Many blasting units do not dose correctly. They open the
dosing valve too widely so that a too high loading factor, i.e. amount of
abrasive to amount of compressed air, is operated, but this does not lead
to a respectively higher abrasive blasting performance. As regards normal
pressure values of 7 – 8 b analyses of used abrasive show that only
30 – 35% burst upon impact so that they cannot be used once again.
Hardness and shape of the used abrasive play a decisive role in this regard.
60% of the abrasive can be used at blasting pressures of 10 – 12 b.
The non-optimal position of the abrasive dosing valve below the blasting
pot is a second important factor for its wrong assessment because the pot
is often installed too far from the nozzle and this causes inevitably a separation
in the blasting hose. This one becomes bigger if the hose cross-section,
the coupling systems and the placing of the hose are incorrect (e.g. many
turns, jams, kinking).
That’s why utmost care has to be taken over dosing the abrasive. The
always increasing costs for disposal of the used abrasives partially mixed
with toxic paints have to be mentioned as well.
7.5 Hoses, lengths of hoses, couplings
The selection of an appropriate hose system is important whereas two different
demands are principally made on the hoses as regards the function:
- up to the blasting pot
transport of the compressed air without any loss of compressed air, if possible
- from blasting pot to nozzle
if possible, transport of the mixture of abrasive/compressed air without
any loss of compressed air under tremendous wearing influence of the transported
abrasive.
The hoses being nowadays common on the market are called compressed-air
hose and are designed just for the transport of compressed air whereas the
oil content of the compressed air that exists just because of the compression
process requires best quality. It is known that oil decomposes bad rubber
mixtures so that parts of the wall of the hose could peel off.
The term “blasting hose” is a feature of the hose quality which
is resistant to abrasion due to its composition. These hoses are, however,
not very much oil-resistant. Blasting hoses are generally made of a mixture
of natural rubber and synthetic rubber.
They are designed in such a way that there is a respectively thick inner
liner and a respectively stiff outer skin; both are tied in a polyamide yarn
inner layer by winding over a punch. The production lengths are 40 m.
The hose must have certain stiffness so that it does not break when laying
across edges like bridge railings, scaffold struts and constructional components.
Each kink means to have a reduction of the cross-section and thus pressure
losses and wear and tear. That’s why is has to be avoided by all means
to lay the hose with bends and turns.
The hose diameter is a very important criterion concerning wear and tear
of the hose and the pressure loss of the mixture flowing out. Sand blasting
hoses are only available at five different internal diameters because of
the complicated manufacture:
Nozzle size up to 5 mm hose diameter 13 mm
Nozzle size up to 6.5 mm hose diameter 19 mm
Nozzle size up to 8 mm hose diameter 25 mm
Nozzle size up to 11 mm hose diameter 32 mm
Nozzle size up to 20 mm hose diameter 42 mm
A thin hose and a big nozzle overload the hose.
A thick hose and a small nozzle overload the nozzle.
The clear span of the blasting hose should be 3 times as big as the boring
of the nozzle, 4 times would be better.
It is quite advisable to select the diameter at one size bigger to avoid
wearing effects. As a rule of thumb, the above-mentioned diameter is to take
in dependence on the nozzle size and the length of the hose.
In principal, hoses longer than 120 m should be avoided. When using such
lengths it is advisable to carry out the first 40-80 m with a bigger diameter.
The use of a bigger hose diameter avoids pressure losses from the blasting
pot to the nozzle. In order to be able to direct the nozzle optimally in
case of objects that are difficult to blast a hand hose of a length of 2
up to 5 m at the most is often used in connection with a jet nozzle at the
end.
The connection of different hose lengths and hose pieces among each other
is done through specially developed bayonet couplings. Those are not introduced
into the hose, but fitted on the hose. The have recessed hose slots and are
kept by tapping screws in the hose. The entire system is designed in such
a way that the claws have a certain shape to connect smaller and bigger hoses
with each other.
Aluminium, malleable iron, bronze and plastics are used for producing the
couplings.
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