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Open Discussion Forum Question 2

2. We have five ethylene cracking furnaces in a Gas cracker plant. The furnaces have "W" configuration coils with inlet at tube number 1 and common outlet of two adjacent coils at tube number 4. The metallurgy of the coil has been upgraded with passage of time. At present, we are using HP40 micro alloy in four furnaces and a combination of HP40 (for cold passes) and 35Cr45Ni (for hot passes) in one furnace. Though we have successfully solved various furnace problems, coil bowing is a chronic problem, which we are struggling to deal with. To solve coil bowing, we have been regularly adjusting the variable spring hangers used to support the coil. However, significant improvement has not been observed. In addition to spring adjustment, we have been monitoring the elongation of tubes during cracking. Recently, we have decided to replace variable spring hangers with constant spring hangers. Ethylene Producer - 7 July 2003

Comment from Texas Ethylene Producer - 28 July 2003

You are on the right track. Counter weights would be even better than constant tension springs since you can adjust the load to match the weight of the coils. With constant tension springs you are depending on a calculated weight of the coils. With counterweights you adjust the weight on the counterweights so the coil just floats before the final weld is made.

Comment for Louisiana Ethylene Producer - 30 July 2003

Elongation of radiant tubes is a common problem in thermal cracking furnaces.

The furnace designers normally do an excellent job of computing the possible radiant coil elongation and based on rigorous stress computations come up with the best possible balancing methods. However elongation results due to many factors.

Different materials have different coefficients of expansion. It is a good balancing act when tube and return bend-material changes are incorporated based on the calculated tube skin temperature profiles on the anticipated maximum operating conditions. The coefficients of expansions of 35Cr:45Ni:Nb < 35Cr:45Ni < 25Cr:35Ni . Thus small additions of rare metals and differences in metallurgical compositions between different production lots may at times give raise to bowing problems. It is up to the end user to examine the compositions carefully before the purchase of materials.

Further similar layout and tube metallurgy furnaces may differ in their elongation rate because furnace heat flux conditions from to another may vary widely. Firebox oxygen conditions are often the big contributor for differential expansions.

Thus furnace area maintenance is vital. Tube skin and wall temperature measurements have to be scheduled and preferably reviewed by operations regularly. Any burner/tip problems need to be attended immediately as this is one of the major contributors for bowing problems.

The radiant coil elongation of about 3 ~ 5 inches per year is not uncommon and have to be adjusted periodically. Specialized maintenance procedure can be developed to minimize bowing and furnace tube life can be enhanced.