Good Nozzle Practices
Blast cleaning system efficiency tips:
Choosing best blast cleaning nozzle for sandblasting productively shouldn’t be difficult, when you know your compressors CFM rating and match it with nozzle bore size!
So, you are looking to optimize performance of your blast cleaning system? What should you be looking at first? The simple answer is look at your compressor’s air output and how to use available air to select the best size of the sandblasting nozzle to maximize blast cleaning productivity. Most efficient blast cleaning systems will need sufficient compressed air to supply continually at least 95 to 100 PSI at the blast cleaning nozzle. There are situations where less forceful blast cleaning may be needed using softer abrasives, under these circumstances, a lower nozzle psi might be acceptable.
Compressor – one of the most important parts of a blast cleaning system.
First and most important thing to remember is that your compressor needs to supply air with constant high volume and constant high pressure. There is no point having high pressure and low volume or high volume and low pressure! Your system will not work efficiently.
Another important point to consider is any pressure drop between compressor and your (new) blast cleaning nozzle. There are several main areas in the blast system that can contribute to pressure drop: (1) length and I.D. of air hose and air lines running from compressor to moisture separator. (2) Moisture separator and (3) Length and I.D. of blast hose. Air supply valve on compressor will point to minimum I.D. of air hose you should use. You do not want to be using a ¾ or 1” ID air hose on a 1-1/2” compressor air control valve, this can cause a pressure drop between 5 and 20 PSI. Air hose should also have a smooth inner wall. A coarse textured inner wall on air hose will also contribute towards a pressure drop.
To compensate for any pressure drop the air compressor control valve can be set over 100 PSI but not to exceed your blast systems (ASME) pressure rating. Air pressure at nozzle can then be checked using a needle gauge. The compressor air control valve can then be adjusted (and if possible locked) to maintain pressure at nozzle of approx. 100 psi. Never subject any blast cleaning equipment to a pressure over 125 psi or the manufacturer’s pressure rating for your system.
What other very important factor will contribute to a system wide pressure drop? You guessed it! The blast cleaning nozzle’s bore. When the bore has worn from its original size to approximately 1/16” over the original bore size, the nozzle will need to be changed for a new nozzle having the original bore size.
Effects of a worn nozzle on blast cleaning performance and productivity:
Abrasive Blast cleaning is an important but costly process in which coatings or contaminants are removed. Anything that can be done to speed up the process and reduce cost through maintaining or improving system efficiency should be seriously looked at.
There are four main ways in which a blast system shows, over time, it is losing efficiency.
- (1) Using more abrasive to achieve same result as system had been getting at an earlier stage.
- (2) Increasing demand on compressor – using more CFM or air.
- (3) Not removing coating or blast cleaning as fast as it was originally.
- (4) Losing nozzle pressure.
What could be causing any or all above problems? The Answer 9 times out of 10, is a blast nozzle with a worn out bore.
When a sandblasting nozzle wears by 1/16” over its original (new) bore size then it should be considered worn and should be exchanged for a new nozzle. When you purchase a new nozzle the venturi is in prime condition and able to provide substantial acceleration to abrasive stream – yes, to over 400 MPH. This gives abrasive particles energy (kinetic) they need to work on a surface to remove coatings or contaminants.
Before increasing CFM or abrasive flow we recommend inspecting sandblasting nozzle for wear or damage. Damage should be visible as cracks in liner wall. Bore wear, is a little more difficult to gauge. There are several ways to check for wear, for more accuracy, use an Everblast nozzle bore gauge. If you don’t have a bore gauge, then a drill bit can be used to give a rough estimate of bore size.
Nozzle Bore Size Selection:
If you choose a nozzle with too small a bore and have plenty of air available you could be sacrificing additional blast cleaning capacity. If you choose a nozzle with too large a bore and don’t have enough compressed air to reach a nozzle pressure of 100 psi you won’t be able to blast clean productively. This is why knowing your compressor’s CFM rating is very important when choosing your sandblasting nozzle.
