TC 9-237 – Chapter 5 – Section I




The equipment used for oxyacetylene welding consists of a source of oxygen and a source of acetylene from a portable or stationary outfit, along with a cutting attachment or a separate cutting torch. Other equipment requirements include suitable goggles for eye protection, gloves to protect the hands, a method to light the torch, and wrenches to operate the various connections on the cylinders, regulators, and torches.


Stationary welding equipment is installed where welding operations are conducted in a fixed location. Oxygen and acetylene are provided in the welding area as outlined below.

a. Oxygen. Oxygen is obtained from a number of cylinders manifolded and equipped with a master regulator. The regulator and manifold control the pressure and the flow together (fig. 5-1). The oxygen is supplied to the welding stations through a pipe line equipped with station outlets (fig. 5-2).

Stationary oxygen cylinder manifold and other equipment
Station outlet for oxygen or acetylene

b. Acetylene. Acetylene is obtained either from acetylene cylinders set up as shown in figure 5-3, or an acetylene generator (fig. 5-4). The acetylene is supplied to the welding stations through a pipe line equipped with station outlets as shown in figure 5-2.

Stationary acetylene cylinder manifold and other equipment
Acetylene generator and operating equipment


The portable oxyacetylene welding outfit consists of an oxygen cylinder and an acetylene cylinder with attached valves, regulators, gauges, and hoses (fig. 5-5). This equipment may be temporarily secured on the floor or mounted on an all welded steel truck. The trucks are equipped with a platform to support two large size cylinders. The cylinders are secured by chains attached to the truck frame. A metal toolbox, welded to the frame, provides storage space for torch tips, gloves, fluxes, goggles, and necessary wrenches.

portable oxyacetylene welding and cutting equipment



Acetylene generator equipment is not a standard included in this manual for information only.

a. Acetylene is a fuel gas composed of carbon and hydrogen (C2H2), generated by the action of calcium carbide, a gray stone-like substance, and water in a generating unit. Acetylene is colorless, but has a distinctive odor that can be easily detected.

b. Mixtures of acetylene and air, containing from 2 to 80 percent acetylene by volume, will explode when ignited. However, with suitable welding equipment and proper precautions, acetylene can be safely burned with oxygen for heating, welding, and cutting purposes.

c. Acetylene, when burned with oxygen, produces an oxyacetylene flame with inner; cone tip temperatures of approximately 6300°F (3482°C), for an oxidizing flame; 5850°F (3232°C) for a neutral flame; and 5700°F (3149°C) for a carburizing flame.

d. The generator shown in figure 5–4 is a commonly used commercial type. A single rated 300-lb generator uses 300 lb of calcium carbide and 300 gal. of water. This amount of material will generate 4.5 cu ft of acetylene per pound; the output for this load is approximately 300 cu ft per hour for 4.5 hours. A double rated generator uses 300 lb of finer sized calcium carbide fed through a special hopper and will deliver 600 cu ft of acetylene per hour for 2.5 hours.


Since considerable heat is given off during the reaction, precautions must be taken to prevent excessive pressures in the generator which might cause fires or explosions.

e. In the operation of the generator, the calcium carbide is added to the water through a hopper mechanism at a rate which will maintain a working pressure of approximately 15 psi (103.4 kPa). A pressure regulator is a built-in part of this equipment. A sludge, consisting of hydrated or slaked lime, settles in the bottom of the generator and is removed by means of a sludge outlet.



Acetylene, stored in a free state under pressure greater than 15 psi (103.4 kPa), can break down from heat or shock, and possibly explode. Under pressure of 29.4 psi (203) kPa), acetylene becomes self-explosive, and a slight shock can cause it to explode spontaneously.


