August 2009,Much of the work I've posted this year relates to a pressure chamber I've made. I'm not prepared to explain the motivation at this point but aspects of the projects are already online.
This is not a "how too" guide on building pressure vessels.
Pressure vessel failure can be very dangerous even at fairly low pressures and isn't something to be taken lightly. There have been fatalities from failures in home made vessels.
The project covers many topics which may be of interest to people working on totally unrelated fields.
The chamber is filament(roving) wound epoxy glass. It has an approximate volume of 800 litres and has been tested to 3 BarA (ie Bar absolute) or around 30 PSIG in obsolete units. The working pressure is two BarA. To put it in perspective this is half as much as I have in my car's tires.
Some pressure vessel comments.If you are building a pressure vessel don't use this a site as guide I'm not qualified to give this sort of advice. The basic engineering equations are online elsewhere if you are into that sort of thing.
There are also freeware structural analysis program available such as "LISA". I found LISA would take more time to learn than I was prepared to put into it.
I do want to point out a couple of traps.
One is not all glass or fibreglass is equal. Just because my chamber holds pressure doesn't mean another one made to the same dimension will also work. The type of glass and resin used is important as is the fabrication method used, the fibre length and orientation and quality of the work.
The would be a vast difference in strength between an epoxy glass filament wound vessel and a for example a polyester glass one made with a chopper gun. If the resin is defective, improperly measured or poorly mixed the thing could be extremely weak.
The other trap is wall thickness must be scaled with size to have a similar working pressure. This was not obvious to me initially and even educated engineers I know got this wrong when asked. A small plastic bottle might hold over ten Bar pressure but you can't say "ok a 1mm PETE wall will always hold ten bar". This also applied to very low pressure items such as balloons or the inflated patterns I use to make me chambers.
When I made my scale model I had to take scaling factor of the wall thickness into account.
The construction of the pressure vessel and the testing leading up to it are posted under the composites heading.
The winding of the vessel is shown in full chamber-winder3.
|The chamber near the end of the first winding session.|
The chamber also needs a lid or hatch capable of withstanding several tonnes of force.
|The lid is 9mm thick vacuum infused exopy/glass|
Several projects under the "Micros and electronics" also relate to it.
I plan to use one or more nokia LCDs to display chamber information.
|This is a test using a SAM7 as the host micro. It is just a static mock up - no real sensors where fitted. The SAM7 can't be a TWI slave so I've changed to using slower AVR micros.|
The TWI/two wire network stuff is primarily for chamber monitoring.Arduinos will be widely used. Mostly I've bought 5V 16Mhz AVR328 based boards but other types maybe also be added.
The pressure plots below are displayed in real time from data sent over a thirty meter TWI.
Some pressure data plots.
|A plot of pressure overnight showing some drift in the mechanical pressure regulator. There was a small leak and the pressure was being actively maintained by the pump. The project could do with an electronic pressure controller.|
|This is the depressurization through a 7.5mm opening. The sample rate it 10 times faster than before. Each grid line is 2 seconds. The slope is a curve as you would expect with the relative pressure dropping to half in around 36 seconds.|
Seals.Making a suitable seal or gasket is a whole research subject in itself. The seal must the air-tight and able to withstand tonnes of force. My unusual geometry means the seal is also under a significant shear load. The gasket should also spread the load to minimize stress in the fibreglass.
I've tried many things and the majority of tests were done using stick-on neoprene. Neoprene sealed well but crept under the shear loads and move towards the rim of the lid. Not only did this cause a gap to form where the strip abutted but it also caused an uneven load on the lid which stressed the composite. It didn't fail or blow out and I'm still using the lid for tests but it will need to be replaced.
The most promising seal is polyurethane. A single part polyurethane sealant is applied and allowed to harden with the lid in place. Unfortunately I've had serious problems with the lid becoming stuck. The trick is to find a good release agent and I know three things which don't work reliably - petroleum jelly, PVA release agent and silicone lube spray.
There are commercial release agents available if you can source them in small quantities.
I've run a few tests (more in progress) and wax and silicone rubber are looking very promising.
Another very promising agent is aerosol water proofing spray, I'm mainly using "selley's waterguard". I think these sprays are similar to "scotchguard". "Mr Sheen" is fairly likely to work along with other silicone and/or wax polishes.
I have PU seal on the scale chamber which works very well and I've used PU on the big chamber but needs more work to find a fabrication method I'm happy with.
I applied a coat of PVA and two costs of waterguard to the chamber. I applied a thick bead of PU to the lid and held it in place for 4 days using an aquarium pump as a low pressure source.
The lid released from the chamber without any problem and is air tight.
More PU will be added later but the first batch is good.
Leaks,leaks and more @#$%^&* leaks.I've had a lot of trouble tracking down small air leaks. Most of my problems trace back to having leaks in the inflated pattern. These directly caused leaks in the fibreglass pattern and secondly caused me to attempt to work too quickly in getting to first layers of glass on. I made mistakes and didn't mix resin properly. This has plagued me even since. I also choose to wind with minimal resin content to save resin and reduce weight - a wetter fibre would have sealed the leaks.
The vacuum failure mentioned above also caused some new leaks.
I have tried many ways to find the leaks so I could patch them. Because of the open nature of the winding - wiping with soapy water doesn't work. I don't have a pool or large body of water to float to chamber in to look for bubbles but borrowing or improvising one is an option.
Many leaks were found by ear - with or without listening aids (tubes). One or two were found by feeling the escaping air against my skin. Some were found by placing a bucket full on soapy (bubble bath) pigmented water inside, pressurising and rolling the chamber around to see where the water leaked out.
One thing I tried which didn't work was to fill the chamber with pigmented soap bubbles.
Another method may have worked but only found a leaking area I already knew about (and possible it was the only leaking area). The method was to burn dozens of incense sticks inside the pressurised chamber and use my nose.
Knowing where the leak is on the outer surface doesn't always tell you where to patch inside.
I've also trying pulling a vacuum around the external leak location to locate the interior defect but without much success.
I also dragged the garden hose into the shed and ran water all over the surface. This did find a leak but didn't show the true extent of it. The water made a big mess so I scaled it down by using a weed sprayer full of soapy water instead.
The sprayer seems to be as good as any other method if you don't mind contaminating the area with soapy water.
Patching...Laying glass and applying resin while inside the chamber is very difficult and messy.
I worked out that doing a wet layer on a plastic sheet and then applying it to the inner surface works fairly well.
When all else fails I pull a slight vacuum inside and suck resin into the damage area (some success). The same method could be used to do a dye trace but I'd rather keep the areas clean if possible.
The supposedly airtight inner layer is not structural. The main strain of holding the pressure is taken by the filament wound exterior.