Tuning up a sandblast siphon tube?

I'm spending enough time at the sandblast cabinet that its poor performance is wasting too much time, & need to make it work better. It's a Cyclone brand generic cabinet with a recirculating siphon blast gun & foot pedal valve. It has a 14cfm nozzle and is supplied by a 24cfm compressor. I'm using a mix of dupont starblast and 60 grit garnet. The air is dry, but the cabinet is in an outdoor shed where I can't control humidity - but at the moment it's bone dry summer in the PNW.

Its problem is what I think is called surging, where it gives a good strong jet that just peels off whatever its aimed at - for about 2 seconds, and then blows air for 10 seconds while another slug of abrasive works its way up the feed tube.

I can feel the mass of abrasive - the nozzle really pushes back in my hand when it's blowing a good abrasive stream, and barely at all when it's feeding poorly.

I've tried varying the pressure from 70psi to 120psi. Seems to work better with higher pressure, but surges more. Below 90psi it doesn't surge but it feeds hardly any abrasive and takes as long to clean a part as with the surging.

I googled for this topic and found only a few useful pages. One is the TP Tools page for an upgrade kit:
http://www.tptools.com/pl/Images,US-30_L,22,2320_USA-Cabinet-Gun-Upgrade-Kit.html
that shows a different style siphon tube, with the bottom of the air tube crimped down I assume to increase the velocity and stir up the abrasive to make it flow better into the suction tube. I made one that looks like it and it works a little better, but still feels like it's feeding way too little abrasive. Of course it's not exactly the same.

Anyway, it'd be helpful to know how it's supposed to work. Wish there was some way to compare how much abrasive was flowing. Looks like this style of gun is supposed to run around 90psi, but what works for you? Any tips on making a better feed tube? Some other web sites say the abrasive has to be dry "or else", am I wasting my time with an outdoor setup?

thx

Bob

It sounds like there is a big slug of grit rising in the siphon tube all at once, then falling back down the tube.

Perhaps restricting the end of the siphon tube with a smaller opening will help in 2 ways - first, it will increase the air velocity through the siphon opening, and second, will hopefully restrict the volume of grit entering the tube so that it remains airborne up the tube.

I have a pressure blaster, and have found that keeping the regulating valve closed pretty tightly and running a "lean" mixture of grit/air seems to keep the hose running free and clear.

I had the same problem with small 24" blast cabinet equipped with a siphon type gun. This is in Silicon Valley where normal humidity is not high. This was with a 12 CFM @ 90 psi compressor without aftercooling. It occurred with any media I tried.

Cured the problem with a large aftercooler equipped with large fan between the air pump and tank which was followed by a refrigerated air dryer between the tank and blast cabinet.

You say your air is "dry" but to what degree? It takes very little moisture (less than you can see) to cause the media to be less than free flowing.

I get "surging" on re-used abrasive that hasn't been properly filtered. Just the littlest spec of something, that isn't little enough will cause problems. Sometimes I have to blow out the line, other times a good wack of the gun will get it. When I filter my media through an old window screen (slow and dusty) I have almost no problems. I sometimes "see" the larger projectile when it finally clears the gun, and yea you can feel it in the gun.

I've also noticed if the media has the least bit of moisture it will make an empty pocket at the base of the siphon tube. A good jar to the base of the container / cabinet will get that if you are impatient.


Good Luck,

Doug S.

Does your siphon tube have an air hole in it where it joins the hose? I have a Zero blast cabinet and it has a pipe nipple Teed into the feed line for an air bleed. The nipple has a line of hole drilled along it's length, and a rubber cap that is slid up and down that nipple to uncover those holes to vary the amount of air that enters the hose along with the air. The proper amount of air seems to be important to carry the media along in the hose to get to the gun. If there is too little air, it behaves like you describe, too much air and it cleans too slowly.

Here is the trick to sandblast siphon tubes...

Sizes are used to demonstrate the point, use whatever you have that has the same approximate relationalship values.

Put a 3/8 OD tube inside a 1/2 ID tube. Make sure they are flush as the end that sticks in the media, but the smaller is a inch or so longer at the other end so it's easy to hook up the siphon tube from the nozzle to the smaller tube.

Either crimp the larger tube or drill and put a cotter key or something so that the two tubes are more or less locked together.

This allows atmospheric pressure at the pickup point no matter how deep the tube is pushed into the media. Don't fix the pickup tube to the side of the cabinet. If a clog develops, pull the tube point it somewhere in the cabinet, put your finger over the nozzle and squeeze the trigger. It'll force air down the pickup tube and clear the obstruction. Put the pickup tube back in the sand and continue sandblasting.

Tools

Have seen this... (600 lb pressure pot)

Damp clumpy sand...... Just takes a little moisture....
Get one of those electric boat bilge drying sticks to keep damp away in cabinet when not in use..

Something like this :


http://www.goldenroddehumidifiers.com/marine_application.htm

Keep that sand dry. I used to live down there.....

So the media is dry and flows freely. OK, going with that. Personally I've never seen a blast booth tht didn't benefit from a heat source in the sand hopper.

There should be an air entranment hole near the pickup end of the siphon. This is intended to "fluff" the media and capture it in a medium velocity airstream so it flows freely up to the gun. Solid media does not flow well. Check this feature and see if it it's clogged or, if it's equipped with a free air connection via a small diameter hose, if the entrainment supply hose is clogged. Generally, blocking the nozzle with yous hand in the sleeve glove and squeezing the trigger will forge a regurgitation clearing the siphon.

Anyway you need a free air source at the pick-up end to entrain the media in an airstream for the siphon to work correctly.

