(206g) Design of Tray Columns for Emergency Blow-Down Situations

The process and hydraulic design of column internals is done at a very early stage of a project. At this time neither blow-down rates nor a pressure safety valve (PSV)-design is available. Therefore, blow-down cases are typically not considered in the tray design. This may lead to the situation that the forces on column internals, due to increased flow rate and pressure drop, are exceeding the mechanical design limits during blow-down.

To eliminate the risk of mechanical damage to the column internals during blow-down situations – which leads to repair costs and additional downtime – a detailed investigation of the blow-down situation is required. The flow rates passing through the column internals during blow-down can be multiples of the normal operating flow rates. Thus, the balance between an acceptable pressure drop at operating conditions, avoiding damage to the trays at blowdown, and an economic design is challenging.

Therefore, for specific applications, Linde Engineering performs fluid dynamic calculations for the blow-down situation of the plant or plant section. As a result, transient flow rates and physical properties during blow-down can be obtained. These are used to perform calculations for every column section to identify its criticality. To obtain transient pressure profiles, additional tray by tray calculations may be performed. Considering the tray and mass-transfer device geometry, the resulting forces on the equipment are calculated. Based on this, FEM calculations of the trays using the 3D CAD/FEM software SOLIDWORKS are performed. Results are the mechanical stress and resulting deformation of the tray segments, bolts and clamps.

The load on clamps and tray segments may cause damage to the parts directly or can cause that tray segments are torn out of their fixation. Every column internal being identified as mechanically overloaded will be further investigated. The design of the trays is adapted to cover blow-down situations without mechanical damage.

A crucial point in the often time-critical situations of a project (execution) is the combination of mechanical and hydraulic design tools and FEM software, which allows finding an optimal design within the available time frame.

The described workflow was successfully followed in the design of trays for a gas plant. A reinforced tray design for the blow-down situation was developed for several columns. The workflow and the obtained results are further discussed in the presented paper.