Another B31.3 Pipe Stress Analysis Question
Another B31.3 Pipe Stress Analysis Question
(OP)
I am a Mechanical Engineering PE in the state of Florida with mostly Industrial Ammonia Refrigeration experience. I have recently had the opportunity to branch out into some other aspects of the pressure piping design world, specifically process piping, and started looking around at various sources of information. I have B31.3 2006 and initially began my search based on the required flexibility analysis.
From other threads on the site, as well as some other sites on the Web, it became clear that simply having a pipe stress analysis package like Bently was a matter of "garbage in, garbage out", and unless one really understands what's going on, it's best to leave this to the professionals (meaning actual pipe stress engineers, not any old yahoo with a Bently or CAESAR II license).
However, you have to start somewhere, so I purchased copies of Rip Weaver's "Piper's Pocket Handbook" and both volumes of "Process Piping Design". They introduced me to my first pass/fail method of culling through piping arrangements for those that definitely have adequate flexibility and those that may need some analysis, an important thing when you could save thousands by honing in on only the items that need analysis.
That's it for the long preamble. My question is this: both books I mentioned have essentially the same exact text on flexibility and minimum leg lengths for the "L" method and its analogs. The chapters also mention guidelines for reaction forces/limits for various equipment types. I am having trouble interpreting these two portions of the text. It would seem to me that piping flexibility is a separate issue from reaction forces at equipment and/or anchor points. My interpretation of the "L" method in the books is that, should you come up with an answer within the criteria, the PIPING is adequately flexible and stresses are pretty-much garanteed gelow allowable from a thermal expansion point of view. However, what does this have to do, if anything, with acceptable reaction forces at anchor points and equipment connections? The text doesn't seem to address how to find reactions, and I must be missing something, but I don't know what it is. Can someone with more experience and familiarity with Mr. Weaver's work please shed some light on this? Am I even approaching this method correclty, i.e. can you provide a better expanation of what "adequate flexibility" means with respect to pipe stress added by the pipe flexing?
Incidentally, as a side question, when he talks about anchoring pipe, what type of support would this be? Is it permitted to weld the pipe itself to a support (I guess if the pipe is not penetrated, it might be OK, at least in Normal Fluid service in B31.3)?
Sorry for the long question, I wanted to give sufficient background for a targeted answer(s). For those who are curious, yes I do realize that the Refrigeration Piping Code, B31.5, requires adequate flexibility as well, but I myself have never performed one of these analyses, and they are uncommon in that industry unless dealing with especially low temperatures or suitability of certain, non-impact-tested materials for lower-temperature service.
From other threads on the site, as well as some other sites on the Web, it became clear that simply having a pipe stress analysis package like Bently was a matter of "garbage in, garbage out", and unless one really understands what's going on, it's best to leave this to the professionals (meaning actual pipe stress engineers, not any old yahoo with a Bently or CAESAR II license).
However, you have to start somewhere, so I purchased copies of Rip Weaver's "Piper's Pocket Handbook" and both volumes of "Process Piping Design". They introduced me to my first pass/fail method of culling through piping arrangements for those that definitely have adequate flexibility and those that may need some analysis, an important thing when you could save thousands by honing in on only the items that need analysis.
That's it for the long preamble. My question is this: both books I mentioned have essentially the same exact text on flexibility and minimum leg lengths for the "L" method and its analogs. The chapters also mention guidelines for reaction forces/limits for various equipment types. I am having trouble interpreting these two portions of the text. It would seem to me that piping flexibility is a separate issue from reaction forces at equipment and/or anchor points. My interpretation of the "L" method in the books is that, should you come up with an answer within the criteria, the PIPING is adequately flexible and stresses are pretty-much garanteed gelow allowable from a thermal expansion point of view. However, what does this have to do, if anything, with acceptable reaction forces at anchor points and equipment connections? The text doesn't seem to address how to find reactions, and I must be missing something, but I don't know what it is. Can someone with more experience and familiarity with Mr. Weaver's work please shed some light on this? Am I even approaching this method correclty, i.e. can you provide a better expanation of what "adequate flexibility" means with respect to pipe stress added by the pipe flexing?
Incidentally, as a side question, when he talks about anchoring pipe, what type of support would this be? Is it permitted to weld the pipe itself to a support (I guess if the pipe is not penetrated, it might be OK, at least in Normal Fluid service in B31.3)?
Sorry for the long question, I wanted to give sufficient background for a targeted answer(s). For those who are curious, yes I do realize that the Refrigeration Piping Code, B31.5, requires adequate flexibility as well, but I myself have never performed one of these analyses, and they are uncommon in that industry unless dealing with especially low temperatures or suitability of certain, non-impact-tested materials for lower-temperature service.





RE: Another B31.3 Pipe Stress Analysis Question
Another excellent book that I recommend is Peng & Peng's Pipe Stress Engineering from ASME presses.
High thermal stresses = lack of flexibility.
Equipment loading from a THERMAL perspective is directly related to pipe flexibility.
Imagine two anchors 100 feet apart north south and only 2 feet part east/west and in hot service.
