Sara,
It really comes down to how your "design" temperature is defined. For many companies, the "design" temperature, which is used primarily for wall thickness calcs along with the "design" pressure is generally a padded number. Different companies have different criteria, but +50F above the maximum operating temperature is not uncommon.
In determining wall thickness for a pipe, that temperature generally doesn't have a big impact. But, for stress, particularly for rotating equipment, an extra 30-50F of padding can have a big effect.
The other thing to keep in mind for rotating equipment in particular - the allowables that we work with are based on deflection more so than stress. Long term exposure to excessive loadings causes unacceptable levels of shaft deflection, leading to early seal failures, leakage, and extra maintenance and down time. Short exposures, as you would see in an "emergency" condition are generally not going to damage the pump in the way we would expect in a pipe or pressure vessel where the exceeding the limits results in an actual over stress of the material.
So, while we very often need to provide additional flexibility in rotating equipment systems, we need to do so with a mind of why we are doing so and the consequences of making a system more flexible than it needs to be.
_________________________
Edward L. Klein
Pipe Stress Engineer
All the world is a Spring