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wasnt sure baout the risks
wasnt sure baout the risks involved at that press tho it sounded high, i was just doing the maths
wasnt sure baout the risks involved at that press tho it sounded high, i was just doing the maths
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Are the qualitys of PVC pipe in question?
Hi Everyone... With safety a concern I went checking the working pressures of certain types of PVC pipe. The lowest working pressure for common 1/2 to 4 inch schedule 40 pipes was 160PSI. I have found some as high as 280 PSI. In the USA the water pressure in my home (common place household water pressures) is 50 to 60PSI. At my work place we have many air operated machines working continuous at 70 PSI on cheapo thin walled plastic air line with cheapo plastic fittings. Also at my workplace we have PVC airlines running down the wall to the machines which are 150 PSI. So making a ballast tank ( possible pressures up to 40 PSI) should be no safety concern so long a high quality PVC is used. But another concern which has been brought to my attenion... The pump will pump WAY less volume at the higher pressures as I don't want to take all day to submerge my boat. I would think 20 to 30 seconds to a full dive would be appropriate. It's a gallon a minute pump at 10 to 15 PSI. Does anyone know of anyplace which sells a 20 PSI cutoff switch with NO low end cut in? And then I could make my pump quit at 20 PSI and set the tank volume up accordingly. I do plan on putting a screw on end (using teflon tape to seal the threads NOT GLUE) on one end of my tank as then I can adjust the volume of the tank without having make the tank shorter all the time to lessen the volume. If anyone knows differently reguarding safety to thee above info I would certainly want to hear about it. As SAFETY is of the HIGHEST concern.
THANX
ScottComment
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Hi spitfiremk3,
You're probably making the
Hi spitfiremk3,
You're probably making the design more complicated than it needs to be since everything will be limited by the pump, I think.
I'm not quite sure what sort of pump you will be using but a gear pump, for example, will not give a pressure of more than about 30 psi. That will give you a tank about 2/3 rds full. Pressure switches will probably not be necessary.
You are probably best advised to buy the pump and do some tests pumping into a container with maybe a pressure gauge attached. If you stand the container on some scales you can monitor the delivery rate of the pump which will reduce to almost nothing at the highest pressure. The highest pressure determines the maximum amount of water you can pump in. You may consider using soft drink (soda)bottles as a test tank. I haven't tried it but I understand that they can safely contain several atmospheres pressure. (Have a look on the web for the various sites on "water bottle rockets" or similar!)
Don't forget baffles in the tank and do be careful when you operate the sub to keep it fairly level fore and aft. One often overlooked problem with the pressure tank system is that you can lose your compressed air if you surface at an extreme angle. No compressed air means no surfacing!
I now use my gear pump in forward and reverse to pump in and pump out for quicker surfacing and to help empty the tank. It is also worth taking the sub out of the water every hour or so to pump the tank out completely. (You can slowly lose some of the air in your tank.)
As far as estimating the tank size goes, I think it has been discussed before in the forum. Skip Assays's method has much to commend it- floating the model at neutral buoyancy then strapping foam under it until you get the right waterline. Or maybe he strapped weights on until it was neutrally buoyant! Anyway you get the idea!
I hope my suggestions don't put you off the pressure tank system which is used a lot here in the UK. I use it and like it.
I use a home made fibreglass tank which forms part of the "equipment tray" so I can move it backwards and forwards when I set up the initial trim.
DavidComment
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Norbert Bruggen equips his 'gummysack'
Norbert Bruggen equips his 'gummysack' system with a pressure switch, although i'm not sure what the pressure rating is. You could try asking him-
AndyComment
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Info update...
I picked up some tubing and brass fittings today pus some hose clamps. This weekend I will put it all together for some tests on my homemade pressure ballast tank. I also have a PSI guage that I will hook up to the tank so I will know how long it takes to fill the tank and at what pressures. I have also learned... Clear flexible plastic tubing ( 40 PSI working pressure) and the semi-clear "harder" but still flexible plastic tubing has a working pressure of 130PSI. I will keep you guys posted on my experiment.
THANX
ScottComment
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Have fun Scott!
On a safety
Have fun Scott!
On a safety note try to keep containers full of water rather than air (less energy stored) and do wear eye protection!
Wrapping the whole thing in a wet towel gives psychological protection also!
Sorry to labour the safety point but US lawyers have a long reach!
(And the pressure tank system is worth it - cheap and simple.)
DavidComment
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Question???
Question... In a pressurized system... How does the boat lose compressed air in the tank when surfacing at an extreme angle? So long as the tank is SEALED, why would it make a difference?
THANX
ScottComment
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Well with the sealed tank
Well with the sealed tank system, you have an air bubble, and thus an exposed water surface.
At the bottom of the tank is located the vent for the pump and valve which allows water in and out. Clearly, if the angle is too steep, it's possible for the air bubble to vent through the pump and valve, which will quickly result in a boat completely out of control, as the tank will then compeletely fill with water.
The risk of this happening is greatly reduced if you have your tank baffled well.
AndyComment
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So you would want to
So you would want to put the tank vent hole in the aft end of the tank... Unless your sub is going to hang vertical in the water... Aft end UP!!! LOL... Thinking... The aft will be the lowest piont when surfacing... A while back I read a story that a WWII damaged U-boat hung vertical in the water for like 24 hours until they got the leaks fixed and the water out.
THANX
ScottComment
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Did Some Teating
Hi... I did some testing today. I used different PVC pipe than in the early photos I posted because I had the end caps on hand. The newly constructed tank hold 80 fluid ounces (totally empty). I takes 40 seconds to fill the tank to 40PSI (when the pump quits by it's pressure sensor). It only take seconds to dump the tank at 40 PSI. The tank will dump 56 OZs, not counting what is left in the lines and the little bit left in the bottom of the tank. The Shur-Flo pump holds the pressure when the pump is not running so its acting as a 1 way valve. I only timed this for 10 minutes though. All I need to do now is make a valve to replace the PSI gauge (in the pic) to dump the water. I am happy with the results as my gold was 64 fluid ounces as a starting point for my sub. The total cost of my ballast system has been around $75.00 (less the valve I need to dump the tank). Not bad for a BIG boat.
THANX
Scott
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Hi Scott,
That is an impressive
Hi Scott,
That is an impressive pump! Producing 40psi it must be some form of positive displacement pump and that also may explain why it works as a non return valve. I think that pump will do very nicely.
Adding to Andy's comments - unfortunately subs pitch forwards and backwards. So you need your connection in the centre at the bottom really. This can take up useful space on the diameter of the tank so I use something rather like the clunk tank used by our aeromodelling friends i.e it feeds in from one end with perhaps a flexible hose internally.
You can really lose your compressed air reservoir as you use your sub. I think it is a combination of sloshing in the tank releasing the odd bubble and a little bit of air actually dissolving in the water. Hence the need to lift the sub out of the water and empty (pump out preferably) every hour or so.
I think this aspect of the pressure tank system is often overlooked. You have no way of topping up the air reservoir while your sub is in the water.
I like your test rig. A little bit of experimentation goes a long way and you look to be nearly there!
DavidComment
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Those pumps use multiple diaphragms,
Those pumps use multiple diaphragms, two or three I believe. So indeed positive displacement, and powerful.
Quite a sizeable tank there, about 5 pounds in weight, so no big surprise it takes a while to fill it up.
As a side note, I had a play with a compressor used for car air horns. these are very inexpensive to purchase (I got mine off of ebay for a couple of quid), and they are of radial design.
They move a LOT of air, but can't reach very high pressure (I'd say 15-20psi tops).
I hooked up a highly scientific ballast tank- a 4 pint milk bottle (plastic, not glass). The pump blew it clear of water in about 5-6 seconds!!
So, this would make a good secondary system, perhaps using aspirated main tanks with a smaller trim tank using a pump or piston tank system.
AndyComment
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1 GOOD THING!!!
One good thing is... In this boat you have alot of room... LOL... But I might NEED 3 guys to help me carry it!!! LOL... How long does it take to fill the tank in your boat(s)? And when I get my boat up and running I might make a tank from copper or at least remove the screw on end from the tank. It will look neater that way. With the scew on end of the tank, you can lessen the volume (just put something solid inside) on demand without making a smaller tank especially for test purposes
Also at 40PSI ballast tank pressure, If over time I "loose" a little pressure, I will have pressure to spare. If it was a 15 PSI tank and you loose a little pressure... You are almost down to nothing. Another good thing at 40 PSI is the tank can be a little smaller in size too.
THANX
ScottComment
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The Shur-Flo pump, 12 volt,
The Shur-Flo pump, 12 volt, 4 amp, 1 gallon a minute at 10 to 15 psi, 40psi max with auto cut off was $42.00 delieved from E-Bay.
THANX
ScottComment
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Another question???
Hi... To those around the world... Does anyone know how to make a GOOD valve (open and closed) that will take 40PSI? I am presently looking at 12v soleniod valves. They cost more than the pump... And the pump does both... As in pumps and works like valve.
THANX
ScottComment
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