Design Rectangular Low Pressure Stainless Steel Pipe
Design Rectangular Low Pressure Stainless Steel Pipe
(OP)
Hello everybody.
I'm tasked to design a 23" X 54" rectangular pipe, about 100 feet long. The pressure is quite low, at about 12 1/2 psi, but the moments produced astound me. I'm looking at 3/8" 304 L Stainless Steel plate with channel stiffeners. If I could do a round pipe, there'd be no problem. It is comparing the wall thickness of the round to rectangular that has me thinking I'm doing something way wrong. The indeterminancy and five foot side is making me consider a career in real estate. Pipe design books seem to only talk about using existing round pipes and configuring them for flow. I need some basics like: what wall deflection is allowed; Does Mc/I still apply (I'm using it); what references may be available; and Is this really as hard as it seems??
I'm tasked to design a 23" X 54" rectangular pipe, about 100 feet long. The pressure is quite low, at about 12 1/2 psi, but the moments produced astound me. I'm looking at 3/8" 304 L Stainless Steel plate with channel stiffeners. If I could do a round pipe, there'd be no problem. It is comparing the wall thickness of the round to rectangular that has me thinking I'm doing something way wrong. The indeterminancy and five foot side is making me consider a career in real estate. Pipe design books seem to only talk about using existing round pipes and configuring them for flow. I need some basics like: what wall deflection is allowed; Does Mc/I still apply (I'm using it); what references may be available; and Is this really as hard as it seems??





RE: Design Rectangular Low Pressure Stainless Steel Pipe
I've seen similar sized and similarly reinforced rectangular pipes used for gas turbine exhausts, at much lower pressure and with somewhat thinner walls, and the designers are always surprised when the pipes develop cracks in less than 30 days of service.
PIPES ARE ROUND FOR A REASON.
Write that down and nail it to your manager's face.
Mike Halloran
Pembroke Pines, FL, USA
RE: Design Rectangular Low Pressure Stainless Steel Pipe
Would you specify a round or square nail for that?
jt
RE: Design Rectangular Low Pressure Stainless Steel Pipe
I appreciate knowing I'm not totally out of it...
I mean, everybody keeps telling me it is no big deal, though none of them seem willing to step up and do it.
Suppose I could just use superglue?
g
RE: Design Rectangular Low Pressure Stainless Steel Pipe
From a design standpoint, I would try using the rectangular plate formulas from Roark to design the plate between stiffeners. Then I think he has frame formulas that can be applied to the stiffeners themselves. Normally, in designing the stiffeners, you wouldn't necessarily use Mc/I with the full plate width. AISC-ASD includes an awkward design method for "wide" flanges on beams that can be applied. It's not uncommon to assume that 16t either side of the stiffener acts with it.
If the pressure is fairly static, and flow is not terribly high, I wouldn't see any big problems, other than lots of stiffeners (and you might want to use a fairly high bending stress for this case). But if pressure is fluctuating or very high flow rates occur, you could get some vibration or unwanted deflection in the panels.
You say it's 304ss- can you use carbon steel stiffeners on it?
RE: Design Rectangular Low Pressure Stainless Steel Pipe
The only thing I remember from the design of rectangular duct is to keep the welds off the corners.
RE: Design Rectangular Low Pressure Stainless Steel Pipe
All things being relative, I'll just state: 70 to 100 cfs is the anticipated flow, so the velocity through this section would be 8 to 12 feet per second. I do wonder how the water will act in such a tall, thin 'pipe.'
RE: Design Rectangular Low Pressure Stainless Steel Pipe
If I remember right, the "L" grades of SS have somewhat lower stresses than the non-L versions. If you're not following a specific code, consider using a dual-spec material & design for non-L properties.
RE: Design Rectangular Low Pressure Stainless Steel Pipe
Thanks, I'm hoping to be able to go into work monday and tell them why this thing may work, but it is going to be much more expensive, thicker, etc. than a round, 48" Diameter HDPE would be.
RE: Design Rectangular Low Pressure Stainless Steel Pipe
Appendix 13 calculates stresses at the side plate midpoints and at the corners. It may require thicker sections than you'd really like, however your pressure is not that high, and the section is not that large. I don't believe it allows for external pressure design, so you would need to avoid those conditions.
Dual certified stainless plates should be readily available if you need the higher allowables. If by HDPE, you mean plastic, yeah, I would figure the stainless, fabricated duct is going to cost more.
Maybe this will be helpful.
Mike
RE: Design Rectangular Low Pressure Stainless Steel Pipe
RE: Design Rectangular Low Pressure Stainless Steel Pipe
Thanks, I'll try to find ASME... and yeah, the HDPE is plastic and carries the water to its ultimate destinations. I just need to get it to where they can connect.
