In piping stress analysis, both the subject terms i.e, Sustained Stress and Expansion Stress are widely used. For all the piping systems which need analysis, the calculated sustained and expansion stresses have to be kept below code allowable stress. In the following tutorial, both of these stresses are clearly explained.
What is Sustained Stress?
Sustained stress is one of the primary stress caused by primary loads’ Weight (Pipe Weight, Insulation Weight, Fluid weight, etc) and Pressure (Internal and External Pressure). This stress is present in the piping system throughout the plant operating life. This means this stress will remain as long as the piping system survives and that’s why it is very important to calculate the piping sustained stresses and keep the stresses below code allowable values.
ASME B31 codes provide equations for sustained stress calculation and also provide allowable values to compare the calculated stress. The allowable stress value and equation for the calculation of sustained stress vary from one piping code to another.
What is Expansion Stress?
Expansion stress is displacement-driven secondary stress generated in the piping system due to temperature change or thermal differential from the installed to operating temperature.
Whenever the temperature of a piping system changes from one value to another, the system starts to move (expansion or contraction). If this free thermal movement is restricted then thermal stresses will be generated in the system.
It is obvious that Piping systems in process or power industries are connected to some equipment or other piping. This means free thermal movement is restricted which causes to increase the thermal stress.
All B31 codes provide an equation for calculating this expansion stress and then compare the calculated stress with allowable values.
Basics of Sustained and expansion Stress in Piping System
The following two video tutorials nicely explain the basics of sustained and expansion stress in a piping system
Video Tutorial 1:
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