pvc pipe under external pressure
pvc pipe under external pressure
(OP)
Hi all
How deep in water could I sink a 20" schedule 20 pipe, assuming the internal pressure of pipe is the atmosphere??
Of course it have not to collapse or be deformed.
How deep in water could I sink a 20" schedule 20 pipe, assuming the internal pressure of pipe is the atmosphere??
Of course it have not to collapse or be deformed.





RE: pvc pipe under external pressure
It would make a very bad submarine, if that's what you're thinking.
What _are_ you thinking?
Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA
RE: pvc pipe under external pressure
Assuming you are in pure water, the internal pressure is 14.7 psi and the 20 diameter PVC pipe of schedule 20 behaves as rigid plastic with properties 5218 psi yield and Young Modulus of 406106 psi, then you can go down to 544.6 feet to get the onset of failure.
Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada
RE: pvc pipe under external pressure
No one can beat you :)
Could you explain me how do yo get this values???
The 544.6 feet deep.
To Mikehallorman , no I do not want to use it as a sub .
I'm XXXLarge size.
My final task is to know if I can make a 1 inch HG absolute vacuum to a 40 inch lenght pipe of such dimension.
Do not worry by the caps , I had solve it yet.
My concern is with the pipe.
Thanks both .
www.cadtutorforum.net
RE: pvc pipe under external pressure
I'm getting about 50 feet for a perfectly round sch40 (.593" wt) pipe, and about 40 feet for one that's a half inch out of round.
Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA
RE: pvc pipe under external pressure
Given a 20 inch diameter pipe of schedule 20, the internal pressure is 14.7 psi, taken to represent atmospheric conditions. You could take this pipe and submerge it to a depth "h" in water of specific weight 62.5 lbf/ft^3 or:
h = P/(rho g)
h = (14.7 psig/62.5 lbf/ft^3)X(12 in/ft)^3 = 406.4 in.
At this depth, the wall is in neither compression or tension since internal pressure exactly balances external pressure subjected by the water. I would call this the neutral axis, wall stresses are exactly zero.
Continue to submerge the pipe. As the water gradient increases with depth, there shall be a point below this neutral axis to which pressure will crush the pipe. By Thick Wall Pressure Vessel Theory, an element of wall does not care if external pressure is crushing it or internal pressure is blowing it outwards. We can treat internal and external pressure by the same equation, I have chosen the Von Mises-Hencky Equation since it relates hoop, radial and longitudinal stresses as a vector gradient for a triaxial state of stress of an element representative of the wall under the loaded condition.
Grinding through the mathematics, the stress gradient applied to Thick Wall Pressure Vessel for hoop, radial and longitudinal stresses becomes:
S = sqrt(3) P [R^2 / (R^2 - 1)] for R = D/d
D = 20.0 in (OD), d = D-2t = 19.25 in (ID)
"t" is the wall thickness, commecial pipe schedule gives 0.387 inch for your 20.0 OD schedule 20 pipeline.
I set the stress equal to material yield, PVC rigid plastic listed at 5218 psi and a Young's Modulus of 2.8 GPa (metric value in Canada). By giggling around the terms in the Von Mises-Hencky Equation, you would find the required pressure to bring the wall to a yield point, hence the onset of failure to be:
R = D/d = 20.0 in / 19.25 in = 1.03896
P=[5218 psi/srt(3)][(1.03896^2 - 1)/1.03896^2] = 221.7 psi
This corresponds to a depth of h' BELOW the previously established neutral point.
h' = P'/(rho g) for P' = 221.7 psi
h' = (221.7 psig/62.5 lbf/ft^3)X(12 in/ft)^3 = 6130 in.
Therefore, total distance as measured from surface, hence the depth you seek is:
H = h + h' = 406.4 + 6130 in = 6536 in = 544.7 ft
This computation will be influenced by the physical values to the PVC material, dependent upon grade. For purposes of illustration and finding this an curiosity, I used generalized values from a DuPont listing. Be careful to check an MTR from your supplier and make the appropriate changes.
Hope this clarifies your apprehensions. This is a very cute problem.
Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada
RE: pvc pipe under external pressure
544.7 feet. I was more worried of spelling mistakes, sorry!
Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada
RE: pvc pipe under external pressure
Under external pressure, the failure mode is not going to be yield, but rather buckling. So, I'm going to completely disagree with Cockroach's approach on this one - you headed in the completely wrong direction.
