Surface World May 2020 Surface World May 2020 | Page 48

TESTING & MEASUREMENT
Furthermore, if the hose is worn and flexible, the air passing through
will become more turbulent, thus adding to frictional losses along its
length.
Therefore, the most influential way a blaster
can reduce pressure loss within the blast
hose is by preserving its condition.
Whilst minimising pressure loss may be
sufficient to improve efficiency in smaller
scale blasting projects, a larger project may
require further adjustments.
A blaster should therefore consider the
combination of the compressor pressure
setting, compressor air flow capacity, blast
pot type, blast hose length and nozzle size.
The blast nozzle is the single most important
part of the blast system, designed to
accelerate airflow to provide the abrasive
with enough energy to clean or profile the
surface being blasted.
Hence, it is critical to use the optimum nozzle
for the project being considered. The
majority of blast work is completed using the
inherently efficient single Venturi design, nozzles are available with a
double venturi design, as well as different bore or orifice sizes.
A nozzle with a larger orifice is thought to consume more abrasive
and more air, causing blasters to have initial concerns about using
larger nozzles.
However, increased nozzle sizes enable larger spray patterns to be
created. This means a larger surface area can be blasted in a shorter
period of time, resulting in faster blasting, reduced air and abrasive
consumption.
As the nozzle size increases, the pressure at the nozzle drops meaning
abrasives impact the surface with less kinetic energy. To counteract this
pressure loss, a blaster can increase the pressure at the compressor.
A larger orifice size enables more air to exit the nozzle which, without
sufficient air volume capacity to support the higher air flow, causes the
nozzle’s pressure levels to fall, resulting in reduced blasting efficiency.
Therefore, air must be replenished to enable the blaster to achieve the
required blast efficiency. This can be achieved by using a compressor
which has a sufficient volumetric flow rating
(usually CFM or LPM) capable of increasing
the air flow to create the required pressure
level at the nozzle.
There is a big misconception that ‘driving’
a larger nozzle with increased CFM is more
costly.
In reality, the expenditure of a blast project
can be reduced significantly by incorporating
the suggested changes.
By increasing the nozzle size and maintaining the
optimum nozzle pressure, a blaster can achieve
significant improvement in coverage speed,
resulting in reduced fuel and abrasive costs per
metre. Consequently, the labour, abrasive and
equipment expenditure fall at a rapid rate.
The advantages of incorporating this
suggested change were highlighted in the
46 MAY 2020
testing commissioned by Elcometer.
Elcometer Abrasive Blast Machines were used to blast a series of
steel panels coated with a 3 layer glass flake epoxy, using three
Each surface was coated
with a tough 3-layer glass
flake epoxy prior to testing.
different compressor pressures (8, 10 &12bar), three different Single
Venturi nozzles (#4¼”/6.35mm nozzle, #6 3 / 8”/9.5mm nozzle,
#8½”/12.5mm nozzle) and air supplied by a 550 CFM 12bar rated
diesel compressor, to a minimum of SA2.5 and a profile of 60-70 μm.
The amount of time taken to blast a predetermined area of 1m2
would then be recorded, before each machine was weighed to assess
the amount of abrasive used.
Parallel tests were also carried out on a range of competitor blast
machines and the performance differences were measured and
recorded.
The testing of various different pressure settings and nozzle
combinations highlighted that higher compressor pressures with larger
nozzles were blasting the surface significantly quicker than lower
compressor pressures. In fact, the set-up widely adopted by the
industry, consisting of 8 bar compressor pressure with a 3 / 8” nozzle,
took 6.93 minutes to blast an area of 1m2. Yet, the optimised 12bar
compressor pressure with a ½” nozzle set up took just 4.42 minutes
to cover the same area - a 36% reduction in time.
The amount of time taken to
blast an area of 1m 2 was
recorded during testing.
CONTINUED ON PAGE 48
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