[PSUBS-MAILIST] Pressure Test

Tue Jul 11 07:59:52 EDT 2017

Steve,Cng tanks have a life of 25 years, and as you said, very robust.  Even buying brand new CNG tanks is cheaper than foam, I doubt I would buy new tanks but I would certainly want  at least 10 years life left.I still want to continue exploring a way to reduce the cost of foam.  CNG tanks are a last resort.Hank 

    On Tuesday, July 11, 2017 2:33 AM, Stephen Fordyce via Personal_Submersibles <personal_submersibles at psubs.org> wrote:

 (phew, having trouble keeping up with all these)Hi Hank,You're totally right in that it probably wouldn't be that much of an issue and there is plenty of redundancy built in.   Typically, the inner lining is good for about half  the working pressure of the complete cylinder.  The trouble is you can't quantify or inspect for it, and with dire consequences of a cylinder failure (the energy stored in even a full SCUBA tank is equivalent to a couple of pounds of TNT), everything is very cautious.
The life expectancy of the composite cylinders I've dealt with (ie. breathing apparatus) is usually 10 years, sometimes 15 years - which is pretty arbitrary and is mostly dictated by a theoretical maximum number of pressure cycles, ie. eventual failure is by fatigue.  Like most pressure vessels of that sort you don't get to take credit for doing only a tiny percentage of the maximum allowable cycles to extend the service life.
The test for underwater use is also pretty arbitrary - submerge a couple of pressurised cylinders in salt water for 3 months and then see if they will handle ~4000 cycles to test pressure without failing.  
Hi Keith,I get new cylinders from a (high quality) factory in China - they're designed for breathing apparatus, and the 2 sizes with the required paperwork for design registration in Australia are 6.8L and 9L.  Ie. ~1.6gal and 2gal - probably not hugely useful for a sub, but may save a bit of weight as an internal oxygen tank.  They are pretty expensive - more than a similar sized SCUBA tank.
Cheers,Steve
On Mon, Jul 10, 2017 at 8:18 PM, hank pronk via Personal_Submersibles <personal_submersibles at psubs.org> wrote:

Steve,That is pretty interesting, but I would think it would take a long time to be an issue.  The aluminum liner is extremely heavy duty in the order of about an inch thick. The carbon fibre and epoxy layer over the aluminum core is also about an inch thick.    When the salt water enters the cracks under pressure, does that mean the salt water is permanently trapped, or does it drain out when on dry land?  What is the life expectancy of a composite scuba tank?  What does the test consist of?Hank 

    On Sunday, July 9, 2017 9:10 PM, Stephen Fordyce via Personal_Submersibles <personal_submersibles at psubs. org> wrote:

 Ah ok Hank, cool.
The trouble with the composite tanks underwater is that when they are pressurised they expand slightly and this can open any cracks in the epoxy (which is only there to support the fibres giving tensile strength).  If you then take them underwater, the external pressure forces water into the cracks, and it can then sit there and do weird things - like corroding the interal alumninium liner.
They are pretty robust though, and I destroyed 3 (albeit new) cylinders after various saltwater and mechanical punishment, none of which failed badly.  Frequent  hydrostatic tests was the best I could come up with for seeing whether one is about to explode - obviously this is not ideal, and there's no data on how quickly corrosion can reduce the strength of the cylinder to the point of failure.
There is some empirical data from composite SCUBA tanks in use over time.  The standards really don't go into much fine detail - they just specify one (rather arbitrary) test to do for a cylinder to be used underwater.
Cheers,Steve
On Mon, Jul 10, 2017 at 12:49 PM, hank pronk via Personal_Submersibles <personal_submersibles at psubs. org> wrote:

Steve,Greg was referring to steel cng storage tanks that are 48 inch ID spheres 3.25 inches thick. I have also been looking at composite cng type 4 tanks as buoyancy modules.  The tanks would be at full pressure (3,600 psi) Hank 

    On Sunday, July 9, 2017 8:44 PM, Stephen Fordyce via Personal_Submersibles <personal_submersibles at psubs. org> wrote:

 Hi Hank,I was under the impression your CNG spheres are composite/fibre-wrapped?  I presume you're aware that using a model based on metal components wouldn't apply well to composite.
I think I posted about it, but I've done a lot of research and some testing on using composite cylinders underwater and it's potentially a bit scary, especially with salt water.  If anyone is interested I can go into more detail.
Cheers,Steve
On Mon, Jul 10, 2017 at 11:53 AM, hank pronk via Personal_Submersibles <personal_submersibles at psubs. org> wrote:

I use the calculator on Psubs written by Alec, it is fabulous because you change the values for different materials.  Hank 

    On Sunday, July 9, 2017 7:17 PM, james cottrell via Personal_Submersibles <personal_submersibles at psubs. org> wrote:

 I still think that a CNG sphere will go way deeper. How did you calculate that?

      From: hank pronk via Personal_Submersibles <personal_submersibles at psubs. org>
 To: Personal Submersibles General Discussion <personal_submersibles at psubs. org> 
 Sent: Sunday, July 9, 2017 4:38 PM
 Subject: Re: [PSUBS-MAILIST] Pressure Test

Very true.I do love the idea, but, weight is a real killer, every pound is super critical.  If I go with a 7500 footer then the buoyancy is cheap because I will use CNG tanks, don't tell Sean ;-)   Then it will be awesome to have it rotate, just think you can rotate so the port is pointing down between your feet.Hank 

    On Sunday, July 9, 2017 2:26 PM, james cottrell via Personal_Submersibles <personal_submersibles at psubs. org> wrote:

 Yes....BUT...being able to get out on your own is never a bad thing.

