Blast School 101
BLAST SCHOOL 101
If pressure blasting was as simple as rinsing-off soap with a garden hose sprayer you would not need this information, but there is more more to it than that.There are at least FIVE important things to learn about blasting a surface:
VOLUME - PRESSURE - NOZZLE WEAR - DRY MEDIA - TECHNIQUE.
Lot’s of pressure and no volume of air and you get a blow job but no cleaning. Lots of volume and no pressure and you get zero cutting action. The right combination of volume and pressure will effectively remove paint, rust, etc. from metal or any other surface that you need to blast clean.
First, and perhaps most importantly a blaster requires air from an external air compressor and that compressor MUST produce a sufficient volume of air to supply a selected nozzle. The blaster tank does not dictate which nozzle you need, it’s about matching the nozzle with the compressor’s ability. The blast tank just holds the abrasive media.
Volume is measured in CFM “cubic feet per minute.” If your compressor does not produce enough air volume you will not maintain correct pressure and the media will not distribute properly.
To determine how large a compressor you need, add the air requirements of a particular nozzle, then add a little more CFM for reserve.
You can find a chart on many websites titled “Compressor – Nozzle Match” – Check it out and see how much air volume you need for the different nozzles. Also, look online for compressor nozzle charts, some charts will show you how much media you will use per hour with each nozzle as well. That way you can figure how much media you will need to clean a known area.
The diameter of the nozzle opening ( nozzle size) determines the amount of air and abrasive you will use during a segment of time.
Nozzle size usually equals how much area you can clean because that equals the size of the grit you can blast and how much of that grit will get blasted.
Blast hose size A good rule of thumb regarding blast hose size is that the internal diameter of a hose should be at least three to four times the diameter of the blast nozzle orifice (hole in the end of the nozzle.) [The exception is steel abrasive - it is so heavy that you have to downsize your hose a little to keep it moving.]
The grit valve (abrasive media valve) at the bottom of the tank determines how much abrasive mixes in with the air going to the nozzle.
SIMPLE RULE to CREATE PROPER MIXTURE: Use a lot less grit than air.
It feels like about 10% grit to 90% air. The air pressure you are using, the size of the nozzle and the kind and size of the abrasive grit you are using also affects the grit valve opening. Since the TexasBlaster is a manual set-up system it’s up to you set the right mixture. There is an optimum air/abrasive mix, but you have to determine that at the outset of each job. Experiment until you get it down.
I suggest starting with the grit valve closed and then opening the valve a little at a time until you attain the right mix. Experienced blasters can hear a steady abrasive flow, too little abrasive causes a high-pitched sound; too much an erratic pulsing sound.
A quarter-inch nozzle usually cleans four times faster than a one eighth-inch nozzle, and uses four times the air and four times the abrasive. That's because the area is proportional to the square of the radius. (high school physics) In other words, larger nozzles clean larger areas more productively, but smaller nozzles clean smaller areas, such as tight corners or welds, more efficiently.
A worn nozzle will slow you down. Even thought the opening is bigger. The nozzle should restrict the passage of air and abrasive. Restricting the passage reduces volume, but it speeds up the air and abrasive mix. Most blast nozzles are shaped like a sideways hourglass - they taper towards the middle. Compressed air speeds up as it passes through the nozzle, accelerating the abrasive in the air stream. As the inside of the nozzle wears the mixture slows down and you loose pressure. You can usually tell when you are not getting as much work done and you will know it is time to replace a nozzle.
Also, just because a nozzle opening is larger does not mean that larger abrasive particles are more productive. Large particles do not always clean faster than small ones. Larger grit abrasives cut deeper but there are fewer particles striking each square inch of the surface. That means nothing hits some parts of the surface, so the blaster operator has to go back over it again and again to clean a surface well.
I suggest you use the finest-size particle that will clean the surface. Test with different size abrasives and find the size that will clean the quickest.
Of course you have to hit the surface with sufficient force to remove unwanted material. That takes air pressure (as well as air volume) to blast a surface clean.
The industry rule of thumb is 80 to 100 psi at the nozzle. With some metal abrasives you need from 120 to 130 psi from the compressor depending on the length of air hose and blast hose. Some abrasives (like sand) pulverize when it strikes a surface if you have too much pressure. That’s why 80 to 100 psi is the rule. Obviously, metal abrasives do not break down like other abrasives.
DO NOT TRY TO GET HIGHER PRESSURES THAN YOU NEED - For 130 psi at the nozzle, you would need 140 psi at the blast tank and most small blasters imported into the U.S. (and there are thousands of them out there) are rated at 125 psi - and pressure should never exceed the rating for the blaster or you can experience a disaster.
Pressure Efficiency: - let’s recall our high school physics class. This lesson will save you time and money and that will come in handy. Sir Isaac Newton (the apple-falling discovered gravity fellow) found that a body in motion will continue to move in a straight line until another force acts on it to stop it or deflect it.
That is as true for abrasives going down the blast line. Each time there's a loop or coil in your blast hose, something stops the abrasive from moving in a straight line and that costs you energy.
Even correctly sized air hose loses two to three pounds of air pressure for every 50 feet of length. A 90-degree turn doubles the loss to five or six pounds of pressure. The more bends or loops the greater the loss. On most jobs you can't avoid bends, but you can make them gradual. With sand and other abrasives, friction against the walls of the blast hose slows the abrasive more than gravity does and you lose velocity and therefore cleaning ability. So the right size of hose is important.
Moisture is a deal killer. Gummy abrasive will stop the show. Use a water seperator or maybe two.
Blasting is more like spray painting than you might think. KEEP THE NOZZLE AT A CONSISTENT DISTANCE FROM THE SURFACE AND KEEP IT MOVING AT A SMOOTH, CONSTANT AND STEADY STROKE.
An inexperienced blaster operator will whip the nozzle back and forth then concentrate in one area, and then blame the blast media for the pits or warping that occurs. Some greenhorns move it in a open wide arc and find a complete lack of productivity. Both are incorrect and frustrating.
To clean metal you want to hold the nozzle generally at a 45 degree angle. Changing the angle one way or the other will change the cutting action when you are removing layers and layers of paint and crud. Try different angles to see the changes in the cutting action.
The distance from the surface can also be critical. The power of the air pressure diminishes quickly when you hold the nozzle farther away from the surface. By adjusting the distance you can blast harder or softer depending on the surface that you are cleaning.
Sand blasting is not complicated. These five factors will help you understand how to blast.
With the correct abrasive and a good nozzle that is the right size and with sufficient air volume (CFM) and 80 to 100 pounds of air pressure you can blast the surface off of almost anything.
We make a darn fine blaster and we hope you will choose to do your blasting with a TexasBlaster! It’s a tool not a toy and it works like it was built to work.
You can order your TexasBlaster right here on-line and save yourself some time and money.