressors is maintained by the pressure difference between the oil and the main engine's fuel injection port.

1. Lubricant

2. Lubricating oil is a very important working medium in screw compressors. It needs to have high oxidation, high flash point, and strong high temperature stability; it must not only have low flow resistance, but also have cooling, noise reduction, lubrication and sealing functions.

3. During the lubricating oil circulation process, due to the high compressor speed and fast oil circulation speed, the lubricating oil is very easy to age under the high temperature conditions with metal catalysts such as copper and iron, and the oil is in a high temperature stirring state. Foam is easily formed. If the oil level is too high (to judge the oil level, it is more accurate to observe the oil level gauge under the loading state of the air compressor), a large amount of foam will enter the oil, which will increase the gas resistance and increase the fuel consumption. In severe cases, it will cause high temperature, Overload injection, etc.

4. The performance and quality of the lubricating oil and its service cycle are decisive for the stable and reliable operation of the equipment. If used improperly, it can cause serious high temperatures in the compressor and even failure of the main engine.　Lubricating oil will be lost over time during the maintenance cycle. At this time, because the amount of oil in the oil and gas barrels decreases, the operating temperature of the equipment will increase. According to the operating oil level indicator, when the z* low limit is reached, it is necessary to stop and replenish the lubricating oil. The continuous replenishment of lubricating oil will not improve its deterioration state. Therefore, if the compressor reaches the oil replacement cycle, it needs to be completely drained and replaced. Special lubricating oil for machine. If the compressor is running in a dusty and high-temperature environment, the oil change interval should be replaced in advance according to the actual situation (the oil change interval is 2000 hours under normal operating conditions, and it is recommended to shorten it to 1000-1500 hours; under normal operating conditions, the oil change interval is For 4000 hours, it is recommended to shorten to 2000-2500 hours; other oil change intervals can be deduced by analogy).

If conditions permit, the compressor lubricating oil can be inspected regularly, and whether it is replaced according to the results of the oil sample. Generally, the compressor equipment runs for about a year. It is recommended to do an online system cleaning for the entire oil circuit system of the compressor. If possible, users should do an online system cleaning before each oil change; online cleaning agents are similar to health products, and their consumers are The whole screw compressor is maintained. With the increasing popularity of screw compressors, due to the lack of knowledge about the maintenance of screw compressors, more and more screw compressors are in sub-health, and the online carbon deposit cleaning agent for screw compressors has also emerged. The advantages of disassembly-free, convenient use, and improved equipment efficiency were quickly accepted by the market. But it should be clear that only when the online carbon deposit cleaning agent is used every time the compressor oil is changed, can the oil circuit coking and carbon deposit chance be reduced. If the main engine head and oil circuit have been severely coked and carbonized, they must be cleaned offline (the main engine, oil pipe, and cooler are removed and soaked or externally cycled for cleaning, and the bottom of the oil and gas barrel is soaked and brushed).

2. Poor operation of temperature control valve

3. The temperature control valve is the center of the lubricating oil circuit of the compressor equipment, and the performance is directly related to the high temperature of the compressor. The temperature control valve mainly controls the z* low injection temperature of the compressor, because the lower injection temperature will cause the condensate to precipitate in the compressor oil due to the pressure dew point, which is prone to emulsification of the compressor lubricant and shortens the lubricant. Life. When the oil temperature measured by the temperature sensor of the temperature control valve is lower than its action value (the action value of the temperature sensing element is 55°C and 71°C, etc.), the lubricating oil will go directly to the main engine from the oil and gas tank through the oil filter; When the oil temperature measured by the valve control temperature probe is higher than its action value, the internal temperature probe thimble moves to push the valve core to open the valve bypass, so that the oil flows through the cooler, and the cooled oil returns to the main engine through the oil filter .

4. The oil filter is blocked

5. The oil filter of the screw compressor is generally a spin-on paper filter. When the filtration accuracy is 12um, there is a 50% impurity separation rate, and when the filtration accuracy is 30um, the impurity separation rate is 99%, which can remove lubricating oil. Most of the impurities in it ensure the normal lubrication of the host. There is a check safety valve at the bottom of the oil filter. With the passage of time, the impurity particles deposited on the filter paper will cause the pressure difference to increase. When a certain pressure difference is reached, the bypass valve opens and the lubricating oil is sprayed in without being filtered Host, this ensures that the host is sufficiently lubricated. However, after the oil is fed through the bypass valve, a large amount of impurities in the lubricating oil are also brought into the machine head. If not treated in time, the service life of the compressor including the bearing will be greatly shortened.

