Study on Selection of Corrosion Margin of Gas Compressor for Chemical Machinery Air Compressor Li Longjun*Zhang Hongbo Hong Junhua (Taizhou Special Equipment Supervision and Inspection Center) Air compressor is a kind of machinery used to compress air, increase gas pressure or transport gas. The gas storage tank is constructed in the air compressor station, and the small air compressor is directly assembled into one.
The air is sucked into the air compressor through the filter, compressed in the air compressor to a predetermined pressure, and then enters the gas storage tank along the exhaust pipe, and then is sent to the use place by the exhaust pipe. The main function of the gas storage tank is to alleviate the pressure fluctuation caused by the uneven and discontinuous exhaust gas, reserve a certain amount of compressed air, maintain the balance between the supply and demand gas volume, and remove the oil and water in the compressed air. With the advantages of simple construction structure, long service life, less environmental pollution and low cost, with the development of industrial production, the application of gas storage tanks in China is more and more extensive.
1 After the promulgation and implementation of the domestic product standard for corrosion allowance (hereinafter referred to as “Simple Toleranceâ€), for the design pressure is not more than 1.6MPa, the volume is not more than 1 000L or the product of the working pressure and volume is not more than 1 L. Cans have been classified as simple pressure vessels.
At present, domestic pressure vessel products mainly implement GB150.1 150.4~2011 standard 0; but for small air compressor gas storage tanks with a volume less than 450L (subject to the "Simple Tolerance" jurisdiction), it is understood that most design and manufacturing units are implemented. JB/T6539-)2 Standard H. The two standards have different requirements for corrosion margin.
The components should be determined based on the expected age of the container design and the rate of corrosion of the metal to the media (and the rate of erosion). However, GB150.14011 also stipulates that the medium is a carbon steel container with compressed air, and the corrosion margin is not less than 1mm. The corrosion margin specified by JB/T6539-)2 is: the corrosion margin is available when the oil tank is internally lubricated. 0.5mm; oil-free lubrication takes a corrosion margin of 1.0mm. The can product is just because the volume is different, the value of the corrosion margin is not the same, it seems unreasonable.
2 Foreign related standards have relatively loose regulations on the amount of corrosion allowance. For example, stainless steel, internal spray and non-corrosive pressure vessels can set the corrosion margin to 0. The corrosion margin of carbon steel containers is determined by the manufacturer. Generally, it is not less than the simple pressure vessel gas storage tank. The same European standard EN286 stipulates that the corrosion allowance should not be less than 0.5mm. 3 In the gas tank corrosion situation, the current common material of the domestic gas storage tank is Q235B, and the volume is slightly larger. Q245R and Q345R. External corrosion includes uniform corrosion and local corrosion. Uniform corrosion refers to substantially the same chemical or electrochemical corrosion on the entire exposed surface of the container wall metal or over most of the area. By collecting samples from some manufacturers' reclaimed gas storage tanks, it was found that only when the outer bottom anti-corrosion layer fell off, there was uniform corrosion. Local corrosion is caused by the damage of the local coating material on the surface, and the local corrosion of the surface. By collecting some samples of the gas storage tanks recovered by the manufacturer, it is found that the outer surface of the coating is intact. * Li Longjun, male, born in November 1971, Senior engineer. Taizhou City, Zhejiang Province, under 318000. No local corrosion occurred.
The internal medium of the gas storage tank is air or oil and gas suspension. Generally, there are some condensed water (oily) at the bottom. The air compressor is generally oil-lubricated, so a protective film can be formed on the inner wall inside the container. The gas storage tank is subjected to anatomical verification of the gas storage tank sample recovered for 4 to 5 years under continuous working conditions, and the inner wall is also substantially non-corrosive. However, if the gas tank is stopped, the inside of the tank is likely to form a large area of ​​uniform corrosion due to the presence of water and other impurities at the bottom. But there is a lack of typical data in this regard.
In the special equipment inspection database, the author randomly selected a part of the thickness measurement data in the periodic inspection of the gas storage tank. The specific conditions are shown in Table 1. Table 1 is used to measure the thickness of the main material volume in the periodic inspection of the gas storage tank. Date of application Date measured wall thickness / mm The upper head of the cylinder head is unknown. It can be seen from Table 1 that there is slight corrosion in the gas storage tank or substantially no corrosion. Regardless of the main material, except for a 40m3 large vertical tank, the corrosion rate is not more than 0.05mm/a. According to data research, the compatibility of air, nitrogen and carbon steel is good, after more than ten years of use. The anatomical examination of the gas storage tank found that there was almost no change in the wall thickness and material properties of the vessel (Definition of Simple Tolerance).
It is understood that the current status quo is that the manufacturing enterprises have a beautiful appearance, and the external surface anti-corrosion measures are quite good before leaving the factory. Most of the small air compressor gas storage tanks adopt the spray-spraying process, and within the recommended service life (simple pressure vessel gas storage) The tank is generally 7-8 years. If there is no artificial damage on the outer wall, it can basically ensure that the external corrosion is 0. And because the compressed air contains oil, the inner surface is not easy to corrode. Foreign materials have been introduced for internal spraying, which can effectively reduce internal corrosion, but the domestic industry has not seen precedents for adopting this technology. Since domestic products have not been found to have internal spraying chemical machinery, the corrosion margin is not reasonable.
No accident cases of gas tank failure due to corrosion have been counted so far.
4 Energy-saving analysis GB150 is most reasonable in terms of the definition of corrosion margin, but at the same time, energy conservation should also be considered.
For example, the small-scale gas storage tanks under the jurisdiction of the Simple Tolerance Regulations, the recommended service life of many design units is 7 years, and if the corrosion rate is 0.05 mm/a, the corrosion allowance is 0. 35 mm. Nearly two-thirds smaller. It is nearly three-third smaller than the JB/T6539-92.
For example, the gas storage tank of the press has an annual total output value of about 1.5 to 1.7 billion yuan. The calculation of the reduction in corrosion margin is based on the wall thickness ratio (average 4% to 5%), and the annual national steel saving value is nearly 70 million yuan. It is quite impressive.
5 Conclusions 5.1 Because the Pressure Vessel Safety Technical Supervision Regulations stipulates that the design life of the pressure vessel should be given 63, and the Simple Pressure Regulation also stipulates that the simple pressure vessel should give the recommended service life, so the corrosion allowance is determined. It is more reasonable to obtain the product of "corrosion rate (mm/a) X recommended (design) service life", and it is not necessary to specify the lower limit value, and the design unit is responsible for checking.
5.2 According to the survey results, under normal working conditions, the corrosion rate of 0.05mm/a is relatively reasonable.
5.3 Small gas storage tanks, if the relevant standards can be adjusted to adjust the corrosion margin, the energy saving effect is very obvious for the whole industry.
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