lowered allowable stress value

[twentythree]

Hi All,

I'm new in HEX design and would like to get some advice.
I was reviewing a heat exchanger and notice that a modified/lowered allowable stress figure was used in the minimum thickness calculation for some of the components.
It is for a shell & tube heat exchanger.
Example:
Component: Tube
Tube side: cooling water
Shell side: liquid ammonia
Design pressure: 9.8 Barg / 20.0 Barg
Design temperature: 100 degC
Material of construction: A179
Design code: ASME BPVC Sec VIII, Div 1, TEMA "R".
And it's designed in 1980, so the allowable stress at the time should read 830 kgf/cm2, but instead 664.0 kgf/cm2 was used instead.
Appreciate if someone could advice why this was probably done?

Thanks.

 

[Bobfromoh]

Dividing 664 by 830 equals 0.8
My guess is that there is a footnote somewhere that says the tube stress
to use in the calcs must be 80% of the listed allowable stress. I have
seen notes like this but you'll have to check your documentation?

 

[twentythree]

Hi,
there is such a footnote mentioning to use 0.8 of the allowable stress, but did not elaborate further on the basis. My guess was could it be as a criteria to exempt for impact testing?
Appreciate if someone could shed some light.