5. High pressure hose and secondary oil return line

6. At present, most of the circulating oil pipes used by screw compressors are rubber hoses. As the use time increases, the hoses are exposed to high temperature and high pressure for a long time, and dew point may appear, which may cause high temperature alarms. When replacing the new oil pipe, make sure that it cannot be lower than the pipe diameter of the original pipe, otherwise it may cause instantaneous high temperature when starting up due to insufficient flow, and equipment shutdown.

　The secondary oil return pipe is located on the upper part of the oil and gas barrel. A copper pipe inside the oil separator core is connected to the machine head through the oil return check valve. Its function is to suck the lubricating oil continuously deposited at the bottom of the oil separator core into the machine head. If the oil return pipe is blocked, the lubricating oil deposited in the oil separation core will enter the rear air pipe along with the separated compressed air, which will not only increase the difficulty of post-processing, but also increase the fuel consumption. In the long run, the equipment is short of oil and high temperature alarms. 4. Recompression and overload of equipment host

5. After the compressor has been running for many years, the internal leakage of the rotor becomes more and more serious due to the wear displacement of the main engine bearing and the mechanical friction of the rotor, that is, the high-temperature oil and gas mixture compressed in the previous compression chamber leaks to the next stage working compression chamber The interior continues to be compressed, causing the main engine exhaust temperature to remain high. During operation, you can listen to the abnormal noise of the oil on the machine head, and use an infrared thermometer to measure the temperature of each part of the host. If there is abnormal high temperature, it needs to be checked carefully. Overload operation of the compressor will also cause high exhaust temperature. Check the loading and unloading pressure set on the compressor panel to see if it exceeds the rated pressure. In addition, a poor capacity control system will also cause the compressor to be unloaded and the intake cannot be closed in time. The valve causes the compressor pressure to rise slowly and the equipment runs at overpressure. When the oil separation core is blocked, the high-pressure oil and gas mixture stays in the oil and gas barrel, and the pressure in the rear pipeline

Sixth, the pressure measured by the sensor is very low. At this time, the compressor will continue to overload operation according to the signal given by the pressure transmission, resulting in high temperature of the equipment.

7. Replacement and application of lubricating oil

After understanding the reasons for the high temperature of the screw compressor, we only discuss the high temperature caused by the lubrication system and improvement measures in this article. Therefore, it is also very necessary to understand the various lubricating oil properties of the screw compressor.

1. Ordinary mineral oil

Working conditions: medium conditions, strong compatibility. Under normal working conditions, 2000 hours of oil change interval is recommended to avoid the formation of coking and carbon deposits, and it is resistant to wear and corrosion.

2. High-quality mineral oil

Working conditions: under severe conditions, under normal working conditions, the recommended oil change interval is 3000 hours, allowing the screw compressor to start at low temperatures (-20°C).

3. Hydrocracking mineral oil

Working conditions: under severe conditions, under normal working conditions, the recommended oil change interval is 4000 hours, allowing the screw compressor to start at low temperatures (-25°C).

4. Semi-synthetic oil

Working conditions: under severe conditions, under normal working conditions, the recommended oil change interval is 5000 hours, and the screw compressor is allowed to start at low temperature (-36℃).

5. Synthetic hydrocarbon oil

Excellent water separation, low volatility, degradable, and strong compatibility, except for polyethylene glycol (polyether) and silicone oil. Under normal operating conditions, the recommended oil change interval is 8000 hours, allowing the screw compressor to start at low temperatures (-40°C).

6. Synthetic ester oil

No carbon residue, with oil circuit cleaning function, degradable, strong compatibility, can be fused with any mineral oil, except for polyethylene glycol (polyether) and silicone oil. Under normal operating conditions, the recommended oil change interval is 8000 hours, allowing the screw compressor to start at low temperatures (-40°C).

