Seismic Analysis of Waste Heat Boiler and Steam Drum Assembly

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Our Client who is a provider of Engineering, Procurement and Construction Services primarily in the Sulphuric Acid Plant and Non-Ferrous Metal Process Industry was contracted to build a Waste Heat Boiler (WHB) in an earthquake-prone zone. Therefore, the main requirement of the design was to check if the WHB could endure the forces arising in an earthquake. The EPC contacted Shirsh Technosolutions to request for seismic analysis of WHB Boiler according to ASCE 7- 05.
The key points expected by the client in the analysis were as follows:-

1) Susceptibility of risers and saddles to seismic acceleration
2) Strength of weld joints at RF pads
3) Stresses at saddles and longitudinal displacement due to thermal expansion
4) Thermal stresses and strains in the steam drum shell

Figure 1. 3D model of Waste Heat Boiler Assembly

Shirsh Technosolutions developed a 3D Linear Elastic Model to check the response of the structure to seismic acceleration. This model was solved using ANSYS Mechanical finite element code. Fig 2 shows the meshed model. The seismic acceleration was considered for longitudinal and lateral cases. In addition to this, the operating pressure and temperatures of the working fluid were applied onto the WHB.

Figure 2. Mesh of Waste Heat Boiler Assembly

The numerical model for WHB consisted of following features:

• Risers and Down-comers
• Sliding Saddles
• Steam Drum
• Tube sheets and Baffles
• RF pads for Riser and Down-comers

The material properties for various components were assigned according to ASME BPVC Section II part D. Furthermore, the WHB assembly was analysed for the required load combinations as stated in ASME BPVC Section VIII DIV.2 Part 5 Table 5.3. Fig.3 shows one of the load cases analysed with all the required boundary conditions.

Figure 3. Boundary conditions for Waste Heat Boiler Assembly

Fig. 4 shows the equivalent stress plot for risers and down-comers for WHB assembly, with higher stresses near the weld regions.

Figure 4. Equivalent Stress Plot for risers and down-comers for Waste Heat Boiler Assembly

The post-processing of the model included the preparation of SCL (Stress Classification lines). The SCLs were constructed to decompose the complex Equivalent Von- Mises stress into its components of Membrane, Bending and Membrane + Bending stress.

Fig 5. Stress Classification line

The SCL made it easier to judge the pass/fail criteria for a particular component by directly comparing the induced stresses with stress limits given in ASME Section VIII Div 2 Part 5
Based on the simulation results for the WHB assembly, with detail analysis done for every component, an FEA report was generated substantiating the WHB assembly.

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