Construction Requirements for Oxygen Pipelines (Part Two)
3. Requirements for pressure and air-tight tests
After the oxygen pipeline is installed, pressure and air-tight tests should be conducted to verify the strength and sealing of the pipe. The pressure and air-tight test should meet the following requirements:
After the oxygen pipeline is installed, pressure and air-tight tests should be conducted to verify the strength and sealing of the pipe. The pressure and air-tight test should meet the following requirements:
(1) The medium used for the pressure test for the oxygen pipeline should be oil-free clean water or dry air, nitrogen. It is strictly forbidden to use oxygen as the test medium. When using nitrogen as the test medium, pay attention to safety to prevent suffocation. When the design pressure of the oxygen pipeline is greater than 4.0MPa, it is forbidden to use gas for the pressure test. The reason is that once the air pressure test is burst, the gas expands sharply and has great power. It entrains pipe fragments and flies around, causing damage to nearby equipment, casualties, which results in great harm. After the hydrostatic test of the oxygen pipeline, it should be dried in time to prevent corrosion. During the hydrostatic test of austenitic stainless steel pipes, the chloride ion content in the water should not exceed 25g/m3, because chloride ions can cause stress corrosion in austenitic stainless steel. Otherwise, corresponding measures should be taken. After the water is exhausted, it should be blown with oil-free dry nitrogen.
(2) The test pressure of the pipeline is calculated based on the design pressure.
(3) When the pipeline is subjected to the pressure test, the hydraulic test pressure is equal to 1.5 times the design pressure, and the pressure of the buried pipeline must not be lower than 0.4MPa. The pneumatic test pressure is equal to 1.15 times the design pressure, and not less than 0.1MPa. The test methods and requirements should meet the requirements of the GB 50235 standard. In the strength test, after reaching the test pressure, stabilize the pressure for 10 minutes and then drop to the design pressure. If there are no pressure drops and leakages, the test will be qualified.
(4) Air-tight tests should be carried out after the pressure test of the oxygen pipeline is qualified. The test medium should be oil-free, dry and clean air or nitrogen, and the test pressure should be equal to the pipeline's design pressure. The air-tight test is stricter, safer and more scientific than the filling gas for detecting leakages. If the leakage rate does not meet the requirements, the leakage must be detected to eliminate hidden dangers and ensure safety.
The air-tight test's requirements shall be carried out in accordance with the provisions of the GB 50235 standard, and the leakage rate shall also be calculated.
After the gas pressure in the pipeline reaches the design pressure, it should be maintained for 24 hours, and the average hourly leakage rate A for indoor and trench pipelines should not exceed 0.25%, because leakages of oxygen is not easy to diffuse indoors or at trenches, resulting in great danger. For outdoor pipes, a rate of no more than 0.5% is considered qualified, because a leakage of oxygen outdoors can easily diffused due to good ventilation and the risk is low. The requirements are lower than before.
The leakage rate A is calculated according to formulas (1) and (2): when the nominal diameter of the pipeline DN is less than and equal to 0.3 m:
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When the pipe nominal diameter DN is greater than 0.3 m:
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In the formula, P1 is the absolute pressure at the beginning of the test. P2 is the absolute pressure at the end of the test. T1 is the temperature at the beginning of the test. T2 is the temperature at the end of the test. DN is the nominal diameter of the pipe.
4. Requirements for the oxygen pipeline before being put into use
After the oxygen pipeline is installed, overhauled, or put into use after a long-term shutdown, the remaining moisture, iron filings, and debris in the pipeline should be purged with oil-free dry air or nitrogen until there is no rust, dust, and other impurities. The purge speed should not be less than 20m/s and not less than the design flow rate of the oxygen pipeline. The flow rate should be great and the flow velocity should be high to ensure the purge effect. Use a white cloth or a white painted target board to check the pipe end. There should be no iron filings, welding slags, rust, dust particles, foreign matter and moisture to ensure safety after supplying oxygen. As a result, requirements for the oxygen pipeline before being put into use have been neglected or are not met well, leading to an explosion, which has been commonplace.
It is strictly forbidden to use oxygen to purge the pipeline to avoid an explosion. When it is difficult to obtain a large purge gas source or there is interference with production, the oxygen pipeline can be blasted and purged, that is, the blasting port is set at the purge port at one end of the pipe. The blasting plate is fixed according to the pressure; it can be rubber asbestos boards or cardboard with single layers or multilayers. The pipe is filled with oil-free dry nitrogen or air, and when the pressure is increased to the blasting pressure of the blasting plate, the blasting plate is broken. The high-pressure airflow is blown from the blasting port. The pressure in the pipeline drops instantly, and the pressure (gauge pressure) at the blasting port is close to 0. The purge gas generates a great flow rate due to the large pressure difference between the inside and outside of the pipe (being equal to the burst pressure) and the rapid expansion of nitrogen or air, even close to the speed of sound, which has great kinetic energy. It can be used to purge the inner wall of the pipe to achieve the purpose of cleaning rust, welding slags, sundries and moisture.
The blasting purge has the advantages of high purge quality, simplicity, safety, reliability, no impact on production, low costs, and short construction periods. At present, it has been widely used. In the implementation, full attention should be paid to the determination of the blasting pressure, selection and reinforcement of the blasting hole, and safety alert around the blasting hole. It should be noted that adding an appropriate amount of degreasing agent to the pipe in the blasting and purging process can achieve the purpose of degreasing, with good results and successful experience.
