Adjusting the Flames for the Oxy-Fuel Cutting
Flame cutting/oxygen cutting requires some source of intense of heat to get the plate hot enough to cut/burn. This source of heat is referred to as the “preheat flame”. Therefore, it’s only necessary to heat the portion slightly ahead of the cutting reaction, not the entire plate.
There are a few plate materials that require “preheating” before cutting to keep them from cracking during cutting. Generally, materials with carbon levels above .25% fall in this category. However, this is not the “preheat” we are referring to.
A number of fuels are common for the oxy-fuel mixture called Preheat. Also, each has some feature to make it attractive.
Up until about 1970 Acetylene was the primary fuel of use. However, its escalating costs, the growth of plasma and the aggressive marketing of other fuels which has resulted in it no longer being a primary choice especially where plasma is also available.
A high heat transfer rate from the flame to the plate is necessary. The aspects the operator can control to achieve the transfer of heat include:
- flame temperature
- quantity of preheat gases used
- tip to work distance
Adjusting the O/F Ration allows for the control of the flame temperature. Raising or lowering the preheat pressures to the torch allows for adjustments of the quantity of preheat. High pressure results in high flowrates, i.e. a high velocity preheat. Low pressures result in low flowrates/velocity. The operator can adjust the O/F ratio by looking at the inner cone of the flame.
In addition, the quantity of preheat necessary varies depending on the operation you are conducting. For a pierce start, a very high flowrate (at the right O/F ratio) is necessary followed by an increasingly lower flowrate (at the right O/F ratio) for the continuing cutting operation. These preheat conditions are Hi Preheat and Lo Preheat. Pierce time is the time the Hi Preheat is on prior to initiation of the cutting oxygen.
Properly adjusting a preheat setting will result in a sharp top edge and a slag free and sharp bottom edge. The mill scale is extremely heavy. In this case, a higher preheat will sometimes be necessary to break the mill scale loose while keeping the leading edge of the cut properly heated.
Too much preheat can cause a rounded top edge as well as a significant increase in slag. This increase in slag frequently results in a cut that will not drop from the base plate. The slag is sometimes so tenacious that the part must be beat out of the plate with a sledge hammer.
The actual recommended quantity (CFH) of oxy-fuel varies with the tip manufacturer and tip purpose.
For practical shape cutting, the quantities typically required are Natural Gas 10-20 CFH and 18- CFH Oxygen and Propane 5-10 CFH and 23-45CFH Oxygen.
These are just the basics of properly adjusting the flames for the oxy-fuel cutting. Brazos Industries continually adds capabilities of new machinery to satisfy growing customer needs. We carry a wide array of steel products for any of your current or future project needs. Contact us with the link below to purchase any of our products in inventory!