Flue Gas Duct Systems and Supporting Structures

Project Overview

The scope of work included structural calculations of flue gas ducts and their supporting structures, preparation of a detailed 3D model and full workshop and erection documentation.
Two separate flue gas duct lines were designed, connecting two gas turbines to one chimney. Each duct line is to be executed in a separate construction phase.
The ducts are made of 10mm thick steel plates, stiffened in both longitudinal and transverse directions. Main stiffening members are HEB sections, while secondary members are IPE sections.
In the inlet zone and internal silencer zones, the duct cross-section is 10.4×8.4m, gradually changing to 6.4×6.4m in the transition zone.
Maintenance and inspection platforms were provided as part of the structure.
The inner surface of the duct is thermally insulated. Mineral wool insulation is protected from hot gases by a thin stainless-steel lining.
Additionally, an external acoustic enclosure was designed in the inlet and silencer zones. The project also included the model and drawings of the internal lining plates.

Design Conditions and Structural Analysis

The structure is located in a seismic area in Germany. The design was carried out according to Eurocodes, including additional internal requirements of the client.
Thermal loads, including temperature gradients, were considered. Due to high operating temperatures, an appropriate number of expansion compensators was required. The structure is supported on bearings allowing controlled movements where necessary.
Because of interaction between both duct lines (one duct line partially supported on the other), a global structural model was created, including both duct lines and their supporting structures between expansion joints.
The following analyses were performed:
– Linear buckling analysis (LBA)
– Nonlinear stability analysis with imperfections (GMNIA)
– Seismic response spectrum analysis for seismic load combinations

Main Design Challenges

– Interaction between both duct lines, including thermal and seismic effects
– Reduction of thermal forces using properly located expansion joints and suitable bearings
– Design of anchorages, especially into the existing foundation slab using chemical anchors with consideration of real reinforcement layout
– Design of connections between acoustic enclosure and duct walls and roof using vibration absorbers
– Analysis of the flexibility of layered acoustic insulation and its influence on the behaviour of the lining plates (3D solid FE model of the duct floor)
– Preparation of plate edges in the model and drawings for all butt welds
– The Tekla Structures model was built using a three-level assembly hierarchy. Nested assemblies were applied to improve clarity of workshop drawings and to clearly present welding sequences for individual fabrication stages.
– Structural tightness was ensured by bolted connections between transport units with fire-resistant silicone sealing.
– The duct was divided into transport units to enable transport and ensure proper structural behaviour.
– Despite the use of expansion joints, additional structural solutions were required to accommodate fabrication tolerances and high-temperature effects.
– The internal stainless-steel lining required high precision and a strictly defined installation sequence to ensure reliable protection of the mineral wool insulation.

Structural Details

The duct is made of 10 mm thick steel plates, orthogonally stiffened with hot-rolled sections: IPE, HEB, and UNP profiles at assembly joints.
Due to multiple structures supported on the duct, local reinforcements were required.
Main stiffeners are HEB frames running around the duct cross-section. Point supports are located on these frames.
All supports are realized using elastomeric bearings to idealize the static system and reduce vibrations.
These special bearings are either directly anchored into foundations or supported on steel support structures over 13m high.
In the inlet zone, the duct is enclosed by acoustic walls and roof. In the remaining part, the roof is made of aluminium trapezoidal sheeting.

Scope of METIB

– 3D structural model in Tekla Structures with geometric verification
– Structural calculations in RFEM
– Connection design in IDEA StatiCa
– Anchor design considering reinforcement layout in IDEA Detail
– FE analyses of thermal insulation flexibility and its influence on lining plate capacity (duct floor)
– Complete workshop documentation with numerical files
– Erection documentation
– Welding details
– Export, sharing, and synchronization of the 3D model on Trimble Connect for ongoing multidisciplinary coordination

Summary

The METIB team successfully met the project challenges. The use of advanced software, a well-defined work methodology and the team’s experience enabled successful project delivery.
The client’s satisfaction and willingness to continue cooperation on future projects confirm the high quality of the engineering work and trust in the design team.