With the development of science and technology, air compressor, as an important power equipment, is widely used in machinery, metallurgy, building materials, electronic power, chemical industry, food, textile and many other industrial fields.
However, in the long-term operation of the compressor, it will inevitably encounter many failures, and frequent loading and unloading is one of them. Frequent actions not only shorten the service life of the unit solenoid valve, compressor inlet valve, unloading valve and other equipment, but also seriously threaten the safe operation of the whole unit.
Cause of failure
What are the reasons for frequent loading and unloading of the compressor?
1) Control reason
As we all know, the discharge pressure of the compressor is the main basis for controlling the loading and unloading of the compressor. If the difference between the two set values becomes smaller, frequent loading and unloading of the unit may occur. When the loading pressure value of the unit increases, the unloading pressure value decreases at the same time; Or simply increasing the loading pressure value and decreasing the unloading pressure value will shorten the loading and unloading cycle of the unit, that is, when the gas consumption of the unit is constant, the closer the loading and unloading pressure values are, the more frequent the loading and unloading will be, and the greater the hidden danger of safe operation of the equipment will be.
2) System causes
If the set value of unit loading and unloading pressure is normal, it can be considered whether there is pipeline blockage between compressor outlet and air storage tank. If the pipeline is blocked but not completely isolated, the compressed air flow will be seriously blocked, and the gas pressure between the blocked point and the compressor is difficult to remove quickly during the compressor loading, which will cause the unloading to reach the unloading pressure value in a short time. However, due to the limited amount of air between the blockage point and the compressor outlet, combined with system losses (such as filter drainage with air, regeneration of the cold dryer) and a small amount of air flowing to the air tank, the pressure quickly drops to the loading pressure value, and the compressor is reloaded again, and so on.
Specifically, there are the following factors:
(1) If the set value of the pressure difference between the loading and unloading of the pressure controller is too small, it may lead to frequent loading and unloading of the unit.
(2) The pressure sampling pipe is blocked or leaked, and the pressure decay is too fast.
(3) The demand for compressed air during production is unstable, sometimes large, sometimes small or discontinuous.
(4) When the compressor unit is unloaded, the smaller pressure valve is not closed timely or tightly.
(5) The load control solenoid valve is faulty. The solenoid valve shall be checked. It may be insensitive to action due to the influence of oil, water and gas or the coil is burnt out.
(6) The pressure sensor controlling the start and stop of the unit is faulty or damaged.
In case of the above problems, they shall be solved in a timely manner, otherwise the related equipment will be damaged and other failures will occur.
Treatment scheme
1. Handling methods for frequent loading and unloading caused by control reasons
This reason can be directly seen from the pressure display during unit loading and unloading. If there is any change, it can be solved through online calibration or removal of the pressure sensor, detailed detection and reinstallation of the sensor. Of course, it is also common for the measurement error caused by the blockage of the pressure sensor measurement pipeline. The common practice is to use compressed air to purge the sampling pipeline.
