Transient Thermal Stress Analysis of BOF Hood Failure

Abstract

The failure of BOF hood tube is a common industrial problem that has received little attention in literature. The major mode of failure as observed by most of the industries is thermal fatigue failure. Earlier people have studied the thermal stress profile in hood system considering constant water temperature. However, in reality there are fluctuations in water temperature due to cyclic thermal load caused by BOF gas. Present study reveals that the water temperature fluctuation is having significant impact in causing thermal fatigue failure. The change in water velocity and tube thickness has negligible effect in thermal fatigue failure compared to that observed for water temperature fluctuation. However, tube material of construction is also having significant impact on its failure.

Appendix

Process Description of Cooling Water Circuit

From the overhead tank (OHT), water comes into the makeup water tank which is used for supplying makeup water to the cooling water circuit. From the makeup, water goes into expansion tank via pump-I. Expansion tank is used in a closed water system to absorb excess water pressure caused by thermal expansion. From expansion tank via pump-2, water goes into the supply header where water flow is divided into several branches as shown in Fig. 1. One part goes into the cooling stack, another goes into the movable hood, and the third one goes into the movable skirt. From the cooling stack and movable hood, water goes into the receiving header, whereas from movable skirt one part goes to the skirt hydraulic and other part goes into flux chute before going into the receiving header. From the receiving header, water passed through fin fan heat exchangers for its temperature to get down through fin fan cooling before finally circulated into the expansion tank. It is important to note here that dissolve oxygen is a major concern as it reacts with the hood tube material to cause corrosion. To control the dissolve oxygen in the water, chemical dosing is done into the water circuit just after the expansion tank. Basically carbohydrazide-based chemical dosing is done into the water which acts as an oxygen scavenger. It reacts with dissolved oxygen directly as shown below [12].

$$\left( {{\text{NH}}_{2} {\text{NH}}} \right)_{2} {\text{CO}} + 2{\text{O}}_{2} \Rightarrow 2{\text{N}}_{2} + 3{\text{H}}_{2} {\text{O}} + {\text{CO}}_{2}$$

(13)

$$\left( {{\text{NH}}_{2} {\text{NH}}} \right)_{2} {\text{CO}} + {\text{H}}_{2} {\text{O}} \Rightarrow 2{\text{N}}_{2} {\text{H}}_{4} + {\text{CO}}_{2}$$

(14)

$$2{\text{N}}_{2} {\text{H}}_{4} + 2{\text{O}}_{2} \Rightarrow 4{\text{H}}_{2} {\text{O}} + 2{\text{N}}_{2}$$

(15)

At room temperature, the first reaction (Eq 13) is much faster than the rest two reactions. However, at elevated temperature release of hydrazine and carbon dioxide becomes significant due to the increasing reaction rate of the second reaction (Eq 14).

It is to note here that carbohydrazide also helps in protecting the hood tubes by forming passive iron oxides required for iron passivation. The reaction is shown as follows:

$$12{\text{Fe}}_{2} {\text{O}}_{3} + \left( {{\text{NH}}_{2} {\text{NH}}} \right)_{2} {\text{CO}} \Rightarrow 8{\text{Fe}}_{3} {\text{O}}_{4} + 3{\text{H}}_{2} {\text{O}} + 2{\text{N}}_{2} + {\text{CO}}_{2}$$

(16)