New Machine Build- Water Table Build - Circulation Question

[AndrewEvans]

So our shop owner has decided that dealing with the smoke from our 5x10 plasma cutting via opening the doors & turning on the fans is a no go (about time, though his reasoning had to do with his heating bill)

He has decided that we are to build a water table, the tank itself, and slats and supports etc are all drawn up and being fabricated at the present time, that is the easy part (though he drew up the tank a bit big - 425ish gallons seems excessive, 3500 lbs later)

Now I am in the process of trying to figure out the right pump for "bladder"/circulation/drainage, as I imagine trying to cut the holes for these items After the fact (when the tank is full of water) would be quite stupid, I have settled on a 1" drain on the bottom, and 2 pcs 3/4" pipe nipples for the circulation/"bladder" function.

I figure I can have a 55 gallon drum (allowing for about 1.5 inches of adjustment) sitting next to the table and get my ball-valve/gravity doing the initial drain into the bladder(drum), come to think of it I may want to use a round shut-off for the drain out of the table.

Now the tricky question, what kind of flow should I be looking for as far as my pump is concerned, the idea is that the water should be circulating constantly draining out at the same rate that the pump puts it back into the tank, supposedly this is to alleviate the hazards of cutting aluminum over the water table.

I have read that this is unneccessary some places, and that it is of the utmost importance other places or that you should place an additive in the water for aluminum cuts and even that you should never under any circumstances run aluminum on a water table

Therefore my questions are as follows:

1. Are we overthinking this and the bladder/pump really only needed for drainage/water level control

2. If circulation IS necessary & therefore we do really need to invest in the circulation aspect, what kind of pump size/flow should I be investing in?

there is no end to the various flows and sizes that I could get and if I need circulation I guess I want to cycle the full tank at LEAST once an hour (800 GPH) or would that be too much? (I can easily get a water level indicator shuttoff so that flow out matches flow in if I need the circulation)

if I dont need the circulation then the question is easy, get the pump that matches what I need for maintenance/cleaning

thank you in advance for your responses

~Andrew

[plain ol Bill]

Andrew circulation is not needed w/ a water table. Is this water table going to be built into the bottom of the table? If so just use compressed air to push water out of the "bladder" up to the table. Very effecient and easy to do. I am going to try and attach a .pdf of a concept for a water table. This was the way I did it - and I love it. My table fills and dumps pretty fast and catches about 95 -98% of the plasma smoke. I generally run with my water no further than 1/4" from the material or even touching the back.
I found this .pdf on Jim Colts yahoo forum several years ago.

[daconfusion]

Awesome attachment. It cautions against water tables and cutting Aluminum.

Do you have similar design slides for Aluminum? Or any design hint slides for plasma CNC systems?

[plain ol Bill]

daconfusion I got a news letter from Hypertherm today that addressed just that subject written by Jim Colt I will cut and copy it in here for you.
HYPERTHERM’S JIM COLT ANSWERS YOUR QUESTIONS


Q: Our shop is using plasma over a water table. After cutting our first aluminum part, we were told by someone that we shouldn’t cut aluminum when using a water table because of the potential for an explosion. Is this a valid concern?


A: When cutting aluminum with plasma over a water table, the byproduct of the process is small particles of aluminum and aluminum oxide. These particles cool as they hit the water, and sink to the bottom of the water table.

Aluminum likes to absorb oxygen, and since water is H20 (hydrogen and oxygen), absorption of the oxygen molecules leaves hydrogen, which rises to the surface of the water in the form of bubbles. The bubbles pop and dissipate into the surrounding air in small quantities. Depending on the style of your water table, this production of hydrogen could be harmless.

There are some water table designs that utilize a submerged tank or chamber that is used to raise and lower the water table rapidly by displacing water with low pressure compressed air.If this type of water table is in use, it would be possible to get some of the aluminum particles inside the lower chamber, where hydrogen bubbles could be trapped. Over time and much use there could be a relatively large bubble of hydrogen, and I suppose if there was some type of ignition source that could occur underwater inside a closed chamber, then there could be an explosion.

Another scenario is on an extremely large cutting table that was used for cutting aluminum. Suppose the operator was cutting aluminum all morning (during steady state cutting, any hydrogen bubbles will be unnoticeably ignited by the arc, others will simply pop and dissipate into the ambient air), and just before lunch the operator loaded a fresh 14’ wide x 20’ long sheet of aluminum on the table. This piece of aluminum just happened to be bowed upward in the center, a great place to trap a bubble of (lighter than air) hydrogen. After lunch, as soon as the torch fires an arc, there could be an explosion, I suppose! A final scenario to watch out for is the distance between the water level and the plate. If the plate is lowered to the level of the water but not submerged, hydrogen bubbles can become trapped between the plate and water level, which could also lead to a potential explosion.


If you are concerned, in many cases a simple aeration manifold made with PVC or iron pipe and compressed air can solve the problem by preventing the hydrogen from collecting. You should contact your table manufacturer for more details on how to insure your table is safe for operation in this manner.


If you are cleaning a water table that was used for aluminum cutting and placing the wet slurry in drums or containers it is a good idea to leave the tops of the containers open, allowing any hydrogen to escape and dilute. So, in most cases with a simple water table on a small machine, it is okay to cut aluminum; however you need to take the right safety precautions as the consequences of any explosion are obviously quite serious.


Jim Colt is an avid fabricator who has worked with plasma cutting systems for more than 30 years. He is the author of many articles and white papers on plasma cutting and a frequent speaker at key industry conferences. If you have a question you’d like Jim to answer please send it to askjim@hypertherm.com.

[CLSmachining]

bump

[jimcolt]

Bottom line: Hydrogen is lighter than air (and water).....so if there is no place for it to become trapped as it rises in a water table...then it tends to simply dissipate in the air. Small water trays or water tables are usually safe with aluminum cutting.


Jim Colt