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How to control the leakage of compressed air within a reasonable range?
Compressed air is one of the most widely used power sources in the industrial field. Because of its many advantages such as safety, pollution-free, good adjustment performance, and convenient transportation, it is widely used in the field of modernization and automatic power. Compressed air is also an expensive source of energy and power. Continuously reducing the overall operating cost of compressed air is an important issue for every factory manager.
Compressed air leaks are almost the most common type of energy waste in factories. The average compressed air leakage accounts for 30% of the entire compressed air volume, which means that tens of thousands of electricity bills are leaked every year. Some leaks are so obvious that not only does it make a lot of noise, it can even be detected tactilely and visually. And some leaks are very concealed. In addition to the small and difficult to hear sounds, "concealed" leaks often occur in environments with high background noise in the workplace. All of the above leaks constitute the source of leaks in the entire system.
Leaks usually occur in these places:
(1) Pipe joints, quick-connect joints;
(2) Pressure regulator (FRL);
(3) Frequently opened condensate drain valve;
(4) Broken hoses, broken pipes;
Leakage is a common phenomenon in the air system. In a normal operating system, it is difficult to avoid leakage. According to the relevant survey results of the US Department of Energy (DOE) and the author's long-term experience, every system has leakage, and nearly 60% of the factories have not taken any measures for leakage in the air system.
Leaks in factories are everywhere. If a factory wants to completely eliminate the leakage, it is almost impossible. What we can do is to control the leakage of compressed air within a reasonable range. This "reasonable" range and the size of the factory has a lot to do with old and new:
(1) For new systems (less than 1 year) or small factories, the leakage rate should be controlled between 5% and 7%;
(2) For systems or medium-sized plants of 2 to 5 years, the leakage rate is between 7% and 10%;
(3) For systems older than 10 years or large plants, the leakage rate is between 10% and 12%;
Leaks don't just directly lead to wasted energy, it also leads to wasted energy indirectly. When the leakage intensifies, the pressure of the entire compressed air system will drop. If the pressure of the air system is to be maintained, additional compressors must be turned on, which will further increase the electricity cost of the entire plant. In some factories, there are a large number of intermittent discharge devices, such as electronic blow down valves, these valves discharge condensate or other waste liquids at regular intervals, and after the waste liquid has been discharged during the discharge time, a large amount of compressed air Leave the compressed air system. At a certain time, there may be multiple discharge valves exhausting at the same time. At this time, the pressure of the entire system will suddenly drop, and even exceed the minimum pressure that the system can accept, causing the entire system to stop production. This is a typical operating accident.
Since the compressed air is produced by the work of the air compressor, and the air compressor is driven by the electric motor, the leakage of the air indirectly means the waste of electric energy.
In practice, three methods are often used to quantitatively evaluate the leakage of compressed air. They are 1. Air storage volume measurement method; 2. Compressor operation measurement method; 3. Ultrasonic leak inspection method; the following are respectively introduced:
1. Determination of gas storage volume
Assuming that an air system is airtight and only leakage is the only way for compressed air to leave the air system, there is the following leakage calculation formula for a compressed air system:
QLeak: Leakage, m3/min
ΔP: differential pressure, bar
P0: absolute pressure, bar
V: Leaked air volume, m3
T: test time, min
2. Compressor operation test method
Turn off all air-consuming equipment in the air system to ensure that all air in the compressed air system leaves the system by means of leaks. Turn on a compressor and run it in the loading and unloading mode (On-Line/Off-Line), and record the compressor pressure set points Pon and Poff and each operating time.
QLeak: Leakage, m3/min
Q: The displacement of the compressor, m3/min
T: Loading runtime, min
t: uninstall running time, min
3. Ultrasonic leak inspection method
The difficulty with compressed air leak detection is that most of the pipes are not easily accessible, they are either installed at high altitudes or hidden in the box, and because air leaks cannot be visually identified, ultrasonic testing is a common method. Ultrasound usually refers to a frequency band with a frequency higher than 20kHz, and the upper limit that the human ear can receive is 16.5kHZ. Using this feature, ultrasonic detection of compressed air leakage can be applied in industrial detection.
Ultrasonic leak detector is a special instrument. Any gas passing through the leak hole will generate eddy current, and there will be part of the ultrasonic wave band. The ultrasonic leak detector can sense any kind of gas leakage. The way the leak is identified by receiving the high frequency "hissing" sound of an air leak.
Ultrasonic leak detectors usually consist of a microphone, filter, indicator and earphones. The amount of leakage is related to the distance of the test and the value of the ultrasonic wave. The ultrasonic leak detectors produced by different manufacturers have different parameter tables.
The steps of ultrasonic leak detection:
1. Tour the entire factory and quickly pick out obvious large leaks in the air system, such as open valves, rags on hoses (some workers cover the rags to silence the leaks), still supplying air, but not activated Machines, drain valves, quick plugs, etc.; during the inspection process, a most suitable detection route can be drawn, and a pipeline diagram can be drawn when possible, which is very helpful for determining the leak point in the future.
2. Use a leak test gun to carefully test all air lines, remember to always wear headphones, and adjust the sensitivity when it is difficult to determine the leak location;
3. Start from the gas supply end, and gradually advance the detection to the use end;
4. It is recommended to subdivide the detection area and carry out one by one to avoid repeated detection or missed detection;
5. After the leak point is detected, mark the position with a label to ensure that the leak label can be hung at the leak point at least until the leak is eliminated (it is recommended not to remove it for re-inspection);
6. Check again after the leak point is repaired, sometimes the repair will lead to a new leak point;
7. Calculate the amount of leakage;
8. Compile leak detection report;
In practice, leak detection services often use a combination of the above methods: Calculate the total leakage of the air system through methods 1 and 2, and the results are used as the basis for management to decide whether to conduct specific leak detection. Method three can measure and mark each specific leak point.
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