(3) When the pipeline is subjected to the pressure test, the hydraulic test pressure is equal to 1.5 times the design pressure, and the pressure of the buried pipeline must not be lower than 0.4MPa. The pneumatic test pressure is equal to 1.15 times the design pressure, and not less than 0.1MPa. The test methods and requirements should meet the requirements of the GB 50235 standard. In the strength test, after reaching the test pressure, stabilize the pressure for 10 minutes and then drop to the design pressure. If there are no pressure drops and leakages, the test will be qualified.
(4) Air-tight tests should be carried out after the pressure test of the oxygen pipeline is qualified. The test medium should be oil-free, dry and clean air or nitrogen, and the test pressure should be equal to the pipeline's design pressure. The air-tight test is stricter, safer and more scientific than the filling gas for detecting leakages. If the leakage rate does not meet the requirements, the leakage must be detected to eliminate hidden dangers and ensure safety.
The air-tight test's requirements shall be carried out in accordance with the provisions of the GB 50235 standard, and the leakage rate shall also be calculated.
After the gas pressure in the pipeline reaches the design pressure, it should be maintained for 24 hours, and the average hourly leakage rate A for indoor and trench pipelines should not exceed 0.25%, because leakages of oxygen is not easy to diffuse indoors or at trenches, resulting in great danger. For outdoor pipes, a rate of no more than 0.5% is considered qualified, because a leakage of oxygen outdoors can easily diffused due to good ventilation and the risk is low. The requirements are lower than before.
The leakage rate A is calculated according to formulas (1) and (2): when the nominal diameter of the pipeline DN is less than and equal to 0.3 m:
When the pipe nominal diameter DN is greater than 0.3 m:
In the formula, P1 is the absolute pressure at the beginning of the test. P2 is the absolute pressure at the end of the test. T1 is the temperature at the beginning of the test. T2 is the temperature at the end of the test. DN is the nominal diameter of the pipe.
4. Requirements for the oxygen pipeline before being put into use
After the oxygen pipeline is installed, overhauled, or put into use after a long-term shutdown, the remaining moisture, iron filings, and debris in the pipeline should be purged with oil-free dry air or nitrogen until there is no rust, dust, and other impurities. The purge speed should not be less than 20m/s and not less than the design flow rate of the oxygen pipeline. The flow rate should be great and the flow velocity should be high to ensure the purge effect. Use a white cloth or a white painted target board to check the pipe end. There should be no iron filings, welding slags, rust, dust particles, foreign matter and moisture to ensure safety after supplying oxygen. As a result, requirements for the oxygen pipeline before being put into use have been neglected or are not met well, leading to an explosion, which has been commonplace.
It is strictly forbidden to use oxygen to purge the pipeline to avoid an explosion. When it is difficult to obtain a large purge gas source or there is interference with production, the oxygen pipeline can be blasted and purged, that is, the blasting port is set at the purge port at one end of the pipe. The blasting plate is fixed according to the pressure; it can be rubber asbestos boards or cardboard with single layers or multilayers. The pipe is filled with oil-free dry nitrogen or air, and when the pressure is increased to the blasting pressure of the blasting plate, the blasting plate is broken. The high-pressure airflow is blown from the blasting port. The pressure in the pipeline drops instantly, and the pressure (gauge pressure) at the blasting port is close to 0. The purge gas generates a great flow rate due to the large pressure difference between the inside and outside of the pipe (being equal to the burst pressure) and the rapid expansion of nitrogen or air, even close to the speed of sound, which has great kinetic energy. It can be used to purge the inner wall of the pipe to achieve the purpose of cleaning rust, welding slags, sundries and moisture.
The blasting purge has the advantages of high purge quality, simplicity, safety, reliability, no impact on production, low costs, and short construction periods. At present, it has been widely used. In the implementation, full attention should be paid to the determination of the blasting pressure, selection and reinforcement of the blasting hole, and safety alert around the blasting hole. It should be noted that adding an appropriate amount of degreasing agent to the pipe in the blasting and purging process can achieve the purpose of degreasing, with good results and successful experience.
5. The painting of the pipeline
The painting of the pipeline is not only for distinguishing the medium in the pipe, but also for safety. It is a safety mark to avoid accidents caused by incorrect filling and misuse. According to the Basic Identification Colors, Identification Symbols and Safety Marks for Industrial Pipelines (GB 7231) and GB 16912 2008 standards, the painting for the oxygen pipeline is baby blue, and other commonly used painting colors of industrial pipelines are that light gray for air pipes, red for steam pipes, pale yellow for nitrogen pipes, brown for polluted nitrogen pipes, red and white rings for hydrogen pipes, silver gray for argon pipes, brilliant green for water pipes, brilliant green and black rings for water pipes, yellow for oil inlet pipelines, yellow and black rings for oil outlet pipelines, red and black rings for heating and thawing gas pipelines, red and nameplates for fire water pipelines, etc.
In the past, the basic identification colors of industrial pipelines were very extensive, and they failed to be distinguished strictly, which caused accidents. For example, the painting color for the oxygen pipeline is sky blue, and the compressed air pipeline is dark blue. The painting's color cannot be distinguished over time. Someone accidentally opened the oxygen valve and used the compressed air for cooling during electric welding construction in the summer, causing a major safety accident in which one person was burnt to death and one person was burned.
In the past, the basic identification colors of industrial pipelines were very extensive, and they failed to be distinguished strictly, which caused accidents. For example, the painting color for the oxygen pipeline is sky blue, and the compressed air pipeline is dark blue. The painting's color cannot be distinguished over time. Someone accidentally opened the oxygen valve and used the compressed air for cooling during electric welding construction in the summer, causing a major safety accident in which one person was burnt to death and one person was burned.
