Surface World May 2020 Surface World May 2020 | Page 46

High Performance Vapour
Degreasing Solvent
Telephone: +44 ( 0 ) 20 8281 6370
Website: www.envirotech-europe.com
TESTING & MEASUREMENT
Optimising your abrasive blast efficiency
The efficiency of an
abrasive blasting project
is measured by two key
parameters: the amount of
surface area which can be
covered in a given time and
the amount of abrasive and
energy required to do it.
As achieving increased efficiency can
significantly reduce the cost of a blast
project, it is critical to understand how these
parameters can be optimised. It is for this
reason that Elcometer commissioned a series
of tests to find out exactly how the set up of
an abrasive blast system can determine the
efficiency of the process.
The testing highlighted one major trend: a set up using higher
compressor pressure and higher CFM capacity, combined with a
larger blast nozzle, yielded by far the most efficient results. This set up
took significantly less time, fuel and abrasive to blast a given area
when compared with more standard set ups.
The Abrasive Blasting Process
The abrasive blasting process requires hardened abrasive to impact a
surface with high energy in order to prepare the surface for a
subsequent coating.
The severity with which an abrasive impacts a surface is dependent
upon the particle hardness and the amount of kinetic energy
(movement) it carries. Whilst the hardness is abrasive specific, the
kinetic energy is determined by both the particle mass and the speed
that it travels.
Mass is defined by the size and weight of the abrasive particle, whilst
the speed is determined by the pressure at the blast nozzle and nozzle
design. Therefore, the key variable affected by the set up of a blast
system is nozzle pressure and hence abrasive speed.
It is widely believed that 7-7.5bar (102-110psi) is the ideal pressure
at the blast nozzle, allowing significant abrasive speed without
becoming too strenuous for a blaster to hold. This ideal pressure can
be achieved through optimising a variety of factors within a blast
system to enable the completion of a project in less time, using less
abrasive.
44 MAY 2020
Minimising Pressure Loss
An Elcometer 102 Needle Pressure
Gauge is designed to measure air
pressure within the blast system.
In the first instance, a blaster should aim to achieve optimal pressure
at the blast nozzle by minimising pressure loss across the blast system.
The laws of Physics dictate that compressed air pressure drops when
the air is moving and considering that generating air pressure is very
expensive, it makes sense to minimise pressure loss across the blast
system (from the compressor to the blast nozzle).
Friction is the main cause of air pressure loss, which predominantly
occurs within the abrasive blast machine and across the length of the
blast hose. Dynamic pressure loss within the blast machine is intensified
if the pipework is narrow or consists of lots of bends and constrictions.
The Boundary Layer Effect (nature’s resistance to flow) causes more
loss to occur around the circumference of a pipe than in the centre,
meaning pipes with a smaller diameter proportionally have more
frictional loss compared to pipes with a larger diameter.
That is why the pipework on Elcometer’s range of Abrasive Blast
Machines has a large internal diameter which, when combined with a
CAD based design minimises the overall flow restrictions, which
results in a significantly reduced pressure loss.
To minimise the pressure loss within a blast hose it is beneficial to
minimise bends and elevations. However, as the scope of a blasting
project largely dictates the positioning of hoses, a blaster often has
little control over the hose pressure loss.
Similarly, the demands of a project may require varying hose lengths,
further affecting the rate at which the pressure loss occurs. The longer
the blast hose is, the more pressure is lost – at a rate of up to 2bar of
pressure across 40 metres of hose.
CONTINUED ON PAGE 46
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