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Oxy-Fuel Torch Tip/Nozzle Preheat Design
Part 2 of a 2 part series
In the previous blog on Flame Cutting we discussed cutting nozzle bore designs and the requirements for various plate thicknesses. In this issue we will discuss the different nozzle designs required for the various fuel gases.
Nozzle Preheat Design
In the US marketplace the four primary fuel gases are 1) acetylene, 2) propane, 3) natural gas, and 4) propylene. There are also some additives mixed with natural gas or propane that are supposed to improve cutting performance; however, these additives do not change the nozzle design for the basic fuel gas. Unlike the cutting Oxygen bore design, the preheat design does not change with plate thickness. Below is a photograph showing the three preheat designs, for the four fuel gases, natural gas and propane using the same design. The difference in design is necessitated by the difference in the flame velocity of the different gasses. In this photo nozzle 1) is an acetylene nozzle, 2) is a propane/natural gas nozzle, 3) is a propylene nozzle, and 4) again is a propane/natural gas nozzle of one piece design.
The acetylene nozzle has a flat front-face and no flame holding skirt. The flame velocity of acetylene is much higher than that of the other fuels and just the interaction between the flames of adjacent properly-placed preheat ports is sufficient to hold the flame on the face of the nozzle at the flow rates/velocities used in cutting. Note that the skirt of #3 is shorter than that of #2 indicating that the flame velocity of propylene is higher than that of NG/Propane so therefore the shorter skirt is sufficient for propylene. The flame holding for the NG/propane and propylene tips come from the proximity of the adjacent flame ports, the interaction of these adjacent ports, as well as interaction of the flames with the skirt wall.
The proper preheat design must be chosen for the preheat gas being used as severe problems can occur by using the incorrect design. Use of acetylene in tips designed for NG/Propane or propylene can result in sustained burn-back or severe overheating of the tip/torch. Use of a non-acetylene gas in acetylene tips will probably not result in a severe problem but will not yield a usable flame as the flame will “jump off” the face of the nozzle at very low flows and will therefore not be usable. Nozzles will tend to run cooler at high preheat flow rates as the higher flow of gases tends to cool the nozzle better than the low flow rates.
In the majority of flame cutting applications the torch is perpendicular to the plate. One exception is that of beveling where the torch/nozzle is generally at angles from 15 to 60 degrees from perpendicular to the plate. This leads to 2 problems 1) is that of increasing plate thickness to be cut and 2) the reduction of the preheat effectiveness. As far as the plate thickness goes think of the cosine of the angle divided into the plate thickness for the increase in actual length of cut. For a 45 degree bevel the cut length increases 1.41 times the plate thickness. So when beveling a 2 inch plate @ 45 degrees choose a nozzle rated for about 3” or more. While the length of cut increases as the bevel angle increases, the effectiveness of the preheat decreases even more dramatically.
Probably the most common bevel error is that of too little preheat. At right is a photo of an attempted 45 degree bevel. Note that the first ½- ¾ inches of the bevel is smooth while the rest shows repeated vertical gouges in the cut surface. Most users find this gouged surface to be unacceptable.
The reason that the first portion was good was a result of the heat put into the plate prior to starting the cut. After the cut “catches up” with this preheated portion of the plate the cut surface deteriorates to unacceptable. The solution to this problem is one or all of the following:
1) reduce the tip to work distance to where the preheat inner cones touch the plate
2) increase the flow rate of the preheat gases to the maximum the tip will allow
3) insure that the O/F Ratio is correct to insure the maximum flame temperature
4) use an auxiliary preheat
This photo shows a three torch configuration with preheats ignited. The resulting part is one with a 45 degree top bevel, a nose, and a 45degree bottom bevel. Torch 1 is the lead torch and requires the most preheat. Note the difference in the length of the preheat cones of torches 1, 2, & 3. Torch 2 is the second torch and will provide the nose/land. This torch requires little preheat and will actually acquire some of its needed heating from Torch 1. Torch 3 will cut the top bevel and in some cases will require more preheat than will torch 2 but usually not more that torch 1.
Again looking at the three torches the preheat cones of Torch 1 are the longest, while Torch 3 has slightly shorter cones as compared to Torch 1 but longer than Torch 2. It is difficult to get too much preheat on the lead-angled torch, but it is relatively easy to get too much preheat on the vertical torch. Too much preheat in this case will often result in tenacious slag on the bottom of the plate that is difficult to remove.
Beveling presents more difficulties than does typical shape cutting, but some of the new equipment on today’s machines helps overcome some of these difficulties.