Accumulator tanks as pressure chambers

Guides and discussions about building water blasters and other water warfare devices such as water balloon launchers.
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SSCBen
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Accumulator tanks as pressure chambers

Postby SSCBen » Thu Nov 10, 2016 12:36 am

A few people back at SSC made backpack blasters with accumulator tanks, also called bladder tanks. Basically, the tank has a rubber gasket separating air and water, so it'll work at any angle. They are used in RVs and homes to smooth out the flow if there is noise and also to avoid turning on a pump. We can use them as expensive and highly durable pressure chambers.

Right now at UT I am getting parts for an experimental water jet system. I plan to buy a big 20 gallon accumulator tank.

In the back of my head I am planning a new water gun that uses the pinch valve Tim found, and I think a 1 to 2 liter accumulator tank pressure chamber would be an ideal pairing.

I started looking into smaller accumulator tanks, but I couldn't find any small ones with an outlet bigger than 1/2" pipe. Some of these tanks also had a bend in the flow path, which is bad for turbulence. Anyone know about these tanks? I don't want to use what I found. (I am on my phone right now, so I'll post links later.)

This 2 gallon tank is tempting. I think it would be much too heavy completely full. It might be best to use it as a precharger by keeping around 1.5 liters of water in it to have a huge air water ratio. The tank weights 5 lbs, which makes it pretty heavy as is. A similar plastic tank might be a better choice.

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Re: Accumulator tanks as pressure chambers

Postby SSCBen » Sat Nov 12, 2016 1:33 pm

Doing the math, I get the impression that the weight for the 2 gallon tank might be acceptable. A dual chamber APH's PCs weigh about 4 lbs. The precharger I made last summer is about 9 pounds, and was really heavy. I'd like a lighter tank, but at least it's not much worse than an APH.

Unfortunately a pinch valve needs higher pressure than the water is under, so using the air in the tank is out. A separate tank for the trigger air is necessary at the very least. I designed a CAP system before that operated like a separate PC water gun and that might be acceptable here. As I recall, the pressure would keep increasing above the regulated pressure during the filling step, making the filling step require more input energy than you get out when firing. This is bad, but latex tubing only returns about 2/3 of the input energy by my calculations, so it's actually not unusual. If the extra pressure was used for the trigger, this would increase the energy efficiency by using the extra energy and also decreasing the input energy. Using the exhaust air as an "air sheath" would further improve the efficiency, but that technology is still in the very early stages for us. (Tests in the 70s prove it works.)

I need to do the math to figure out how many shots you would get out of such a system. The temperature/specific internal energy could keep increasing, but the air mass would decrease, and I imagine that eventually would lead to sufficient pressure drop for the gun to not work.

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Re: Accumulator tanks as pressure chambers

Postby jSpazz » Mon Nov 14, 2016 4:02 pm

I am an electrical engineer but I work with hydraulics a bit. I'm not familiar with the kind of setups you are referring to though, and can't speak to what kind of turbulence you would experience.

The accumulators I've worked with have rubber bladders inside them that are precharged with gas, and then the chamber is filled with liquid. The liquid compresses the bladder as it is pumped in, and then the bladder pushes out on the liquid as it exits the chamber. Since hydraulic accumulators are usually designed to be used with hydraulic oil, the bladders are normally charged with nitrogen. With water there would be no risk of explosion, so charging with air should be fine.

If you were intending to simply use an accumulator as a pressure chamber, you would not have to worry about losing air when firing since it never leaves the bladder. If you are intending to use air pressure for other purposes, you are probably correct in the assumption you would need a separate air tank.

Depending on which kind you use, you have to be sure to charge the bladder before filling the chamber with water. If you don't do this you can kink and damage the bladder.

As to the amount of power you can get out of one, I don't know how efficient they would be compared to other construction methods you guys have come up with. They are usually not intended to be the main energy source for a hydraulic system.

I realize I'm probably telling you stuff you already know, but I figured I would still chip in.

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Re: Accumulator tanks as pressure chambers

Postby SSCBen » Sun Nov 20, 2016 3:04 pm

jSpazz, I appreciate your post. I don't know much about these tanks and learned a few things, like to not charge the tank with water if the air side is not pressure.

I bought an "expansion tank" at Home Depot for about $40. See the attachment for a comparison with a pinch valve. I also ordered a fitting to attach an external air tank to the air side (Schrader valve to 5/32" tubing), which should come in soon.

The tank is not that heavy, but bulky and awkwardly shaped for a water gun. I believe these tanks have promise, but the common 2 gallon size is not ideal for our needs. The 4 gallon size one might make a great backpack gun, but it would be annoying to fill due to how the tanks work.

I have been doing the math needed to design a CAP system that operates like a normal separate PC water gun. Likely I'll post some analysis here in the coming weeks.
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Re: Accumulator tanks as pressure chambers

Postby Tim » Fri Nov 25, 2016 1:28 pm

Hey Ben,

"A few people back at SSC made backpack blasters with accumulator tanks..." - I recall Dusty's Water Gun and Andrew's AR-1.1 "Typhoon". Were there others?

"I started looking into smaller accumulator tanks, but I couldn't find any small ones with an outlet bigger than 1/2" pipe." - Andrew used a 1.32 gallon (5L) Accumulator, but you'd have to consult the seller regarding the connection specs. It might have a BSP thread instead of NPT, but 1/2" & 3/4" BSP is "compatible enough" with NPT to get a leak-free seal because they both have the same Pitch (TPI) in those two sizes.

