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"Polypropylene / fiberglass tanks are bulging: how to add stiffeners?"



2005

Q. Hello,
We are in plastic fabrication business. Some of the tanks manufactured by us in PP + FRP are bulging dangerously. We provide MS stiffeners embedded in FRP to all the tanks but they still are bulging. Size of tanks are 18 x 3 x 3 high with 5 mm PP + 3 mm FRP.

Parag Patil
- Mumbai, India
^


"Design of Weldments"
by Omer W. Blodgett
from Abe Books
or

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(Ed. note: I fondly remember this as the very 1st book I made use of after graduation from engineering school ... the one that took me from theory to real-world practice :-)

2005

A. You didn't include units of measure, Parag, but first off I feel your materials are bit too thin. I would not design such a tank with less than 1/4" (6 mm +) thickness for the polypropylene and 5/16"-3/8" (8-10 mm) for the fiberglass. If the tank is 18 feet x 3 feet x 3 feet, the bracing can be done with a couple of structural tubing belly bands, but one or two toe braces on the long side may be better. If you mean 18 m x 3 m x 3 m, such a tank size would be impractical to brace with girth bands (belly bands) alone, and toe bracing (together with girth bracing) is probably a much better idea -- but you definitely want to up the material thickness to at least 1/2" (12 mm+) for the polypropylene and 3/8" (10 mm) for he FRP.

An old book, "Design of Steel Weldments" by Omer Blodgett =>
describes how to approach the design problem of bracing open-top rectangular tanks, and has been used as the basis for plating and anodizing tank designs by many suppliers for many decades. Although I'm sure that more advanced approaches have been described in the literature since (Finite Element Analysis didn't even exist back then), you would be well served to get a copy while it can still be found.

The real world problem will probably be that these tanks may cost several times the amount that was budgeted for them :-(
While that would be painful to hear, even worse would be to spend still more money playing around building completely unsatisfactory tanks. If these tank measurements are in feet it's possible that an experienced plastic fabrication shop foreperson can figure it out; if the dimensions are in meters, please retain an experienced engineer, as this is way beyond seats-of-the-pants design :-)

Good luck!

Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
^


2005

A. Hi Parag,

As Ted pointed out, some belly bands might well be necessary but that's not my cup of tea. Also he assumed you meant that the given 18 x 3 x 3 figures were metric dimensions ... but I've never, ever encountered a tank of that length (40 odd years in the plastic fabrication business and making the world's first successful dual laminates in the early 60's ... Alcore Fabricating) and even rarely in 'feet'.

The philosophy I used was to have a) a heavy longitudinal stiffener at 1/3 of the tank height plus, of course, a top flange. .... I wasn't much a lover of belly bands but just good adequate longitudinal stiffeners ... and they should do the trick.

Ted was also right to surmise that the material thicknesses were too thin ... it's NOT the PP thickness (heck, you could have used 1/8") but the fibreglass... and at a rough guess that should be min. 1/4" (6 - 7 mm) thick using alternate glass woven roving or cloth with matte in between.

It's the fibreglass that'll give you the structural strength ... and restrain the PP from expansion and buckling (presuming a good bond). Doubtless (I most sincerely hope) you acquired the PP sheets with the glass fibres already thermally bonded to it!

Didn't you check that tank before shipping it?

freeman newton portrait
Freeman Newton [dec]
(It is our sad duty to advise that Freeman passed away
April 21, 2012. R.I.P. old friend).

^


June 1, 2009

A. If the tank is dual laminate you may be able to use some heavy bore plastic pipe welded to the inside of the tank for extra stiffening.

