×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Design Rectangular Low Pressure Stainless Steel Pipe
6

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??     

RE: Design Rectangular Low Pressure Stainless Steel Pipe

2
That's a big pipe, and the net force on each face is substantial.  The problem becomes worse, of course, when the pressure changes.

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

Mike-

Would you specify a round or square nail for that?

jt  blllttt

RE: Design Rectangular Low Pressure Stainless Steel Pipe

(OP)
Thanks,
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

I would anticipate that the design could become unreasonable.  Even if it's rectangular at each end, it might be worthwhile to transition to round in between.

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

Look for a book that covers designing duct work.    

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

(OP)
The "pipe" has to stay rectangular throughout its length.  No, the stiffeners (channel/angle/hollow setions, etc) will also be SS  to avoid corrosion.  May use 316L SS for the stiffeners for the slight additional strength, haven't yet checked availability. I'm not convinced they would change the design much anyway.  The pressure will be about 11 psi (pushing out) when full, with possibly 12 psi (pushing in) while emptying.  It will be vented, but in the field sometimes the undesirable things we couldn't quite do in the lab happen.  I'm planning on an 8-inch vent line at the high point of the line, but a fast evacuation with the help of a plugged vent could result in very low pressure in the 'pipe.' At the elevation where this will be located, atmospheric is about 12.2 psi, so that is what I'm using for all calculations.  I'll see what I can find from Roark, and thanks for saying "16t."  
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

Normally, in a round pipe or vessel, pressure tends to round it out, whereas vacuum tends to buckle it.  But here, it's stiffened either way, and it won't make a great deal of difference if it's vacuum or pressure.

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

(OP)
Yes, the 304L SS is lower, Fy=30,500 psi, but it is easier to weld as in less steps before, during and after.  It has about 0.03% Carbon whereas the regular 304 has about 0.08%.  If nothing else, I'm learning about stainless steel.  316L or 316 is much stronger, but cost about 1 1/2 times as much, so my first, thick-walled, 316L design was shot down.  
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

eeh68, if available, you might take a look at ASME Sec VIII, Div 1, Appendix 13. It deals with pressure vessels of non-curcular cross section. If you don't have software that runs these calculations, they are not too difficult for a simple rectangular section as you describe. The appendix also has calculations for a stiffened section as you describe.

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

If you can have "dead space," how about a big round 60" pipe for pressure, with a thin rectangular flow liner with support plates/discs along the length ?  {check for flow-induced vibration}

RE: Design Rectangular Low Pressure Stainless Steel Pipe

(OP)
Mike,
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

(OP)
arto etal,
     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

eeh68-

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

(OP)
jt,
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

When we made our duct we made channel sections and welded the side plates to same.   The the corner was rolled with about a 1"-2" straight section beyond the tangent line of the rolled corner.  This was to take the bending moment at each corner out of play, in other words the weld doesn't see this moment.       

  

RE: Design Rectangular Low Pressure Stainless Steel Pipe

I'd be inclined to make the channel sections in quarters, each having just one short bend, to the outside, so all the corner seams are bolted, or plug welded, with all the fasteners on the outside.

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

(OP)
Sorry, this pipe isn't necessarily the subject of the inspection, though it would be inspected.  The things requiring inspection are 'upstream.' The gates at the mouth of this new, pressurized pipe, and a free discharging spillway above and ultimately to the side of the new pipe.  Also the concrete box culvert the pipe will be in along with its existing waterstops.  I think the inspection frequency is every 6 months, though the users may inspect it more frequently.  We obviously have more than one entity involved, actually several within our own office.  The fifty year life I mentioned, is probably less than expected.  We generally look at 200 year lives for the things we design and build...This particular pipe is out of the ordinary for us, and many of you have helped me see it isn't exactly ordinary for anybody.
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

eeh68-

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

(OP)
JT
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

Hi eeh68,

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

(OP)
vesselguy,
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

eeh68,
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

You could at least make the pipe as round as possible, radiusing the corners until the short side disappears.

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

(OP)
Thanks Mike and everybody else.
     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

(OP)
Mike and JT,
   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

Now that you've got the radius up a bit, another option presents:  roll angles or tees into half-rings.  Or just use heavier plate.  There's probably a balance to made there if you care to screw around with the problem long enough.  

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 became a long discussion!
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

(OP)
GenB
    You mean there is something more fun than engineering?
Hard to believe...

RE: Design Rectangular Low Pressure Stainless Steel Pipe

Believe it, I am doing this for fun,
(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

(OP)
I did understand that.  It is my stamp and signature going on it. But I've gotten some excellent advice and sincerely appreciate it.

g

RE: Design Rectangular Low Pressure Stainless Steel Pipe

(OP)
P.S. What Code???
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

According to the banner ad from Pauling.com on top of this page they have the software to cover rectangular pressure vessels.  All their software is code orientated generally ASME or API.

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

(OP)
Thanks UncleSyd,
     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

Look at the points of inflection in the moment diagram welding at these points will take the bending out of the welds.

RE: Design Rectangular Low Pressure Stainless Steel Pipe

(OP)
Hi Guys,
    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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources