QUESTION ABOUT CAESAR II 2

[juliocesarp]

Hi, my name is Julio Cesar and I'm from Colombia (south America). I'm working with Caesar II and my question is about the static output analysis. I want to know why the operational cases aren´t used for make the code stress check( code stress verification) and the sustained and expansion cases are used for code stress verification.

Thanks for your time...

 

[drywall]

Juliocesarp,

If you perform an analysis under ASME B31.3 you will find that the operating case does not have a code stress check. Nevertheless, it is useful for restraint loads and displacement. It is better that you directly contact COADE. query@coade.com

 

[juliocesarp]

Thanks, that is the reason. I have another question about the the Code; if the code doesn´t have an allowable stress for the operational case, what can I do to check if in the ope case I have an stress value higuer than in the sus and exp cases?

 

[drywall]

Keep in mind that the program uses different formulas for the different cases and that sometimes the calculations for the operating case will result in higher values than for the other cases. When this happens it doesn't mean it is a catastrophe; it may be all right. These values are important to consider but, as you have said, CAESAR II is using the sustained and the expansion cases for the Code checks. You will need to look at the loads on your anchors, limit stops, etc. In my opinion, no one will make the call on that without looking at everything.

 

[juliocesarp]

Thanks a lot, I'm starting in this field and I´m learning the basics of pipe stress analysis.

So, thanks for your cooperation.

 

[pjdobson]

juliocesarp:

The formulas that Caesar II uses comes straight from the Code book. The B31.1 and B31.3 both have equations to calculate stresses for three different conditions: 1) Dead weight (sus); 2) Expansion (operating); and 3) Occasional. It has been awhile since I have used Caesar II, but if I remember correctly, Caesar II should automatically develop the sus and exp cases for you. The exp case is the operating case, and the most important thing is that the exp stress that Caesar II calculates for your particular piping system is less than the Code allowable. drywall is correct in that you need to use the loads to evaluate your supports/anchors. The Caesar II manual does a good job of explaining what the different conditions are and what the program checks for.

 

[juliocesarp]

I really appreciate your help...

Thanks

 

[drywall]

Well, I really think juliocesarp could better benefit from addressing the question to COADE. However, let me give you my thoughts:

CASE 1 (OPE)W+T1+P
CASE 2 (OPE)W+T1+P
CASE 3 (SUS)W+P1
CASE 4 (EXP)L4=L1-L3
CASE 5 (EXP)L5=L2-L3

Operating Checks come from B31.8 and while they are not Code checks for B31.3 they are for B31.8. But the information is still useful for B31.3.

 

[richay]

Juliocesarp,

You should search the CAESAR II Forum on the COADE website for this subject. There is quite a bit of information on this topic.

Basically the B31.1/.3 Codes evaluate two stress states "primary" and "secondary". Primary stresses are produced by "force based" loads that lead to collapse. Secondary stresses are produced by displacment strains, and while somewhat self-limiting, can lead to (fatigue) failure. The Code philosophy is that if the primary (Sustained) and secondary (Expansion) stresses are below their respective allowables, then the piping system is acceptable from a "stress point" of view. This is why these Codes don't evaluate the operating condition, from a "stress" point of view.

pjdobson - I disagree with the statement that "exp" is "operating". The Expansion case is a "range" between two other states (load cases).

Richard Ay
COADE, Inc.

 

[drywall]

My apologies for my abominable mistakes above:

CASE 1 (OPE)W+T1+P1
CASE 2 (OPE)W+T2+P1
CASE 3 (SUS)W+P1
CASE 4 (EXP)L4=L1-L3
CASE 5 (EXP)L5=L2-L3

This is an example from a recent CAESAR II printout for B31.8 where Operating is a Code check and CAESAR II will also do it for B31.3. Try it!

 

[doberdorks]

BTW, of course, depending on your temperatures, the default Caesar II load cases may not encompass the full range of thermal stresses. ..

-SLH

 

[JohnBreen]

Hello Julio Cesar,

As you are a "beginner" I would recommend that you study the books discussed in this thread:

thread378-189629

The software follows the requirements of the B31 Codes faithfully. You really must understand what it is that the B31 Codes require and it is important that you understand WHY the Codes include the equations that they provide for calculating primary and secondary stresses.

The B31 Codes have equations for minimum required wall thickness and these equations control the circumferential (aka, "hoop stress") stresses (these are primary stresses). The sustained LONGITUDINAL stresses due to weight (dead weight and live weight) are added to the sustained LONGITUDINAL stresses due to internal pressure (these are primary stresses) and the sum of these is compared to a Code maximum allowable stress for this loading case. The bending stresses due to thermal expansion / contraction are evaluated using the "stress range" concept. The SUM of the pipe stresses due to the temperature excursion from the installed temperature to the MINIMUM temperature and the temperature excursion from the installed temperature to the MAXIMUM temperature is compared to the Code maximum allowable stress RANGE (this stress range is a secondary stress). As you can see, there the B31 Code requires you to consider these stresses SEPARATELY and the Code provides a limit for each.

As you can see from the loading cases suggested in the postings above, the software calculates the T+W+P (operating) case (primary and secondary stresses) and then it calculates the P+W case (primary stresses) and then it subtracts the primary stress case (W+P) from the operating case (T+W+P) to arrive at the thermal excursion case (T - secondary stress). The Code provides a limit for the W+P case and it provides a limit for the T case.

It is important that the piping engineer understand why the Codes require the secondary stresses to be considered separately from the primary stresses. It is also important that the piping engineer understand how and why stress intensification factors are used in the Code stress equations. It is important that the piping engineer understand the concept of thermal stress range and why the Code equation for allowable stress range is formulated the way that it is. For all these reasons it is important that the beginning piping engineer reads the Codes completely and reads the suggested books on the topic of piping design and stress analysis.

Good luck in your career.

Regards, John.