Sucking hole in media

[Mte320]

I have BIG problem with my homemade blast pot. It continuously sucks a hole in the media, I have 2 water seperators on the system. The main reason is likely moist in air. I tried with aluminiumoxide and normal blasting sand "yes the forbidden one". and the same thing happens.

Does the shape of the pot have any influence on the result, my pot is a old 100l compressor tank. That is very flat at the bottom. And its a bit rusty in the bottom where the sand is!

 

[rtbcomp]

I get that with fine and dusty media. Try crushed glass or J-blast supa-fine.

The shape of the pot will influence how much is left before you have to re-fill it. A cone-shaped hole forms in the grit as it's used up, so a conical pot will encourage it to empty completely, whereas you'll be left with quite a bit in a pot with a flat bottom. The shape shouldn't affect grit flow until the pot is almost empty though.

 

[Mte320]

I get that with fine and dusty media. Try crushed glass or J-blast supa-fine.

The shape of the pot will influence how much is left before you have to re-fill it. A cone-shaped hole forms in the grit as it's used up, so a conical pot will encourage it to empty completely, whereas you'll be left with quite a bit in a pot with a flat bottom. The shape shouldn't affect grit flow until the pot is almost empty though.

Could it be that the hydrovane put so much moist in the air that the media is wet within minutes in the blast pot. ??

 

[brightspark]

if u have a moisture seperator on the pot u will soon see if your getting water through your airline and causing any problem. the pot will run fine for a few minutes from cold then as the air warms up thats when u start getting condensation which will be evident at the pot seperator

 

[rtbcomp]

It's unlikely to be moisture in the compressed air as there is no flow through the pot (or at least not until you get a hole through the media!). Moisture in the compressed air usually causes problems at the mixer at the bottom of the pot.

I get the same effect using a syphon gun with fine grit and there is no contact with compressed at all until it reaches the gun.

The media that dare not speak its name may contain moisture even though it's supposedly dried in a kiln. I've had sopping wet bags of the stuff from Wickes and B&Q.

 

[Mte320]

Both seperators is putting quite a bit water out on the floor. With the fine aluminiumoxide the pot works for a few minutes if i put brand new media in. And the its back to the same problem. I just tried with the sand and it works a bit longer and the the problem comes. The choke valve helps for 2 sec. and then nothing happens.

Does steel grit have more or less problems with moist then other types?

 

[brightspark]

Both seperators is putting quite a bit water out on the floor. With the fine aluminiumoxide the pot works for a few minutes if i put brand new media in. And the its back to the same problem. I just tried with the sand and it works a bit longer and the the problem comes. The choke valve helps for 2 sec. and then nothing happens.

Does steel grit have more or less problems with moist then other types?

chilled iron can be a nightmare with moisture . it sticks to the side of the pot and pipes then goes rusty and solidifies all together , then u have to try and break it all up and get it out of the pot what size jet and cfm comp are u using

 

[rtbcomp]

Try some grit in a bucket with a hole in the bottom and see if it flows through. If not no amount of messing with the compressed air will sort the problem.

 

[Mte320]

40cfm 3mm carbide nozzle.

But it dont suprise me that it is a moist problem. The weather here in denmark is really good at the time. And the humidity is SKY HIGH.
I tried to look down the pot with a flash light and i think i can see water drops on the mixer.

The reason i asked about the the steel grit is that im thinking about changing over to it. Mainly because of the many passes it can do

 

[brightspark]

Try some grit in a bucket with a hole in the bottom and see if it flows through. If not no amount of messing with the compressed air will sort the problem.

thats what i thought when i made my own metering valve . it worked fine just opening the valve slowly and flowed through great then when pressurised the pot in use it wouldnt flow properly i increased the bore size and it works perfectly now . theres a lot of variables to look at for the cause and its not allways simple

 

[Mte320]

The mixer valve on my pot is 1/2" slide valve, simply because it fits directly in compressor tank

 

[brightspark]

is your pot simlar to this one pipework wise /these are the type i use now

 

[MattF]

If you're thinking it might be moisture from the compressor, whack a longer hose on between the compressor and pot and dunk the hose, coiled, in a bucket of cold water, before the moisture trap. Should help to condense any moisture out of the air.

 

[Mte320]

Yes it is exactly the same!!

 

[rtbcomp]

If you close the valve that feeds air to the bottom of the pot and air still comes out of the nozzle then you have a hole through the media in the pot.

I use a cut-down propane bottle as a funnel to fill my pot and with well worn grit a vertical column of falls into the pot, leaving most of it behind - I've even known it to form overhangs!

"pressurised the pot in use it wouldnt flow properly" What I think happens is that when the pot pressurises it holds the grit against the sides of the pot and stops it flowing, altering the nozzle size, operating pressure and so on can affect the pot pressure and cause the grit to behave differently. Grit size, moisture and dust content also affect this behaviour.

I have a de-pressuriaing valve on my pot and when I open this briefly grit falls back down and it carries on OK.

Mostly I have my pot within kicking distance!

 

[Mte320]

I actually strapped a sanding mashine to the side of the pot. Then connected it to timer that starts it 3 times a minute. But even that dont help

The pot is quite rusty inside, maybe thats a problem?

 

[rtbcomp]

The pot needs shaking rather than vibrating, I suspect vibrations might cause the grit to compact.

I don't think rust will be a problem unless it's flaking off and blocking the outlet.

 

[Mte320]

I tried the siphon gun with the fine media last night. And nothing happens!!

Im thinking about buying a dryer. Just have a little hard to believe that problem will disappear from one moment to another!!

 

[rtbcomp]

I run my syphon at 4 bar and it works perfectly, I have to kick the bucket of media from time to time to collapse the hole formed. Place the suction tube just under the surface of the media, if you push it too far down you get a solid plug formed and it blocks up.

Try compressing a handful of media and see if it forms a "snowball", if so it may be damp or too dusty.

Try drying some media in an oven before splashing out on a drier, I experimented with drying out grit-sand and the cost of elctricity + sand was greater than buying J-Blast.

I've also noticed that sometimes it just doesn't want to work at but when I've swept up ad sieved the media to use again it all goes perfectly. Could be using the grit has dried it out a bit.

 

[Mte320]

UPDATE!!

I think i have solved the problem. I have bought a cheap old air dryer. Installed it, and try blasting with the "forbidden media" and the problem was all gone.
Now i will clean out cabinet and try with the fine media and see how it goes.