Catastrophic pressure pot failure...my wife almost became a widow!

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I'm glad you are OK Curtis. Your post will be a good wake up call for others to look at this and other safety measures that we may want to revisit in our shops.
Let us know what kind of equipment you decide to go with.
 
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So glad you are OK.

My very old cast aluminium paint pot has cast recesses for the clamps, but I have no idea what it is rated to. Mind you I only cast at 2 bar (29psi) and the pressure valve is set to 3 bar (43.5psi).
 

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Like the rest I am glad you are ok. I am looking at getting a pot soon and after reading what happened to you I will use some chain to cross over the lid and hold the clamps from moving.
David
 
MesquiteMan said:
I do think, however, that I will be investing in better equipment.
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I spent some time last night looking for better equipment...after SHMBO informed me that my casting days are over until I make some changes. First one is the HF pot will now be a vacuum pot.

I am interested in what people decide is better.

Tom
 
Generally you want 3-5 times the pressure you plan to use in your vessel as a safety factor.

I think that it would be wise to change your method and decrease your pressure because I doubt you will find anything reasonable in the pressure vessel market in the size you need to make it worthwhile. As the safety factor goes up, the size goes down and the price increases exponentially.
 
Looks like it exploded at the weakest link, to me.
had this happen to me on a reserve tank on air conpressor.
The bottom blew out the (welds) and warped just like yours. THANK GOD I WAS IN THE HOUSE WHEN IT HAPPENED. I had just hooked it up to test it and turned the air on and left it to fill.
I think by adding heigth to your pot lid by adding (POT SAVERS) you altered your clamping angle. thus causing failure,plus the high pressure. I would never go over 60#s
I would also weld on pot savers and maybe add fixture to hold clamps in place (such as rod from 1 to 3 and 2 to 4 or a metal ring around the outside of clamps.
 
Hello Curtis,

All I can say is I am glad you are safe and sound. It is just too close for comfort. Take care, MesquiteMan.
 
Glad you were out of the line of fire. Thanks for showing everyone what can happen if you are not careful. I recently did my first cast using 60lbs. and after seeing what can happen I will be very careful not to excede that.
 
Curtis,
I am glad you were not hurt. My pressure pot starts leaking around the seal at 65psi and I was concerned how to prevent it. Now I will just accept it as an added safety feature. Thanks for posting the pictures.
john
 
jeff said:
Wow, Curtis. Incredible story. Glad you're ok.

You have some heavy benches in your shop? How about setting the pot under one of those while you're casting?

I bet the lid is warped because the clamps didn't disengage simultaneously. It doesn't take much of a deformity to set in motion a few millisecond chain of destructive events. One clamp slips a little, the lid deforms slightly, then boom! Lot of force on that thing! What's that, a 12" lid? Every PSI is 113lb?

Did I see 80PSI in another post? 9000lb on that lid.
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Obviously there is an engineering formula here some place. Can you let some of us non-engineer analyticals in on the math? :confused:

Curtis: Glad you were not hurt. I don't think I would try to repair the lid. Your chances of a repeat performance may be greater with the lid weaker than original. Thanks for sharing the warnings.
 
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Curtis, glad you're alright, Perhaps you need to consider a ring completely around the clamp area so the pressure of clamping is evenly distributed, that way there would be no distortion of the lid, whether from uneven clamping tension, or from pressure exerted from the inside while under pressure, I think you're right though with the amount you cast I think a commercial grade pot might be in order.
 
Curtis, First I am so very glad that all you got out of this was a good scare. I agree with you that it is the modification and lack of holding power of the glue that allowed the lid to basically slide out from under the clamp. the deforming of the lid woudl be due to not all clamps loosing there hold. it looks to me like only one or two lost there grip and then the lid was free enough to bend.

