Is stress range calculated correctly?

Most pipe stress software calculates stress-range from the “Installed-position” to the “first-hot position”.

Is the stress difference between the “installed-position” and “first-hot position” equal to the stress-range every for system – even if the software says that it is?

Pipe stress theory and piping codes require that stress range is calculated for a full cycle of movement. A cycle of movement is from cold-to-hot and from hot-to-cold.

By definition, a cycle of pipe movement requires that the cool-down half-cycle is equal and opposite to the warm-up half cycle.

Pipe stress software which calculates the stress-range between “installed-position” to “first-hot position” makes an implicit and fundamental assumption. i.e.

The cool-down cycle is assumed to be a perfect mirror image of the first warm-up cycle.

A fundamental implication is that the pipe must return precisely to the installed-position on which the calculation is based.

If pipe support friction prevents the pipe returning to its installed-position, then the warm-up movement and cool-down movement are different. The stress differences for warm-up and cool-down are therefore different. If so, the combined cold – hot – cold movements are not a cycle.

If the stress differences between warm-up and cool-down are different, the warm-up stress difference is not a stress range. The calculation is then invalid.

Most pipe stress software will not report an over stress or indicate that the results are invalid!

As I was told by a well known software supplier many years ago, the software is based on the assumption that all users are experts.

Read more here. Is Stress Range Calculated Correctly?

Disclaimer. The above is my personal opinion. No warranty is implied.

A Weighell May 2021