Half the Peak Stress? 1

[StressGuy]

I'm reading a statement and I'm having trouble wrapping my head around it. I'm reading Dr. Charles Becht's "Process Piping" book, 2nd edition. There's an insert 7.1 (it looks like it was also insert 7.1 in the first edition).

In the insert, which is about vibration, there is a line that "...Code flexibility analysis equations calculate about one-half of the actual peak stress..."

I'm having trouble understanding what is meant by this phrase. Can someone enlighten me or point me to another reference?

Edward L. Klein
Pipe Stress Engineer
Houston, Texas

"All the world is a Spring"

All opinions expressed here are my own and not my company's.

 

[LSThill]

Mr. Edward L. Klein, Pipe Stress Engineer, Houston, Texas,
Contact the author: Dr. Charles Becht's IV at Becht Engineering Co. ASME B31.3 PROCESS PIPING SECTION COMMITTEE,

http://www.bechtengineering.com

Leonard@thill.biz

http://www.thill.biz

 

[hacksaw]

two aspects of this are that he might be referring to the peak shear stresses as opposed to the principle stresses. you have to sort that out from the context or from the author directly. the second issue is that B31 focuses on the controlling stress rather than some combination of all of the stresses, so it is possible that stress per the code only accounts for half the actual stresses. the nuclear codes on the other hand work with the combined stresses. the key is that the code only defines the minimum requirements.

 

[JohnBreen]

Hi Ed,

What Dr, Becht is referring to is the use of B31 Stress Intensification Factors (SIF's). As you know, the SIF's were based upon the fatigue testing done at Tube Turns in the early '50s. The SIF's reflect the ratio of the number of cycles-to-failure for the pipe/fitting weldment as compared to a parallel test (at the same time on the same machine) of a piece of straight pipe with a butt weld. But the straight pipe weldment really has a stress intensification of somewhere North of 1.9 so when used as a basis of comparison the pipe/fitting weldment should be multiplied by about two.

If your compare the stress intensifiers used in Section III of the B&PV Code (NC-3600) you will notice that the comparable factors are about twice those used in the B31 Codes. So, as far as calculated elastic stresses are concerned, the B31 secondary stresses are about one half of the actual peak elastic stresses.

Best Regards, John.

 

[bvi]

The reason is described in section 8.4 for that book.

 

[StressGuy]

I've read section 8.4 and I must admit, as I read more, I find my knowledge level shrinking. Why are we only calculating a value that his half of the true stress in the system, particularly if other codes like Section III seem to have SIF's to calculate the full stress.

I mean, I've used the liberal stress allowable on rare occasions for piping systems and obtained stresses in the upper 30,000 to lower 40,000 psi ranges. As I read these references, the actual stress is therefore pushing 80,000 psi, which is above the tensile strength for a common material like A106B. Not to mention being in excess of twice yield to insure elastic shakedown. Yet Caesar says the stresses are within code.

So, how can I judge a system adequate if the tools I'm using aren't giving me the "correct" answer?



Edward L. Klein
Pipe Stress Engineer
Houston, Texas

"All the world is a Spring"

All opinions expressed here are my own and not my company's.

 

[JohnBreen]

Hello all,

This thread has now grown to address several concepts:

Sustained load bending stress

Secondary (displacement) bending stress range

So-called "liberal" allowable bending stress range

Well, take a deep breath as I am not know for my economy of words.

Perhaps it would be of some benefit to begin with a review of 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. Caesar II (C2) uses 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 equation for calculating the maximum allowable stress range, SA. The B31 Codes say that it “MAY” be added into the “0.25Sh” 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 elect 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). Since Sh (neglecting the possibility of creep) is set at 2/3 Sy for both the hot and cold conditions, we can calculate the hot yield stress as Sh*1.5 and we can 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). But the Code uses 1.0Sh for the sustained loadings of weight and longitudinal pressure and this leaves 1.25*Sc + 0.25*Sh for the allowable thermal expansion (displacement) stress range (bet you wondered where that came from). 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 vagaries), it follows that the rule in the paragraphs cited above allows the analyst to put the unused (difference between calculated sustained longitudinal stresses and the allowable 1.0*Sh) portion to use in increasing the allowable thermal expansion (displacement) stress range. You will recognize that the “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 judgement. 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 that 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”.

The B31 Codes always compare the primary stresses that result from "sustained" (longitudinal pressure and bending due to weight) to an allowable magnitude from Appendices "A". No "range" is involved in this "allowable stress". The B31 Codes must be used in context with their allowable stresses ans stress ranges - i.e. use the B31 stress (stress range) calculations WITH the appropriate allowable stress (stress range) from the same specific B31 document. This will assure safe and reliable design. Never use the "allowables" from some other ASME Code with your B31 analysis.

Regarding secondary stress range calculations (i.e. expansion stress range or displacement stress range), it should be remembered that these calculations address fatigue as the major failure mechanism. Stress Intensification Factors for various fittings came from cyclic (fatigue) testing done at Tube Turns in the early 1950's (why we use SIF;s with primary (sustained) stress calculations puzzles me). The "liberal" allowables are only intended for these secondary stress range calculations where the entire "range" (as discussed above) is involved.

Of course, all the above is just my opinion and does not reflect the opinion of ASME or any Code Committee.

Best regards, John.

 

[StressGuy]

John,

I have become aware of most of what you had said in your post (actually, I had picked up much of it from you earlier here and at the Coade forum). Actually, this is part of what has lead to my confusion.

I'm still having trouble reconciling the stress range that has a basis in +/- yield for secondary stresses, yet at the same time, the calculations are only giving me about half the stress that's actually there.

Should I be content that these actual peak stresses fall within a "range" of +/- tensile strength?

Edward L. Klein
Pipe Stress Engineer
Houston, Texas

"All the world is a Spring"

All opinions expressed here are my own and not my company's.