What is the need for pressures of 85 and 90 P.S.I.?
I have not been keeping up with all the casting info etc. and the last I knew of it was much lower pressures that where being used.
One final suggestion for everyone using pots. maybe a plywood box to place over your pots when they are under pressure. just set it down over the pots so that if they do something like this the box takes the initial hit.
I knew once of a kid that worked in a water heater plant. his job was to pressurize every tank to insure they where safe at XXX pressure. the chamber had a sliding door type shield but as repetitious work goes he got tired of opening and closing it. It took only one tank failing to open up his entire abdomen. He lived but spent quite a bit of time wishing he hadn't. It was pretty much guaranteed that eventually a tank would fail. He was basically playing an extended game of russian roulette.
 
THarvey said:
Obviously there is an engineering formula here some place. Can you let some of us non-engineer analyticals in on the math? :confused:

Curtis: Glad you were not hurt. I don't think I would try to repair the lid. Your chances of a repeat performance may be greater with the lid weaker than original. Thanks for sharing the warnings.
Click to expand...

Glad you are okay. It definitely should make people stop and think. As someone mentioned above, actually placing the pot into something might be a good idea. Wooden box or similar would definitely take momentum out of a flying object.

The 9000+ force is calculated:

12" diameter = 6" radius

area = Pi*radius^2

Therefore, 6*6*3.14 = 113 sq. inches

(113 in^2) * (80 psig) = 9,043 lb force
 
Well good news is that you are OK. Bad news is that you pot is shot and you had a scary warning. Might have been a good thing.

Why are you using 120% of the designed max. pressure? Would you shoot a gun with shells that are 120% of the max. proof load. Max. load is not intended to be the working load. That pot is designed to run at 40-60 psi. not 95 psi. Most working loads are less than 75% of the max load to give a built in safety factor. If you are going to keep abusing the pot like that I agree you need to build a safer system around it to contain any failures.

These pots from HF are decent but not that great quality. That goes for almost everything in HF. Works but not the best materials or the best workmanship. If you are after high pressures like you seem to be interested in you may want to look into a more expensive pot that is designed to handle the pressure. Next time you might not be so lucky and we would miss you and your family would really miss you. Just my two cents worth.
 
PP

Glad you weren't standing next to your Pot when it failed.

I was thinking the same thing as CaptG. Your recessed cart is a nice idea but building a hinged lid would be a smart modification.

If I were to do this I think a hinged wooden top would work fine. The top would not be so tight as to help hold the Pot lid on but have enough of a gap to allow the pressure to release during a failure.
 
I too have a few questions. Why do you need to cast at such a high pressure??? :confused: When making the clamp modifications would it not be better to have a ring welded equal to the size of the top??? :confused: This would distrubute the downward force of the clamps better. I would not even trust weld-bond. I have had that stuff break free of aluminum. If you use that stuff the surfaces must be free of paint and any residue. Would casting at a lower pressure been less of a dramatic effect or is that something that can not be calculated???:confused:

The use of a pressure relief valves is only there to prevent over pressurising a tank. If you have a pressure guage on your tank and can read the pressure plus if you have a cutoff on your compressor and have it set at your working pressure would that not be enough???:confused: I ask this question because you mentioned the relief valve did not release and is in working order. You have it set at a slighly higher setting than what you cast at.

Anyway glad you are OK and it was not a worse outcome. I am sure you will be making some safety modifications in the future.
 
Yep, I recall when this happened to me. Relatively new pot, used only a few dozen times. Clamped tight in all directions. The pot was only holding about 40 Lbs of pressure if that. The pot exploded about 2-4 min after pressurizing. I could clearly see that the pot expanded itself out of round in a direction between the clamps, telling me I wish there were 6 clamps instead of only 4. The lid would have been pulled out from 2 clamps as the pot expanded causing the lid to jump off, the two remaining clamps trying to hold on which in turn causes lid to bow as well. My lid did not go more than a foot from the pot. The pot jumped just enough to land on it's side making a nice big liquid spill. I destroyed everything and started fresh.
 
workinforwood said:
Yep, I recall when this happened to me. Relatively new pot, used only a few dozen times. Clamped tight in all directions. The pot was only holding about 40 Lbs of pressure if that. The pot exploded about 2-4 min after pressurizing. I could clearly see that the pot expanded itself out of round in a direction between the clamps, telling me I wish there were 6 clamps instead of only 4. The lid would have been pulled out from 2 clamps as the pot expanded causing the lid to jump off, the two remaining clamps trying to hold on which in turn causes lid to bow as well. My lid did not go more than a foot from the pot. The pot jumped just enough to land on it's side making a nice big liquid spill. I destroyed everything and started fresh.
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Jeff again I ask the question would have the pressure relief valve done any good???:confused: Not sure if you have one on your pot or not. I think I am doin my casting outdoors from now on.
 
