Summery:

Due to sudden drop in temperature through bottom dish end to skirt support  leads to un even thermal expansion which leads to the very high thermal stresses. Theses high stresses further leads to crack initiation at the critical location due to cyclic loads. To reduce the rate of drop in temperature at the dish end to skirt junction, Hot box slot is provided. Hot Box acts as air pocket in a high temperature skirt supported vessel. It helps to reduce the thermal gradient across the thickness as well as thermal expansion. The following analysis are performed to access the structural integrity of the hot box;

Steady state thermal analysis is performed at design temperature in order to find the temperature distribution. Furthermore, along with the temperature loading, maximum allowable working pressure and  operating weight are considered. The acceptance criterion is as per ASME Sec VIII, Div.2, Part.5, para 5.3.

Scope:

The purpose of the present study is to find an optimized skirt Hot-box junction geometry which will minimize thermal stresses and improve the structural integrity.

Methodology:

2-D modeling of axisymmetric structures is used with few assumptions, like a non-axisymmetric loading ( Examples of such loading are bending, shear, or torsion ). As support skirt is fixed by anchor bolt at base, proper restraint of structure is obtained by setting ux=0, uy=0 and uz=0 at base.

Evaluation for Protection against Local Failure:

According to ASME Sec VIII, Div.2, Part.5, para 5.3, protection against local failure has been checked using elastic stress analysis method.

Three loading conditions are considered to assess the structural integrity of the hot box.

ü  Case-1 : Maximum Allowable Working Pressure + Operating Weight

ü  Case-2: Maximum Allowable Working Pressure + Operating Weight + Wind load

ü  Case-3: Maximum Allowable Working Pressure + Operating Weight + Thermal expansion load

The algebraic sum of the three linearized primary principal stresses is used  as per design Criteria is based on ASME Sec.VIII, Div.2, Part.5.3

                                         S1+S2+S3 < 4Sa

Where;

Sa = Allowable stress based on the material of construction and design temperature.

S1,S2, and S3 are primary principal stresses

Primary principal stresses are calculated at each location.  The sum of the three linearized primary principal stresses are compared with allowable limit, and utilization factor is calculated. From the calculations, it is observed that utilization factor is less than the one. So, it is satisfying condition of the protection against Local Failure.