Preventing Coke Drum Bulging and Cracking

Coke Drum Innovation

Chicago Bridge & Iron Company (CB&I) has fabricated, erected and repaired Coke Drums for over 50 years. CB&I is well aware of the problem of bulging and cracking in the vicinity of the circumferential weld seam experienced with many Coke Drums. These bulges and cracks are due to the severe operational thermal cycling often experienced in Coke Drums over the life of the vessel.

Example of Bulging

Background

For many years there have been ongoing studies to find ways to lengthen Coke Drum operating life. Thermal cycles occur during filling with hot (approx. 900 ° F) product and during water quenching after the coke has formed. As the surface of the fluid (product or water) rises, a severe moving thermal gradient occurs in the vessel shell as the fluid (hot or cold) is pumped into the vessel.

Thermal and stress analyses of this condition show that the higher strength weld metal in circumferential seams has a stiffening effect which increases stresses leading to distortion and cracking. The effect is similar to having a bellyband around the vessel. Vertical welds do not introduce the same problem since there is a continuity of metal properties in the vertical direction. In order to address this effect, some Coke Drum specifications have required that weld metal yield strength be within a close percentage of base metal yield (e.g. 10%). Because of the many variables involved, such as actual yield strength of base metals and weld metals, and variation of yield strength with temperature, this approach may not provide a practical solution.

Several other methods of minimizing this problem include:

• To profile (grind smooth) circumferential welds to reduce the stiffening and stress concentration effect.
• Use of more expensive grades of material with higher Chrome content.
• Increased vessel shell thickness to lessen the peak stresses effect.
• Increased duty cycles to stave off metal fatigue.

Each of these options has resulted in a significant increase in the installed cost of the Coke Drum with limited success to date.

CB&I’s Solution

CB&I’s solution to this problem is an idea which would eliminate the circumferential seams altogether in the area of concern. Orienting the shell plates with their long direction vertical accomplishes this, providing an increased (up to more than 40 feet) shell length without a girth seam. This section of the vessel can be located in the areas that experience the most severe thermal cycles.

One CB&I Plaza
2103 Research Forest Drive
The Woodlands TX, 77380
Phone:(832) 513-1636
Fax: (713) 466-3182
Attn. Coby Stewart
E-mail: cstewart@CBIepc.com

For more information, visit our website at: www.chicagobridge.com

Concept Development:

Development of the concept has been completed. There are no design or Code issues. CB&I knew clad plates over 40' long in the thickness ranges desired could be obtained. CB&I has the required manufacturing facilities and know-how to form these plates within the required tolerances. This has been confirmed by forming a test plate (40' x 8' x 1.5") in CB&I's Houston manufacturing facility. The results showed that the formed plate met the necessary tolerances to fabricate and erect the plates in this manner.

CB&I is prepared to provide Coke Drums built with this innovation. CB&I will:

• Provide fully assembled Coke Drums to your plant ready for installation.
• Provide field erected Coke Drums for logistically difficult locations.
• Install assembled Coke Drums.

Advantages:

There are two significant advantages in a Coke Drum built with this innovation

• Minimal circumferential seams reduce the opportunity for bulging and cracking.
• Replacement sections are easily subassembled to allow retrofitting of existing Coke Drums.

This innovation along with other Coke Drum innovations (i.e. such as skirt to shell attachment welds) will provide vessels with a longer operating life and lower repair costs.