Although acetylene is nontoxic, it is an anesthetic, and if present in a sufficiently high concentration, is an asphyxiant in that it replaces oxygen and can produce suffocation.

a. Acetylene is a colorless, flammable gas composed of carbon and hydrogen, manufactured by the reaction of water and calcium carbide. It is slightly lighter than air. Acetylene burns in the air with an intensely hot, yellow, luminous, smoky flame.

b. Although acetylene is stable under low pressure, if compressed to 15 psi (103.4 kPa), it becomes unstable. Heat or shock can cause acetylene under pressure to explode. Avoid exposing filled cylinders to heat, furnaces, radiators, open fires, or sparks (from a torch). Avoid striking the cylinder against other objects and creating sparks. To avoid shock when transporting cylinders, do not drag, roll, or slide them on their sides. Acetylene can be compressed into cylinders when dissolved in acetone at pressures up to 250 psi (1724 kPa).

c. For welding purposes, acetylene is contained in three common cylinders with capacities of 1, 60, 100, and 300 cu ft. Acetylene must not be drawn off in volumes greater than 1/7 of the cylinder’s rated capacity.

d. In order to decrease the size of the open spaces in the cylinder, acetylene cylinders (fig. 5-6) are filled with porous materials such as balsa wood, charcoal, corn pith, or portland cement. Acetone, a colorless, flammable liquid, is added to the cylinder until about 40 percent of the porous material is saturated. The porous material acts as a large sponge which absorbs the acetone, which then absorbs the acetylene. In this process, the volume of acetone increases as it absorbs the acetylene, while acetylene, being a gas, decreases in volume.

acetylene cylinder construction


Do not fill acetylene cylinders at a rate greater than 1/7 of their rated capacity, or about 275 cu ft per hour. To prevent drawing off of acetone and consequent impairment of weld quality and damage to the welding equipment, do not draw acetylene from a cylinder at continuous rates in volumes greater than 1/7 of the rated capacity of the cylinder, or 32.1 cu ft per hour. When more than 32.1 cu ft per hour are required, the cylinder manifold system must be used.

e. Acetylene cylinders are equipped with safety plugs (fig. 5-6) which have a small hole through the center. This hole is filled with a metal alloy which melts at approximately 212°F (100°C), or releases at 500 psi (3448 kPa). When a cylinder is overheated, the plug will melt and permit the acetylene to escape before dangerous pressures can be developed. The plug hole is too small to permit a flame to burn back into the cylinder if escaping acetylene is ignited.

f. The brass acetylene cylinder valves have squared stainless steel valve stems. These stems can be fitted with a cylinder wrench and opened or closed when the cylinder is in use. The outlet of the valve is threaded for connection to an acetylene pressure regulator by means of a union nut. The regulator inlet connection gland fits against the face of the threaded cylinder connection, and the union nut draws the two surfaces together. Whenever the threads on the valve connections are damaged to a degree that will prevent proper assembly to the regulator, the cylinder should be marked and set aside for return to the manufacturer.


Acetylene which may accumulate in a storage room or in a confined space is a fire arid explosion hazard. All acetylene cylinders should be checked, using a soap solution, for leakage at the valves and safety fuse plugs.

g. A protective metal cap (fig. 5-6) screws onto the valve to prevent damage during shipment or storage.

h. Acetylene, when used with oxygen, produces the highest flame temperature of any of the fuel gases. It also has the most concentrated flame, but produces less gross heat of combustion than the liquid petroleum gases and the synthetic gases.


a. General. Oxygen is a colorless, tasteless, odorless gas that is slightly heavier than air. It is nonflammable but will support combustion with other elements. In its free state, oxygen is one of the most common elements. The atmosphere is made up of approximately 21 parts of oxygen and 78 parts of nitrogen, the remainder being rare gases. Rusting of ferrous metals, discoloration of copper, and the corrosion of aluminum are all due to the action of atmospheric oxygen, known as oxidation.

b. Production of Oxygen. Oxygen is obtained commercially either by the liquid air process or by the electrolytic process.

(1) In the liquid air process, air is compressed and cooled to a point where the gases become liquid. As the temperature of the liquid air rises, nitrogen in a gaseous form is given off first, since its boiling point is lower than that of liquid oxygen. These gases, having been separated, are then further purified and compressed into cylinders for use. The liquid air process is by far the most widely used to produce oxygen.

(2) In the electrolytic process, water is broken down into hydrogen and oxygen by the passage of an electric current. The oxygen collects at the positive terminal and the hydrogen at the negative terminal. Each gas is collected and compressed into cylinders for use.



Always refer to oxygen as oxygen, never as air. Combustibles should be kept away from oxygen, including the cylinder, valves, regulators, and other hose apparatus. Oxygen cylinders and apparatus should not be handled with oily hands or oily gloves. Pure oxygen will support and accelerate combustion of almost any material, and is especially dangerous in the presence of oil and grease. Oil and grease in the presence of oxygen may spontaneously ignite and burn violently or explode. Oxygen should never be used in any air tools or for any of the purposes for which compressed air is normally used.