RE: pvc pipe under external pressure
As for your comment, best to review the theory. I suggest Advance Mechanics of Materials, Boresi & SideBottom, pg 169 or Design Stress Analysis, Timoshenko & MacCullough, chapter 4. Also try the University of Tennessee at Martin, Lecture 15 in the course Engineering 473 touches on the method relative to interference fits. I would imagine that Roark or Mark deal with the subject, probably drops the mathematical theory for the practicioner however.
I stand by my analysis.
Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada
RE: pvc pipe under external pressure
I'm looking at my copy of Advanced Mechanics of Materials 5th ed by Boresi, Schmidt, and Sidebottom. On page 169 they begin section 5.2, "Castigliano's Theorem on Deflections." Is that the section you were referring to? I suspect you have a different edition.
jt
RE: pvc pipe under external pressure
thread378-103771
thread794-95854
RE: pvc pipe under external pressure
Leaving the failure mode (buckling vs yield) aside for a moment, I believe you should revisit the "submerge it to 406 inches to achieve zero gage pressure" approach. I'd argue that as soon as the pipe is submerged to any depth, it is under external gage pressure. You start on the surface with the cylinder open which achieves 0 psig or 14.7 psia. Now close the cylinder and begin to submerge it. At a depth of 1 foot, you'd have the weight of the water, 62.4 #/ft^2 or 0.43 pounds per square inch. Now, if you are playing in the "gage" sandbox, you started at 0 psig and now have 0.43 psig external added for a grand total of 0.43 psig acting on the walls of the cylinder. If you prefer to play in the "absolute" sandbox then you started with 14.7 psia in the cylinder countered by the weight of the water and the air above the water for a total of 0.43 psi + 14.7 psi. So you wind up with 14.7 psi internal and 15.1 psi external for a differential pressure of 0.43 psi pressing in on the cylinder.
jt
RE: pvc pipe under external pressure
I thinkt hat jte has covered the gage vs. absolute thin well enough, so I will focus solely on the failure mode(s). In each design, there are many different failure modes that need to be considered. In no particular order (and not necessarily complete) they are:
1) Plastic collapse
2) Local failure (exceeding local strain limits)
3) Hydrostatic material failure
4) Buckling / Instability
5) Progressive distortion
6) Fatigue
7) Brittle fracture / crack growth
8) Material degradation over time (creep, radiation damage, hardening, etc)
So, I think that cockroach has adequately covered the first failure mode. However, there are many others (as shown in my list) that need to be considered.
Hyposmurf, what have you done to evaluate these other failure modes? If you need the forum to point you in the right direction regarding calcualtions, we will be glad to oblige.
RE: pvc pipe under external pressure
Of course your comment and that of TGS4 would unnecessary complicate the mathematics. As stated in my analysis, with the absence of more information, like tension placed on the pipeline, sagging inbetween supports or laying on the bed (inelastic foundation), etc, etc, the problem would be unsolvable. I have made assumptions based on a reasonable approach to the problem and came up with 545 ft.
But how could you model the problem using fatigue, much less crack propagation? We don't have and would never hope to have material properties to assist us with modelling these phenonema.
I would hope that HypoSmurf begin with the computation set forth in this forum, research the discussion and direct his efforts accordingly. Is this not the point of the forum, healthy academic discussion?
Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada
RE: pvc pipe under external pressure
For example, in this particular case, if hyposmurf were to say that the pipe would not be repeatedly submerged, then I could say that, regardless of the state of stress, it would not fail due to fatigue in one cycle. Similarly, I would not be concerned about progressive distortion. Likewise, if material degredation is not a concern, then time-dependant properties are likely not significant.
However, all of the other failure must be dealt with. From my perspective and based on my engineering experience and judgement, the buckling failure mode would likely be the governing failure mode. B31.3 has formulae to address external pressure and the buckling/instability failure mode.
"But how could you model the problem using fatigue, much less crack propagation? We don't have and would never hope to have material properties to assist us with modelling these phenonema." To use a very legalise approach to this, ignorance is no excuse. Unless you can achieve positive confirmation that such failure modes will not occur (such as my discussion above), then as an engineer you are obligated to "get" the information necessary for you to make a sound judgement. To simply throw your hands up in the air and say that it is impossible ("never hope to have") is a complete cop-out. It's that kind of attitude that results in failures and deaths (Challenger and Columbia to name a few recent examples).
Finally, cockroach, please tell the forum why you neglected the buckling/instability failure mode. Do you have additional information on PVC that the rest of us may not be privy to?
RE: pvc pipe under external pressure
In similar fashion, buckling instability in my judgement, is not a factor. My transition points between straight collapse, elastic and transitional, show pressures around the yield point. Again, without additional information I would suggest yield to be predominant mode of failure.
