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Old 03-07-2018, 02:30 PM   #1 (permalink)
 
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Default My $80 80 psi pressure pot

I'm starting down the casting road to oblivion and started gathering tools and supplies. This should be listed as a hobby unto itself. My first purchase was a 2 1/2 gallon paint pot I found on Walmart's website. It was listed for $79.99 with the pot, tubing, regulator, valve, and paint sprayer and free shipping. It was also advertised as made of Stainless Steel. What a Deal!!! Well sort of. The regulator is broken along with the attached gauge. The pot is painted making me doubt it is made of stainless. I emailed the seller who checked with their supplier who "verified" it is made of "Stainless Iron". They have offered to rebate me $8 or I can send it back. I countered with a request for a $15 rebate.

Now the good news. It is marked with a maximum working pressure of 80psi. I tested it last night to 75 psi w/o popping the pressure relief valve and w/o leaks. I plan to upgrade the pressure relief valve with a US made ASME coded relief valve, add a better liquid filled gauge, and use the regulator on my compressor to pressurize it. So for a final cost of $65-$72 I'll have a casting pot to start with good to ~80 psi. I'll use it to 50-60 psi.

Here is a link to the item.

https://www.walmart.com/ip/10L-Indus...&wl13=&veh=sem


More to come.

Danny
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Old 03-07-2018, 03:04 PM   #2 (permalink)
 
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Replacing that relief valve is a good idea.

If you're not planning to use that high of a working pressure, it might be good to replace it with one closer the the working pressure you intend to use.
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Old 03-07-2018, 04:15 PM   #3 (permalink)
 
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I wouldn't put too much stock in the "Maximum Pressure", they can basically put whatever they want without much basis in fact or safety margin. I'd be surprised if the tank is substantially different than the HF tank, which has a "Max" of 60 psi. I am no expert in pressure vessel code/design/construction, but I know that ASME pressure vessels have strict requirements for design, construction, and testing, which provides a reasonable assurance that they won't explode up to the MAWP (with a substantial safety margin). These tanks don't offer that. Basically, what I am saying is, if you wouldn't operate a HF tank (stamped 60 psi) at 75 psi, then you shouldn't operate the one stamped 80 psi there either.

I'm not saying to throw it away, or that you're involved in some sort of high risk activity like base jumping. There is a lot of anecdotal these types of tanks operate fine for long periods of time (without modification to the lid retainers) in the 50 to 60 psi range (or higher). Just don't think there is something magical about the max pressure, it probably won't explode at 81 psi but there is also a relatively unknown probability that it will blow at 79.

All of the above said, I have a HF tank that I have not used but plan to someday, will operate in the 40 to 50 psi range with some additional safety considerations. Just my 2 cents...
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Old 03-07-2018, 05:31 PM   #4 (permalink)
 
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MY plan is to do some reverse Engineering and some testing as well. I did hook up a long hose around a couple corners prior to adding pressure last night to be safe. I use to review pressure vessel designs earlier in my career as well as safety designs and vent and flare systems up to 20,000 psi. Even 80 psi of compressed gas in a 2 1/2 gallon pot will kill you if it fails. ASME requires a test to 1.5 times the maximum operating pressure, (safely with water). I will reverse that and operate at my Test pressure divided by 1.5. I was impressed with the weld quality on the tank and hardware. The fittings and valves were garbage. Thanks for the advice and concern.

Danny

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Old 03-07-2018, 06:16 PM   #5 (permalink)
 
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Sounds like a reasonable approach, I had a sense that you had a background in this sort of thing, but wasn't sure. I am a Chemical Engineer and have worked in operations at chemical plants for a while, so I don't get too far into the equipment design stuff but probably see more than the average guy. When I first looked into the paint tanks I was surprised how these inexpensive tanks are sold with essentially no evidence to support their pressure "rating". Anyway, I hope I can tag along with some of your casting adventures!
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Old 03-07-2018, 07:09 PM   #6 (permalink)
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I looked on the web site, but they say "50" lbs there.
I think, like HF, that they are constantly changing items as they (walmart) order similar products from different suppliers on some occasions.

Congratulations though on your purchase. Looking forward to seeing the results of your work here!
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Old 03-07-2018, 07:22 PM   #7 (permalink)
 
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I’m a Mechanical Engineer working upstream for Chevron. Use to design and install a lot of pressure vessels. The optimum operating pressure is listed as 50 psi. The tank is marked at 80 psi. I assumed it would be a 50 psi pot too.


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Old 03-08-2018, 05:43 PM   #8 (permalink)
 
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I love the pic of the box it comes in. Is that what sealed the deal? :)

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Old 03-08-2018, 06:34 PM   #9 (permalink)
 
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I bought a California Air Tools labeled pressure pot online through the Home Depot website. It came in a CAT labeled box but the pot has no specific markings. Has a big curved sticker on the lid that says “Max operating pressure 90 psi”.

Pressured it up, pop-off let loose at 40 psi. Began to adjust pop-off so it tripped at 85 psi and took it up to 80 psi and let it set overnight. Did great... the first time. Began to plumb for casting and was having issues holding pressure. Sealed up every possible plumbing leak and still not good.

Check lid seal and it was leaking, no damage to the seal but deformity to the outer ring of the lid. Hmmm. Went online to CAT website and downloaded manual (none included with my unit). Manual said not to pressurize above 60 psi. Called them asking about a new lid and they called back indicating they do not sell lids by themselves, never recommend their pots above 60 psi and should return to Home Depot.

At this point I reformed the outer ring and pot holds well at 55 psi with pop-off at 60. Obviously something is askew I n the labeling and such, but decided not to chase it.


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Old 03-08-2018, 07:03 PM   #10 (permalink)
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Thanks Dan and Mike for clarifying the data. It bugs me when something is written one way and then actually made much more durable, or vice versa! I understand that there are those that will, without common sense, way overload things and end up with a Darwin award - and therefore the company overbuilds and understates for pure safety purposes.

I am not an engineer, but I do tend to overbuild things, and they do last. I grew up on a farm, and with broken machines such as plows, disks, trailers, axles, my dad discovered I had a knack (at 10, 11 and 12 years of age) for correctly diagnosing the real underlying problem that caused a break at a different place; then he discovered I could arc weld better than he could. He took me off the tractor and put me to shop maintenance. What I fixed didn't break again. The high school Shop teacher came and talked my dad into making me join the FFA so that I could go to school district FFA welding contests!

I don't doubt your ability knowing your backgrounds. Thanks for clarifying the data.
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