Having read the post again I can answer the dimension dilemma I think. I believe it must be a rectangular tank. 3 mm thick FRP sounds disastrous for a rectangular tank of these dims. Heavy heavy stiffening will be required -- may work out cheaper to add a lot more layers of CSM to the walls. 3 mm works out at 2 Layers of 0.6 kg CSM. When I have designed rectangular tanks I was looking at 16 Layers up to 22 depending on the spacing between stiffeners required and this was for tanks. You really need to revisit your calculations as they really do not sound right for the application. Steel stiffeners may work for you but you will need a lot both horizontal and some vertical. As a guess I would suggest 10 stiffeners horizontally and maybe 1 stiffener each meter vertically. Rectangular tanks are real nightmares; the flanges are probably already fitted so you are limited to where to put the stiffening -- if flange clashes it may mean adding more laminate to the entire tank to avoid the clash. I designed a PP rectangular tank which started bulging I solved this by adding the internal stiffening as per my previous post. You are more than likely going to have to entirely redesign the tanks. I wish you luck

Chris Allen [returning]
- Belfast, Northern Ireland
^



August 15, 2014

Q. Dear All,
I want to weld a rectangular tank by PP sheets with 20 mm thickness.
Tank dimensions are as below: 3 m * 1 m * 1.5 m (L*W*H).
Would you please help me to choose the right horizontal stiffeners for this tank?

Thank you in advance

Jalal Jafari
surface and electroplating technologies - Isfahan, Iran
^


August 2014

A. Hi Jalal. I'd probably use two (2) 4 x 3 rectangular structural tubing belly bands (also called girths or horizontal stiffeners) located 23" and 42" down from the rim, plus a 3" angle iron rim. This is not a considered design by a professional engineer, just a free "feel" for typical designs from my experience.

Good luck.

Regards,

Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
^


September 3, 2014

A. Dear Mr. Jalal.

Especially for P.P. Pickling tank there should be M.S. cage surrounding the tank based on its size. Generally for your size of tank we use 100 mm x 50 mm x 3 mm thick RHS duly fiber coated and insert the P.P TANK inside it. For your size of tank vertically you should have at least 6 supports @ 500 mm distance and Horizontally you should keep 3 support. first support at top of the frame, second one @ 500 mm from there and third one again @ 500 mm. you will get best result of it.

Regards.

Ilesh G Vyas
Gunatit Builders
supporting advertiser
Manjalpur, Vadodara, Gujarat

gunatit builders
^


September 6, 2014

Q. Dear All,
Thank you for your answers.
As this is our first experience in tank fabricating.
Our base plate has 4 cm oversize from each side. How we can install vertical stiffeners? Would you please give me some references or Photos?

Thank you again

Jalal Jafari [returning]
- Isfahan, Isfahan, Iran
^


September 2014

A. Hi again Jalal. This is not a really large tank, and as long as you are not putting really heavy weight loads on its rim, I do not think vertical stiffeners are required. Rather I think you'll be okay with just horizontal stiffeners.

36482

Usually you make a 1/4" thick polypropylene "cover" for the steel stiffeners to protect them from corrosion, and weld it to the tank wall to hold the steel stiffeners at the right height.

This is, as I say, not a large tank, but it's not a really small one either and it's always better to start small since this is your first effort. Good luck.

Regards,

Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
^

36482-2

September 8, 2014

Q. Dear Ted,
Thank you again for your nice support.
Which side of steel profile must be flange side? smaller or bigger?

Can we use screws to join steel profiles at 90 degrees together or just we must weld steel profiles?

Thank you in advance
Jalal

Jalal Jafari [returning]
- Isfahan, Iran
^


September 2014

A. Hi Jalal. Steel members are much stronger against bending "the long way" than "the short way". So the 3" dimension would be touching the tank wall, and the 4" dimension would be perpendicular to the tank wall. I can't imagine a practical bolting scheme for the steel tubing; cut them at a 45° angle and weld them please.

In America we do not design structures in so casual a way. Rather, a plastic fabrication shop starts small, and from long experience gradually develops a "seat of the pants" feel for good and safe designs; and they retain an engineer for designs that are not within their experience.

Regards,

Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
^


September 12, 2014

Q. Dear Ted,
Hope to be fine.
Thank you for your details and information.
So before we weld the top frame [rim] of PP, we must install steel stiffener and fix them on the right height and then top frame [rim] of PP?

Thank you again.

Jalal Jafari [returning]
- Isfahan, Iran
^


September 2014

A. Hi again. Yes, that would seem to be the way to do it. Alternately, you can lay the tank upside down, and slide the steel stiffeners down to their position (although if I understand you, the bottom plate on your tanks may be bigger, and may prevent that approach).

Regards,

Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
^

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