2. Handling methods for frequent loading and unloading caused by system reasons
The treatment of system causes is relatively complex, which can be judged by the difference between the compressor outlet pressure and the dryer outlet pressure. If there is a difference or the difference is too large (general filter ≤ 0.6bar, cold dryer ≥ 0.2bar), it can be judged that the pipeline or equipment is blocked. As there are many equipment in this section of pipeline, it is found through long-term maintenance and observation that the equipment that is often blocked are:
1) Oil and water removal filters (the dust removal filter is located behind the cold dryer, and the probability of blocking is very small): including filter core; Filter blowdown solenoid valve; Front filter screen of blowdown solenoid valve. Among them, the filter screen in front of the blowdown solenoid valve is likely to be blocked, which can be solved by cleaning or replacement. However, the blow down solenoid valve may be jammed in the valve body, burnt coil, or damaged timer, which can be solved by replacement. The filter screen in front of the blow down solenoid valve of the water removal filter was blocked in the dust removal compressor of a factory, which could not be discharged in time, so that the sewage filled the filter, causing frequent loading and unloading of the compressor. The maintenance personnel cleaned the filter screen and found it normal;
2) Switching valve at inlet A and B of adsorption drying tower inside the cold dryer. At present, many enterprises use adsorption dryers for post-treatment of compressed air, which generally requires switching between tower A and tower B. If the switching valve is not switched in place, the gas flow may also be blocked. If the inlet change-over valves of drying tower A and B of the cold dryer are of pneumatic switch type, the failure of the pneumatic switch type valve generally occurs in the control of air source and valve body. The maintenance personnel can judge whether the control air source is normal by checking whether there is water in the filter pressure reducing valve or whether the air source pipeline falls off; You can also manually operate the pneumatic valve to determine whether the valve body acts flexibly and whether the cylinder seal is good. If the filter pressure reducing valve is blocked by the water in the control air source, the compressor will be loaded and unloaded frequently because the switch valve cannot be controlled. In view of this situation, the sampling point of the control air source can be changed to the outlet pipe of the cold dryer to solve the problem of water in the control air source;
3) If the temperature of the precooler and evaporator in the refrigeration dryer is not adjusted well, it is also easy to cause ice blockage at the inlet and outlet of the evaporator, thus hindering the gas flow. Ice blockage or improper temperature regulation of evaporator can be judged by observation. If there is "sweating" phenomenon in the compressed air pipe in the cold dryer; Or if the parameter setting of the cold dryer does not meet the equipment design requirements, it should be adjusted in time to avoid unnecessary losses caused by freezing the equipment. This kind of situation occurs more frequently during the seasonal change period, which is generally solved by properly adjusting the evaporator temperature;
4) The wrong installation of the one-way valve at the outlet of the dedusting filter or the falling off of the valve stem and valve body will also cause frequent loading and unloading of the compressor. In the above factory, it happened once that the compressed air could not flow into the air receiver, and the gas in the air receiver could flow back to the chiller dryer, resulting in frequent loading and unloading of the unit, because the valve was inadvertently installed in the wrong direction after repairing the internal leakage of the one-way valve at the outlet of the dedusting filter.
3. Frequent loading and unloading due to unstable gas consumption
In this case, the method of adding air storage tank or increasing the volume of the original air storage tank is generally adopted. If the addition of air storage tank cannot meet the production needs, it is recommended to replace the original power frequency unit with a variable frequency compressor. In addition to solving the hazards caused by frequent loading and unloading, the variable frequency compressor has other advantages:
(1) Barometric stability
As the variable-frequency screw compressor generally uses the stepless speed regulation feature of the frequency converter, it can start smoothly through the controller or the PID regulator inside the frequency converter; In case of large fluctuation of gas consumption, it can quickly adjust the response.
Compared with the upper and lower limit switch control of power frequency operation, the pressure stability is improved exponentially.
(2) Start without impact
As the inverter itself has the function of soft starter, the starting current is within 1.2 times of the rated current, and the starting impact is very small compared with the power frequency starting which is generally above 6 times of the rated current. This kind of impact not only affects the power grid, but also greatly reduces the impact on the entire mechanical system.
(3) Variable flow control
The compressor driven by power frequency can only work at one displacement, while the variable frequency compressor can work at a wide range of displacement. The frequency converter controls the exhaust volume by adjusting the motor speed in real time according to the actual gas consumption. When the gas consumption is low, the compressor can also sleep automatically, which greatly reduces the loss of energy.
(4) Better voltage adaptability of AC power supply
The frequency converter with overmodulation technology can still output enough torque to drive the motor when the AC power supply voltage is slightly low; When the voltage is slightly high, it will not lead to high voltage output to the motor. For the occasion of self generation, variable frequency drive can show its advantages.
(5) Low noise
Most working conditions of the frequency conversion system are lower than the rated speed, and the mechanical noise and wear of the host machine are reduced, extending the maintenance and service life. If the fan is also driven by frequency conversion, the noise during compressor operation can be significantly reduced.
The maintenance of the compressor is the objective requirement of the compressor's own movement, which is particularly important for the compressor (device). Only by insisting on regular inspection and timely replacement of relevant parts can the normality of mechanical work be ensured, so as to ensure the continuity and stability of enterprise production and bring higher benefits to the enterprise.