A typical oxygen cylinder is shown in figure 5-7. It is made of steel and has a capacity of 220 cu ft at a pressure of 2000 psi (13,790 kPa) and a temperature of 70°F (21°C). Attached equipment provided by the oxygen supplier consists of an outlet valve, a removable metal cap for the protection of the valve, and a low melting point safety fuse plug and disk. The cylinder is fabricated from a single plate of high grade steel so that it will have no seams and is heat treated to achieve maximum strength. Because of their high pressure, oxygen cylinders undergo extensive testing prior to their release for work, and must be periodically tested thereafter.

oxygen cylinder construction


a. General. The gases compressed in oxygen and acetylene cylinders are held at pressures too high for oxyacetylene welding. Regulators reduce pressure and control the flow of gases from the cylinders. The pressure in an oxygen cylinder can be as high as 2200 psi (15,169 kPa), which must be reduced to a working pressure of 1 to 25 psi (6.90 to 172.38 kPa). The pressure of acetylene in an acetylene cylinder can be as high as 250 psi (1724 kPa). A gas pressure regulator will automatically deliver a constant volume of gas to the torch at the adjusted working pressure.


The regulators for oxygen, acetylene, and liquid petroleum fuel gases are of different construction. They must be used only for the gas for which they were designed.

Most regulators in use are either the single stage or the two stage type. Check valves must be installed between the torch hoses and the regulator to prevent flashback through the regulator.

b. Single Stage Oxygen Regulator.

The single stage oxygen regulator reduces the cylinder pressure of a gas to a working pressure in one step. The single stage oxygen regulator mechanism (fig. 5-8) has a nozzle through which the high pressure gas passes, a valve seat to close off the nozzle, and balancing springs. Some types have a relief valve and an inlet filter to exclude dust and dirt. Pressure gauges are provided to show the pressure in the cylinder or pipe line and the working pressure.

Single stage oxygen regulator


In operation, the working pressure falls as the cylinder pressure falls, which occurs gradually as gas is withdrawn. For this reason, the working pressure must be adjusted at intervals during welding operations when using a single stage oxygen regulator.

The oxygen regulator controls and reduces the oxygen pressure from any standard commercial oxygen cylinder containing pressures up to 3000 psi. The high pressure gauge, which is on the inlet side of the regulator, is graduated from 0 to 3000 psi. The low or working pressure gauge, which is on the outlet side of the regulator, is graduated from 0 to 500 psi.

c. Operation of Single Stage Oxygen Regulator.

(1) The regulator consists of a flexible diaphragm, which controls a needle valve between the high pressure zone and the working zone, a compression spring, and an adjusting screw, which compensates for the pressure of the gas against the diaphragm. The needle valve is on the side of the diaphragm exposed to high gas pressure while the compression spring and adjusting screw are on the opposite side in a zone vented to the atmosphere.

(2) The oxygen enters the regulator through the high pressure inlet connection and passes through a glass wool filter, which removes dust and dirt. The seat, which closes off the nozzle, is not raised until the adjusting screw is turned in. Pressure is applied to the adjusting spring by turning the adjusting screw, which bears down on the rubber diaphragm. The diaphragm presses downward on the stirrup and overcomes the pressure on the compensating spring. When the stirrup is forced downward, the passage through the nozzle is open. Oxygen is then allowed to flow into the low pressure chamber of the regulator. The oxygen then passes through the regulator outlet and the hose to the torch. A certain set pressure must be maintained in the low pressure chamber of the regulator so that oxygen will continue to be forced through the orifices of the torch, even if the torch needle valve is open. This pressure is indicated on the working pressure gage of the regulator, and depends on the position of the regulator adjusting screw. Pressure is increased by turning the adjusting screw to the right and decreased by turning this screw to the left.

(3) Regulators used at stations to which gases are piped from an oxygen manifold, acetylene manifold, or acetylene generator have only one low pressure gauge because the pipe line pressures are usually set at 15 psi (103.4 kPa) for acetylene and approximately 200 psi (1379 kPa) for oxygen. The two stage oxygen regulator (fig. 5–9) is similar in operation to the one stage regulator, but reduces pressure in two steps. On the high pressure side, the pressure is reduced from cylinder pressure to intermediate pressure. On the low pressure side the pressure is reduced from intermediate pressure to work pressure. Because of the two stage pressure control, the working pressure is held constant, and pressure adjustment during welding operations is not required.

Two stage oxygen regulator

e. Acetylene Regulator.


Acetylene should never be used at pressures exceeding 15 psi (103.4 kPa).

This regulator controls the acetylene pressure from any standard commercial cylinder containing pressures up to 500 psi (3447.5 kPa). The acetylene regulator design is generally the same as that of the oxygen regulator, but will not withstand such high pressures. The high pressure gage, on the inlet side of the regulator, is graduated from 0 to 500 psi (3447.5 kPa). The low pressure gage, on the outlet side of the regulator, is graduated from 0 to 30 psi (207 kPa). Acetylene should not be used at pressures exceeding 15 psi (103.4 kPa).


a. General. The oxyacetylene welding torch is used to mix oxygen and acetylene in definite proportions. It also controls the volume of these gases burning at the welding tip, which produces the required type of flame. The torch consists of a handle or body which contains the hose connections for the oxygen and the fuel gas. The torch also has two needle valves, one for adjusting the flew of oxygen and one for acetylene, and a mixing head. In addition, there are two tubes, one for oxygen, the other for acetylene; inlet nipples for the attachment of hoses; a tip; and a handle. The tubes and handle are of seamless hard brass, copper-nickel alloy, stainless steel. For a description and the different sized tips, see paragraph 5-10.

b. Types of Torches. There are two general types of welding torches; the low pressure or injector type, and the equal pressure type.

(1) In the low pressure or injector type (fig. 5-10), the acetylene pressure is less than 1 psi (6.895 kPa). A jet of high pressure oxygen is used to produce a suction effect to draw in the required amount of acetylene. Any change in oxygen flow will produce relative change in acetylene flow so that the proportion of the two gases remains constant. This is accomplished by designing the mixer in the torch to operate on the injector principle. The welding tips may or may not have separate injectors designed integrally with each tip.

mixing head for injector

(2) The equal pressure torch (fig. 5-11) is designed to operate with equal pressures for the oxygen and acetylene. The pressure ranges from 1 to 15 psi (6.895 to 103.4 kPa). This torch has certain advantages over the low pressure type. It can be more readily adjusted, and since equal pressures are used for each gas, the torch is less susceptible to flashbacks.

equal pressure type general purpose welding torch


a. The welding tips (fig. 5-10 and 5-11) are made of hard drawn electrolytic or 95 percent copper and 5 percent tellurium. They are made in various and types, some having a one-piece tip either with a single orifice or a of orifices. The diameters of the tip orifices differ in order to control the quantity of heat and the type of flame. These tip sizes are designated by numbers which are arranged according to the individual manufacturer’s system. Generally, the smaller the number, the smaller the tip orifice.

b. Mixers (fig. 5-10 and 5-11) are frequently provided in tip tier assemblies which assure the correct flow of mixed gases for each size tip. In this tip mixer assembly, the mixer is assembled with the tip for which it has been drilled and then screwed onto the torch head. The universal type mixer is a separate unit which can be used with tips of various sizes.

5-11. HOSE

a. The hoses used to make the connection between are made especially for this purpose.

(1) Hoses are built to withstand high internal the regulators and the torch pressures.

(2) They are strong, nonporous, light, and flexible to permit easy manipulation of the torch.

(3) The rubber used in the manufacture of hose is chemically treated to remove free sulfur to avoid possible spontaneous combustion.

(4) The hose is not impaired by prolonged exposure to light.


Hose should never be used for one gas if it was previously used for another.

b. Hose identification and composition.

(1) In North America, the oxygen hose is green and the acetylene hose is red. In Europe, blue is used for oxygen and orange for acetylene. Black is sometimes also used for oxygen.