Flexibility: If you pipe 50', elbow 2', elbow 50' the stress will be high. The system is very rigid. All the expansion has to go into that 2' perpendicular leg. Make the routing 25', elbow 10 feet, elbow 50 feet, elbow 8 feet, elbow 25'. Now you have 18' (10+8) of perpendicular leg to take the growth and you have a much better chance of passing thermal stress range.
For anchor loads just think of a cantilever beam, 2' long or 18' long. To take up 4" of growth, the free end of the beam has to move 4". Does it take more force to deflect the 2' long piece or the 18' long piece?
- Steve Perry
RE: Another B31.3 Pipe Stress Analysis Question
Thanks for the very helpful reply. It certainly makes sense to put it in terms of a moment arm being longer to reduce a force on the fixed support, whether it is equipment or other. I would still love some clarity on what those reactions are or how to calculate them, as I can see many instances where this would be useful (i.e. pipe is anchored to roof stand that is anchored to bar joist. Am I going to distort joist when the system starts up?)
Please indulge me a little further if you will. As you explained it, the basic concept is very clear. However, it raises some other questions. I imagine that an unrestrained pipe has zero (or very low, since I guess there is no truly unrestrained piping when considering friction, connections, etc.) stress due to thermal expansion. Therefore, an adequately flexible system would not introduce additional thermal stresses that, in simple terms, would push the pipe stress anywhere in the pipe above allowable. Building on that premise, depending on the other factors (pressure, static loads), how do we know that a little bit is not too much? If the static and pressure loads put the stress right on the edge of allowable, then no matter how flexible the system is, thermal stresses will put you over the top, right?
Where I'm going with this is that it sounds like whatever you do, designing an adequately-flexible system according to simple rules, placing supports in the right spot, etc. in no way gaurantees your stresses will be below allowable. In other words, if you're not sure you have plenty of additional wall thickness, flexible or no, your theoretical stress may be too high. Is that a fair assessment? If so, then what is reasonable? If I follow that logic, a flexibility analysis, while necessary, is not anywhere near sufficient, if you get my meaning, and without gettign too crazy, once complete with the pass/fail method of flexibility analysis, you're still at a loss as to how much is too much thermal stress without knowing your other stresses. (I say "without getting too crazy" because I understand that for low pressures (300 psig, say), most of the time sch 40 pipe has several times the necessary wall thickness so you're probably good). Can you speak to the practical side of this?
If anyone can give guidance on calculating reactions as well, I would appreciate it. Please feel free to respond to any and all of my questions because I am the type who benefits from a broad variety of perspectives and knowledge.
P.S., Steve, I just bought Peng and Peng's book, as well as the MW Kellogg Design of Piping Systems. Pretty reasonable on Amazon. Thanks again. I look forward to perusing both when they arrive next week.
RE: Another B31.3 Pipe Stress Analysis Question
The first 10 rules of pipe stress,
Rule #1
Adding a restraint increases stress and forces, but with the advantage of reducing movements. Therefore never add a restraint, if you can deal with the expected movement. A free pipe will expand with temperature without creating stress. A restrained pipe will not expand, but create great stress.
You get a lot more flexibility with beam bending deflection, rather than axial elongation or contraction. Always try to provide as much beam bending as possible. Unfortuanately providing beams for bending is contrary to the ideal pipeline practice, which is making straight lines from point A to point B. Straight line pipe develops high axial forces that are difficult to restrain. If you throw in a few 90s, axial forces reduce considerably, but its true bending stress goes up. The ideal practice is to balance the two, using a pipes cross_sectional area to full advantage for axial stress and its section modulus to full advantage to carry bending stress such that the combined stresses are less than allowables. Then getting the beam deflections to occur in areas that are not critical to movement; ie far away from equipment. Increasing bending stress, reduces those powerful axial forces.
Unfortunately attaching pipe to equipment always seems to be a requirement. Some of that equipment acts like anchors, because they are extremely rigid in relation to long skinney pipe. As a consequence the equipment does not allow movement at the connection and therefore must take the loads caused from completely restraining the pipe.
Some lighter equipment, especially pumps and compressors, are very sensitive to movement, because they easily distort under relatively light loads resulting in shaft misalignments and extreme bearing wear. They don't provide much of an anchor thereby allowing relatively large pipe movements, so it is extremely critical to have very flexible pipe configurations in the vicinity of these types of equipment, so the pump will move the pipe by providing only a slight anchor load, rather than the pipe moving the pump straight into the motor.
Rules 2 to 10,
Read Rule #1 nine more times.
While most anchors and guides and equipment flanges are typically considered to "full anchors", they can have some degree of flexibility, if the equipment is relatively weak, as a pump vs. a fat heat exchanger, so a "full anchor" at a pump will not be nearly so full at a pump and you need to recognize a full anchor at a heat exchanger may provide 20,000 lbs worth of force, while the same full anchor of the same piping configuration provided by a pump flange may only provide 150 lbs, because actually the pump flange will deflect, twist and distort the pump, so its important to recognize the difference of the degree of anchor that whatever is on the other side of the pipe flange can actually provide and actually model that equipment as much as you need to to get a good representation of how rigid that anchor point really is. The software today will let you do that. It wasn't so easy back in the old days where you had a choice of anchor or no anchor.