I do have PVC properties not typically available to most engineers in the field. To suggest ignorance as an excuse or believe another failure may occur, again, let's see your numbers complete with the model.
Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada
RE: pvc pipe under external pressure
Mx = L / sqrt(Ro*t)
assuming an unsupported span of 20ft
Mx = 87.64
Therefore, Ch = 0.55*t/Do
Ch=0.0103125
Fhe=1.6*Ch*E*t/Do
Fhe=123.75 psi
Since Fhe/Fy<0.552, Fhs=Fhe/FS
Assuming a FS of 3, Fha=41.25 psi
Calculating Pa = 2 * Fha * t / Do
Pa=1.547 psi
------------------------------
Assumed values:
Do=20
t=0.375
E=4e5
------------------------------
That would mean that you could (with a design margin of 3) place the 20"NPS Sch. 20 pipe to a depth of 43 inches. Reducing the design margin to unity gives a depth of 129 inches (give or take a few since the pressure would not be uniform).
So, let's see you API collapse equations...
RE: pvc pipe under external pressure
You are of course assuming that the cylinder is perfectly round. If there is an ovality then buckling would override the other parameters.
The factor of safety for buckling is thus much higher.
Suggest Timoshenko Theory of Plates and Shells is referred to for the subject.
RE: pvc pipe under external pressure
Thanks all , for pay attention to this task I have to do.
Hope we get at the end the proper result.
Please keep it on run.
Thanks again .
www.cadtutorforum.net
RE: pvc pipe under external pressure
20 20 in od of inner tube
sch 40 sch 20 tube gage
0.593 0.375 in wall thickness
PVC PVC material
4.00E+054.00E+05psi Elastic modulus
1.54E+051.54E+05psi Shear modulus
0.30 0.30 Poisson's ratio
5.22E+035.22E+03psi yield point for tube material
25.08 6.13 psi critical buckling pressure for a _perfect_ tube
per eqn 2.6 of ref 2
0.5 0.5 in radial deviation from circle at worst point
20.08 5.69 psi critical pressure for collapse of imperfect tube
ref 1: "Tubing Limits for Burst and Collapse", Tech Note, CTES, L.C., Conroe TX www.ctes.com
ref 2: "Effect of Initial Eccentricity on Collapse Pressure of Circular Beam Tubes", S. Yadav
Fermi National Accelerator Laboratory, Batavia IL www.flab.gov
Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA
RE: pvc pipe under external pressure
I don't believe Timoshenko's "Theory of Plates and Shells" really goes into the elastic stability issues. Try his "Theory of Elastic Stability" for that. However, the only copy of that which I have seen (in a library) was badly outdated. And the general shell formulas that are given in "Theory of Plates and Shells" are given in a more condensed form in "Formulas for Stress and Strain".
The shell buckling formulas in "Formulas for Stress and Strain" are similar to those typically used for steel tanks, if that helps.
One thing to be aware of is that most of this buckling information that is available was derived for steel (think submarine research, in particular.) If your material properties vary markedly from steel, you may not get the accuracy you'd expect from an allowable-stress type equation. And buckling equations are prone to wide variations in the first place.
One other resource is the PVC pipe people. I know the AWWA standards for ductile iron pipe give charts or other design data for wall thickness based on burial depth. I suppose this information is also available for PVC. You may or may not find it in a useful form, though. The external pressures in buried piping are different from your application.
RE: pvc pipe under external pressure
One other thing that occured to me as I was discussing this with collegues- at room temperature, PVC creeps. So, now you're talking about creep buckling, a regime that I am a little unfamiliar with. Howeve, I know enough that I would be uncomfortable designing in that regime.
RE: pvc pipe under external pressure
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RE: pvc pipe under external pressure
One thing to add if you get over about 8O ft in the Gulf Mexico you will have to add a little temperature correction, I know I do or better did.
http://ww
RE: pvc pipe under external pressure
"Pipe Ovality Effects
Pipe deflection will reduce flattening resistance, and lower critical flattening pressure"
The equation for buckling quoted is a Timoshenko and can be found in the following reference.
1) Materials Selection Manual for sewers, pumping mains and manholes, Water
Research Centre Buckinghamshire: 1993.
2) Pipe Materials Selection Manual , Water Research Centre Buckinghamshire: 1988.
Buckling will be the controlling criteira in design rather than yield. This is mainly because the pipe will not be perfectly round. Buckling is not something that is easily predicted because of imperfections in shape. Such imperfections have little impact on properties such as "yield".
RE: pvc pipe under external pressure
Anybody have any ideas on the creep buckling that I mentioned earlier? Do you think that it woudl actually be a factor in PVC pipe?
