Syphon blast gun issue

[drummerdimitri]

Been having trouble getting the triggerless syphon blasting gun to work how I want it to in my DIY cabinet.

The issue is with media suction. At first, no matter how far in or out I adjusted the airjet with respect to the gun's body, I could not get the media to feed at all.

At first I thought there was something wrong with my setup, but then I decided to adjust another variable being the pressure.

I had initially adjusted the air pressure regulator at 120 psi and although the air stream was substantial, the only blasting I did was air.

It was then I decided to strap a vacuum gauge to the abrasive port to measure the actual suction being generated, and to my surprise it was way below what it should be at around 7 inches of mercury.

I read online that an ideal number for such blast guns is 16 inHg so I started backing the pressure down and went as low as 60 psi and I was able to reach that magic number and the gun started working properly.

As soon I as start increasing the pressure back up though, the suction started falling off again.

My question is the following: Why can't I blast at higher pressures, say 120 psi without losing suction since my compressor can deliver 42 CFM at that pressure continuously even though the nozzle and airjet combo only needs 34 CFM.

It bothers me to know I can clean parts faster with higher pressure and with more aggression but not being able to do so.

I'm using a 1/4" Boron Carbide nozzle with a 1/8" airjet, the correct combination so does anyone have an idea of what might be the issue here?

I've been suspecting the gun to be at fault since when I received it, the nozzle was way off and not at all concentrically aligned with the gun's blasting end so I did some machining to get it fixed and I did but the result is still a bit poor.

Will be checking the airjet for any defects as that would be affecting performance.

In the meantime, if anyone has any experience with such guns, please do share your thoughts.

 

[OccupantRJ]

Does your blast nozzle have a funnel shaped entry hole on the inner side of the gun?

 

[drummerdimitri]

Does your blast nozzle have a funnel shaped entry hole on the inner side of the gun?

It absolutely does and it installed correctly with an O-ring seal on the inner shoulder.

 

[OccupantRJ]

I suspect the higher pressure is overpowering the design of the gun. 120 psi is likely pulverizing the media anyway if that matters to you. I run at a flowing pressure of 100 psi during actual blasting using aluminum oxide, and on my second cabinet around 60 psi for glass beads. I can get my shop built gun alox gun to 14 inches vacuum. The glass bead gun is a factory gun similar in design to yours but with a different type nozzle.

 

[drummerdimitri]

I suspect the higher pressure is overpowering the design of the gun. 120 psi is likely pulverizing the media anyway if that matters to you. I run at a flowing pressure of 100 psi during actual blasting using aluminum oxide, and on my second cabinet around 60 psi for glass beads. I can get my shop built gun alox gun to 14 inches vacuum. The glass bead gun is a factory gun similar in design to yours but with a different type nozzle.

I think you're right, there is a sweet spot with these guns and 120 psi seems to be way above it.

I'd be better off blasting at 90 psi with a larger airjet and nozzle combo for faster cleaning rates.

 

[OccupantRJ]

I think you're right, there is a sweet spot with these guns and 120 psi seems to be way above it.

I'd be better off blasting at 90 psi with a larger airjet and nozzle combo for faster cleaning rates.

I agree. It is worth a try.

 

[GeoBruin]

I'd be better off blasting at 90 psi with a larger airjet and nozzle combo for faster cleaning rates

I think that's the way to go. If you've got more air, run a bigger jet/tip. 120 psi is way too high. You'll obliterate your media into dust. I started running more aggressive media (crushed glass vs glasses beads) for rust removal and it allowed me to step the pressure down even lower (60 - 80 psi) while still working just as fast.

 

[finn]

I think that's the way to go. If you've got more air, run a bigger jet/tip. 120 psi is way too high. You'll obliterate your media into dust. I started running more aggressive media (crushed glass vs glasses beads) for rust removal and it allowed me to step the pressure down even lower (60 - 80 psi) while still working just as fast.

This.

The media pickup works on a Venturi principle. If you run too much pressure, the airflow chokes in the nozzle, creating back pressure at the media pickup point.

The two options to counteract this is either back down the air pressure, or open up the nozzle orifice.

 

[Klokwerk]

This.

The media pickup works on a Venturi principle. If you run too much pressure, the airflow chokes in the nozzle, creating back pressure at the media pickup point.

The two options to counteract this is either back down the air pressure, or open up the nozzle orifice.

This helped. To me it seemed counter intuitive but after dialing my pressure back it helped. For my situation I think I'm also fighting a poor siphon tube design, the one that comes with my new Vevor 40 gal benchtop.

 

[finn]

This helped. To me it seemed counter intuitive but after dialing my pressure back it helped. For my situation I think I'm also fighting a poor siphon tube design, the one that comes with my new Vevor 40 gal benchtop.

**** / TP tools sells a retro fit pickup tube assembly, as well as replacement guns.

I have a really old HF cabinet that needs a new Venturi. Tube and gloves. It’s on my list, but in reality I don’t need it, since I have. Larger **** blast and a nice Econoline cabinet.

Don’t need three.

 

[Klokwerk]

Thanks for the tip...checking it out!
....and 2 minutes later, that's exactly what I need....thanks! Ordering now. Or maybe try to make one myself?

 

[txvwnut]

To media blast you do not need a ton of pressure, I run my cabinet between 40 and 50psi. When I was doing large media blasting at my friends shop we would run about 30psi as anything higher would warp sheet metal.

 

[Steve_P]

With a siphon gun, I use 80 PSI for AlOx, and 40-50 PSI for glass beads. High pressure kills the media and makes much more dust.