Some codes allow welding supports to pipe, some don't. Usually its not a good idea and it should be avoided when possible. Pipeline codes do not allow welding directly to pipe. We like flexibility and not welding to pipe is a good way to get some. For example, we would prefer to use a butt stop up of a pipe with a full encirclement wear plate pushing against a chunk of concrete, rather than welding on a wear plate, then welding the plate to the support. Butt directions are chosen to direct movements away from sensitive equipment.
Hope that gave you some ideas about the difference between "anchor-anchors" and "equipment-anchors". "Anchor anchors" from supports, or specially designed concrete anchor blocks are considered to be much more rigid than the anchors provided by equipment flanges.
I always tell people wanting to really understand pipe stress to get a length of stiff copper wire and keep it on your desk. Bend it into a small-scale shape of your pipe configuration. You'd be surprized how pushing the ends together and pulling them apart and watching how it bends can give you a good understanding of how axial force and bending deflection can work together to reduce combined stress. In fact, I've heard rumors that is how they used to do pipe stress back in the really, really old days.
"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
http://www.youtube.com/watch?v=hpiIWMWWVco
"Being GREEN isn't easy." Kermit
http://virtualpipeline.spaces.liv
RE: Another B31.3 Pipe Stress Analysis Question
Not wishing to push their product, good though it is, Coade who own Caesar II hold very good seminars over 4 or 5 days, which give an excellent grounding to stress in petrochem generally. their web site is www.coade.com.
RE: Another B31.3 Pipe Stress Analysis Question
This challenge will be exacerbated by people (typically Clients) who will tell you "...The layout is what it is. There is no room to make the changes you want to make. Your calculation must be wrong. Stress engineers are too conservative. We have done this before a hundred times and nothing has blown up. Come up with something that looks like this...".
Agree on Peng and Kellogg.
Regards,
SNORGY.
RE: Another B31.3 Pipe Stress Analysis Question
I was hoping someone could address one of my more fundamental questions a little more directly. I appreciate the responses on reaction forces, and can see that it's just going to take reading and understanding to ge the hang of those.
I would love to get a more thorough answer to the question I pose above, basically, what good is a pass/fail method for determining which pipes need full stress analysis if you have no information that your pipe wall is conservative with respect to your other stresses (static loads, pressure, and wind/seismic if you have to get into that)? Is Rip Weaver giving us anything of value with his published method? If so, exactly what is it (and what are its limitations)?
Thank you all again.
RE: Another B31.3 Pipe Stress Analysis Question
Consider first of all that for most low pressure low temperature piping it's the conformance to previous successful installations that will suffice to justify them.
Concerning the reactions, the only possible method with a simplified approach is the cantilever beam model suggested by StevenHPerry. You have an expansion in the long leg and this is taken in bending by the short leg (and of course this only works for simple 'L' configurations). Elbow flexibility however may add a lot to it, and it should not be difficult to account for it, but I don't know of an accepted method to do so. Also, piping handbooks should have tables to calculate the reaction forces for different typical configurations (I have the Crocker & King, and it does).
Well, that's not really right. Thermal stresses are a very different beast from the stresses due to mechanical loading. The proof is that increasing the thickness will generally increase the thermal stress and possibly also increase the total stress.Thermal and mechanical stresses should be kept separate from an allowable stress perspective, and indeed, if I recall correctly, B31.3 requires a separate check, where the allowable for expansion is reduced as a function of the actual mechanical stress (but is not zero when the mechanical stress equals the allowable).
The limit on expansion stresses is not given by the usual a fraction of yield criterion, you can go well beyond yield without necessarily harming the pipe strength. Understanding this part of the story is fundamental before going on with any real world flexibility analysis on an expansion critical piece of piping.
prex
http://www.xcalcs.com : Online engineering calculations
http://www.megamag.it : Magnetic brakes and launchers for fun rides
http://www.levitans.com : Air bearing pads
RE: Another B31.3 Pipe Stress Analysis Question
Increasing the thickness of a straight pipe does not increase axial thermal stress; that stays the same, axial thermal stress is always = α * ΔT * E.
But increasing wall thickness will increase thermal FORCE, as that is
= x-sectional area * Thermal Stress
= x-sectional area * α * ΔT * E.
"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
http://www.youtube.com/watch?v=hpiIWMWWVco
"Being GREEN isn't easy." Kermit
http://virtualpipeline.spaces.liv
RE: Another B31.3 Pipe Stress Analysis Question
I can related to be a newbie maybe more than these career types who forget more about pipe stress than I'll ever know..
First thing, if you didn't already understand it, is the bending stress is /independent of wall thickness/.. That is a difficult concept for a beginner to understand, and based on your statement above, I just wanted to highlight that and make sure you were aware.
I take it from your earlier post that you are seeking a threshold for "is there a quick check test for when I should computer analyze for pipe reaction forces?".. You won't get a solid answer on that, but I will advise that if you keep the BENDING stress low relative to the TYPE of anchor point, then it will be fine.. Use 6 ksi max for steel nozzles, and 1.5 ksi max for cast iron nozzles and pumps. For steel and concrete anchors the weak point will likely be the anchor detail used. Use 12 ksi max for process industry type welded anchors. You should always model rotating equipment unless it is very flexible by simple observation. I use an old guided cantilever chart to rough figure the bending stress.. Add up the total amount of pipe in the plane perpendicular to the thermal growth direction, and calculate (look-up on a nomograph) the bending stress.
