Grit Blasting and Moisture

[awemawson]

I've had a mini project over the last couple of days grit blasting and re-painting some tractor rear wheels. Now my blasting set up has featured on other projects here - it's a Hodge Clemco 1440 pot that take 75 kgs of grit, and a 140 cfm road compressor

[awemawson]

Now the grit blasting went well on three 'wheel sides' but on the fourth the grit supply was intermittent - it would feed a bit each time I closed the 'dead mans handle' trigger then stop. This has happened before and it's a right pain so I decided to get to the bottom of it.

The blast pot contains the grit (NOT sand) and has a metering valve at it's base containing a thick rubber tube that is squeezed to restrict the flow of sand. Sand falls by gravity into the air stream, but is also is assisted by pressure that builds up in the pot when the trigger is pressed. A clever 'mushroom valve' in the top of the pot is pushed up to seal when the trigger is pressed, and when the trigger is released the mushroom falls allowing extra grit to flow into the pot from a reservoir on top.

A good illustration is here:

http://www.hodgeclemco.co.uk/wp-content/themes/Hodge%20Clemco/IETMs/Pots%20+%20PPE/1440.html

So I pulled it all apart for inspection. Never done this previously, just used it  Nothing wrong, all seemed well, but it gave me a chance to completely empty the pot of grit through the now open bottom hole.

Then it struck me - that grit doesn't flow through my fingers as I'd expect - it sticks - it's DAMP 

[awemawson]

Now the compressor has a three stage dryer system, each having their drain cocks deliberately cracked open to let separated water escape. The day was dry and hot. The Hodge Clemco has a drier incorporated, again with its drain slightly cracked open to prevent water building up.

So the moisture must be in the grit - time to investigate. I know I did have some damp bags. When I bought the blaster it came with something of the order of two tons of grit. I had to make 5 trips with my open trailer and the heavens opened on one trip. The bags are 'vented' that is to say they are not completely sealed. I thought I'd eliminated the damp bags years ago.

So then it dawned on me - I have a 'Speedy Moisture Meter' which I used for foundry sand to get the moisture content correct - why not sample a few bags and put some science into the equation 

[awemawson]

These moisture testers are rather cunning. A weighed sample of the subject is put into the body of the instrument, and a measured volume of crushed calcium carbide is put in the lid. The lid is clamped on, the whole lot shaken up, and the calcium carbide reacts with the water in the sample to produce acetylene gas. In the base is a pressure gauge calibrated in  percentage moisture content.

So my sample from the pot read 3.5%
A sample from a new bag of finer grit that I'd been given read 0%  (*)
A bag from my pile of bags gave 3.5%
The next 10 bags from my pile gave 0%

So to cut a long story short I'd been unlucky and chosen a damp bag 

When you are dressed up in the blasting helmet and gloves, and are loading bags into the pot 'on the fly' you don't really get a chance to feel how it flows, but comparing the damp bag with a dry bag it is actually quite obvious with ungloved hands.

An 8x4 of hardboard was laid out on the concrete outside the workshop - the damp bag contents spread out in the fine warm day we are having today, and raked about with a garden rake. Within 20 minutes it was down to 1.5%,and leaving it over lunch my sample showed no perceptible moisture at all. Reading specifications from grit suppliers web sites it looks like a maximum of 0.2% is what it should be


(* I am fortunate in having a professional grit blaster renting parking space at the farm and he kindly gave me the finer grade grit left over from blasting bits of Hastings Pier which is rising phoenix like after it's disastrous fire of a few years back)

[vtsteam]

Could be the sand picks up moisture on a hot day due to condensation. The mass of the sand stays cold. Bad sand could do it too -- any with even a tiny amount of entrained salts will be hygroscopic. Or it could conceivably be left over moisture from the rain even a long time ago.

It is very hard to dry just about anything in a pile or stack, it's amazing how much it has to be spread out to dry in the sun on even a hot dry day.

I had a couple tons of wood chips in a covered pile which got damp one day after a rain -- it took spreading them in shifts on a tarp to a 1" thick layer for hours to get them to dry -- maybe a hundred pounds at a time. It took a week to get them back to their original state.

[vtsteam]

oops, cross posted. I see it was only one bag.

I used to dry wood chips on the stove in a cast iron pan and measure weight before and after to get moisture content.

[awemawson]

Steve,

This is not sand but Calcium Silicate manufactured grit. Blasting with sand has been made illegal in the UK for some years due to silicosis problems induced by the free silica in the sand.

Crushed glass is becoming quite popular but I've never had the opportunity to use it. To be honest the grit I'm using is not the grit of choice for blasting steel work - it's meant for stonework and brickwork. But as I have a large pile of 37 kG  sacks of the stuff I'll use it up before buying anything else. It must be old stock, as the large sacks are now also banned for handling reasons 

Large areas of my farmyard are becoming quite covered in the stuff 

[Jonny]

Interesting I have had no end of the same trouble since changing media in a siphon based setup.
Do have a cheap pressure pot but never used it in 9 years.

True shouldn't use any form of sand but have been using kids playing sand since 1999 until last year. Rarely a problem after manually drying.

Changed gun, hose and pickup to a pro from http://www.angloscotabrasives.co.uk/
Same symptoms. Slightly better was junking above elbow and 15mm valve for a smoother stainless jobby but still have problems. Have to fire blast gun down the siphon outlet every few mins forcing it up the pipe. I also have to use bare minimum media not exerting extra pressure in the bend and pipework.
It had crossed my mind before the heat been passed in to the media then cooling off generating moisture. It was ok at first then bought two more large bags and chucked the lot in - worked.


If I leave the lid open its usually fine or better when surrounding air is not as damp ie summer.
Never checked dampness since getting the three bags 60 to 120grit aluminium oxide but pretty certain your right.
Firing up compressor generates heat that will fill up the extra old compressor tank then via 1/2"bsp regulator filter and moisture trap, yards of 15mm copper plumbing tube via another trap. Air is still warm in second tank filled from empty unless I leave a good 10 mins.

I'll let you know tomorrow I'll try drying some media.

[PekkaNF]

Moist grit might be a problem, but I have had more problems with moist compressed air. Even after a moisture trap. Clogs the bottom of pot easy.

Pekka

[Arbalist]

I think that's the problem I had Pekka. Mostly my fault as I don't drain the compressor as often as I should.

[awemawson]

I solved my problem by buying 1 ton of crushed glass. It flows much more easily than the stone grit that I was using, and is more tolerant of moisture in the air.

I'm told though don't use it for exposed oak beams - the tiny glass particles embed in the crevices in the wood, and sparkle. Apparently they are pretty well impossible to remove. Presumably other grits do the same but don't reflect the light in the same way.

[Arbalist]

I need to buy some new media when I set up again. I used to use AO which was fine but notice MM sell glass beads. Anyone tried them?
I used to run my old blaster from a small 2.5 HP 25L which was half the minimum recommended size for my cabinet and it worked fine for the small items I needed it for. Like most though my compressor is very noisy, anyone tried using one of those "noiseless" ones to run a small cabinet. I know the figures say the output is too low but then so was the one I've been using over the past 12 years!  perhaps it's doable if I reduce the nozzle size? As I say I'm only blasting small stuff, not houses!

[vintageandclassicrepairs]

Hi All,
Blasting with glass can also be a health hazard
Pressure needs to be kept low (60 lb ??) to prevent the glass breaking up and
releasing the dreaded silicates 

John

[awemawson]

I use a positive pressure Apollo helmet when 'free' blasting what ever media I'm using. In the blast cabinet it's under negative pressure from an exhaust system, so either way the nasties are kept at bay