Fault performance
Below the loading pressure → the air compressor starts to load and exhaust → the pressure rises rapidly → a few seconds to a dozen seconds to reach the set unloading pressure → air compressor unloading to stop the exhaust → rapid pressure drop → several seconds to ten seconds The pressure drops to the set loading pressure → the air compressor starts to load the exhaust again, so it starts again.
The danger of frequent loading and unloading of air compressor
Undoubtedly, such failures will cause serious harm to the air compressor, mainly:
1 The severe test is carried out on the moving parts involved in the loading and unloading of the air compressor, such as the intake valve and its valve block, the minimum pressure valve, the relief valve, etc., the life will be greatly shortened, and then cause more other failures;
2 Although frequent loading and unloading starts and stops frequently, the impulse force on the motor and the main engine is not so large, but such frequent replacement of light and heavy load and frequent changes of lubrication conditions are very obvious to the bearing damage of the motor and the main engine;
3 Frequent loading and unloading impact on the gas path and oil filter device will lead to oil core and oil filter failure in advance;
4 power consumption increases, the unit energy efficiency is extremely low
Possible Causes and Analysis of Frequent Loading and Unloading of Air Compressors
First of all, we must rule out some factors that are noisy:
1 There is no gas storage tank or a small gas storage tank, and the air compressor has a large exhaust volume and a small gas consumption;
2 The amount of gas used in a single gas is large, and there is not enough buffer tank;
3 The pressure setting of the unloading is too close, such as loading 6.9 bar and unloading 7.0 bar;
Second, understand the control principle of screw machine loading and unloading:
The control logic of the screw air compressor loading and unloading is to trigger the loading and unloading action by the set pressure value, such as 6bar loading and 7bar unloading. It is different from the mechanical structure of the pressure switch of a small piston machine. The screw machine monitors the pressure through a pressure transmitter, and the pressure transmitter provides an electric signal of 5~20 mA to the controller, which is then converted and judged by the controller. The output electrical signal controls various solenoid valves to perform the loading and unloading action.
The sampling point of the pressure transmitter of the screw machine is generally set after the air cooler, that is, the exhaust port of the screw machine. The exhaust port of the screw machine is connected to the pipe network, and the pressure of the pipe network is not lower than the required pressure is the sole purpose of the air compressor work. Therefore, when the pressure sampling point detects that the pressure is equal to or lower than the set loading pressure, the air compressor must be loaded. Conversely, when the monitored pressure is equal to or higher than the set maximum pressure, the air compressor must be unloaded.
The operation of the air compressor loading is: the air compressor inlet valve is opened, a large amount of intake air is compressed, separated, the minimum pressure valve is opened, and the air pipe network is discharged after cooling;
The operation of the air compressor unloading is: the intake valve is closed, only a small amount of intake air, the bleed valve action bleed internal pressure (that is, the pressure in the gas path before the minimum pressure valve is vented, generally only the oil circulation is retained. The pressure required, when the unloading, the pressure in the oil and gas barrel is generally only 2bar, so that it can only bear the lower back pressure and reduce the energy consumption when loading again. The minimum pressure valve plays a key one-way check function at this time. The finished compressed gas in the pipeline is not allowed to flow back to the oil and gas tank to be vented.
It can be seen from the analysis of the above principle that the loading and unloading of the screw air compressor depends on the pressure change at the pressure sampling point, so the following causes of frequent loading and unloading of the air compressor are easy to understand:
1 Is there a check valve between the air compressor outlet and the air tank?
At this point, it is expressly required in all screw machine training that no check valves should be installed. This is because of the piston machine era, the installation of check valves is standard operation, some "masters" and even design institutes, and still have this design. To be clear: not to say that the check valve is installed, the screw machine must be started frequently, which is related to the sealing of the minimum pressure valve of the screw machine. If the minimum pressure valve seal is good, the frequent start is not obvious. But the reality is that most of the smallest pressure valves are impossible to seal at all.
If a check valve is installed between the air compressor and the air tank, it means that the pipe between the air compressor minimum pressure valve and the check valve will form a factual air storage space. This will cause the pressure sampling point to fail to monitor the pressure change of the pipe network (the check valve will block the return flow of the pipe, and the pressure transmitter actually monitors the pressure of the gas in this pipe).
When the unit is unloaded, the pressure of this section of the pipeline is higher than the pressure in the oil and gas tank (as described above, the pressure of the oil and gas tank is released, only about 2 bar), and the minimum pressure valve cannot be done. To the complete check seal, it will inevitably leak to the oil and gas barrel, and the volume of this pipeline is small, and the pressure is quickly lowered due to leakage (so the closer the check valve is to the air compressor exhaust outlet) The more frequent the loading and unloading, the closer the volume of the pipeline is.
When the unit detects that the pressure drops, it must be reloaded. When the pressure of the oil and gas tank reaches the minimum pressure valve opening pressure, the air flow flows through the air cooling to the pipe network, which quickly increases the pressure of the pipe in which the pressure changer is located until it is opened. The check valve is connected to the pipeline of the pipe network. At this time, the pressure of the pipe network is only slightly lower, the natural pressure quickly reaches the unloading pressure, and the unit performs the unloading action, which is the reason why the screw machine is frequently unloaded.
The above occurs when the output of the air compressor is greater than the gas consumption. If the air compressor and the gas consumption are equal or the pressure cannot be filled, the frequent loading and unloading phenomenon will not occur. This means that the air compressor can't reach the unloading pressure until it is loaded. Therefore, there is no such thing as frequency conversion and some air compressors with capacity adjustment function, that is, this will not happen without venting of the unloading. However, it is harmful to install the check valve after the screw machine.
2 When there is no check valve installed, why is this phenomenon?
Through the above installation of the check valve, the cause of frequent loading and unloading, divergent thinking, all other reasons that may cause the formation of a partially closed pipeline near the pressure sampling point may cause this to happen.
For example, the air compressor outlet is connected to the filter. If the filter is clogged or severely soaked (the airflow can still pass, only enough back pressure is needed), because the flow rate of the airflow through the filter becomes slow, it is easy to air compressor to filter. The pipeline of this device (where the pressure sampling point is here) reaches the set upper pressure limit, and then the air compressor is unloaded. In fact, the pressure of the main pipe network may not be reached. The high-pressure air of this partial pipeline continues to flow to the two low-pressure ends (through the filter to the pipe network, the minimum pressure valve to the oil and gas drum), as described above, because the volume of the pipe is small, and the pressure will be extremely short. drop rapidly.