6. Polyethylene glycol (polyether) oil

7. Degradable, no carbon deposit, low volatility, etc., and poor compatibility. Under normal working conditions, the recommended oil change interval is 8000 hours, and the screw compressor is allowed to start at a low temperature (-50°C), specifically for the screw compressor host using polyethylene glycol (polyether polyester).

8. Synthetic silicone oil

Replacement-free for ten years, suitable for fuel injection screw, no carbon deposits, sludge or carbon paint are produced, no lubricating oil by-products remain in the pipeline system, and good compatibility with various metal materials and seals. The appearance is a colorless and transparent water-filled liquid. In addition, the viscosity index is extremely high, allowing the screw compressor to start at low temperature (-65°C), specifically for the original screw compressor main body using silicone oil.

So, can screw compressor lubricant products with different base oils be compatible? Please refer to the table below.

How to solve the grease stain in the compressor system

In the working process of the compressor, as the compressed air absorbs work, which causes the temperature of the compressed air to rise, heat will be transferred from the compressed gas through many ways, including the housing, lubrication system, heat dissipation system and so on. In the actual work of the compressor, 10%-20% of the work is converted into heat energy, and this heat energy is absorbed by compressed air and lubricating oil

　The higher the compressor discharge temperature, the shorter the service life of the oil. Because of the high temperature, more lubricating oil will be in the gaseous state, and it is difficult to increase the oil content. Too high temperature will inevitably reduce the service life of lubricating oil, as well as the service life of all seals and pipelines.

The excessively high oil temperature during the operation of the equipment will reduce the gas transmission coefficient and increase the power consumption, and even cause the bearing loose ball accident. Too high temperature will also cause the lubricating oil to undergo thermal decomposition under the catalysis of metal, generating free carbon, colloid and moisture harmful to the work, which accumulates over time. If these substances encounter instantaneous high temperature, they will lead to the formation of carbon deposits. In severe cases, the main engine screw will bite. dead.

Therefore, the solution to the grease, gum or carbon deposits in the compressor system is cleaning.

At present, most air compressor after-sales service units use online cleaning agents, acidic cleaning agents, alkaline cleaning agents and solvent-based cleaning agents. Acidic and alkaline cleaning agents are generally called water-based cleaning agents. Solvent-based cleaners and water-based cleaners have their own advantages and disadvantages:

1. Oil-based cleaning agents are generally used as online carbon deposit cleaning agents. There are two types, one is additives, which can be added proportionally to the compressor oil circuit to be maintained for random operation, mainly to decompose and clean the light oil system of the oil circuit system; the other is to clean the oil, drain the old compressor before maintenance Oil, add the cleaning oil directly, wait for 30 minutes to 2 hours to run, and then add new oil to the system. If the system colloids or carbon deposits are serious, it is recommended to use water-based cleaners or solvent-based cleaners to immerse and clean the machine.

2. Water-based cleaning agent is a cleaning product based on water, which is safe for the environment. Generally, it is recommended to circulate cleaning. The effect of immersion cleaning is relatively slow. Compared with organic solvent-based products, it has many advantages, such as low price and convenient use. Disadvantages: Need a lot of water to rinse, the alkaline cleaning agent reacts with the oil stains to generate easily soluble compounds to dissolve the oil stains, and then rinse them with water. Care must be taken during cleaning to prevent the formation of pitting corrosion due to intergranular dechromization of the alloy.

3. Solvent-based cleaning agents are cleaning products based on organic solvents. Solvent-based cleaning agents are roughly divided into two types: one is composed of two layers of liquid, the upper layer is covered by a water-based protective liquid, and the lower layer is a solvent-based cleaning with a strong penetrating agent Agent; the other is a pure solvent-based cleaning agent refined with a powerful penetrant and synergist. The latter is more effective in cleaning. The disadvantage of the latter is low boiling point and faster volatilization; the advantage is that there is no residual moisture after cleaning, rapid drying, and no need to clean with water.

Note: A lot of internal oil will be brought out when the radiator is cleaned, so after cleaning it, you need to add new oil to supplement the space inside the pipeline and heat exchanger before restarting to prevent high temperature caused by oil shortage at the moment of startup.