Thanks all for the replies. Based on that so far, neither my abrasive nor my air are as dry as they could be. The compressor has an aftercooler but not a separate dryer. I'll check out a heater for the hopper.

The abrasive does seem to flow freely, doesn't just clump and clog. For immediate relief, I'll try the series of holes in the top of the feed tube, to admit air to help the abrasive flow more evenly.

Bob

I'd like to dig this thread up to hopefully generate additional feedback. I built a cabinet from 3/4" ply with a hopper do blast wheels at work. I only have about 15-20 CFM to work with, so my jet/nozzles are the 4/8 CFM small ceramic set from TP Tools. I am using the $20 Campbell Hausfeld siphon gun and tube from Tractor Supply, but I presume it's just as good or bad as all the rest of the cheapos on the market. My cabinet is fed through a 1/2" hose that connects through 1/4" quick connects, and though I am looking to get some 3/8" quick connects, I noticed the air feed on the gun (and most others in this class) is 1/4" so it may be a while before I can graduate to the pricey ones with larger inlets. I have a 1/2" water separator from harbor freight, followed by a 2" Sch 40 PVC 2 stage dryer that begins with an over glorified separator and terminates at a large desiccant filter before passing through my regulator. From there, it Tees into the siphon air inlet, and another quick connect to link up the pressure pot. I know the pot is way more efficient, but the need to continually screen and refill makes it way less TIME efficient.

I just purchased a 1" black iron flange, nipple, 45° elbow, 1" - 3/4" bushing, gate valve, 3/4" - 1/2" bushing, and a 1/2" MPT/barb. I saw some videos on YouTube of guys that picked up the Tacoma Company upgrade kit which essentially converts a HF blast cabinet into a workable unit, but the meat of this upgrade is the gravity fed metering valve that supplies a measured amount of media to the venturi stream. Though I wish I were financially stable enough to purchase the upgrade outright, my reality has left me in the position of trying to reverse engineer it or do without it. That said, I am going to try this regardless and will share my results, but I'm hoping maybe some else has tried something like this with some success.

Maybe I just got lucky, but I recently bought the Horrible Freight blasting cabinet and have been using it with great success. I sealed all the joints with caulk as I was assembling it and mounted a motor and fan from an upright vacuum on the back of the unit to create negative pressure. This works so well that the gloves are fully inflated and standing straight out when I turn on the vac. In addition you can see air bubbling up through the sand (yes i'm using play sand) at the bottom when the vac is running. I think these were key to my success, sealing the joints and a strong negative pressure.

Apart from dry grit, another trick is to put a metered air line in the sand ''pit'' to keep it fluid therefore more easily sucked up by the vacuum.

Keith, I hope you're venting to somewhere other than your open shop air for your safety. I got a strong shop vac, but can't do the glove test till my new gloves arrive in the mail, but as it is now, when blasting, I can feel the air draw across my arm hairs through the large rips in my current Horrible Fright blast gloves.

Sami, I had that same fleeting thought today, but this would only work if that metered air were triggered from say a foot pedal which I don't yet have. Otherwise, air would continue to push media into the feed, and then back out once it gets resistance.

I connected all my fittings today and drilled the necessary holes in the hopper, but the media feed was only momentary. I think I might feed a recovery air line into the 1" line to hopefully assist with the siphoning and hopefully make for a steady fluid media stream to my gun. Here's some pics.

Limy Sami said:

Apart from dry grit, another trick is to put a metered air line in the sand ''pit'' to keep it fluid therefore more easily sucked up by the vacuum.

Click to expand...

Sorry, I think I misunderstood, do you mean to have the air blowing indefinitely to fluff the media near the suction port?

MikeOfAllTrades said:

Sorry, I think I misunderstood, do you mean to have the air blowing indefinitely to fluff the media near the suction port?

Click to expand...

That's it, just enough to keep it moving so it doesn't pack around the pick up, causing the pulsing.

Actually in the middle of this endeavor myself. I picked up a replacement gun for my harbor freight cabinet from Sandblasters, Sandblasting Guns, Gloves, Parts & Equipment | Sandblaster Parts

Using a solenoid valve and foot switch to control flow. I have my pickup located externally. I welded a pipe nipple to the bottom of the cabinet, on the hinged drain. There's a tee screwed to that with a barb on one side for the suction hose. Currently the other side of the tee is open.

Experiencing similar pulsing, I'm planning on spending some time putting an air bleed on it, hopefully it solves the problem.
I can post some pictures if anyone wants to see my setup


Sent from my iPhone using Tapatalk

kineticmx said:

I can post some pictures if anyone wants to see my setup

Click to expand...

I'd love to see your pics!

Thanks for bringing this thread back to the top. I've been fighting the same issue, which I think is partially due to debris in the media. I just ordered a sifter from TP Tools to clean it up. The air entrainment is interesting, I'm running a Waterloo blast cabinet with a second tube and ball valve that runs down to the base of the siphon tube. Now I'm going to have to fiddle with the valve to see if I can get it to feed better.

Graham Gott said:

Thanks for bringing this thread back to the top. I've been fighting the same issue, which I think is partially due to debris in the media. I just ordered a sifter from TP Tools to clean it up. The air entrainment is interesting, I'm running a Waterloo blast cabinet with a second tube and ball valve that runs down to the base of the siphon tube. Now I'm going to have to fiddle with the valve to see if I can get it to feed better.

Click to expand...

I think I'm going to give Sami's suggestion a whirl and pipe in a constant metered air line to blow air across (or at, but not into) my inlet port. Some years ago, I worked with dry bulk trailers and when unloading, they used aerators at the base of each of the hoppers to loosen packed materials enough to pass through their dump valves and into the feed stream. It's the same exact concept as what Sami suggested.