ASME B31.8 TABLE 841.1.18-1 Temperature Derating Factor

[NATIVES]

ASME B31.8 TABLE 841.1.18-1 has temperature derating factor for temperatures ranging from 250F to 450F with 50F increment.

GENERAL NOTE below the table says : For intermediate temperatures, interpolate for derating factor.

I noticed a graph of Temperature vs derating factor make a straight line. So why not select factors at 250F and at 450F (just 2 points) to interpolate for any temperature in between to determine derating factor? ?

Any reason why we have to iterpolate in between each 50F block?

 

[racookpe1978]

Errors are too likely when interpolating between extended ranges.

Sure, if you treat the interpolation arithmetic as "automatically correct all the time" the same result will always result, and no one will make a mistake selecting the proper material strength for his.her pressure vessel (today, and back in the slide rule error (er, era).) However, is it not smarter to force the selection for an error band into the smallest practical upper and lower limit? And, temperatures are NOT explicit and exact across the entire vessel! Selecting from a reasonably small band forces the designer to consider the ranges of temperatures fro inlet to outlet, at exposed nozzles and deep within the heated zone of a PV that is fed hot fluid one place, and venting colder fluid elsewhere.

We see already people who "design by the pretty computer picture". No red spots = good design, right?