Stress corrosion of partially immersed heat exchanger u-tubes

conference paper

English

In general, every heat exchanger is designed individually. Various aspects, such as inlet and outlet temperatures, flow rates and stream pressures are considered in design process. Pursuit of optimal reliable design, tailored to specific needs is necessary for manufacturers in order to keep up with competition. However, optimal design can be very sensitive to changes in working conditions. In this paper, stress corrosion cracking of heat exchanger u-tubes as a consequence of decreased medium level in shell space is presented. Object of the study is heat exchanger with u-tubes used in petroleum industry. There is a mixture of gas oil and hydrogen in tube space. This mixture also contains sulphur chloride and ammonia therefore corrosion aided stress cracking presents serious threat. Inlet temperature is 350 C and medium flows at the pressure of 4.27 MPa. Shell space is partially filled with cooling water pressurized to 0.88 MPa, which is turned into steam with outlet temperature up to 200 C. At previous setup, heat exchanger was working without any major problems for several years. Just a few tubes were blinded due to leakage. Nevertheless, need for enhanced flow of hot stream at present setup resulted in drop of water/steam interface, so the u-tubes were not fully submerged. With these new conditions, great leakage occurred soon and the heat exchanger had to be shut down. During revision, severely damaged tubes were observed. There were many cracks on u-tubes. After further investigation, it was found out that the cracks growth had been driven by stress corrosion. As the main cause for this, increased stress due to irregular temperature distribution on partially immersed u-tubes is considered. This paper presents simple parametric model for calculation of such thermal field induced stress. Results showed that it is not reliable to let the u-tubes cross water/steam interface. For presented heat exchanger, replacement of damaged u-tube bundle is recommended. Entirely new heat exchanger should be considered too as it presents more durable solution, although it would be more expensive. In both cases, it is necessary for the u-tubes to be fully submerged in cooling water.

heat exchanger, u-tube, thermal analysis, stress analysis

LÉTAL, T.; NEKVASIL, R.

2010

25. 8. 2010

Process Engineering Publisher

Praha

978-80-02-02248-0

Materials of the 19th International Congress of Chemical and Process Engineering CHISA 2010 and the 7th European Congress of Chemical Engineering ECCE-7

First Edition

1

1205

1205

1