RE: Design Rectangular Low Pressure Stainless Steel Pipe
I guess I've not given the limitations. This 'pipe' conduit or what-have-you is limited to the rectangular shape and cannot exceed 60" high, and preferably 24" wide though the stiffeners are going to make it wider. It is going into a 4ft wide by 5ft tall concrete box culvert and both the inside of the pipe and the remaining area outside of the pipe must be accessible for people (smaller than me!) to go through and inspect. i've got a ton of 'better ideas' but the shape, the location, etc. are being dictated. so all i need to do is come up with a design that will work and last at least fifty years.
g
RE: Design Rectangular Low Pressure Stainless Steel Pipe
One thing I haven't seen mentioned here is the possibility of rounding the corners. Do you abosolutely need to have square corners or can you bend the plate and give the corners a 2" radius? That would do wonders for your thickness calc's.
jt
RE: Design Rectangular Low Pressure Stainless Steel Pipe
Would it?? We expect to have the steel bent into shape so rounded corners are a given (at least in my mind). My drawings all show rounded corners and I've seen that as a potential problem due to the high moments at the corners. I've been thinking I need to put some sort of plate from the corners to the stiffeners, but the post from 'unclesyd' had me worried about doing so, though I assume he is referring to longitudinal welds. I'm still at the stage of handling the moment and deflection at the center of the five foot span and researching stainless steel. Apparently I lied to y'all in that 304L apparently isn't any easier to weld and perhaps has no place in this project. Anyway, as is apparent from my first post, I'm in over my depth, and our office library doesn't have too many references that approach this kind of a problem.
Thanks
g
RE: Design Rectangular Low Pressure Stainless Steel Pipe
RE: Design Rectangular Low Pressure Stainless Steel Pipe
But I find the requirement for external inspection access somewhat inconsistent with the fifty year design lifetime.
Inspection suggests that you expect it to fail. But if it does, how will you repair or replace it? And if it were not accessible, and failed, wouldn't you still detect the failure the same way? How often will it actually be inspected? What happens when it leaks?
Mike Halloran
Pembroke Pines, FL, USA
RE: Design Rectangular Low Pressure Stainless Steel Pipe
If it leaks, we fix it somehow...that is a possibility that has been disrupting my sleep. Fortunately this pipe leaking would be inconvenient as opposed to life-threatening. It is part of a larger system that if it failed probably still wouldn't be life threatening, but would be a big problem...you might see it on the news!
I haven't been trying to hide what the overall project is, just didn't think it really mattered for the specific questions I've had. This pipe is being placed in the outlet works of a small dam. there is 25 feet of head, and a fairly good sized pond impounded behind the dam and dike system. the referenced inspections are mainly for the dam. even though dams look like little more than piles of dirt, they are actually fairly complex structures, and a little bit of water in the wrong places can cause horrible damage.
g
RE: Design Rectangular Low Pressure Stainless Steel Pipe
Yes, it would. Go outside and pop the hood on your car/truck. Look at how the radiator header on the top or bottom is fabricated. See how they put a radius at the corner instead of a sharp bend? From another perspective, imagine if you could increase the bend radius to... 11½". Don't you think that would be a stronger design? So, qualitatively, any move from a 0" radius sharp corner towards your ideal 11½" radius is beneficial. If you could run a quick plane strain FEA you'd see just how significant a small radius is. Another example: Look at a 1 gallon milk bottle. It has flat sides with rounded corners. They're rounded for a reason...
jt
RE: Design Rectangular Low Pressure Stainless Steel Pipe
Thanks. I have no problem accepting that the rounded corners will be stronger, I just do not have a way of calculating exactly how much. I may be overdesigning as I have the "stiffeners" carrying everything with their spacing being determined by the flat plate stresses.
RE: Design Rectangular Low Pressure Stainless Steel Pipe
I don't have time to read all the threads on this topics but I found this problem very interesting.
I know you have a culvert space limitation to deal with but round pipe is the only way to go for your pressure and size.
My suggestion to you is to use 2 or 3 round pipes, staggered and tie into a header/disributor at each end to handle the flow rate and delta-P. The round shape will solve your stress problem. As for the access issue, you may be able to get ride of this depending on the corrosion rate and work around it. What do you think?
RE: Design Rectangular Low Pressure Stainless Steel Pipe
This was my first 'solution' but the guy who would have to crawl through the pipes disagreed. I may approach it again with the idea of an upstream manhole, but the area is really, really tight.