(2) The hose is a rubber tube with braided or wrapped cotton or rayon reinforcements and a rubber covering. For heavy duty welding and cutting operations, requiring 1/4-to 1/2-in. internal diameter hose, three to five plies of braided or wrapped reinforcements are used. One ply is used in the 1/8-to 3/16-in. hose for light torches.

c. Hoses are provided with connections at each end so that they may be connected to their respective regulator outlet and torch inlet connections. To prevent a dangerous interchange of acetylene and oxygen hoses, all threaded fittings used for the acetylene hook up are left hand, and all threaded fittings for the oxygen hook up are right hand. Notches are also placed on acetylene fittings to prevent a mixup.

d. Welding and cutting hoses are obtainable as a single hose for each gas or with the hoses bonded together along their length under a common outer rubber jacket. The latter type prevents the hose from kinking or becoming tangled during the welding operation.



Always have suitable fire extinguishing equipment at hand when doing any welding.

When setting up welding and cutting equipment, it is important that all operations be performed systematically in order to avoid mistakes and possible trouble. The setting up procedures given in a through d below will assure safety to the operator and the apparatus.

a. Cylinders.


Do not stand facing cylinder valve outlets of oxygen, acetylene, or other compressed gases when opening them.

(1) Place the oxygen and the acetylene cylinders on a level floor (if they are not mounted on a truck), and tie them firmly to a work bench, post, wall, or other secure anchorage to prevent their being knocked or pulled over.

(2) Remove the valve protecting caps.

(3) “Crack” both cylinder valves by opening first the acetylene and then the oxygen valve slightly for an instant to blow out any dirt or foreign matter that may have accumulated during shipment or storage.

(4) Close the valves and wipe the connection seats with a clean cloth.

b. Pressure Regulators.

(1) Check the regulator fittings for dirt and obstructions. Also check threads of cylinders and regulators for imperfections.

(2) Connect the acetylene regulator to the acetylene regulator and the oxygen regulator to the oxygen cylinder. Use either a regulator wrench or a close fitting wrench and tighten the connecting nuts sufficiently to prevent leakage.

(3) Check hose for burns, nicks, and bad fittings.

(4) Connect the red hose to the acetylene regulator and the green hose to the oxygen regulator. Screw the connecting nuts tightly to insure leak proof seating. Note that the acetylene hose connection has left hand threads.


If it is necessary to blow out the acetylene hose, do it in a well ventilated place which is free of sparks, flame, or other sources of ignition.

(5) Release the regulator screws to avoid damage to the regulators and gauges. Open the cylinder valves slowly. Read the high pressure gauges to check the cylinder gas pressure. Blow out the oxygen hose by turning the regulator screw in and then release the regulator screw. Flashback suppressors must be attached to the torch whenever possible.

c. Torch. Connect the red acetylene hose to the torch needle valve which is stamped “AC or flashback suppressor”. Connect the green oxygen hose to the torch needle valve which is stamped “OX or flashback suppressor”. Test all hose connections for leaks at the regulators and torch valves by turning both regulators’ screws in with the torch needle valves closed. Use a soap and water solution to test for leaks at all connections. Tighten or replace connections where leaks are found. Release the regulator screws after testing and drain both hose lines by opening the torch needle valves. Slip the tip nut over the tip, and press the tip into the mixing head. Tighten by hand and adjust the tip to the proper angle. Secure this adjustment by tightening with the tip nut wrench.


Purge both acetylene and oxygen lines (hoses) prior to igniting torch. Failure to do this can cause serious injury to personnel and damage to the equipment.

d. Adjustment of Working Pressure. Adjust the acetylene working pressure by opening the acetylene needle valve on the torch and turning the regulator screw to the right. Then adjust the acetylene regulator to the required pressure for the tip size to be used (tables 5-1 and 5-2). Close the needle valve. Adjust the oxygen working pressure in the same manner.

Low pressure or injector type torch
Balanced pressure type torch


a. Shut off the gases. Close the acetylene valve first, then the oxygen valve on the torch. Then close the acetylene and oxygen cylinder valves.

b. Drain the regulators and hoses by the following procedures:

(1) Open the torch acetylene valve until the gas stops flowing and the gauges read zero, then close the valve.

(2) Open the torch oxygen valve to drain the oxygen regulator and hose. When gas stops flowing and the gauges read zero, close the valve.