Johnson Pump has a half-gallon accumulator. The threaded Tee that it comes with can be removed, but again, you'd have to ask the seller for the actual tank connection specs.

Perhaps best of all is the Groco PST-1. This 1-gallon tank has a confirmed 3/4" NPT connection.

Although accumulators for hydraulic oil systems may not come pre-charged, the ones above will have a pre-charge. Nonetheless, you can fine-tune this pressure. Charging with Nitrogen would be best because temperature fluctuations will have a lesser effect on pressure compared to charging with air.

VR,

Tim
Last edited by Tim on Mon Nov 28, 2016 5:53 pm, edited 1 time in total.

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Re: Accumulator tanks as pressure chambers

Postby SSCBen » Fri Nov 25, 2016 9:48 pm

I do not recall any others at SSC who used these tanks.

Unfortunately, there are two volumes reported for these tanks: the overall volume (water and air) and the maximum water volume. It can be hard to know which is being used in the product literature. From what I've seen manufacturers and sellers assume that one half of the overall volume is available for water. The Groco tank is very similar to the tank I have, actually, as they report the water volume and I reported the total volume. This can be verified by looking at the dimensions of the tanks, which are similar. (Will add link later.)

I have a smaller British tank bookmarked at work, si good to know about the compatibility of the threads. I will add a link later when I am there. I also found an accumulator for fire protection that has a better volume and can post it later. I am not certain the price of the latter is acceptable, however.

The Johnson Pump accumulator looks promising. I might not have the time in the coming weeks to contact the manufacturer, so if you do, let me know what you can find out about the attachment.

Tim wrote:Charging with Nitrogen would be best because temperature fluctuations will have a lesser effect on pressure compared to charging with air.


Why would this be true? For an adiabatic process the difference between air and N2 should be fairly negligible, as their ratio of specific heats is about the same. (If the process is reversible adiabatic.)

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Tim
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Re: Accumulator tanks as pressure chambers

Postby Tim » Fri Nov 25, 2016 10:40 pm

Hi Ben,

Nearly every racecar, plane, and heavy equipment tire is filled with nitrogen rather than regular air. There are several benefits of using nitrogen. However, for our purposes, the benefits are likely negligible. I was just thinking that if you have easy enough access to it, go with nitrogen.

VR,

Tim

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Re: Accumulator tanks as pressure chambers

Postby Tim » Thu Dec 01, 2016 6:05 pm

UPDATE:

I contacted SPX Flow Technology in Sweden (owner of the Johnson Pump brand) regarding the connection on their 2L (half-gallon) accumulator tank. Cecilia Trumbäck informed me that “The metal connection going into the tee is 3/4” and BSP.” Since my last post, I found several other water accumulator manufacturers (most in Italy), and all of their 2L accumulators have a 1/2” BSP connection. The other 2L accumulators are also rated for lower pressures than the 12 bar boasted by Johnson Pump.

As for 5L accumulators, all of the European manufacturers supply them with a 3/4” BSP connection. However, not all 5L accumulators are created equal. The best 5L accumulator I found (in terms of specs and availability) is the ACS 5 manufactured by CIMM. It can be purchased from Acorn Engineering via their ebay store for shipment to the US. This accumulator is rated for 10 bar operation, whereas many others are only rated for up to 8 bar.

Another differentiator for the above 2L and 5L accumulators is that they both employ a replaceable rubber bladder rather than a fixed rubber diaphragm. Based on what I’ve read, the bladder type allegedly has a greater usable water volume (given the same tank size).

VR,

Tim

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cdmt

Postby CDMT » Sun Mar 12, 2017 7:41 am

Image

I just noticed that you guys had been discussing accumulator tanks. The main benefit to these is reliability. This is what made 2 years ago and it was a lot of fun. Yes it took a tremendous amount pumps to get up to pressure (110-115 psi) but look on peoples faces was worth it. It was amazing that the chinese toy squirt gun pump could handle the pressure. I should have reinforced it though both internally and externally since I eventually snapped the piston after a few weeks and dented the outer tubing. I never cleaned up the design since I moved on to the smaller Jabsco accumulator tanks that you can get at West Marine.

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Tim
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Re: Accumulator tanks as pressure chambers

Postby Tim » Tue Mar 14, 2017 7:11 pm

cdmt,

Awesome! Thanks for sharing. How did you adapt the "chinese toy squirt gun pump" to the PVC? Do you have any pics of your Jabsco blasters?

By the way, are you in the military? Just curious about the blousing band in the background.

VR,

Tim

CDMT
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Re: Accumulator tanks as pressure chambers

Postby CDMT » Wed Apr 05, 2017 2:51 pm

Sorry it took so long to reply it's been very busy for me lately. As to your questions amazingly the Chinese toy squirt gun wasn't glued much like the smaller stream machine where you can remove the forward nozzle. It's thread pattern roughly matched to garden hose fittings. All I needed was a little Teflon tape. Unfortunately they are now gluing their nozzles on. The blaster below in the picture below was just thrown together to test the Jabsco tanks versus the Shurflo ones I had been using. In the future I will be experimenting with the pressurization of the tanks outside the internal bladder and streamlining the design to make it both more efficient and more compact. Also I will change out both the nozzle and trigger valve. Yes I am in the US Coast Guard.

Image

https://www.westmarine.com/buy/shurflo- ... k--7877905

https://www.westmarine.com/buy/jabsco-- ... ecordNum=1


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