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Oxy-Fuel Torch Tip/Nozzle Design & Selection
Part 1 of a 2 part series
When designing or selecting an oxy-fuel machine torch cutting nozzle/tip one must consider several factors. The following are several, but not all, items to be considered 1) thickness of the steel to be cut, 2) type cutting bore (cylindrical bore or high speed/divergent bore), 3) preheat gas to be used, 4) type cutting to be conducted (shape cutting, bevel cutting, etc.), and 5) characteristics of material to be cut (material composition, primed plate, mill scale, etc.). There are other parameters to be considered but these seem to be the major ones. Of all of the factors in Flame Cutting, the nozzle, and the stream of cutting Oxygen exiting from it, is by far the most important item in the process. Neither the torch nor the pressure control devices associated with it have much to do with the actual cutting.
Cutting convention today finds that it is necessary to increase the cutting nozzle bore diameter as the plate thickness increases. The primary reason for this increase is the fact that the quality/velocity of the cutting stream decreases as the distance from the nozzle exit increases. The velocity of the cutting Oxygen stream is almost always at least sonic (Mach=1) and expands to supersonic either in the nozzle or downstream of the nozzle.
The term “laminar” is sometimes used to describe this stream but is not an appropriate term to use for a supersonic stream. The length that the stream remains “good” is a function of the bore diameter. With this supersonic stream there is an L/d relationship between the length of good cutting stream (L) and the nozzle diameter (d); therefore, the actual length of this stream increases as the bore diameter increases.
While one can use an “oversized” nozzle to cut “thin” material, this will usually result in a reduction in quality as well as an increase in cutting Oxygen consumption. As the bore diameter increases the quantity of Cutting Oxygen also increases. For a fixed operating pressure the flowrate/quantity of cutting Oxygen increases with the square of the diameter, d2, so small increases in bore diameter results in large increases in flowrate.
Therefore, going oversized is not a good idea as it results in a significant increase in operating cost. Going severely “oversized” also reduces the squareness of the cut. Typically the cut becomes an inverted ”V” with the bottom of the kerf wider than the top of the kerf.
Today there are two basic bore designs 1) cylindrical bore and 2) divergent bore (also called high speed bore) (see figure 1). The cylindrical bore is most commonly used for hand cutting and machine cutting thin materials (1/8-3/8 in.) while the divergent bore is typically used for material thicker than 3/8 in. Since most CNC cutting machines today are equipped with both plasma and gas cutting, most customers cut thin material with plasma and switch from plasma to gas at ¾ to 1in. thick materials. The switch point is typically determined by the number of plasma torches installed, the power of these torches as well as the number of oxy-fuel gas torches installed.
The operating difference in these two style oxygen bores is the pressure required for best operation. The cylindrical bore operates best at about 40 psig while the divergent bore operates best at pressures in the range of 75-100 psig. The actual best pressure is a function of the ratio of the exit size to the throat size, so the pressure is determined by the manufacturer of the tip depending on the ratio they choose. These pressures are pressures as measured at the torch inlet. Cutting tips will typically operate satisfactorily at pressures +/- 10 psig of the design pressure. Some operators find these special conditions after an evaluation for their particular conditions. It should be noted that kerf width will increase as the cutting oxygen pressure is increased.
Properly used divergent bore nozzles have faster cutting speed capabilities as compared to cylindrical bore nozzles and in spite of having higher operating pressures generally consume less oxygen per foot of cut. The chart below is a general comparison of cutting speeds for these two types of nozzles.
While the actual speeds are not impressive by plasma standards, the percent difference between the two styles is impressive. Since almost all of flame cutting is conducted at speeds less than 20 ipm, an increase of 1 ipm results in a significant percentage increase in production.
In part 2 of this article, I will discuss nozzle preheat designs for the various different fuel gases and requirements for beveling as opposed to shape cutting. We will also discuss several conditions that can create cut quality problems.