As for checking the wall thickness to see if it is adequate for other stresses such as pressure, you can do a quick check for wall thickness for pressure using the formula in B31.3. Don't forget mill tolerance and corrosion allowance. The amount of allowable stress allocated by the B31.3 committee for pressure has nothing to do with the cycling bending stress.. The code treats them differently, and they are NOT additive.. If you want to see this demonstrated, then open a model and severely reduce the wall thickness in one your pipes that passed bending stress.. It will still pass the code check, but if you look at the operating condition output the max stress will be too high.. The lesson is design for pressure first, and flexibility second. Pipe stress programs calculate bending stress.
To design for static loads, just go with a minimum span chart and your sustained stresses will be minimal. For seismic or wind put in guides as appropriate. For rough estimating, you can apply a horizontal force at the midspan between the guides and estimate the bending stress (or forces) due to the wind/seismic to decide where to put the guides.
my 2 cents anyway.
RE: Another B31.3 Pipe Stress Analysis Question
If bending stress is independent of wall thickness, how do you calculate I?
Stress = M*c/I
SP
- Steve Perry
RE: Another B31.3 Pipe Stress Analysis Question
Steve, you are of course correct I should have said "allowable bending stress" is independent of wall thickness.
The main point being extreme newbies assume you can just make the wall thicker and therefore lower the stress and fix the problem.. hardy har har.
for example, for a guided canti-lever,
Se = 6ERd / l^2
R=OD of pipe
l=length of leg absorbing
d = displacement
Se = expansion stress range
RE: Another B31.3 Pipe Stress Analysis Question
I am now keying in on an important point that I read above but it didn't quite click in pipenpumps's reply. I will use B31.3 as a reference point.
If I understand correctly, the allowable stress from Table A1 for a material is used for determining a minimum wall thickness (really max stress) due to static loads and pressure. The ALLOWABLE STRESS DISPLACEMENT RANGE is used in a completely separate flexibility analysis, and is at LEAST f(1.25Sc + 0.25Sh) from equation 1a. If Longitudinal stress is not at max allowable (at hot temperature), then this can actually be higher using equation 1b. The two analyses have nothing to do with eachother.
Is that correct?
I am a little confused as to why longitudinal stress is what's focused on in the code for the stresses due to sustained loads. The hoop stress is always twice the longitudinal from the pressure component for a closed cylinder. What I think the reason is for ignoring hoop stress once a suitable pressure-containing thickness is selected is that the only contributing factor to hoop stress (in any significant way) is pressure. So long as the wall thickness can support the pressure, everything after that (sustained loads, moments, etc.) causes an increase in longitudinal stress, making it the necessary focus of analysis for sustained loading allowable stress (as opposed to the hoop stress).
Is that correct? If so, why does the code specifically refer to longitudinal stress when maybe it ought to more appropriately refer to the principle stresses? Don't bending moments introduce shear in some arrangements, making the principle stresses higher than longitudinal and circumferential stresses?
I seem to be replacing every question with two more, but your help is greatly appreciated. I can tell that there is no way to be of any use on this subject without a good deal of reading and the coincedent struggle to understand.
Thanks as always for your responses.
RE: Another B31.3 Pipe Stress Analysis Question
Believe it or not, the codes generally like to keep things as simple as possible, hence they focus on limiting each individual component rather than principle stresses, although they will accept a combined stress analysis using Tresca (again prefered for simplicity), or Von Mises, if you chose that combined stress route.
"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
http://www.youtube.com/watch?v=hpiIWMWWVco
"Being GREEN isn't easy." Kermit
http://virtualpipeline.spaces.liv
RE: Another B31.3 Pipe Stress Analysis Question
"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
http://www.youtube.com/watch?v=hpiIWMWWVco
"Being GREEN isn't easy." Kermit
http://virtualpipeline.spaces.liv
RE: Another B31.3 Pipe Stress Analysis Question
The code allows the pipe to yield due to the thermal expansion. The code thermal expansion stress range is based on 7,000 heat up and cool down cycles. Remember the fatigue life curve from mechanics of materials? That is basically what is going on. Because thermal expansion is "self relieving" stress it is ok to let the pipe yield.. Once the pipe yields the stress "goes away" so to speak...So it doesn't continue to yield the pipe. This is not so for gravity or wind. The good thing about gravity on a simple horizontal pipe run, though, is the pipe will take on a cantenary curve, sagging until it reaches an equilibrium.
It is very important to understand these basic concepts. For example, how do you model wave forces on a pipe? As occasional loads? Ask yourself how many waves will be hitting the pipe..
RE: Another B31.3 Pipe Stress Analysis Question
RE: Another B31.3 Pipe Stress Analysis Question
"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
http://www.youtube.com/watch?v=hpiIWMWWVco
"Being GREEN isn't easy." Kermit
http://virtualpipeline.spaces.liv
RE: Another B31.3 Pipe Stress Analysis Question
You seem to be minimizing the importance of considering material yielding in the analysis. From a practical standpoint, most (all?) pipe stress programs are incapable of considering it so it's not something that is typically discussed. Yielding would typically have a relatively small effect on strain, but it could redistribute loads in ways that cannot be anticipated. I could see how it might also have a significant effect on the modal dynamic analysis.
