Types of Nozzle

The type and quality of the nozzle can significantly impact the production and operational outputs of abrasive blast cleaning units.

Two main types of nozzle design have existed in the industry:

1. Straight bore nozzles.
2. Venturi style nozzle.

This training lesson will examine both types of nozzle design and the various specialist nozzles used for abrasive blast cleaning.

Straight Bore Nozzle

This nozzle is sometimes referred to as a conventional style nozzle.

1. At pressures of 100psi (7bar), the velocity of the abrasive leaving this type of nozzle is approximately 200 miles per hour (89 m/s).
2. The straight-bore blast delivers full impact only in the centre section.
3. This is illustrated in the photograph of a previously coated steel plate.

Straight Bore Nozzle

This illustration shows an abrasive blast test with a straight-bore nozzle held approximately 46 centimetres (18 inches) from the surface.

1. The full impact is approximately 12.5 centimetres, which is under 5 inches.
2. There is a complete and partial impact.
3. The partial impact will require additional blast cleaning to remove all the coating, which will take extra time and cost.

Venturi Shaped Nozzle

In the early 1960s, venturi-shaped nozzles were developed.

1. This design allowed a more uniform impact on the entire pattern.
2. These nozzles have had a significant impact on the abrasive blast cleaning industry, becoming very popular and widely used in surface preparation.

Venturi Nozzle

This illustration demonstrates an abrasive blast test using a venturi nozzle held approximately 46 centimetres (18 inches) from the surface.

1. The full impact is 15 centimetres or nearly 6 inches.
2. There is no partial impact with the venturi nozzle, only full impact. The full impact is more effective and efficient.
3. This illustration demonstrates how this type of nozzle is now extensively used for abrasive blast cleaning, especially on large surfaces.

Venturi Shape Nozzles

The venturi shape nozzle has a large entrance mouth.

1. The large entrance mouth allows better flow of abrasive through the nozzle by eliminating turbulence and friction found in straight-bore nozzles.
2. The Venturi style nozzle is the primary nozzle used in industrial work and large-scale contracts where high volume blast cleaning is performed.

Nozzle Types

This image illustrates the difference between the straight bore nozzle and the venturi shaped nozzle.

1. The venturi shaped nozzle is more efficient than the straight-bore nozzle.
2. An operator can check the nozzle type (before it is fitted) by looking down the funnel.

Nozzle Comparison Chart

This comparison chart illustrates the differences between cast iron, ceramic and tungsten carbide nozzles.

1. The life expectancy between nozzle types varies tremendously.
2. It is always worth reviewing the nozzle's life expectancy before selecting it.
3. Long-life nozzles will be initially more expensive, but may be more economical over time.

Boron Alloy Nozzle

A relatively inexpensive nozzle.

1. This nozzle is used extensively for site work but is prone to mechanical damage.
2. Life expectancy is 40-60 hours, depending upon the type of abrasive used.
3. Typically, these nozzles are used a lot longer than recommended. Additional abrasives are then used.
4. The nozzle is generally held in place by a lightweight nozzle retaining nut and a nozzle gasket.

Abrasive Blast Cleaning on Site

This video demonstrates abrasive blast cleaning on a site structure.

1. Boron alloy nozzles are generally used with expendable abrasives.
2. The environment dictates that these nozzles get a lot of abuse and have to be changed regularly due to damage from being dropped onto a steel surface, etc.
3. Spare nozzles should always be available.

Spot Blasting

When spot blasting or blasting at low pressure, you must select the correct nozzle for the specific task.

1. Smaller nozzles, designed for smaller hoses, are ideal for spot blasting and repairs.
2. Bigger nozzles and hoses will not give the operator the same level of control.

Boron Alloy Nozzle

This illustrates a boron alloy nozzle secured to the blast hose by an aluminium retaining nut and gasket.

1. This retaining mechanism is safer than a nozzle held with a tightening clip, which is often found on site equipment.
2. This set-up is generally found in factory abrasive blast chambers, where a better maintenance program is typically in place.

Less abuse is also observed in factory blast equipment than in site blast cleaning equipment.

Silicone Carbide (lined nozzle)

A silicone carbide nozzle is a very long life type nozzle with an expected life of up to 2,000 hours.

The nozzle is lightweight and will generally outperform most nozzles.

Boron Carbide Nozzle (illustrated)

Extremely abrasive resistant.

Generally very expensive but can last a long time if looked after and not abused.

Special Nozzles

There are specific applications where very tough abrasives, such as aluminium oxide and silicon carbide, are required.

