The Shop > Finishing

Grit Blasting and Moisture

(1/3) > >>

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  :scratch: 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  :bugeye:

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  :ddb:

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  :bugeye:

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.

Navigation

[0] Message Index

[#] Next page