MesquiteMan said:
Yeah, but Johnnyjarhead had one up to 200# with no problems! I don't think it was the pressure. I still think it was the "pot savers". I guess that is what I get for trying to save my pot, huh? :)
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Please remember that he used a brand new pot That was modified by the removal of all safety equipment (not advised)and after a failure in a hose causing someones injury he stopped. He also took the pot out of service by placing the lid in a log splitter and watching it twist like a pretzel. I run 3 pressure pots here and after 2 years in service they get either destroyed on purpose or turned into vacuum pots. All metals flex, but they do hit a point that will cause fatigue. I have to agree with you that it was most likely the alterations made to the lid that caused the failure, if the screws are allowed to dig into the metal, they should grip enough to slow down the blast and allow a leak. I don't think I'm going to complain about my pot having its little hiss, after all, Alumilite sets up quick enough that we don't need to have the pressure on that long.
Curtis, I'm just happy that you were out of the area when that happened, not only could the parts have caused you injury, but the concussion from the blast could have caused other personal damage if you were closer.
 
How could it have? Now..I do not have one on my pot because it always want to leak, but that is besides the point. If you put pressure in a pot, the valve will not go off until you exceed the value set on the valve. So if I load it with 40 Lbs and the valve is set for 50 Lbs, the valve will not go off. Now, the pot is loaded to 40, the hose is removed from the pot and it is simply sitting there all on it's own. The pressure in the pot is not going to increase to 50 and set of the safety valve, it can't do that on it's own. Even if the pot was in the window with the sun shining on it, it couldn't increase 10 Lbs of pressure over 3-4 minutes. So the answer is that the relief valve would not have done a thing to prevent it.
My believe is that some pots are not 100% round. It got dinged around somewhere or something at sometime happend to it. Now if it isn't perfectly round, then air pressure is going to make it even more out of round. Eventually with the flexing of the pot being used, it will reach a breaking point. I think it's pretty simple in theory and why should it have to be complex? I believe that if the pot had 6 tie downs instead of 4, it would be a drastic improvement. I don't think you will see any pot companies rushing to go do that, because what we are doing with a pot is not what it was meant to do. I do not believe a pot will just pop like this on it's first go, even if it's a bit out of round. But, after a dozen flexes, that's like bending a wire back and forth several times. The only cure I could foresee would be to constantly measure your pot with some calipers prior to each use. If you kept measuring it each time, one side is going to be growing a hair at a time and the other side will be shrinking a hair at a time, not necessarily noticable to the eye. Each time it would grow a hair more on one side. As the pot warps, it is doing the same thing to the lid. I am believeing that Curtis' adjustments to the lid are a good idea. It will extend the life of the lid. I don't believe what he has done had any impact on the life of that pot, other than the over-presurizing part..but ya'll know what I mean. I think the over-pressurizing part simply made the pot fail faster and more dangerously, but otherwise, that pot was destined to fail prematurely.

Best idea I gained from this post was making a box to put the pot into. That's a good solution. Just stack up some cinder blocks and put the pot inside just like a fireplace. They do the same thing when working on big trucks. They always put my tire inside a thick metal cage before they inflate it. They always put my brake chambers in a solid thick metal box and cut the retaining clamp with a torch. Otherwise, the spring in those brakes can go right through a cinderblock wall! So why should working with a pressure pot be any different?

ah...crossed posts with Neil! Pretty much what he says, I agree!
 
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Ok so I re-ran my numbers. If there are any ME's, CE's, AE's or Phy's here, look them over and correct me if I overlooked something. This is a rough look at the Force v pressure on the lid radius.

