Accident prevention of the hottest pressure vessel

Pressure vessel accident prevention

pressure vessel is a kind of special equipment widely used in the national economy and people's life. Once a leakage or explosion failure accident occurs, it will have disastrous consequences, endanger people's life safety and cause serious losses to people's property. Therefore, it is very important to do a good job in the safety of pressure vessel.

accident example

the medium temperature pipe of the secondary reformer of a 400000 t synthetic ammonia plant in a plant uses a low alloy steel seamless pipe made of II11. During the construction hydraulic test, it burst under 20MPa pressure. At that time, the temperature was about 0 ℃, and the burst crack was about 850mm, almost without expansion and deformation. After the accident, the professional technicians of the investigation team carefully investigated and analyzed the cause of the accident. The investigation includes the following steps

1. Anatomy of accident tube

after dissection, it is seen that there is a 220mm long longitudinal original crack defect at the burst medium temperature tube, which is perpendicular to the tube surface. It is preliminarily analyzed as an accident caused by the original defect

2. Material test

after the chemical composition analysis, mechanical property test and fracture toughness test of the material, the data obtained show that the chemical composition and mechanical properties meet the requirements, the crack opening displacement of fracture toughness (5) the tensile stress-strain curve value is also greater than the required value, and the metallographic structure is tempered sorbite. The test shows that the raw material basically meets the requirements

3. Burst test

this test is to carry out ultrasonic testing on the spare pipe. The results show that when the tube has no macroscopic defects, all indicators are normal; Once the pipe has cracks, delamination, or uneven wall thickness, it will affect the blasting strength to varying degrees

4. Fracture analysis

test results show that the flawless tube shows good toughness and strength; If there is serious original crack, if there is no Wei Qiao corrugated pipe, the bursting pressure is significantly lower than the yield pressure, and the fracture is special

5. Steel pipe mechanism process analysis

Where did the defects come from during the important strategic opportunity period that was still in development? This is the central problem of accident analysis. The existence of its corrosion black film shows that it is not newly cracked during the hydraulic test, but has long existed; The original fracture is cleavage type, and cleavage fracture will not occur during high temperature rolling; The metallographic structure is normal, there is no heating, and there are decarburization and oxidation phenomena caused by aging test, indicating that the original defects cannot be formed during high-temperature rolling; II11 tube is hot-rolled, and the tube embryo is not peeled, but pickled and abraded, forming the original defects of hot rolling

according to the above analysis, the original crack is the root cause of the accident. The formation of the original crack is due to the fact that the tube embryo is only damaged by pickling and grinding, but not by peeling. In the finishing process, the original crack is caused by the maximum cold deformation. Therefore, as long as the installed pipes are subject to 100% ultrasonic flaw detection (or plus surface inspection), the defective parts are removed and qualified pipes are added, they can be used safely to avoid accidents