1-What is the difference between a backfire and a flashback? (top)
A backfire occurs when the flame momentarily burns back into the torch tip and makes a loud popping sound. Sometimes the flame relights by itself and you can continue working. Other times the flame does not relight and you have to shut off the torch valves. A backfire is normally caused by using the wrong tip pressures, by overheating the torch or by touching the tip to the workpiece. A sustained backfire is more serious. That’s when the flame burns back into the tip and keeps burning inside the tip or the torch at the mixing point. There is usually a high-pitched squealing sound and sometimes sparks coming out of the tip. It is extremely important to shut off the oxygen valve first as quickly as possible, then the fuel gas valve, to prevent damage to the torch. The torch should be allowed to cool and then the set-up should be verified before relighting. Improper working pressures or defective equipment can be the cause of sustained backfire.
A flashback occurs when the flame passes right through the torch and into the hose due to the presence of a mixture of oxygen and fuel gas in the system. The flame burns so fast that you don’t have time to shut the torch valves before it passes through the torch. A flashback can burst the hoses or progress back to the regulators and cylinders with devastating consequences. Always purge your hose lines, one at a time, before lighting the torch to make sure there is no mixed gas in the hoses.
2-What is the difference between a check valve and a flashback arrestor? (top)
A check valve is a spring-loaded valve that allows flow in one direction but prevents flow in the reverse direction. It is used to reduce the possibility of forming an explosive mixture of oxygen and fuel gas in the hose in case one or the other gas backs up in the system. They are normally mounted at the torch inlet connections, but models exist for installation at regulator outlets as well. Check valves must be tested frequently to make sure they are not leaking in the reverse direction due to dirt accumulation on the valve seat. It must be understood, however, that a check valve will not stop a flashback in the system if a mixture of oxygen and fuel gas is somehow present and ignites. The flashback flame will pass right through a check valve.
A flashback arrestor is a safety device that incorporates a porous metal filter (or other means) to stop the flashback flame and prevent it from traveling farther upstream in the system. Most flashback arrestors also have a built-in reverse flow check valve to help prevent a gas mixture from forming in the first place. Various models of flashback arrestor are available that perform other functions as well. Some incorporate a pressure relief valve to protect equipment from the pressure wave from a flashback, and others include a thermal cut-off to stop the flow of gas if the flashback arrestor overheats due to a fire. Models are available for mounting at torch inlets or regulator outlets.
3-Where should a flashback arrestor be installed? (top)
Flashback arrestors can be installed at the torch inlets, at the regulator outlets or both. Installation at the torch inlets has the advantage of being the farthest downstream, and so protects the hose and the operator the most. Installation at the regulator outlet is a cleaner location that is less subject to damage and fouling from dirt that can come from the hose. Regulator mounted units can also be larger, with greater flow capacity for high flow applications, without hindering the easy manipulation of the torch. Most Provinces in Canada have regulations that dictate the required location of flashback arrestors in the system. Users must follow the regulations in effect in their Province.
4-Can we put flashback arrestors on both the torch end and the regulator end at the same time? (top)
You can put flashback arrestors on both the torch and the regulators at the same time. Be careful, however, to check the maximum flow rate required for the jobs to be done. The tip flow and pressure charts provided by most manufacturers do not take into consideration the addition of hose line accessories in the system. Adding too many accessories in the hose lines, using too small a hose diameter, or using very long hose lengths can cause the pressure at the tip to be too low. This can cause backfiring and overheating if not corrected. You will have to compensate for the pressure drop caused by the flashback arrestors by increasing your operating pressures on the regulators.
5-What pressures should be set on the regulators when doing oxy-fuel welding or cutting? (top)
The proper pressure settings for the oxygen and fuel gas regulators depend on what work is being performed and with what equipment. Each cutting or welding tip has recommended operating pressures. The Air LiquideTM POCKET GUIDE FOR OXY-FUEL GAS WELDING, CUTTING, HEATING, literature stock number 1140-322A, provides settings for various styles and sizes of BLUESHIELDTM tips.
6-What tip size should I use for a specific application? (top)
The Air LiquideTM Products Catalogue, as well as the POCKET GUIDE FOR OXY-FUEL GAS WELDING, CUTTING, HEATING show recommended tip sizes corresponding to various metal thicknesses.
