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.