Force = Pressure x Area
pressure = lbs / inch^2
Area = Pi x r^2
r = D/2

on a 9.5" lid
r = 4.75
Area = 70.882 in^2

Force @ 40psi = 2835.28lbs
Force @ 45psi = 3189.69lbs
Force @ 50psi = 3544.10lbs
Force @ 55psi = 3898.51lbs
Force @ 60psi = 4252.92lbs
Force @ 65psi = 4607.33lbs
Force @ 70psi = 4961.74lbs
Force @ 75psi = 5316.15lbs
Force @ 80psi = 5670.56lbs
Force @ 85psi = 6024.97lbs
Force @ 90psi = 6379.38lbs
Force @ 95psi = 6733.79lbs
Force @ 100psi = 7088.2lbs

On a lid with a radius of 4.75" there is roughly an increase of 354 pounds of force for every 5psi

on a 10" lid
r = 5
Area = 78.53975 in^2

Force @ 40psi = 3141.59lbs
Force @ 45psi = 3534.29lbs
Force @ 50psi = 3926.99lbs
Force @ 55psi = 4319.67lbs
Force @ 60psi = 4712.39lbs
Force @ 65psi = 5105.08lbs
Force @ 70psi = 5497.78lbs
Force @ 75psi = 5890.48lbs
Force @ 80psi = 6283.18lbs
Force @ 85psi = 6675.88lbs
Force @ 90psi = 7068.58lbs
Force @ 95psi = 7461.28lbs
Force @ 100psi = 7853.98lbs

On a lid radius of 5" there is roughly an increase of 392 pounds of force for every 5psi applied.
 
MesquiteMan said:
In previous cases, the JB weld has helped hold the "pot saver" and thus the clamp to the lid since it is rigid and actually creates a good bond. In this case, the hot glue just did not have the strength to keep the "pot savers" from slipping off the lid, allowing the clamps to open and launch the lid. I am not sure how that explains the severely warped lid, though.
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Curtis,

Glad you are okay.

I think your problem was caused by the hot glue. It is not rigid enough to hold that pressure. The pressure probably compressed it and allowed minute movement that started a domino effect failure.

As mentioned by others...a small movement is like an avalanche.

Hope more people learn from your experience.
 
If you are going to put it in a box then poke some holes in it to help the air release faster. You don't need to pressurize another vessel.

All this now leads to the next question. How about safety measures when filling??? Do you fill it from a distance and how??? What kind of safety gear do you wear when filling??? We all have shut valves at the pot itself. This does make you think.
 
Glad you're alright. All the discussion about this clamp or that clamp is still asking for trouble. The bottom line is the pot is not rated for that pressure period. after 25 years as a manufacturing Eng. you do not want to over pressurize a vessel because it will blow eventually.
One other thing I am curious about, was the pot full or was there very little in the pot. The reason for this question is if there were a lot of blanks, racks, anything solid in the pot the amout of air in the pot would be decreased. With less air compressed to reach any pressure there is less energy should something go wrong. As in testing tanks that are typically filled with water and then a small air space can be pressurized. If the tank explodes the rsults are much less exciting which is a good thing!
Bob
 
Curtis:

I previously made a hasty reply to your original post without reading through the whole thing and did not see the following paragraph. It's my very bad habit, sorry. But after reading through it now, I have to say 1) I am glad you did not get hurt and 2) the fault was entirely yours. What you did was silliness beyond comprehension. There was absolutely nothing wrong with your HF pressure pot, at least not until you modified it! Sorry for the blunt language in 2), but I can find no other way to express my reaction. What you did was essentially to disable a critical safety feature built into the pot. The relief valve was set to relief at something close to the 80 psi because that's the max safe pressure for the pot and you modified it to relief at 95 psi., way beyond its safe operating pressure. Was there any surprise the lid would pop?

Steve



MesquiteMan said:
I installed the "pot savers" and then pressured it up. I went a little past the 80# where I normally cut it off (95# actually) but have done this many times in the past with no problems. I do have a pressure relief valve but I have it adjusted so it does not start leaking until 95# or so. In this case, it did not leak at all. Everything was fine and the pot was holding air. It blew 10 minutes or so later.
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Why do I need to cast at such high pressure? Becasue I WANT to cast at such high pressure! You have not seen some of the things that I have cast and gotten excellent penetration that was not possible at lower pressure when tested. I normally do not cast at 95 but I do routinely cast at 80.