RE: pvc pipe under external pressure
Once the buckle was detected (you couldn't observe it directly because of elbows), everyone remembered the loud bang/ clang/ pop they heard at the time. No one had reported it or thought it unusual, because our shop rings with loud noises all day long.
It was my fault, for not making the calculation before the event happened.
I have since tried to further correlate the equations I used with some real world situation, e.g. took a boat apart looking for a buckle that wasn't there, at least not yet. The equations may be a little conservative, but are so wildly sensitive to wall thickness and ovality that it's hard to make correlations with any precision.
Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA
RE: pvc pipe under external pressure
Nowhere in the original post from HypoSmurf is "span" or the mere mention of pipe support entertained. Properly, buckling, traverse loading, what have you, is beyond the scope of computation until additional information is required. Maybe the entire pipeline is fully supported and laying on the lake bed, i.e. no collapse. Then again, maybe a support will be placed every six feet. Who knows? So the solution is incomplete regarding buckling.
I know one thing for sure. This morning I stood on a piece of 20 diameter pipe, schedule 20 over in my buddy's yard. Granted the pipe was on the ground, just blocked from rolling away from me. I'm a little guy, 62 kg. The pipe was strong enough to hold me, even bouncing up and down on it moderately. I think your "129 inches" is ridiculus, that's only 4.66 psi. Oh yeah, you unnecessarily assumed 20 feet span between supports. Bad assumption.
But reviewing your earlier posting(s), was that not the point you where trying to make?
Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada
RE: pvc pipe under external pressure
Coackroach, did you not happen to read the postings by MikeHalloran and stanier? I think that you are the one who is comletely out to lunch on this one. An external pressure of 222 psi is competely ridiculous! If you were to introduce even slightest perturbation (you do remember that elastic stability is based on perturbation methods, don't you?) to the wall, in the form of out-of-roundness, or even a pressure gradient between the top of the pipe and the bottom of the pipe, the PVC pipe would collapse (elastically I might add) at an external pressure of less than even 1/10 of the 222psi that you predict.
Cockroach, unless you can come up with a calculation that completely refutes the methodology and results from MikeKalloran's 4 May 05 21:58 posting, I will simply reject your assessment on this item. Elastic stability does not seem to be your strong suit.
BTW - if you were paying attention, the 4.66 psi external pressure that I calculated was based on a design margin of 3.
RE: pvc pipe under external pressure
By now TGS4, you can see we are measuring depth on the order of hundreds of feet, not a few hundred inches. Using the yield collapse equation, 20 diameter pipe of size schedule 20:
let M = D/t = 20.00 in/0.375 in = 53.333
P = 2S[(M-1)/M^2] = 2(5218 psi)[52.333/53.333^2] = 192 psi
h'=P/(rho g) = [192 psi / (62.5 lbf/ft)] X (12 in/ft)^3 = 5308 in
This is the depth below the point where wall stress is zero, already deduced as 406 in.
H = 5308 + 406 in = 5714 in = 476 ft.
I had reported 232.4 psi in an earlier post for collapse rather than 192 psi because of a reference to yield stress of 6316 psi for rigid polyvinylchloride material in a state of compression. Plastic properties vary in the literature because of manufacturing methods, HypoSmurf would need to obtain a valid MTR with shipment of product in order to validate his computation.
This computation is completely independent of the assumption of span. So TGS4, I rest my case.
Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada
RE: pvc pipe under external pressure
I think I have figured out where you went wrong - you (inocrrectly) assumed that the "collapse" equations refer to elastic instability buckling. They do not - they refer to failure due to the pipe/vessel reaching the yield stress.
As you will find in any basic theory of elasticity textbook, the Young's Modulus will be a factor in a buckling analysis. Check out Euler's formula - where for columnar buckling the critical load:
Prc = (pi^2 * E * I)/(L^2)
So, unless you can find an ELASTIC BUCKLING equation that refutes what I and MikeHalloran stated, I again reject your assessment on this item. Elastic stability still does not seem to be your strong suit.
General statement - "For external pressure, the governing failure mode will be elastic buckling" Ipsa res loquitor.
RE: pvc pipe under external pressure
The answer is one end free and the other end fixed with a load parallel to the centreline. Is this the case we are considering here? I don't think so!
Anyway, I think this subject has been beat up enough. Take care. :)
Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada
RE: pvc pipe under external pressure
The PVC pipe manufcturer Harvel has a duct product with ratings given for internal pressure and external pressure loading. The duct product has thinner wall than for your size 20", but it would be interesting to compare lab tests of their .219" wall product rating to some of the predicted pressures of the previous posts.
www.harvel.com
The Harvel product rating is only 24" wc, that is inches of water column and not psi. If you want to factor the Sch20 0.375" wall thickness with a 2X or 4X plus Harvel's 5X design factor, then it would be only approximately 17 psi.