RE: Design Rectangular Low Pressure Stainless Steel Pipe
Well you guys must have a good reason to insist on crawing through the pipe for inspection. If that is the driving force then you have no choice. Anything can be built as long as you got lots of money.
RE: Design Rectangular Low Pressure Stainless Steel Pipe
As for inspection, you can now buy pipe inspection snakes, comprising a TV camera and light source on the end of an umbilical of arbitrary length, with little caterpillar tracks and a steering mechanism. You don't even need a particularly compact one for that pipe.
Mike Halloran
Pembroke Pines, FL, USA
RE: Design Rectangular Low Pressure Stainless Steel Pipe
I think this thread has about died.
I do appreciate your suggestions and comments. We'll see if once they realize just what this solution will cost them, whether they will want to do it or not... There are other, very different possibilities that we've suggested.
It has been a great review of the principles that got me excited about engineering oh so long ago.
gene
RE: Design Rectangular Low Pressure Stainless Steel Pipe
Still there?
I was originally planning on using a 4" radius at each corner, but I'm taking your suggestions and doing the whole thing round. Rough calculations on the sides certainly are an improvement since this eliminates nearly 2 feet. I would like to refine those calculations. I don't have a FEA program (other than a rather shabby one I wrote in '94 while in school) but even if I did, I'll need something I can show; step by step, to upper management people, so if you know of any 'by-hand' methods for dealing with the round to straight, etc. I'd certainly appreciate it.
How would you deal with the stiffeners? If I attach them, top and bottom, to the center of the round, will that cause a stress concentration? Should I come up with stiffeners to follow the rounded portion? Keep my rectangular frame and add plates from pipe to frame? or just leave the top and bottom alone now that they will behave like a pipe?
Thanks for your advice. I'm not really expecting you to do the entire design, but guidance is greatly appreciated.
RE: Design Rectangular Low Pressure Stainless Steel Pipe
I wouldn't attempt to support the curved plate on tangents- that's just asking for trouble, probably even with local doublers.
Mike Halloran
Pembroke Pines, FL, USA
RE: Design Rectangular Low Pressure Stainless Steel Pipe
It can be done. If you must have the rect. pipe you will have to stiffen very closely. my guess is that you need a 3/4 x 2 stiffener ring every 12 in.; you will habe to calculate that, if you do not have software, the pipe/vessel will force you into being a millionair in real estate.
Believe it, I am doing this for fun,
real estate is much easy and profitable.
genb
RE: Design Rectangular Low Pressure Stainless Steel Pipe
You mean there is something more fun than engineering?
Hard to believe...
RE: Design Rectangular Low Pressure Stainless Steel Pipe
(B&PV NGINEERING).
Tha's your problem with the Code, you do not read and read and read.
Again, real estate is much easy and profitable.
genb
RE: Design Rectangular Low Pressure Stainless Steel Pipe
g
RE: Design Rectangular Low Pressure Stainless Steel Pipe
What code covers rectangular pipes?
Besidesm who would be clueless enough to even try to use one? Oui, C'est moi. Mais ce n'est pas mon idée.
RE: Design Rectangular Low Pressure Stainless Steel Pipe
We have about 50 absolutely rectangular vessels operating at 70 psig at 600°F. All have the ASME stamp. The earlier vessels have a lot stay rods with later ones have no stays and use internal components to reinforce the flat sides.
The duct work mentioned in my previous post has not stamp but was designed according to the code.
RE: Design Rectangular Low Pressure Stainless Steel Pipe
If we start doing this on any kind of a basis (at this point, this appears to be the first (and only) in over 100 years) we will need to get some ASME materials and get our Mechanicals to show us how to do it.
For What it is Worth, I talked to the fabricator and his original plan was to fold two pieces into 'L's and weld along the top and bottom edges. Thanks for the heads up on the inadvisability of doing that. Now, we are looking at the full radius top and bottom, so that moved the welds to the midline of the long edges anyway...plus, the long edges are shorter. I'm still concerned with exactly what is happening where the straight edge begins to bend into the radius. It isn't a fixed connection, or a hinge, or ?? So I have free body diagrams all over my desk. Anyway, thanks.
RE: Design Rectangular Low Pressure Stainless Steel Pipe
RE: Design Rectangular Low Pressure Stainless Steel Pipe
Just wanted to close this by telling you we've decided to not use the rectangular pipe. Thanks for your help in this decision. We are moving the spillway and that immediately solves many problems. We will still have a square/rectangular transition section as we move from a 3' by 3' square opening into a 48" diameter ROUND pipe. Once everybody saw just how involved and expensive the rectangular pipe would be, the decision to spend the money to build a new spillway was less painful.