A serious problem for petrochemical companies is the wear and tear undergone by the steel tubes used in the production of ethylene, a raw material used in making polymers like polyethylene and polypropylene, used, for example, in making
A serious problem for petrochemical companies is the wear and tear undergone by the steel tubes used in the production of ethylene, a raw material used in making polymers like polyethylene and polypropylene, used, for example, in making packaging and parts for the automobile industry. Produced from a family of refractory (heat resistant) steels, these tubes suffer from wear from the action of the high temperatures and of the gases used in synthesizing polymers. Accordingly, the quality of these tubes is a fundamental factor for avoiding the constant changing of the tubing and bringing production to a halt. It was to get a precise understanding of this wear and tear that a team from the Technological Research Institute of the State of São Paulo (IPT) decided to develop a new methodology for testing, capable of assessing the behavior of the heat resistant metallic alloys under the conditions met in the production of the polymers.
The environment where these tubes in a serpentine shape work is in the inside of furnaces for pyrolysis (decomposition by heat, without the presence of oxygen). In the production of ethylene, the outer surface of the tubes reaches temperatures of up to 1,150 degrees centigrade. The result is the appearance of two phenomena, called fluency and carburetion. In the former, deformations occur as a result of the tensions caused by heating. In the latter, there is a marked degradation caused by the diffusion of carbon atoms in the inside of the tubing. Generally speaking, the tubes are designed for a useful life of 100,000 hours (about 11 years), but, due to carburetion, this period is reduced by as much as 50%. Nowadays, to guarantee the original useful life, the manufacturers are increasing the thickness of the wall of these tubes, which makes the product dearer.
The researchers were able to count on support from FAPESP, which funded the project under the Partnership for Technological Innovation Program (PITE). The partner company is Engemasa, a refractory steel tube maker from São Carlos (SP). It was not, however, the first industrial concern to be involved in the project. When it was conceived, in 1997, the partnership took place with Aços Villares, which used to have a factory making these tubes. While the project was being assessed by the Foundation, the company closed down this unit’s activities. The researchers from the IPT then offered the project to Fundinox, a division of Sulzer Brazil. The company accepted, and research was started in May 2000. A little while later, Fundinox was bought by a major German manufacturer, which did not show any interest for the work.
Change in partnership
“As the project was already under way, FAPESP agreed to it going on without a partnership with a company, until we knocked on the door of Engemasa”, says Mário Boccalini, a researcher from the IPT’s Metallurgy Division. The work was concluded in April this year, and the results show in detail the general laws covering the two phenomena that affect the tubes. “We made technological progress that is without precedent in Brazil. We managed to reproduce accurately the physical and chemical conditions that occur in the furnaces, and, from then on, we were able to get a better understanding of fluency and carburetion”, explains physicist Zehbour Panossian, the coordinator of the PITE project and head of the IPT’s Corrosion and Protection Group.
To understand the significance of this project’s tests and its results, the two phenomena that affect the tubes need to be known. According to Boccalini, fluency is a deformation caused by two kinds of strains: the pressure caused by the gas that circulates within the tube, and the longitudinal tension brought about by the weight of the tubes. They are usually around 10 meters long and from 5 to 20 centimeters in diameter. When submitted to high temperatures, they may experience deformations of up to 10 centimeters in their length. “This is harmful because it affects the soundness of the tubes, making them fragile and subject to cracks. The tubes therefore lose their capacity for deformation and become more susceptible to the occurrence of catastrophic fracture (breakage of the tube)”, Boccalini explains. When this happens, the material has to be repaired or exchanged, bringing the company serious losses.
To carry out the tests that would make it possible to assess the behavior of the alloys – made up of iron, chrome, nickel and a small 0.8% addition of carbon – when submitted to carburetion, the researchers made a synthetic atmosphere with methane and hydrogen in the inside of a furnace containing cylindrical test bodies of 10 millimeters in diameter by 70 millimeters in length. The temperature inside was kept at as much as 1,150 degrees centigrade, and each test (there were 25 in all) lasted about 100 hours. After the small cylinders left the furnace, they were measured to identify the variation in mass. Next, they would undergo a microstructural analysis using several instruments.
In these analyses, layers of metal were removed to find out up to what point and in what quantity the carbon atoms had penetrated the alloy. “In some cases, 5% of the mass of the tube was made up of carbon, which went so far as to spread itself for 5 mil