(3) When the above operations are performed properly, both high and low pressure gauges on the acetylene and oxygen regulators will register zero.

c. Release the tension on both regulator screws by turning the screws to the left until they rotate freely.

d. Coil the hoses without kinking them and suspend them on a suitable holder or hanger. Avoid upsetting the cylinders to which they are attached.


a. Leakage of gas between the regulator seat and the nozzle is the principal problem encounter with regulators. It is indicated by a gradual increase in pressure on the working pressure gauge when the adjusting screw is fully released or is in position after adjustment. This defect, called “creeping regulator”, is caused by bad valve seats or by foreign matter lodged between the seat and the nozzle.


Regulators with leakage of gas between the regulator seat and the nozzle must be replaced immediately to avoid damage to other parts of the regulator or injury to personnel. With acetylene regulators, this leakage is particularity dangerous because acetylene at high pressure in the hose is an explosion hazard.

b. The leakage of gas, as described above, can be corrected as outlined below:

(1) Remove and replace the seat if it is worn, cracked, or otherwise damaged.

(2) If the malfunction is caused by fouling with dirt or other foreign matter, clean the seat and nozzle thoroughly and blow out any dust or dirt in the valve chamber.

c. The procedure for removing valve seats and nozzles will vary with the make or design.

d. Broken or buckled gauge tubes and distorted or buckled diaphragms are usually caused by backfire at the torch, leaks across the regulator seats, or by failure to release the regulator adjusting screw fully before opening the cylinder valves.

e. Defective bourdon tubes in the gauges are indicated by improper action of the gauges or by escaping gas from the gauge case. Gauges with defective bourdon tubes should be removed and replaced with new gauges. Satisfactory repairs cannot be made without special equipment.

f. Buckled or distorted diaphragms cannot be adjusted properly and should be replaced with new ones. Rubber diaphragms can be replaced easily by removing the spring case with a vise or wrench. Metal diaphragms are sometimes soldered to the valve case and their replacement is a factory or special repair shop job. Such repairs should not be attempted by anyone unfamiliar with the work.



Defects in oxyacetylene welding torches which are sources of gas leaks must be corrected immediately, as they may result in flashbacks or backfires, with resultant injury to the operator and/ or damage to the welding apparatus.

a. General. Improved functioning of welding torches is usually due to one or more of the following causes: leaking valves, leaks in the mixing head seat, scored or out-of-round welding tip orifices, clogged tubes or tips, and damaged inlet connection threads. Corrective measures for these common torch defects are described below.

b. Leaking Valves.

(1) Bent or worn valve stems should be replaced and damaged seats should be refaced.

(2) Loose packing may be corrected by tightening the packing nut or by installing new packing and then tightening the packing nut.


This work should be done by the manufacturer because special reamers are required for truing these seats.

c. Leaks in the Mixing Heads. These are indicated by popping out of the flame and by emission of sparks from the tips accompanied by a squealing noise. Leaks in the mixing head will cause improper mixing of the oxygen and acetylene causing flashbacks. A flashback causes the torch head and handle to suddenly become very hot. Repair by reaming out and truing the mixing head seat.

d. Scored or Out-of-Round Tip Orifices. Tips in this condition cause the flame to be irregular and must be replaced.

e. Clogged Tubes and Tips.

(1) Carbon deposits caused by flashbacks or backfire, or the presence of foreign matter that has entered the tubes through the hoses will clog tubes. If the tubes or tips are clogged, greater working pressures will be needed to produce the flame required. The flame produced will be distorted.

(2) The torch should be disassembled so that the tip, mixing head, valves, and hose can be cleaned and cleaned out with compressed air at a pressure of 20 to 30 psi (137.9 to 206.85 kPa).

(3) The tip and mixing head should be cleaned either with a cleaning drill or with soft copper or brass wire, and then blown out with compressed air. The cleaning drills should be approximately one drill size smaller than the tip orifice to avoid enlarging the orifice during cleaning.


Damages inlet connection threads may cause fires by ignition of the leaking gas, resulting in injury to the welding operator and/or damage to the equipment.

f. Damaged Inlet Connection Threads. Leaks due to damaged inlet connection threads can be detected by opening the cylinder valves and keeping the needle valves closed. Such leaks will cause the regulator pressure to drop. Also, if the threads are damaged, the hose connection at the torch inlet will be difficult or impossible to tighten. To correct this defect, the threads should be recut and the hose connections thoroughly cleaned.

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