Although material nonlinear behavior is an effect that is typically ignored, maybe it shouldn't be, especially with nonlinear pipe supports. Do you have first hand experience running side-by-side comparisons? The codes consider it in their stress allowables which tells you it's an important consideration... but it's ignored in other aspects of the analysis. That seems inconsistent and incorrect, even if it's the "norm" on how things are typically done. How big of an impact it would have on a typical piping analysis would be an interesting study. That's all I'm suggesting.
RE: Another B31.3 Pipe Stress Analysis Question
"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
http://www.youtube.com/watch?v=hpiIWMWWVco
"Being GREEN isn't easy." Kermit
http://virtualpipeline.spaces.liv
RE: Another B31.3 Pipe Stress Analysis Question
But these nonlinear reactions are not unrelated and decoupled as you suggest. Whether or not a gap closes or friction breaks depends entirely on the load at that support point, and load distribution is affected by nonlinear material behavior at elevated temperatures.
I'm a structural engineer with access to SAP2000. I spent a little time earlier to construct a small line/beam element piping model in SAP with nonlinear supports and compared results with and without material nonlinear behavior. In my example with delta 900F thermal load, consideration of nonlinear material behavior (plastic hinges) affected anchor moments over 65% in a poorly supported system, as well as calculating the wrong sign in one direction, and a 40% differential in moments in one direction in a more realistically supported system. Fortunately, at least in my small example, ignoring material nonlinear behavior was conservative (material nonlinear behavior reduced anchor loads for the most part), but the load distribution changes were significant. They were not some minor "sharpening of the pencil" differences.. Modal results surprisingly weren't much different(15%). With load differentials that large as a result of nonlinear material behavior, I could imagine some scenarios when it might be unconservative not to consider material yielding when calculating equipment or support loads. A 10% or 15% difference is one thing, but 65% difference and changing loading directions? That's a scary big difference. I guess we should be grateful that ductile pipe is so forgiving and that conservatism is built into the codes.
Another consideration would be changes to modulus at elevated temp which I ignored in my little test. With nonlinear pipe supports it could be unconservative to ignore it. That's the deal.. whenever we're modeling gaps and friction, changes to load distribution can have a huge impact on the analysis.
I've learned a lot from you in this forum biginch. The fact that such an experienced knowledgeable piping engineer as yourself has not given much consideration to material nonlinear behavior confirms my hunch that few engineers have thought to consider it.. I think the reason is because piping stress programs don't offer it, so engineers tend to think it's not important. ASME acknowledges yielding "shakedown" in their stress allowables.. perhaps they should be consistent by giving clear guidance on the effects of yielding when calculating piping loads.
RE: Another B31.3 Pipe Stress Analysis Question
"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
http://www.youtube.com/watch?v=hpiIWMWWVco
"Being GREEN isn't easy." Kermit
http://virtualpipeline.spaces.liv
RE: Another B31.3 Pipe Stress Analysis Question
If B31 piping codes assume that yielding will take place at elevated temperatures as reflected in the stress allowables, then why no consideration of this same yielding for calculation of piping loads on equipment, support, flanges, etc. It's inconsistent to use it for stresses while ignoring it for calculation of loads and deflections. Unless I'm missing something big, I don't believe my observation is controversial.
RE: Another B31.3 Pipe Stress Analysis Question
Consider the effects of loads caused by displacements, rather than displacements caused by loads. Maybe somewhat foreign to a structural engineer. If displacement stops, loads do not get any larger. Now add creep, "the tendency of a solid material to slowly move or deform permanently under the influence of stresses. It occurs as a result of long term exposure to high levels of stress that are below the yield strength of the material." (quote from Wiki) The piping code has allowances for both. Consider the effects from thermal loads wherea E is reduced, loads do not necessarily go higher if the rate of the reduction of E with temperature is sufficient, even though perhaps temperature is still increasing and expansion of the material continues. And I think its the effects of creep, essentially "yield below yield stess", that are the most relavent to pipe stress, rather than actual yield at stresses above the "yield point", perhaps the effects that you are identifying with.
A displacement stress range is calculated due to thermal conditions. Under certain conditions, such as a relatively low allowable stress pipe, used in low cycle service conditions, the allowable stress range may be greater than the allowed longitudinal stress, but (I believe at a maximum of around 96%) still less than the basic yield point of that pipe.
Maybe some quotes from the code will help too.
"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
http://www.youtube.com/watch?v=hpiIWMWWVco
"Being GREEN isn't easy." Kermit
http://virtualpipeline.spaces.liv
RE: Another B31.3 Pipe Stress Analysis Question
http://www.sstusa.com/99janmar.php
Relevant quote: "Generally, the flexibility allowable stress-range was permitted to approach two times yield"
I could swear that I've read similar comments from ASME code committee member John Breen in other posts. Maybe a dumb structural guy like me can't comprehend things like you say, or maybe nonlinear analysis is not your strong point yet you insist on defending your position without basis.. in my very subjective and less experienced-than-you opinion.
I'd love for other knowledgeable piping engineers like prex or John Breen or DSB to weigh in on this conversation.
