Design Rectangular Low Pressure Stainless Steel Pipe 7

[eeh68]

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??

 

[vesselguy]

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?

 

[eeh68]

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.

 

[vesselguy]

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.

 

[MikeHalloran]

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

 

[eeh68]

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

 

[eeh68]

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.

 

[MikeHalloran]

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

 

[GenB]

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

 

[eeh68]

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

 

[GenB]

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

 

[eeh68]

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

 

[eeh68]

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.

 

[unclesyd]

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.

 

[eeh68]

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.

 

[unclesyd]

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

 

[eeh68]

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.