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Water Pressures

Water Pressures

Water Pressures


I am trying to get some rough calculations. We are trying to calculate the pressures on a PVC pipe that is 12" in diameter, 300 ft. underwater.

I know that at ground level we have 14.7 psi.
Every 10 meters down, you double that.

Essentially, at 300 feet below, you experience about 148 psi. of pressure pushing inwards in every direction.
Now this changes because a PVC pipe is a cylinder, basically an infinite arch. PVC pipes have pressure ratings but they are inward forcing outward, I am trying to calculate the other direction.

We are still trying to find out what kind of pipe we need when it comes to technical dimensions, (Material, temperatures, water tight, inner wall thickness, etc.)

Here are a couple calculations

Tensile Strength, psi @ 73°F 7,450 ASTM D638
Tensile Modulus of Elasticity, psi @ 73°F 420,000 ASTM D638
Flexural Strength, psi @ 73°F 14,450 ASTM D790
Compressive Strength, psi @ 73°F 9,600 ASTM D695
Izod Impact, notched ft-lb/in @ 73°F 0.75 ASTM D256

I should be able to calculate the failure point using the above and the dims of the pipe (12.75" OD, .187" average wall thickness for 12" duct pipe.) CL100 in 12" has a typical wall thickness of .311".


RE: Water Pressures

Bad news:

CODE -->

12.75	12.75	in	od of inner tube	
			tube gage	
0.187	0.311	in	wall thickness	
PVC	PVC		material	
4.00E+05	4.00E+05	psi	Elastic modulus	
1.54E+05	1.54E+05	psi	Shear modulus	
0.30	        0.30		Poisson's ratio	
5.22E+03	5.22E+03	psi	yield point for tube material	
6.375	6.375	in	outer radius	
6.188	6.064	in	inner radius	
6.2815	6.2195	in	mean radius	
2.9	13.7	psi	critical buckling pressure for a _perfect_ tube	
0.500	0.500	in	radial deviation from circle at worst point	
0.013045	0.013045			yield/modulus
546.4567326533	4349.7317922835			
-2.15E+02	-4.38E+02			
212.7242118038	428.245032953		p	
2.5688506636	10.1571097329		p	
2.57	10.16	psi	critical pressure for collapse of imperfect tube 

You may need to use a stouter material, or use a much smaller tube.

The numbers above were produced by a spreadsheet that I do not wish to distribute,
based on these papers:

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

The reference to an inner tube came from an investigation into collapse
of a jacketed tube under hydrostatic test of the jacket, which was the whole
point of doing the analysis in the first place.

Mike Halloran
Pembroke Pines, FL, USA

RE: Water Pressures

Thank you

RE: Water Pressures

I ran a goal seek on 148 psi collapse pressure, with 0.25" max out of round, which is possible if machining from billet, but may or may not occur in a pipe you can find. Hope it's been stored with the axis vertical.

Using the same 12.75" OD, you will need a wall thickness of at least .790" in PVC.

That will of course be heavy, but there is another consideration; have you figured out how much your pressure hull must weigh in air in order to be neutrally buoyant? Hint: it's the weight of an equivalent volume of water, or seawater, depending on where you are submerged.

Steel submarines are commonly ballasted with many tons of lead in order to allow them to submerge. That's probably another subject you'll need to read up on.

Mike Halloran
Pembroke Pines, FL, USA

RE: Water Pressures

Wow, Mike I'm surprised so little pressure will toast a 12" PVC pipe.

HunterMoore; Is this for an ROV?

Keith Cress
kcress - http://www.flaminsystems.com

RE: Water Pressures

Keith, I got interested in the subject when my shop was doing their normal leak check with air at 15 psi on the jacket of an exhaust pipe for a jet boat. Everybody in the shop, hell everybody in the building, heard a loud bang, but nobody could find the source, so we all went back to work.

At sea trial, the turboshaft engine didn't like the backpressure from the collapsed tube, between two elbows of a J-shaped pipe where it couldn't be seen, so we got to make another one for free. The inner tube was 13" OD x .075" wall, stainless steel. The outer tube was 14" OD, same material, mirror polished.

Mike Halloran
Pembroke Pines, FL, USA

RE: Water Pressures

Mike; I'm starting to see why hydraulic electrical explosive metal forming seems to work so well.

HunterMoore; Take a look at oil filled underwater systems. You could make the pipe out of shellacked cardboard and not have it collapsing.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Water Pressures

Yes it is a ROV,

I am not sure what you mean by oil filled systems, any more info? I would like to look into it.

Hunter Moore
Project Neptune

RE: Water Pressures

We built electric motors for sub- use.
They were vacuum filled with high purity mineral oil and had elastomer pressure equalization bladders.
They didn't mind 14,000' of head on them.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

RE: Water Pressures

What IR and Ed said.

Surprisingly you can put most electronics in pressurized oil and they wont care and all sorts of problems go away.

There's a thread in ET somewhere that went into this pretty deeply.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Water Pressures

Thank you very much, this sounds like a great way, do you think it is possible to just waterproof any electronics in the sub and fill it with one of these oils? What oil would I use?

EdStainless- Anymore info on the electric motors? Our next project was finding a relatively small motor to put onboard.

Hunter Moore
Project Neptune

RE: Water Pressures

thread240-141108: Schottky diode failure under pressure

Disregard our ignorance in the first few posts.

As for oil type. I'm not sure really. It probably doesn't matter much as the hydraulics of it doesn't care at all.

Secondary conditions may be more important, like if you want to retrieve the board and clean it to alter it. In that case something not poisonous and washable in your sink probably makes more sense. Cooking oil perhaps.

Doing this is not a slam dunk, you do have to think it through and make sure you're not doing anything stupid. Stupid, would for instance, be using aluminum capacitors. They enclose space that can include air making them likely to fail if compressed by external pressure. There may be some other items that will have the issue too. You need to consider all your components.

Do note that if your pipe is small enough you can take your 'cylinder' to a fire extinguisher shop where they do hydro testing of fire extinguishers. They have a safe pressure testing vessel that they can set accurately to any test pressure under about 12,000psi. The pressure testers have to be able to accommodate a 70cuft CO2 tank which is like a very large welding tank.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Water Pressures

There are a number of considerations in choosing the fluid:
> Safe disposal/toxicity/environmental impact
> Leakage impact to environment
> Corrosion properties
> Cleaning off the electronics afterwards
> Heat transfer

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

RE: Water Pressures

Thank you, I will look for a fire extinguisher shop, I have a 5 gallon bucket of hydraulic oil, used because it doesn't compress.

Anything else I should know?

Hunter Moore
Project Neptune

RE: Water Pressures

We used mineral oil, high purity, low viscosity, and no additives.
It was safe and easy to handle, with no health issues (you could drink it), and it didn't attack plastics or rubbers.
Such as this

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

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