I will say it again, THE POT DID NOT FAIL. The failure was caused by the pot savers slipping off of the pot due to the use of an inadequare adhesive. Simply a rush decision on my part. The other pot sitting next to this one has been in service for 3 years with the pot savers installed for 1 year with absolutely no problem, always being used right at 80 psi.
 
OK, I have to say it...

Curtis, thank you for sharing this with us. It may save a life.

You didn't have to share it, but I'm sure you felt it was best for all of us to do so. You have taken some licks because of sharing it...and there surely will be some jokes once the shock dies out, boom boom...

In my mind, what happened is important, why it happened is important, and simply knowing that it CAN happen is important.

I have thought many times about putting "pot savers" on my pot after seeing yours last month. I would have NEVER thought about adhesives, or the potential for failure. I probably would have just put them on and left them loose, in order to simplify alignment.

In fact, your choosing to share may have saved MY LIFE.

I'm sure all of the comments made herein were intended in the same regard, but I shutter to think what info might not be shared next time, through embarrassment, or fear of ridicule.

Curtis, thank you! You are a stand up guy.

Tom
 
Ok, simplest solution is to have a plywood box with a hinged lid. Have the pot sitting in the box, load and put your lid on. Connect your hose to the container, close and latch lid. All you need is a hasp you can put a bolt thru. Reach in a hole large enough to get your hand into to turn on your valve, or to be safer, use a valve on the other end of the hose. Pressurize and then cut off valve. If lid goes, box will retain it.

If it were me, I'd not go much over 40 psi without something like this. HF stuff is pretty crude no matter what anyone thinks. And YOU could have defective container/bracket.
Just my opinion!
 
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MesquiteMan said:
Why do I need to cast at such high pressure? Becasue I WANT to cast at such high pressure! You have not seen some of the things that I have cast and gotten excellent penetration that was not possible at lower pressure when tested. I normally do not cast at 95 but I do routinely cast at 80.

I will say it again, THE POT DID NOT FAIL. The failure was caused by the pot savers slipping off of the pot due to the use of an inadequare adhesive. Simply a rush decision on my part. The other pot sitting next to this one has been in service for 3 years with the pot savers installed for 1 year with absolutely no problem, always being used right at 80 psi.
Click to expand...

I have read every page of this thread and examined the PIX and the explanations. This is a safety issue so I think I will chime in. I own one of these HF paint pots, other pressure tanks, and several compressors. I am not a casting expert but I have used tanks and compressors for over 40 years. So I am not unfamiliar with this situation.

Looks to me like the pot DID fail. The top blew off. The top is part of the pot. Regardless of the mods made and the assignment of blame to the mod, the pressure unit failed.

It is clear that the pot was operated over its safe rating. There is a safety margin built in and it appears that margin was exceeded counting on mods to make an unsafe condition safe. That is a potentially deadly gamble.

For awhile it worked. But that by no means establishes that it is safe. Damaged tiles worked many times on shuttle landings until one time when it burned up in the atmosphere.

The lesson is that if one needs to use very high pressures, then you need commercial equipment designed to handle such forces... not Harbor Freight home painting pots modified beyond their recommended capabilities.

People doing casting should take note that something that should not happen during casting... just happened. Make sure that you aren't doing the same things or you are at risk. Forewarned is forearmed.

Overpressurizing any kind of pressure vessel is very dangerous. Removing or altering any pressure relief or other safety device is very dangerous. The result can be fatal.

Please find another way. Pushing the envelope for a pen blank makes no sense. I just buried a friend who pushed the envelope on his Harley. He had done it a thousand times before without any problem until 3:05 PM on Saturday Sept. 12th when his so called "pot blew up". It was a senseless and unnecessary death for our very best rider... and our Safety Officer. Failing at pushing the envelope plays no favorites and the best can die just as easily as the worst.

Your wonderful talent is worth much more to us than a higher degree of bubble compression.

My $.02 for whatever it's worth.
 
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