Btw BigInch, I'm extemely grateful that you post here at this eng-tips forum. I've learned a LOT from you in many areas of piping engineering, and I hope to learn more from you in the future.
RE: Another B31.3 Pipe Stress Analysis Question
I can't speak for Mr Hapt; I'm certainly not in his league and I think he's saying its complicated for him to interpret the code as well, so I won't. I just see what's in the 31.3 code today, as quoted above and I know nothing of B31.1. What I don't see there right now is 2 * yield, but perhaps that could occur at higher temperatures where allowable stress might be a greater percent of a high temperature yield stress, and wider temperature ranges than what I'm used to seeing. With relatively cool temperatures within a narrow range, Sc and Sh would be equal and at low cycles f = 1.2, so theoretically that's a max of 1.5 * Sa. Sa is typically around 0.6 Sy, so I'm seeing 1.5 * 0.6 = 0.9 Sy.
Do you see something different?
I'm willing to listen to what anyone else wants to say, as long as its not a cut and paste job of a hundred and one references.
"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
http://www.youtube.com/watch?v=hpiIWMWWVco
"Being GREEN isn't easy." Kermit
http://virtualpipeline.spaces.liv
RE: Another B31.3 Pipe Stress Analysis Question
If you continue reading in that section, the code elaborates as to WHY this yielding is acceptable.. because it relieves stresses.
We're all trying to learn here, but I think my original question/observation asking why loads aren't calculated with consideration of yield when stress allowables consider yielding (and creep), is a question that sort of got lost when the question of whether yielding is permitted at all was raised. If the piping codes acknowledge and account for yielding in the stress allowables, it's inconsistent not to consider yielding (and creep in some cases?) when calculating piping loads on equipment/supports/flanges, and in calcuation of deflections. Why is yielding almost always ignored in piping stress analysis when calculating these loads and deflections?
RE: Another B31.3 Pipe Stress Analysis Question
As to why its not considered further, when you reach localized yield at a point in the pipe, what are the support loads doing? Although some nonlinear changes in stress happen, arn't the support loads remaining relatively the same, or at least increasing at a much lesser proportion to strain than before yielding occured.
"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
http://www.youtube.com/watch?v=hpiIWMWWVco
"Being GREEN isn't easy." Kermit
http://virtualpipeline.spaces.liv
RE: Another B31.3 Pipe Stress Analysis Question
My little test problem confirms that your instincts are good that yielding will typically not increase support and equipment loads. But we're often dealing with NONLINEAR pipe supports, so the closing of gaps or friction can make a huge difference in the analysis. I'd guess that in most designs consideration of yielding would reduce loads, but in some designs it could increase calculated loads. Once it's decided that nonlinear gaps and friction are important, and they often are, then EVERYTHING that affects loads and deflections also becomes important as it all can impact the analysis in a huge way. That includes nonlinear yielding and P-Delta effects too.
Another eng-tips poster (LHill or some similar monikor) linked to a software program from Paulin Research called something "gold", I can't remember the exact name. The piping software advertisement made a very valid point that I rarely/never see acknowledged among piping engineers - friction acts differently in different directions between heatup and cooldown. P-Delta analysis is required to consider this phenomena. It's clearly unconservative to ignore it. I don't know whether that software is any good or not, but why has this not been a "big" issue in the past? Because in structural engineering, nonlinear P-delta analysis is commonplace, yet in piping engineering it's unheard of. This relates to my original question - why is material yielding explicitly acknowledged in the calculation of B31 code stresses, yet ignored when calculating piping loads and deflections? I still haven't heard an answer to that question other than attempts to minimize the importance of that question.
RE: Another B31.3 Pipe Stress Analysis Question
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RE: Another B31.3 Pipe Stress Analysis Question
I'll refrain from going fully into it, and will simply try to state two points on which your respective arguments seem weak to me.
BigInch, if you don't see allowance for yield in B31.3, then you are wrong. This is demonstrated by eq. 1b) you quoted: assume SL, the longitudinal stress due to sustained loads, is close to zero and that Sc=Sh (low temperature), then SA=2.5fS; now if you can take f=1.2 in this situation, SA=3S or something that may be close to 2Sy.
ZippyDDoodah, in your crusade for calculating expansion stresses with due account for nonlinearities (yield, supports, etc.), you seem to forget, if I correctly understand your point, that codes are there to provide a good equilibrium between public safety and economics. If a code designed for relatively simple and non critical systems like B31.3 (but the same holds for B31.1 and many other codes) was enforcing a plastic analysis for piping systems, then safety would not necessarily be improved, because of the complexity of such analyses, but the cost would certainly increase.
Of course we know that engineering methods are improving quite fast these days, so this point of view could notably change in future versions of the code (perhaps not before ten years, as code committees are, correctly, quite conservative).
However note also that the concept of elastically calculated stresses, on which B31.3 is based, does not imply that the calculated stresses should stay below yield, but only that a linear behavior is assumed also beyond yield, and the method, on the basis of well founded theoretical considerations, may well give safe results, not always necessarily realistic, but safe.
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RE: Another B31.3 Pipe Stress Analysis Question
"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
http://www.youtube.com/watch?v=hpiIWMWWVco
"Being GREEN isn't easy." Kermit
http://virtualpipeline.spaces.liv
RE: Another B31.3 Pipe Stress Analysis Question
Zippy, I'm sure I can't answer the question to your satisfaction either. This is still an applied science, not a pure science, so not everything works out. To put it another way, if the analyses were not sufficiently accurate the code requirements would be changed.