You can further optimize your blast cleaning nozzle selection by choosing a liner material to maximize productivity for type of abrasive being used. For example you wouldn’t want to be using a tungsten carbide nozzle with aluminum oxide abrasive. The tungsten carbide nozzle just won’t last.
|Liner Material||Steel Shot / Grit (hours)||Expendable Abrasives (hours)||Aluminum Oxide (Hours)|
|Silicon Nitride / Carbide||600-1000||400-600||50-100|
All figures is above table are indicative & given for comparison purposes only. Actual nozzle service life will depend on variables such as blast pressure, media dimensions and profile.
Sandblasting Nozzle Threads:
Sand blasting nozzles are available with several thread specifications. The most common are:
- 1-1/4" NPSM straight threads. Generally found on all contractor nozzles in USA. Note these are straight threads not tapered.
- 2" UNC or 50 mm threads. These are coarse threads and found on most contractor nozzles outside North America. However these coarser threads are catching on in popularity in USA, as threads are less likely to get damaged and are easier to wind in and out of nozzle holders.
- 3/4" NPSM threads. These are found on smaller nozzles for cabinet and spot blasting. These are straight threads not tapered.
There are a few manufacturers who are selling equipment with NPT threads - **** note these are tapered threads and are not widely used in blast cleaning industry. You should be careful if you are using equipment with tapered threads that you match with nozzles with tapered threads. If you have holders with tapered threads you will be limited in your choice of replacement nozzles. Fortunately most of the major manufacturers of blasting equipment use NPSM straight threads where there is an excellent choice of nozzles available.
Sandblasting Nozzle liner geometry:
95% of all nozzles sold for the contractor market are fitted with a venturi style liner. Venturi liners add velocity to air/abrasives mixture as it passes through the nozzle. This additional velocity means abrasive particles have more energy (Kinetic) to work on surface to remove coatings and or contaminants.
Tips for Installing Sandblasting Nozzles:
Suggested Good Nozzle Practice.
- Your new carbide lined sandblasting nozzle is supplied with a nozzle washer. Place threaded end of nozzle up against end of abrasive blast hose in nozzle holder. There should be 100% surface contact between nozzle washer and top of hose. (See picture below) When installed correctly the sandblasting nozzle washer will help prevent wear at blast cleaning nozzle entrance.
- Always inspect equipment & replace worn hose, nozzle holders, washers & carbide sandblasting nozzles.
- To maintain productivity always replace your carbide sand blasting nozzle when inside bore has worn to one size larger than originally supplied. (1/16" /1.59mm)
- To avoid injury/harm when blast cleaning, always follow all statutory & other established safety rules covering blast cleaning operating procedures.
- To avoid injury/harm when blast cleaning, always follow all statutory & other established safety rules covering the selection & use of equipment & media.
- When using and cleaning sandblasting / blast cleaning nozzles do not bang/impact them as this may crack or fracture the carbide liner.
- Please remember the blast cleaning nozzle is a tool and should be protected against damage like any other tool.
- Always check carbide lined sandblasting nozzles before using for excessive wear, cracked or broken liners. Always replace nozzles when you see the liner has been damaged, cracked or otherwise compromised.
- Before screwing nozzle & holder together, clean threads of grit etc. Make sure you use the supplied washer and replace when worn. The washer insures a good seal between the bottom of the blast cleaning nozzles and hose.
Important guide lines for installing your new nozzle:
To securely install nozzle in a holder there must be at least 70% of threads engaged in nozzle holder - a minimum of 8 threads on 1-1/4” NPSM or minimum of 4 threads on Contractor / 50mm style sandblasting Nozzle. The more threads engaged in holder the stronger the sandblasting nozzle installation. In engineering terms, the more threads that are engaged the more secure the connection.
Hose should be cut squarely & installed in holder so as to allow bottom of nozzle with washer to seat firmly against hose. There must be 100% contact with washer at bottom of sandblasting nozzle & sandblasting hose. Hose installation in holder should allow blast cleaning nozzle threads to engage in holder as recommended above.
Your quality sandblasting nozzle is supplied complete with a nozzle washer. This washer will provide a seal between end of sandblasting nozzle & blast hose when nozzle is installed correctly. Always use sandblasting nozzle with a washer.
Cut away showing nozzle installed in nozzle holder flush up against hose. There should be 100% contact between bottom of nozzle (washer) and hose surface.