      From: hank pronk via Personal_Submersibles <personal_submersibles at psubs. org>
 To: Personal Submersibles General Discussion <personal_submersibles at psubs. org> 
 Sent: Sunday, July 9, 2017 4:12 PM
 Subject: Re: [PSUBS-MAILIST] Pressure Test

Greg,That was my original plan, I think it is a great idea for a shallow diver.  But if your going real deep then chances are it will be lowered into the water from a boat or barge, so need for the swivel.  If I end up being less ambitious, and build an Elementary 7,500 I will do just that.Hank 

    On Sunday, July 9, 2017 1:57 PM, james cottrell via Personal_Submersibles <personal_submersibles at psubs. org> wrote:

 Hank,
I was thinking that it might be possible to mount a sphere within a steel frame that allows it to swivel up and down like a gun turret in a bomber. That way, the forward viewport could be incorporated into the hatch and the hatch/ port could be in the "up" position to get in and out and then in the "down" position during the dive for viewing. All internal components could be mounted on an internal frame that hangs from a bearing so that the pilot is always upright. The main ballast system could be a pair of port/ starboard pontoons.
For a shallow diving sub, the hatch/ port could be a large dome for good viewing. A self draining "sail" could also be added around the hatch/ port. If Darth Vader had a sub it would look like that one!
Greg

      From: hank pronk via Personal_Submersibles <personal_submersibles at psubs. org>
 To: Personal Submersibles General Discussion <personal_submersibles at psubs. org> 
 Sent: Sunday, July 9, 2017 3:27 PM
 Subject: Re: [PSUBS-MAILIST] Pressure Test

Greg,Not a bad idea.  I have lots of time, I still need to finish Elementary 3000.  I am not planning to buy heads until fall of 2018.  I only bought the vertical lathe because I got a SMOKIN deal and what are the chances of finding another  machine like that in the Canadian prairies. Hank 

    On Sunday, July 9, 2017 12:23 PM, james cottrell via Personal_Submersibles <personal_submersibles at psubs. org> wrote:

 Hank,
You may want to contact these guys before spending a lot of $$$
Australia Pressure Vessel Heads 2011 Pty Ltd

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Australia Pressure Vessel Heads 2011 Pty Ltd
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Greg C

      From: hank pronk via Personal_Submersibles <personal_submersibles at psubs. org>
 To: Personal Submersibles General Discussion <personal_submersibles at psubs. org> 
 Sent: Sunday, July 9, 2017 1:23 PM
 Subject: Re: [PSUBS-MAILIST] Pressure Test

Greg,Hmmm, just googled EN 26 and it considered machinable even  hardened and tempered, but what does that mean?  Hank 

    On Sunday, July 9, 2017 11:12 AM, hank pronk via Personal_Submersibles <personal_submersibles at psubs. org> wrote:

 Greg,I asked EE if they could make HY heads and they can but need a quenching tank.  I can not find a supplier that can supply large enough pieces of HY steel  to press a hemispherical head.  EE does not supply anything but 516-70 .  I can ask them but would need to source the material first.  I would need to be sure I can even machine that material, I kinda doubt it?  Hank 

    On Sunday, July 9, 2017 10:36 AM, james cottrell via Personal_Submersibles <personal_submersibles at psubs. org> wrote:

 Hank,
Do you know if EE can fabricate your hull from EN26 steel? The hull that Ron A. had fabricated was only 2.5" thick and went full ocean depth with plenty of safety factor. Using the same grade steel in a thickness designed for only 13,000 ft might save a lot of weight.
Greg C

      From: hank pronk via Personal_Submersibles <personal_submersibles at psubs. org>
 To: Personal Submersibles General Discussion <personal_submersibles at psubs. org> 
 Sent: Monday, June 26, 2017 7:54 AM
 Subject: Re: [PSUBS-MAILIST] Pressure Test

Alan,That really turned out nice, how did you mould the polyurethane? that stuff sticks like mad. I would leave the white gasket, it looks fine.  Either the gasket thickness is not even or the seat is not perfectly matched to the Lenz. 

    On Sunday, June 25, 2017 10:30 PM, Alan via Personal_Submersibles <personal_submersibles at psubs. org> wrote:

 Pressure tested my light to 1000 psi for an hour using the water blaster 
& no leaks! I didn't have a pressure relief valve in the system as I do with
my air pressure set up & it was a pain getting the chamber lid off. I had
thought the water blaster would have leaked a bit of pressure but it didn't.
The sealing system for the wires was a 3 layered process; encapsulating the
wires in the epoxy, then coating the insulation for an inch up from where they came 
out of  the epoxy, & over the epoxy with a 2 part heat activated pvc glue that 
they use on inflatable boats. This glue worked a lot better on the pvc insulation
than several others I tried like E61000 (supposed to be better than shoe goo),
3M 5200 & polyurethane.
Over this, to tidy it up & act as a cable support, I moulded polyurethane.
In the attached photos the white ptfe gasket that the lens sits on looks wet
but it is just the compression of the lens against the gasket. I don't like this
from a cosmetic point & may change it. The 2 objects with the light are the
male mould for the polyurethane cable support & the silicone mould for the 
same.
Alan

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