Currently the way things are done relies on the analyst's judgement for situations that are not fully addressed in some/most modern pipe stress analysis programs. For example, if I have large moment across a flange or a valve (near code allowable), I'm definitely going to look into fixing or analyzing it further because it is more critical than a simple pipe. e.g. what is the valve material, is it ductile? does manuf. have max design loads? Where friction would result in a less conservative result, such as a nozzle load, you would want to model in friction to get the additional axial forces to make the supports break the static friction forces and pop.
I appreciate where you are coming from, because all the GOOD engineers go through the same process.. asking why. But sometimes you just have to follow the recipe if you want the cake to come out right..
RE: Another B31.3 Pipe Stress Analysis Question
Whenever a piping network is modeled with nonlinear gaps and friction which is commonplace, analysis considerations which could easily affect the closing of gaps or breaking of friction.. those considerations, including nonlinear yielding and p-delta on the beam element model, would seem to become even more important. My little SAP piping model indicated substantial changes to load distribution when considering plastic hinges in a high temperature system, although in that example the hinges "softened" the loads on the anchors. Current thinking seems to be that yielding and p-delta effects are not very important, but my guess is that these considerations could have a profound impact on the results of many designs, even if it's believed to be too "costly" to consider them with current technology.
Apologies if my comments came off as a "crusade". I originally intended it as a one-off comment/observation until I had to further explain and defend my original statement in subsequent posts.
RE: Another B31.3 Pipe Stress Analysis Question
Can you build a model example that proves your contention?
"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
http://www.youtube.com/watch?v=hpiIWMWWVco
"Being GREEN isn't easy." Kermit
http://virtualpipeline.spaces.liv
RE: Another B31.3 Pipe Stress Analysis Question
If you are specifically criticizing B31.3's way of calculating end reactions, then you should show an example where B31.3 gives an unsafe result. This of course is far from being impossible, but then we would have something to discuss about in detail. Also you should recall that checking the strength of equipment against piping end loads is outside the scope of the piping code, and that, if those loads are excessive, you can always try to recalculate them with a finer method, staying outside the scope of the piping code.
As I already pointed out in my previous post, a code has two main goals: safety and economics. The first one may be attained also with very unrealistic results, it doesn't matter to the code, provided they are safe: so to tell that a code gives unrealistic but still safe results is not acceptable as a criticism to the safety of that code.
Of course an unrealistically safe result might impair the economic side of the picture, but then, to criticize the code on that one, you should estimate the change in cost of design, fabrication, testing and also define a representative set of piping systems, from the simplest to the more complex, each one with its frequency of occurrence in the industry...: quite difficult as you see.
To say the same with other words: the fact that you find big changes in the end loads (or whatever else) when using an approach more sophisticated than the minimum required by code, doesn't mean at all, as a consequence, that the code should be revised.
prex
http://www.xcalcs.com : Online engineering calculations
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RE: Another B31.3 Pipe Stress Analysis Question
This book covers the requirement of flexibility 31.3. It covers factors, formulas, and considerations to be taken during the analysis.
RE: Another B31.3 Pipe Stress Analysis Question
1) B31.3 governs the piping system. Component loads are to be evaluated in accordance with their own codes, such as BPV Section VIII, API 650, API 610, et cetera.
2) Although some yielding in the piping system is acceptable due to its ductility, it is completely unacceptable to cause equipment nozzles to yield. In fact, the nozzle allowables for API 610 pumps are designed to prevent excessive casing deflection even within the elastic range. The piping system must be designed to not only stay within B31.3 allowables, but to minimize the loads on nozzles to the level of acceptability determined by their codes.
3) Deflection of nozzles within the elastic range is included in the analysis (although some client standards insist that vessel nozzles be treated as rigid). These days, I typically use FEA for nozzles on pressure vessels, put the resulting flexibility into the piping system, then extract the loads to put on the nozzle and examine it in FEA to meet the BPV code allowables.
In summary, we don't need to account for yielding of equipment nozzles because it is part of our job to ensure that it does not happen.
RE: Another B31.3 Pipe Stress Analysis Question
Crusader, you misunderstood my point. I never suggested yielding at nozzles. I suggested that yielding in other parts of the piping system could have a significant effect on equipment loads and in calculation of piping loads on supports and possibly dynamic reactions.
I have examples where consideration of yielding resulted in significant changes to load distribution (300% change to anchor moments in one direction). However, the changes in those small examples reduced anchor loads, so ignoring the yielding was conservative from an anchor load standpoint, although the loads were distributed elsewhere in the system. However, with large changes to load distribution with nonlinear pipe supports, and nonlinear pipe supports are commonly used, the risk that that gaps could close or not close resulting in less conservative results would seem to be an obvious concern. That I don't have an example handy (I'll post again after I have some time to experiment) doesn't mean that it's not a big deal, or that these concerns are merely academic. My earlier comment was that it would be interesting to see a study. The effects of yielding are real, and piping codes acknowledge yielding in their stress limits. It's inconsistent to ignore this phenomena in calculation of piping loads on sensitive equipment and on supports unless one "knows" that ignoring yielding will always result in a conservative design.
