Pressure pipe design - thermal stress range

[sbnz]

Hi,

What is the exact definition of "thermal stress range" in pressure piping codes B 31.1 or B 31.3? Does it mean:

a) the thermal stress range from "installation" temperature to either maximum or minimum operating temperature, Or

b) the thermal stress range from "minimum" to "maximum" operating temperature?

For example, take a typical case, as follows:
average or installation temperature = 20 deg C
minimum ambient temperature = -10 deg C
maximum operating temperature = 200 deg C

Is the thermal stress calculated for a stress range of 210 deg C (from minimum to maximum) or 180 deg C (from temperature of installation to maximum operating temperature in this case).

regards,

 

[DSB123]

sbnz,
Answer to your question is option b) (i.e. in your example 210 deg C temp range)

 

[C2it]

DSB123 is quite correct in defining the thermal stress range for your conditions. This is the cyclic limit the system can see in terms of fatigue.

However, for land based plant you would be quite justified in using installation to max operating (assuming it's a hot system) when defining thermal and operating loads on equipment and supports.

 

[JohnBreen]

........so in summation:

For use with the B31 Code for Pressure Piping:

The full thermal range is the coldest temperature that the piping will experience to the hottest temperature that the piping will experience. It would be the summation of the installation temperature to coldest temperature and the installation temperature to the hottest temperature. Keep in mind there might only be a few full cycles and many, many partial cycles and that is why the B31 Code provide an equation for calculating the equivalent total cycles (and this becomes the indicator for the "f" value in calculating the allowable stress range.

Also remember that the B31 Codes currently require the use of the materials modulus of elasticity at 70 degrees F when calculating the thermal STRESS range for these thermal (displacement) ranges.

When you calculate the loadings (forces and moments) on the equipment you would use the OPERATING temperature and the modulus of elasticity at the operating temperature. You may find that this makes a great difference in very hot systems and very cold systems.

You can see why it might be necessary to make quite a few loading cases in your analyses.

Regards, John

 

[BigInch]

The OP question wasn't clear in that regard, as the Minimum Operating Temperature[/] was left undefined. It specified only a minimum ambient temperature.

"Make everything as simple as possible, but not simpler." - Albert Einstein (1879-1955)
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