1. Traditional nozzles would wear away fairly rapidly with these types of abrasives
2. Special nozzles with a cyclone liner are available to handle these types of abrasives.
3. These nozzles are also the venturi type to assist with the full surface impact, as discussed earlier.

Nozzle Selection

This illustration shows the various types of nozzles available to abrasive blast cleaning operatives.

1. The size and type of nozzle will vary depending on the task to be performed.
2. Nozzles significantly affect the efficiency of abrasive blast cleaning, so it is essential to review the type of nozzle to purchase and use.

Specialist Nozzles

Specialised nozzles are also available.

1. These are generally made from the same materials as venturi nozzles. Specialised nozzles are available in short and angled sizes.
2. Other specialised nozzles are available for wet abrasive blast cleaning and other tasks.
3. This picture demonstrates the various types of nozzles used with wet abrasive blast cleaning equipment.

Wet Abrasive Blast Cleaning

This video demonstrates wet abrasive blast cleaning.

1. Various non-metallic abrasives can be used with wet abrasive blast cleaning.
2. Specialised purpose-made wet abrasive blast nozzles are used for this process.
3. Water is mixed with the abrasive in some of these nozzles.

Wear and Tear

Nozzles from reputable suppliers should be well designed and have precise inlet, orifice, and outlet dimensions.

1. Wear and tear are inevitable in all nozzles. As a result of the process, nozzles will need to be replaced.
2. A nozzle inspection gauge or visual inspection can be used to assess the nozzle's wear and tear.
3. Visual inspection of the nozzle is easy.

Nozzle Inspection

This animation demonstrates the use of a Pressure Blast Analyser Gauge for checking a nozzle for wear and tear.

1. The inspection is a quick, easy test that should be conducted before commencing the work.
2. Worn nozzles are not effective and should not be used.
3. Always have spare nozzles available.

Wear and Tear

A worn nozzle will consume more air, leading to a drop in nozzle pressure and, in turn, reduced efficiency.

The larger the orifice:

1. Increased abrasive use.
2. Greater air consumption.

This adds up to more costs.

If you manage the nozzles correctly, you will save money!

Air Consumption Table

This table illustrates the air consumption, in cubic feet per minute, of blast cleaning nozzles at various pressures.

1. The use of such charts helps calculate the pressure requirements for different-sized nozzles.
2. The nozzle number corresponds to the nozzle diameter.
3. A No.3 nozzle has a 3/16" nozzle orifice.
4. A No.5 nozzle has a 5/16" nozzle orifice.

Inspection of Nozzle

The user should always ensure the following:

1. The nozzle gasket is not worn.
2. An externally fitted nozzle holder is used.
3. The right size nozzle is used.
4. The nozzle pressure is correct. A hypodermic needle can check this

Nozzle Pressure Testing Kit

A nozzle test kit contains the pressure gauge and hypodermic needles.

1. The hypodermic needle is attached to the pressure gauge and inserted into the blast hose just behind the nozzle.
2. This test will give an instant pressure reading.
3. The same test can be performed near the compressor or at mid-length to measure any pressure drop.

Pressure in the Blast Hose

This animation illustrates the use of a hypodermic needle gauge to check the pressure in the abrasive blast hose.

1. The needle must be angled to follow the flow of air and abrasive.
2. This is a simple test and is worthwhile conducting, as it gives an immediate indication of the pressures.

Curved Nozzle

It is possible to attach 40-degree curved nozzles to blast clean difficult areas.

These nozzles are attached to a steel jacket and thread and are boron carbide lined.

Angle Nozzles

45-degree angle nozzles have been used successfully on structures where traditional (and curved) nozzles cannot reach.

1. These nozzles are lined with tungsten carbide. However, due to the acute angle, the nozzle will wear away considerably faster than traditional tungsten carbide lined nozzles during use.
2. The blast hose type and diameter must be correctly set for this type of blast nozzle.

Multi-Nozzle Wand

When it is essential to abrasive blast clean extremely tight locations, as illustrated, a special 'wand' type nozzle with double, triple, orifice nozzles is available.

1. The nozzle can use most abrasives and only requires a ¾" (19mm) clearance.
2. The process requires adaptation of the blast hose and is relatively slow compared to traditional blast cleaning methods.

Summary

This training unit reviewed the various types of blast cleaning nozzles used for abrasive blast cleaning.

For traditional nozzles, we reviewed the straight bore and Venturi types and the advantages of the latter.

We have discussed the various types of 'new' nozzles that are extremely hard wearing and used for tough abrasives.

We also discussed nozzles suitable for awkward locations and tight spaces.

Nozzle wear and tear and blast hose pressure testing were reviewed.