Design Conditions for Stress Analysis 2

[shaff]

Hello,

I am having confusion with respect to temperatures which I need to select for a pipe stress analysis. Do I have to consider the Minimum Design Temperature (zero DegC) for stress analysis or is it enough to take only installation temperature (21degC).

My temperatures conditions are

T1: 84 DegC (max design temp)
T2: 0 DegC (minimum Design temp)
T3: 52 DegC (operating temp)

or the below conditions are ok?

T1: 84 DegC (max design temp)
T2: 21 DegC (Installation temp)
T3: 52 DegC (operating temp)

When I change temperature T2 from 0 to 21 Deg, expansion case stress ratio reduces drastically (from 92% to 7%).

My opinion is that minimum design temperature is for selecting the material so that it doesn't experience the brittle fracture. And upto 21 DegC there is no thermal expansion so we do not need to consider the 0 (zero) DegC for stress analysis.

Thank You

 

[TGS4]

You need to consider the full range of temperatures. The thermal expansion between 0C and 21C is just as physical as the expansion from between 21C and your maximum temperature.

 

[XL83NL]

If your system can get colder than 21 deg C, even when not operation, it may generate stresses caused by thermal displacement. As these stresses are secondary type stresses in B31.3 (assuming youre working under B31.3), they need to be considered as TGS4 rightfully pointed out. Most stress analysis programs have an input for the installation temperature and auto-consider this for evaluating thermal stress ranges. Its typically called Tamb, or similar.

 

[pennpiper]

Just my opinion:
- Design Temperature if a fictitious number and should only be used for pipe wall calculations.

- The Maximum/Minimum Operating Temperature is a real temperature and is the proper factor to use for Thermal expansion/contraction calculations for Piping systems.

If you use the Design Temperature then you are 'over designing' the system resulting in added cost.


Sometimes its possible to do all the right things and still get bad results

 

[BigInch]

The installation temperature is the temperature you consider as the point of zero thermal stress. You must examine thermal expansion or contraction stresses created by excursions from the installation temperature. So from 21C to 0C tension develops in a restrained pipe as the pipe contracts and the expansion case is from 21C to 84C where compression is developed as the result of restrained expansion.