Practcal Guid to ASME B31.3 by Glyn Woods & Roy B. Baguley 6

[cantiqa78]

To All Respected Foru Members,

I just read "Practical Guide to ASME B31.3" by Glyn Woods and Roy B. Baguley. In Chapter 3 especially in the section of Allowable Stress Range, it is mention that SA by Equation 1(a)
on ASME B31.3 as "SYSTEM" Allowable Stress and SA by Equation 1(b) as "COMPONENT" Allowable Stress.
So Can anyone here give an enlightment on why that Eq.1(a)called by "System" Allowable Stress and Eq 1(b) called by "Component" Allowable stress?
I usually check my Thermal Stress with Eq 1(b) first, and then if i want a conservative result i will use Eq 1 (a) depend on the job/project.
Thank you.

 

[JohnBreen]

Hello cantiqa78,

I think that you basically have the right idea. What Glynn Woods is alluding to is the fact that the allowable stress range “for the whole system” (without regard to SL at any location) will remain the same at every location (at every component) when applying equation (1a). However, if you calculate the magnitude of SL at various locations you will note that the SL stress will change from location to location on the piping system depending upon how well the system is supported (more supports will usually mean shorter spans and reduced magnitudes for SL). So if you use equation (1b) - which includes a calculation of SL - to calculate the allowable stress range you will usually calculate a higher allowable stress range AT SOME LOCATIONS (where the SL is lower). This latter equation is sometimes referred to as the “liberal” SA (although B31.3 DOES NOT refer to it as such).

Perhaps it would be of some benefit to review the meaning of the term “liberal allowable stress range”. First of all, this is not a term that is used by the ASME B31 Pressure Piping Codes. We sometimes use the term as a sort of “short-hand” for referring to the increase in thermal (displacement) allowable stress range that is allowed in B31.1 (paragraph 102.3.2(D)) and B31.3 (paragraph 302.3.5(d). In these paragraphs, the B31 Codes allow the positive difference between the calculated combined longitudinal stresses (due to weight (bending) and longitudinal pressure) and the maximum allowable stress, Sh, to be added to the term “0.25 Sh” (the second term) in the SA equation for calculating the maximum allowable stress range, SA. The B31 Codes say that it “MAY” be added into the “0.25 Sh” term, thereby making it optional. If the analyst elects to ignore the additional allowable stress range the decision (all other things being equal) would be judged “conservative”. If the analyst elects to employ this additional allowable stress range, the resulting allowable stress range is sometimes termed “liberal”. It is, of course, all semantics. The Code allows the additional allowable stress range for a very good reason.

The concept underlying this rule is very well described in the famous book by S.W. Spielvogle (Piping Stress Calculations Simplified, Fifth Edition, 1955). Spielvogle explains that the B31 rules intend for the analyst to be able to use the ENTIRE RANGE of stress from the material yield point at the operating (hot) temperature to the material yield point at the ambient (cold) temperature (less a factor of safety to address certain vagaries). Since Sh (neglecting the possibility of creep) is set at 2/3 Sy for both the hot and cold conditions, we can go to the Code Appendices "A" and find the "allowable stress" for Sh and the "allowable stress" for Sc for the material and calculate the hot yield stress as Sh*1.5 and calculate the cold yield stress as Sc*1.5. Taken together the total allowable stress range for the combined loadings of weight (bending), longitudinal pressure (tension) and thermal expansion (displacement) would be (1.5*Sc) + (1.5*Sh), or 1.5(Sc + Sh). This range of allowable stress has been reduced slightly to allow for the vagaries of material and for other "real world" inaccuracies. The Code philosophy would then permit the total allowable stress range (after the factor of safety is applied) for all the combined loading described above to be 1.25(Sc + Sh) (if, in this discussion, we neglect the stress range reduction factor ,”f”, for simplicity). However, since the Code uses 1.0Sh for the sustained loadings of weight and longitudinal pressure that must be subtracted form the “usable range” and this leaves 1.25*Sc + 0.25*Sh for the allowable thermal expansion (displacement) stress range SA. Because the Code intends for the entire strength of the material (from hot yield to cold yield) to be used (except for the “adjustment” made for material and other vagaries), it follows that the rule in the paragraphs cited above allows the analyst to "add back" the UNUSED (difference between calculated sustained longitudinal stresses and the allowable 1.0*Sh) portion thereby increasing the allowable thermal expansion (displacement) stress range by that UNUSED portion. You will recognize that the "unused" or “excess” sustained case allowable stress will vary across the system being analyzed and that the variation will directly reflect how well supported the system is (bending stresses will have the greater effect). This variation in “excess” sustained case allowable stress from node to node in the model will (when the “liberal” option is used) result in the allowable stress range, SA, being different at every node when the Code compliance report is viewed.

So, one might ask, why would an analyst opt to not use the “liberal” allowable stress range for comparison to calculated expansion (displacement) stress range? This is an engineering judgment. For example, if the sustained stresses were calculated as 80 or 90 percent of Sh and the system were operated in the material’s creep range, the designer might want to take the conservative decision to not use the “liberal” allowable stress range when evaluating thermal (displacement) stress ranges. Another example might be offered as a case when the system is in severe cyclic service (see B31.3 paragraph 300.2 for the definition) and the designer is looking for a longer fatigue life. Going the “conservative” route might also appeal to the designer (or owner) if the system would be operating within the pressure/temperature variations described in paragraph 302.2.4 in B31.3 or paragraph 102.2.4 in B31.1. If we have some degree of uncertainty, we employ an additional measure of conservatism. As the saying goes, “when in doubt, build it stout”.

Regards, John.

 

[JohnBreen]

..........and, I guess it helps to understand that the "allowable stress range" is the basis of comparison of (the maximum limit for) the calculated displacement stress range (a secondary stress). The total displacement stress range (e.g., the thermal expansion range from the minimum temperature, through the "installed temperature", up to the maximum temperature) is not a stress "at any steady state". The sustained stress due to weight (dead weight and live weight) and pressure (a primary stress) is a steady state stress. It sometimes surprises me to find how many people do not understand that steady state "thermal stresses" (i.e., at operating temperature), per se, have no allowable stress limit defined by B31 Codes.

The book on B31.3 by Woods and Baguley (CASTI Publishing) and the book on B31.3 by Dr. Becht (ASME Publications) do a great job in explaining it all.

Regards, John.

 

[cantiqa78]

Sir John B.,
Thanks for the valuable explanation that you give.i always like to read your explanation in many forum....including the discussion about Liberal Allowable Stress at COADE Forum...=P
Well yes now i can see why it call as "System" and "Component" Allowable Stress, I see when Sl is include in Thermal expansion check the Allowable Stress Range will in variation magnitude depend on your Sl in your Component/support, but when you use the conservative way (Eq 1(a)) the Allowable Stress Range will be the same for all the piping system.
and about the Limit of Stress at operating Temperature, maybe that's why you and others committee members put this Appendix P in B31.3...........
Thank you.............