On a related note, Paulin Research points out with an example that it's often unconservative not to account for changing friction directions during heatup and cool-down, yet that too is commonly ignored. Pipe rack deflections of 4" or more under seismic and wind load are often commonly ignored in pipe stress analytical models too. It's not a "criticism" of the piping codes to raise these sort of questions in how things are typically done and whether or not such practices might results in problematic designs. Skip past their FEA offering and page down to 'path dependent friction': http://www.pclgold.com/info/PCLGold2009.PDF
RE: Another B31.3 Pipe Stress Analysis Question
I have PRG's suite of programs, including PCLGold, but I have not yet convinced myself that one can model in that program as quickly as in Caesar (a definite concern with project man-hour budgets being cut to the bone these days), nor do I feel that the aforementioned nonlinear effects would exceed the safety factors in the B31.3 code and the equipment code allowables. As you are no doubt aware, the safety factors are there to account for the things we do not know, cannot anticipate, or cannot easily quantify. I think this fits in the latter category, although the advance of technology allows us to calculate things today that would not have been practical yesterday. NozzlePro versus WRC-107 is a good example of this.
RE: Another B31.3 Pipe Stress Analysis Question
I appreciate all of your input. It looks like we're well away from the original topic and point of my post. Is there any way this in-depth discussion could be done in another thread? I am trying to get help and advice on performing a code stress analysis, and while it is helpful to know its limitations, I'm not anywhere near being able to make use of this discussion.
Sorry, I'm not terribly familiar with forum rules, but these topics are far more in-depth than the help I was seeking. If someone else has something further to input regarding that, I would be grateful.
Please don't think I'm not appreciative of your time and effort, as I most certainly am.
RE: Another B31.3 Pipe Stress Analysis Question
On the original topic, I am a buzzard coming late to the kill, in that I can add little to the excellent advice you've been given so far. You've already read some piping design books. Peng's book is excellent. Kellogg's is dated but definitely worth reading. The Casti guidebook is a great adjunct to the code itself. I would recommend Caesar training. I also recommend the use of Paulin Research Group's suite of finite element analysis software after you have mastered pipe stress analysis somewhat. Welcome to a fascinating field.
RE: Another B31.3 Pipe Stress Analysis Question
I have never used PCLGold, so I have no idea whether it's any good. But they make numerous valid points about limitations in current pipe stress programs. SAP2000 is almost certainly less user friendly than PCLGold for creating piping models (and it lacks piping libraries, code calculations, spring hanger module etc), so I understand what you're saying with the need to be able to construct an analytical model quickly. Programs with high-end analytical capabilities often tend to be too time consuming to be used in practical piping design work.
Paulin Group also makes some good observations with regards to applicability of FEA and their templates appear to be easy to use. But my concern for now is more basic and limited to global beam/line element analytical modeling. Most piping stress programs ignore TOO much with their limitations imo. No P-delta, no load sequencing, no ability to consider yielding, limited control over the mass model and other limitations. Can all of these real-world effects be safely ignored for most designs? Especially designs where nonlinear supports are considered? Until someone takes the time and trouble to build analytical models for typical pipe network designs and compare side-by-side, we'll never know for sure. PCLGold took a nice step in that direction with their example which demonstrates that it's unconservative to ignore load path dependent friction effects.
RE: Another B31.3 Pipe Stress Analysis Question
Again, not to detract from your contribution, as I appreciate your input, but this way off topic. It has almost nothing to do with my post and no one is helping me with my question, they're responding to yours. Please start a new thread. Thank you.
RE: Another B31.3 Pipe Stress Analysis Question
The B31.3 code has you consider weight and pressure stresses independently from thermal stresses. As you are undoubtedly aware, how/where you support your piping is largely dependent on space and nearby steel. Hopefully you have others in your office to learn from and coordinate with for advice in this area.
In addition to B31.3 code stress requirements, you also will need to check to ensure that piping forces and moments do not overload connecting equipment such as compressors, equipment which is not explicitly dealt with in the B31 code. Typically you would create an "operating" load combination for equipment load checks, a combo which should include weight + thermal load(s) and compare calculated loads against the applicable equipment standard, or if you're unsure, contact the equipment vendor directly to discuss allowable piping loads. Peng's book has a lot of advice for you in that area too.
RE: Another B31.3 Pipe Stress Analysis Question
OP, I recomend adding L.C. Peng's book "Pipe Stress Engineering" to your library collection.
http://www.eng-tips.com/viewthread.cfm?qid=248080
I have read this book at least three times over and find it to be an invaluabe recource for my work.
Just as a point of interest I thought I would mention that I had the pleasure of taking the ASME course on B31.1 Piping design and fabrication taught by none other than Ron Haupt. He has a self admitted "liberal interpretation of the code" on the matter of local yielding and initial shakedown. Ron also himself admitted that he was but one voice on the commitee and that other members may have a different interpretation of the same code.
At the end of the day the code is not a design guide and is not a replacement for sound engineering judgement. It is a set of design and fabrication rules meant to prevent failure.
Just my two cents worth.
A question properly stated is a problem half solved.
Always remember, free advice is worth exactly what you pay for it!
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