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Oxyfuel cutting - process and fuel gases
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Diagram of oxyacetylene cutting
process |
The oxyfuel process is the most widely applied industrial thermal
cutting process principally because it can cut thicknesses from 0.5mm to
2,500mm, the equipment is low cost and can be used manually or mechanised.
There are several fuel gas and nozzle design options that can
significantly enhance performance in terms of cut quality and cutting
speed.
Process fundamentalsThe cutting process is illustrated in
Fig. 1. Basically, a mixture of oxygen and the fuel gas is used
to preheat the metal to its 'ignition' temperature which, for steel, is
700°C - 900°C (bright red heat) but well below its melting point. A jet of
pure oxygen is then directed into the preheated area instigating a
vigorous exothermic chemical reaction between the oxygen and the metal to
form iron oxide or slag. The oxygen jet blows away the slag enabling the
jet to pierce through the material and continue to cut through the
material.
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Fig.1. Diagram of oxyacetylene cutting
process |
There are four basic requirements for oxy-fuel cutting:
- the ignition temperature of the material must be lower than its
melting point otherwise the material would melt and flow away before
cutting could take place
- the oxide melting point must be lower than that of the surrounding
material so that it can be mechanically blown away by the oxygen jet
- the oxidation reaction between the oxygen jet and the metal must be
sufficient to maintain the ignition temperature
- a minimum of gaseous reaction products should be produced so as not
to dilute the cutting oxygen
As stainless steel, cast iron and
non-ferrous metals form refractory oxides ie the oxide melting point is
higher than the material, powder must be injected into the flame to form a
low melting point, fluid slag.
Purity of oxygenThe cutting speed and cut edge quality are
primarily determined by the purity of the oxygen stream. Thus, nozzle
design plays a significant role in protecting the oxygen stream from air
entrainment.
The purity of oxygen should be at least 99.5%. A decrease in purity of
1% will typically reduce the cutting speed by 25% and increase the gas
consumption by 25%.
Choice of fuel gasFuel gas combustion occurs in two distinct
zones. In the inner cone or primary flame, the fuel gas combines with
oxygen to form carbon monoxide and hydrogen which for acetylene, the
reaction is given by
2C2H2 +
2O2  4CO + 2H2
Combustion also continues in the secondary or outer zone of the flame
with oxygen being supplied from the air.
4CO+2H2+3O2 4CO2+2H2O
Thus, fuel gases are characterised by their
- flame temperature - the hottest part of the flame is at the tip of
the primary flame (inner cone)
- fuel gas to oxygen ratio - the amount of fuel gas required for
combustion but this will vary according to whether the flame is neutral,
oxidising or reducing
- heat of combustion - heat of combustion is greater in the outer part
of the flame
The five most commonly used fuel gases are
acetylene, propane, MAPP (methylacetylene-propadiene), propylene and
natural gas. The properties of the gases are given in the Table. The
relative performance of the fuel gases in terms of pierce time, cutting
speed and cut edge quality, is determined by the flame temperature and
heat distribution within the inner and out flame cones.
AcetyleneAcetylene produces the highest flame temperature of all
the fuel gases. The maximum flame temperature for acetylene (in oxygen) is
approximately 3,160°C compared with a maximum temperature of 2,810°C with
propane. The hotter flame produces more rapid piercing of the materials
with the pierce time being typically one third that produced with propane.
The higher flame speed (7.4m/s compared with 3.3m/s for propane) and
the higher calorific value of the primary flame (inner cone)
(18,890kJ/m3 compared with 10,433
kJ/m3 for propane) produce a more intense
flame at the surface of the metal reducing the width of the Heat Affected
Zone (HAZ) and the degree of distortion.
PropanePropane produces a lower flame temperature than acetylene
(the maximum flame temperatur in oxygen is 2,828°C compared with 3,160°C
for acetylene). It has a greater total heat of combustion than acetylene
but the heat is generated mostly in the outer cone (see Table).
The characteristic appearance of the flames for acetylene and propane are
shown in Figs.2 and 3 where the propane flame appears to be less
focused.. Consequently, piercing is much slower but as the burning and
slag formation are effected by the oxygen jet, cutting speeds are about
the same as for acetylene.
Propane has a greater stoichiometric oxygen requirement than acetylene;
for the maximum flame temperature in oxygen, the ratio of the volume of
oxygen to fuel gas are 1 to 5 for acetylene and 4.3 to 1 for propane.
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Fig.2. Ocyacetylene gas jet and nozzle design |
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Fig.3. Propane gas jet and nozzle
design |
MAPPMAPP gas is a mixture of various hydrocarbons, principally,
methylacetylene and propadienne. It produces a relatively hot flame
(2,976°C) with a high heat release in the primary flame (inner cone)
(15,445kJ/m3), less than for acetylene
(18,890kJm3) but much higher than for
propane (10,433kJm3). The secondary flame
(outer cone) also gives off a high heat release, similar to propane and
natural gas. The combination of a lower flame temperature, more
distributed heat source and larger gas flows compared with acetylene
results in a substantially slower pierce time.
As MAPP gas can be used at a higher pressure than acetylene, it can be
used for underwater cutting in deep water as it is less likely to
dissociate into its components of carbon and hydrogen which are explosive.
PropylenePropylene is a liquid petroleum gas (LPG) product and
has a similar flame temperature to MAPP (2896°C compared to 2,976°C for
MAPP); it is hotter than propane, but not as hot as acetylene. It gives
off a high heat release in the outer cone (72,000kJ/m3) but, like propane, it has the disadvantage of
having a high stoichiometric fuel gas requirement (oxygen to oxygen ratio
of approximately 3.7 to 1 by volume).
Natural GasNatural gas has the lowest flame temperature similar
to propane and the lowest total heat value of the commonly used fuel
gases, eg for the inner flame 1,490kJ/m3
compared with 18,890kJ/m3 for acetylene.
Consequently, natural gas is the slowest for piercing.
Table : Fuel Gas Characteristics |
| Fuel Gas |
Maximum Flame
Temperature °C |
Oxygen to fuel gas
Ratio (vol) |
Heat distribution
kJ/m3 |
| |
|
|
Primary |
Secondary |
| Acetylene |
3,160 |
1:5 |
18,890 |
35,882 |
| Propane |
2,810 |
4.3:1 |
10,433 |
85,325 |
| MAPP |
2,927 |
3.3:1 |
15,445 |
56,431 |
| Propylene |
2,872 |
3.7:1 |
16,000 |
72,000 |
| Hydrogen |
2,834 |
0.42:1 |
- |
- |
| Natural Gas |
2,770 |
1.8:1 |
1,490 |
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