7-What is an oxidizing or a carburizing flame? (top)
An oxidizing flame is produced when excess oxygen is supplied to the flame. A carburizing flame results from an excess of fuel gas in the flame. Each different fuel gas requires a different amount of oxygen to burn completely. In the case of acetylene, when one volume of oxygen is mixed with one volume of acetylene a neutral flame with a well defined inner cone (primary flame) is produced. If more acetylene is added it produces a carburizing flame with an inner cone that has a visible acetylene feather and an orange coloured outer envelope (secondary flame).
8-Why is a propane cutting tip a two-piece tip? (top)
Propane only produces one half of the heat that acetylene does in its primary flame (inner cone). Propane also requires four times the volume of oxygen that acetylene does and is a much larger molecule than acetylene. In order for a propane tip to supply the same amount of heat from the inner cones of its pre-heat flames as an equivalent acetylene tip, it has to allow about five times more total volume of gas flow through it than the acetylene tip. This means you’d have to drill many more holes in a single piece propane tip to achieve the required flow. It is not economical or mechanically feasible to drill so many holes in a solid copper tip so propane tips are made in two pieces that allow many slots to be milled into the centre core of the tip to provide the required flow and adequate heat. If acetylene is used in a two-piece propane tip it is much too hot and can cause overheating of the tip and possible backfiring into the tip.
9-What are typical cylinder pressures for oxygen and acetylene? (top)
The typical acetylene cylinder is filled to a pressure of 250 psig at 70 degrees Fahrenheit. Typical oxygen cylinder pressure is 2217 psig at 70 degrees Fahrenheit. However, cylinder pressure will vary with the temperature. As the temperature goes down the pressure inside the cylinder will go down. A cylinder left outside in winter will show a lower pressure than a cylinder that is brought inside and allowed to warm up. Similarly, a cylinder outside in the sun in the summer will register a higher pressure. In both cases the amount of gas contained in the cylinder has not changed, it is only the pressure that has.
10-What is the difference between acetylene, MAPPTM and propane? (top)
All three are hydrocarbon fuel gases that produce heat when burned with oxygen. Each one has different properties that make it better suited to particular metalworking jobs. Acetylene is a very hot gas that burns with a concentrated heat in its primary flame near the tip. This makes it better for fast starts and cutting of thinner material. Acetylene is slower for general heating applications. MAPP’sTM primary flame is almost as hot as acetylene’s, for fast starts, but it’s secondary flame has twice the heat content of acetylene. This makes it more effective than acetylene in cutting heavier material, and for heating applications. Propane has most of its heat content in its secondary flame, so starts are slow when cutting. It is best suited to thicker material and wide area heating. As far as safety goes, acetylene is unstable and requires a special cylinder to store it. Acetylene gas is dissolved in acetone solvent in the cylinder to keep it stable. Acetylene is lighter than air and forms explosive mixtures with air or oxygen over a very wide range of concentrations. Both MAPPTM and propane are heavier than air and are stored as liquids in their cylinders. They have a narrower range of explosive limits in air and oxygen than acetylene making them safer to handle. For more complete information on fuel gases consult the FLAMALTM Gases brochure, Air LiquideTM literature sheet 1140-263E.
11-How much time can I cut with a size 23 acetylene cylinder and a size 22 oxygen cylinder? (top)
The cubic capacities of oxygen and acetylene cylinders are found in the Air LiquideTM Products Catalogue. A size 23 acetylene contains roughly 130 cubic feet of acetylene, and a size 22 oxygen contains 125 cubic feet of oxygen. The POCKET GUIDE FOR OXY-FUEL GAS WELDING, CUTTING, HEATING, AL literature number 1140-322A, tells us that an 802 size 0 tip will consume approximately 45cfh of oxygen and 12 cfh of acetylene to cut ¼ inch material. The limiting factor, in this case, is the oxygen which will last approximately 3 hours, whereas the acetylene will last approximately 11 hours.
