Sep-2024

Novel approach for assessing integrity of FCC unit cold wall riser with finite element analysis (RI 2024)

The fluid catalytic cracking (FCC) unit is considered the heart of any modern refinery.

M P Singh, Namrata Keshri, Vikas Sharma and Kaushalendra Kumar
IOCL Mathura Refinery India

Article Summary

The riser-reactor is one of the most important pieces of equipment in an FCC unit in a refinery. In most cases, the riser of the FCC unit is designed with a cold wall arrangement (i.e. internal refractory lining). A hot spot on the shell due to an internal anomaly in the refractory lining is seldom seen in the riser, especially with a refractory lining older than 8-10 years. (In this context, a ‘hot spot’ refers to a small area with high stresses, which can exceed allowable limits if not measured and controlled accordingly.)

In one refinery, the FCC unit riser had hot spots on its shell, which were a cause for concern for its operators and maintenance team. To predict the creep life of the riser-reactor section of the FCC unit with hot spots at identified regions for safe plant operation, a thermal displacement analysis of the riser-reactor section was carried out in the following cases:
- Thermal + pressure + DW (without cooling effect on hot spot region)
- Thermal + pressure + DW (with cooling effect on hot spot region)
The material properties of the shell material (SA 516 Gr 70) of the riser are in Table 1.

Modelling Strategy for finite element model
The riser-reactor section was modelled using 3D elements. A minimum of three layers of elements were maintained at the shell and nozzle. Welds were modelled in continuation of the geometry for full penetration and consistent material weld type.

Density has been modified to compensate for the mass of the refractory lining. The lifting trunnion and spring supports were also modified according to the site conditions and observations (Figure 1).

Sample finite element Model of Riser
Boundary conditions defined for analysis as:
• Constraint defined at the top end of the cold wall riser connected to the reactor stripper (Figure 2).
• Shell around the feed nozzle was fixed and constrained around lateral movement.
• Gravity load was applied in the vertical direction of the riser.
• For thermal load, a surface shell temperature of 250°C was taken into account.
• For the hot spot case, a surface film condition with a temperature of 400°C was applied on the inner surface of the shell at the specified location to achieve a hot spot temperature of 350°C (Figure 3).

Considering the assumptions taken during finite element (FE) analysis, a comparison of Von Mises stress values were compared in both cases with or without external cooling: υWith external cooling, the maximum Von Mises stress at the hot spot region was 141 MPa, which is less than the allowable stress of 236 MPa.

Without external cooling, the maximum Von Mises stress at the hot spot region without the cooling effect was 198.1 MPa, which is less than the allowable stress of 236 MPa (Figure 4).

In both cases, the stress values were found under the limit. However, considering the similar stress exposure, the creep life for the shell in Case A was observed to be three years. For Case B, where external cooling has been used to reduce the stress, the creep life of the shell has been shown to be about 15 years.

Conclusion
The subject case study is one method of analysing ‘hot spots’ through finite element analysis. This method can be chosen as one of the engineering solution for assessing high-stress regions in equipment being operated in hot spots condition. However, the adopted method has made certain assumptions, which may impact the result obtained through the study if altered or not chosen carefully. Hence, the method and study should be chosen carefully when analysing the problem statement.

This short article originally appeared in the 2024 Refining India Newspaper, which you can VIEW HERE