Stress Analysis for an LPG tank load line
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- 3 April 2018
- by ludovico
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For an important customer in Northern Europe, we have performed stress analysis on different circuits concerning an LPG storage tank, whose particular load line is shown on picture below.
On this circuit there are many particularities to take into account: first, usual for tanks, is to consider an imposed settlement of the tank (fig.2) due to sinking of foundations. This phenomenon was discussed with the manufacturer for a better understanding of the data. Indeed, it is not always clear how much this settlement leads to a permanent deformation and how much it leads to an elastic deformation proportional to the load on the tank.
Second aspect to examine was the behavior of the tank under operating temperature (-30°C), that leads all the structure and supports on the roof to follow the thermal contraction of the tank itself (fig.3).
A further investigation was performed to manage stresses due to vibrations. A liquid flash from the flow control valve to the nozzle could occur, therefore, we proceeded in compliance with the Energy Institute Guidelines for the avoidance of vibration induced fatigue failure in process pipework.
Critical was the design of the line, because we have aimed to the maximum rigidity of the circuit, without compromising the thermal flexibility. Supports have been designed to be feasible. It was not this case, but in the presence of process analysis with complex softwares able to check the type of the operating flux, further consideration may be discussed on the mechanics of the tube.
Since an unreinforced pipe-to-pipe connection with high SIF (Stress Intensification Factor) has been indicated on the line specification, very thin thicknesses proved to be a further complication. SIFs tends to become too high if the thickness of the header is thin. When SIFs reach values close to 10, the design becomes more critical and delicate. Therefore, joints become matter for discussion and deeper analysis.
The client has requested explicitly Intergraph Caesar II as a software to deal with, in compliance with EN13480 code, but we can provide advice with other software and codes too.