12-Can I use the same regulator for acetylene as for Propane? (top)
Air Liquide acetylene cylinders have a unique CGA 410 valve outlet, whereas other gas suppliers use the same CGA 510 cylinder valve fitting for both acetylene and LPG cylinders (propane, propylene, MAPPTM, etc.). The use of acetylene is limited by law to a maximum working pressure of 15 psig (regulator gauges are red-lined above 15 psig). Above this pressure acetylene can become unstable and an explosion can occur. Liquefied petroleum gases (LPG), on the other hand, may be used at any pressure up to their cylinder pressure (approximately 100 psig at 70 degrees Fahrenheit). If a regulator designed for propane or other LPGs is used on a competitor’s acetylene cylinder with a CGA 510 fitting, and the pressure is raised to 50 or 60 psig, there could be an explosion. Only use regulators for the gas service marked on the regulator.
13-How thick can I cut with a DuoTM Kit? (top)
The DuoTM Kit is supplied with a Style 802 size 0 tip which is designed to cut ¼ inch thick material. The torch can be used to cut thicker material, however, if a larger tip size is installed. Consult the POCKET GUIDE FOR OXY-FUEL GAS WELDING, CUTTING, HEATING (AL literature stock number 1140-322A) to obtain the fuel gas flow rates for specific sizes of tips. Remember that in heavy cutting or heating applications you should not exceed a withdrawal rate of 1/7 of the cubic capacity per hour of any size acetylene cylinder (otherwise you must manifold the cylinders).
14-Can you cut with propane with an Air LiquideTM kit? (top)
Not with the standard kits that are intended for use with acetylene only. Air LiquideTM does offer, however, the BLU-DUOKITLPG for use with fuel gases other than acetylene. This kit contains the proper regulator, cutting tip and welding hose for propane and MAPPTM use.
15-What is the warranty on Air LiquideTM gas apparatus? (top)
One year from the date of purchase by the original owner.
16-What is the difference between Style 801, 802 and 803 tips? (top)
The style 801 is a light pre-heat tip, with four pre-heat flames, for use in hand cutting thin material that is clean and free of scale, rust and oil. The 802 is a medium pre-heat tip, with four pre-heat flames, for general purpose hand cutting of dirtier material. The pre-heat holes in an 802 are bigger than the holes in an 801 tip to supply more gas flow. Style 803 tips have six pre-heat holes to supply a heavy preheat for cutting heavily scaled, dirty or oily material. Remember that the center hole in a cutting tip is the cutting oxygen hole and the surrounding holes are to supply the pre heat flames.
17-What type of gas hose should I use? (top)
Page 86 of the Air LiquideTM Product Catalogue lists the different types of gas hose. Grade “R” hose is an economy hose for oxy-acetylene applications only. This hose is neither oil resistant nor flame resistant. It is best suited to clean, indoor, bench work where it will not be exposed to oily conditions or welding slag and sparks. Grade “RM” hose is also designed exclusively for oxy-acetylene use, but it has the advantage over Grade R of having an oil resistant and flame resistant outer cover. This makes it suitable for general-purpose use in most welding shops where it might be dragged across dirty floors and subjected to welding sparks. Grade “T” hose is an all-purpose hose designed for use with any of the common fuel gases, including acetylene. Both the inner tube and outer cover are flame and oil resistant. Grade T is recommended for shops that use more than one type of fuel gas and also for all heating applications. Use of the wrong Grade of hose can cause premature degradation of the hose due to chemical attack by the fuel gas used or the working conditions (oil, sparks, abrasion). Check hose frequently to make sure it is not spongy, dried out, cut deeply or burned. Replace worn hose with the proper Grade for the job conditions.
18-What size of gas hose should I use? (top)
The proper size of gas hose depends on the maximum gas flow rates required by the job (tip size) and by the hose length (distance between the regulators and the torch). Pressure drop occurs in the hose as the gas passes through it. Setting 50 psig on the regulator will not deliver 50 psig to the tip if the hose is too small or too long. The Air LiquideTM POCKET GUIDE FOR OXY-FUEL GAS WELDING, CUTTING, HEATING (AL literature stock number 1140-322A) provides recommended operating pressures for BLUESHIELDTM tips. These are the pressures required at the tip. The POCKET GUIDE also contains hose pressure drop charts that indicate the amount of pressure drop caused by different lengths and sizes of hose. Adjust the regulator settings by adding the pressure drop shown in the chart, for a given size and length of hose, to the recommended operating pressures of the tip being used.
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