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The Analysis of Corrosion of Concentrated Sulfuric Acid in Carbon Steel Pipelines (Part One)

Posted: 09/12/2021 09:26:40 Hits: 68

Abstract: 98% sulfuric acid is a highly corrosive strong mineral acid. It has great oxidizing properties, dehydrating, acidity, water absorption and is hard to volatilize. However, at room temperatures, a dense oxide film can be formed on the surface of carbon steel to inhibit the occurrence of corrosion due to its good oxidizing properties, and the corrosion rate decreases with time. Therefore, carbon steel is often used as a material for equipment used for storing concentrated sulfuric acid. However, when it is used as a pipeline material for conveying media, the effect is just the opposite.

Alkylated oil is a high-octane gasoline additive under the action of a catalyst (sulfuric acid or hydrofluoric acid), which has an anti-knock effect and produces a clean product after combustion. Sulfuric acid is used to produce alkylated oil for the Ningxia Petrochemical's alkylation plant, and 98% sulfuric acid needs to be supplemented in the reaction process. In order to ensure that the sulfur content in the alkylated oil does not exceed the standard value, continuous supplementation of concentrated sulfuric acid is one of the key factors to reduce the sulfur content.

In the Ningxia Petrochemical's alkylation plant, the design material of the supplemented concentrated sulfuric acid pipeline is 20 steel, that is, carbon steel. According to the great oxidizing property of concentrated sulfuric acid, a dense passivation film will be formed on the surface of the pipeline's medium to prevent corrosion of carbon steel pipes. However, since the start of the alkylation unit, frequent leaks of sulfuric acid have occurred in the supplemented concentrated sulfuric acid pipeline, which has caused great safety hazards to people. Corrosion and leakages of alkylated acid pipelines are mainly concentrated in the following parts: the part with the changed flow direction of the medium, around the welding seam of the pipe fitting and the changed flow direction of the medium.

1. Corrosion and leakages of acid pump inlet and outlet pipelines
1.1 Corrosion, leakages and locations of inlet pipelines
In the early morning of June 2, 2019, after starting the 2229-P-702 new acid pump, the team discovered that sulfuric acid was leaking at the welding seam of the pump inlet pipeline. After disassembling the inlet pipeline as a whole, it was found that there was intensive scouring at the reducer of the inlet pipeline and inner wall of the flange, and the wall thickness was reduced; the welding seam corroded and leaked. However, there was slight corrosion at the inner wall of the inlet pipeline of DN100.

2229-P-702 Severely corrosion of flanges of DN40 and reducers of inlet pipelines and slight corrosion of the pipeline of DN100

On July 30, 2019, during the operation of P-701B, a leak occurred at the reducer of the inlet pipeline. After disassembling the inlet pipeline as a whole, it was found that there was obvious scouring of the inlet pipeline and inner wall of the flange; the wall thickness was reduced, and corrosion and leakages happened before the welding seam.

1.2 Corrosion, leakages and locations of outlet pipelines
At 24:20 on August 10, 2019, the first elbow of Unit 400 of the outlet pipeline corroded and leaked. Since it was the first time that corrosion and leakage occurred on the outer part of the elbow, the thickness of the outer elbow near the first elbow was measured, and the thickness was only 1 mm. The two 90° elbows were replaced.

At 10:00 on June 29, 2019, the first elbow of Unit 200 of the outlet pipeline corroded and leaked. The elbow here was a socket-welded 90° elbow, and the outer bending side of the elbow was corroded and thinned. It was found that the on-site construction and the design did not match by checking the design drawing, and the original design was a butt-welded 90° elbow. After disassembling and inspecting the insulation, it was found that a total of 3 elbows were not consistent with the design. On July 18, 3 socket welding elbows that did not conform to the design were replaced. The replaced socket welding elbows were thinner, and the thinnest thickness of the wall was only 1 mm.

At 11 o'clock on August 21, 2019, corrosion and leakage happened at the reducing tee at the root of the vent valve of unit 400 of the outlet pipeline. The flow direction of the medium and the corroded parts are shown in the previous figure.

On April 24, 2019, the pipeline behind the outlet flange of 2229-P-701A corroded and leaked. The outlet flange has a specification of DN25 and CL300, and the pipeline behind the flange ø60×5. There was a sudden change in diameter. After measurement, it is found that the minimum thickness of the pipeline at the corroded perforated part was 1.6 mm and the maximum was 4 mm. The overall thickness of the pipeline from the nipple of the pump outlet to the check valve of the pump outlet was 4.0 mm to 4.5 mm, and the nipple at the rear of the pump outlet flange was partially corroded.

At 19:20 on July 31, 2019, the 0.5cm behind the welding seam of the 200 unit elbow corroded and leaked. After the leaking part was cut, it was found that the welding seam on the inner wall of the pipe was not completely welded, and there was obvious scouring of media on the inner bending side.

On June 2, 2019, the acid line flowmeter FT-4101 suddenly had a flow rate of 0, but the acid pump was not holding pressure. It was judged that the hand valve plate before and after the flowmeter had fallen off, and the internal leakage of the auxiliary line was serious. The acid line was handed over on September 18, and it was found that the valve plate of the hand valve in front of the flowmeter fell off; the valve screw was in the open state and the valve plate was in the closed state; the valve screw was corroded and disconnected. The outlet pipeline of 2229-P-702 has not leaked. It is used as an intermittently operating pump. According to calculations, after the start of alkylation, it will run for approximately 1,120 hours as of June 2019. The pump outlet pipeline's specification is 114*6 mm. It can be calculated that the flow velocity of sulfuric acid in the cross-section of the pipeline of DN100 is 0.3399m/s. The wall thickness of the straight pipeline measured in July 2019 was 6 to 6.2 mm, and the thinnest part of the wall thickness of the 90° elbow was 4.8 mm. The corrosion rate is approximately 3.8mm/a.

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Next: The Analysis of Corrosion of Concentrated Sulfuric Acid in Carbon Steel Pipelines (Part Two)

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About the author

Teresa

Teresa is a skilled author specializing in industrial technical articles with over eight years of experience. She has a deep understanding of manufacturing processes, material science, and technological advancements. Her work includes detailed analyses, process optimization techniques, and quality control methods that aim to enhance production efficiency and product quality across various industries. Teresa's articles are well-researched, clear, and informative, making complex industrial concepts accessible to professionals and stakeholders.