The 192 psi applied to a one foot length of the 20" pipe would calculate to 192 x 20 x 12 = 46,080 lbs / ft length.
That is a bit more than your 62 kg. Could you picture a 20" ring of PVC supporting a 46,080 lb truck? No way.
The length of the pipe under water is subjected to the buoyancy of the empty pipe. If the pipe is vertical, then there is not a bending effect, but a column loading effect. If the pipe is horizontally oriented and restrained at the ends, then it would be like gravity case of weight causing bending - except bending would be in the upward direction.
RE: pvc pipe under external pressure
Depending on the manufacturer, wall thickness is expected to vary. I used a commercial pipe size table to obtain the cross sectional geometry. As others have pointed out on numerous ocassions, ovalness need be a consideration. Again, HypoSmurf would need to address this, the calculation is extremely simplistic and would need considerable project development to bring to reality.
As usual, UncleSyd brings an excellent point to the table, thermal gradient with water depth. The material properties for the PVC pipe would be expected to change somewhat at water depths measured on the order of hundreds of feet. HypoSmurf would need to consider material selection, hence obtain a superior MTR from his supplier.
Your observation about weight supported by the pipe. Remember my 62 kg mass is not over the entire surface area of the pipe, it is localized over the contact area of my foot. My foot contacts the surface of the pipe, ideally as a tangent to a circle, that is, at a point. What I am getting at is the issue raised of localized stresses and deflections. The 20 diameter schedule 20 pipe composed of "rigid PVC" material is surprisingly strong.
It is far easier to over analyze this problem. I would suggest that HypoSmurf is only interested with the order of magnitude depth a cross section of 20 diameter schedule 20 pipe could handle given atmospheric bore pressure. That is it, all we know, everything else is a complete unknown.
If anyone goes to the field and starts laying underwater pipeline based on this discussion, well, I give participants of this forum credit, I think we're all smart fellows here. Wouldn't you agree?
Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada
RE: pvc pipe under external pressure
Guess a horizontal pipe lying on a flat lake , one or both end are conected to the atmosphere.
I never guess that this question would make a so interesting discussion.
Now I will change my question.
I need to do a chamber with a 40" lenght pipe, the same pipe section diameter and material.
Both end caps are flat plates , tick enough to not to be in cosideration , guess 4" or more what ever is need .
The caps have a small grove and an "o" ring to seal it .
The caps are have 4 threaded bar to join it with the pipe, of course 8 nuts do the final job.
Like a old way of air filter.
Or as the caps where blind flange and the pipe a 40 inch length gasket , all arrange with the belonging stud or bolts and nuts.
All the array is vertical stand.
The atmosphere is normal or about 28.79174 inch Hg
The temperature is ambient
Now the final question , how low I can make vacuum inside the chamber????
Hope it does make a new and nice discussion , as the previous.
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RE: pvc pipe under external pressure
External pressure conditions are a little more complicated than internal pressure. For one thing, the surface area that pressure is acting on is larger for a given vessel(or pipe). There's a pretty good discussion of this situation in Roark's but the following may be of some help:
q = (t/R)*(Sy/(1+(4Sy/E)(R/t)^2))
q= external pressure
t=thickness
R=Outer radius
E=Young's Modulus
Sy=yeild strength
What do you think about , for the given values,
t=thickness t 0.375
R=Outer radius R 10
E=Young's Modulus E 406106
Sy=yeild strength Sy 5218
I got
t/r 0.0375
r/t^2 711.1111111
4Sy/E 0.051395448
(1+(4Sy/E)(R/t)^2) 37.54787447
(Sy/(1+(4Sy/E)(R/t)^2)) 138.9692512
(t/R)*(Sy/(1+(4Sy/E)(R/t)^2)) 5.21134692
So the final answer is 5.21 PSI .
Any comments???
www.cadtutorforum.net
RE: pvc pipe under external pressure
How does your original post relate to these last two. Why did'nt you just say in the beginning you wanted to build a vacuum chamber.
At sea level you'll have 15psi external pressure.
Connect a vacuum pump to one of the flanges.
Do not use PVC pipe.
Regards
pennpoint
RE: pvc pipe under external pressure
Download the following +GF+ handbook, Chapter 3:
http:/
Turn to page 8, and do the math for answers to both of your questions: Both pressure and vacuum.
CRG