A METHOD OF PRODUCING A PROTECTIVE GAS AND A GAS MIXTURE THEREFOR

Abstract: The present invention relates to a method of producing a protective gas consisting of 0.0010-0.100 percent by volume nitrogen monoxide, 0-90 percent by volume helium, 0-5 percent by volume hydrogen, 0-50 percent by volume carbon dioxide, and the remainder argon. The invention also relates to a gas mixture for producing the productive gas. The method of producing the protective gas is characterized by dissolving carbon monoxide in condensed argon to a concentration of 0.0010-0.100 percent by volume, calculated on gas phase, vaporizing the resultant solution and mixing remaining constituents in the vaporized solution in such proportions as to obtain a protective gas of the predetermined composition. The gas mixture used for producing the protective gas is characterized in that it consists of 0.0010-0.100 percent by volume nitrogen monoxide in condensed argon, calculated on gas phase, and is preferably held under an overpressure.

Description: A method of producing a protective gas and a gas mixture therefor The present invention relates to a method of producing a protective gas consisting of 0.0010-0.100 percent by volume of nitrogen monoxide, 0-90 percent by volume helium, 0-5 percent by volume hydrogen, 0-50 percent by volume carbon dioxide and the remainder argon, and also to a condensed gas mixture for the production of said protective gas. In electric gas-arc welding processes there is often used a protective gas in the form of a gas mixture consisting of or comprising argon and nitrogen monoxide. The protective gas mixture or protective gas may also include carbon dioxide, helium and/or hydrogen. To large consumers of protective gases, these gases containing nitrogen monoxide, there is at present delivered a gas pre-mixture, a master gas, consisting of a nitrogen monoxide in argon under high pressure, this gas being delivered in conventional steel bottles. The proportion of nitrogen monoxide present is normally from 1-5 percent by volume, for instance 1.8 percent by volume. The desired protective gas is then produced by mixing the various gases with the gas pre-mixture to the composition desired. The amount of nitrogen monoxide present in the final protective gas is normally at most 0.030 percent by volume. Since nitrogen monoxide is a poisonous gas, it is desirable to use lower proportions of nitrogen monoxide in the master gas than those used hitherto. This results in higher transportation costs, since it is necessary to transport to the consumer more gas bottles containing master gas at unchanged protective gas consumption. Another drawback with the master gas is that when preparing a protective gas it is necessary to supply ad¬ ditional argon in order to reach the desired final con- centration of argon in the protective gas produced. Accordingly, one object of the present invention is to provide a gas mixture of nitrogen monoxide in argon with a lower proportion of nitrogen monoxide than has hitherto been used in the master gas without increasing the transportation costs as a result hereof and without occasioning excessive switching between gas sources (nitrogen monoxide containing gas) . Another object of the present invention is to provide a gas mixture of nitrogen monoxide and argon for the production of a protective gas which does not require the additional admixture of pure argon. Still another object of the invention is to provide a gas mixture of nitrogen monoxide and argon in liquid form in order to considerably reduce the number of gas-mixture deliveries to the consumer. A further object of the invention is to provide a method for producing a protective gas which in addition to con- taining nitrogen monoxide and argon also contains helium, carbon dioxide and/or hydrogen, by using simpler mixing apparatus. These objects are achieved with a gas mixture which comprises condensed argon in which 0.0010-0.100 percent by volume, calculated on gas phase, of nitrogen monoxide is dissolved. Argon is a liquid between about -185.9 and -189.2 degrees Celsius at atmospheric pressure, whereas nitrogen monoxide, NO, has a freezing point at about -164 degrees Celsius. It has now surprisingly been found that nitrogen mon¬ oxide can be dissolved in liquid argon at atmospheric pressure in an amount of at least up to 0.1 percent by volume calculated on gas phase, and that the vaporized gas mixture has essentially the same composition as the liquid phase. The invention will now be described in more detail with reference to an exemplifying embodiment thereof illustrated in the accompanying drawing, the single Figure of which is a graph in which the nitrogen monoxide concentration, in ppm, is plotted as a function of time during which removals of gas mixtures takes place. A pressure container which accommodates about 800 kg liquid argon was filled with liquid argon, whereafter nitrogen monoxide was dissolved in the argon until the carbon monoxide concentration had risen to about 300 ppm. This dissolution can be achieved by dissolving in condensed argon a master gas consisting of nitrogen monoxide in argon, e.g. 1.8 percent by volume NO in argon. 60 liters of the gas mixture were taken from the container each minute over a period of 8 hours in each working day. This gas outtake was intended to simulate the consumption of protective gas in a gas arc welding process. The weight of the gas mixture remaining in the container, both in liquid phase and gas phase, is given in the Figure. The experiment was continued for 30 calendar days. Samples of the composition of both the gas and liquid phases were continuously taken. The weight of the remaining gas mixture is plotted on the right-hand abscissa axis. The Figure also shows the concentrations of nitrogen monoxide in argon as a function of time. The concentration of nitrogen monoxide is plotted in ppm on the left-hand abscissa axis of the graph. It will be seen from the graph that the nitrogen monoxide concentration in the gas phase is almost equal to the nitrogen monoxide concentration in the liquid phase. Furthermore, it is evident that the concentrations in the two phases are substantially indepen¬ dent of time and of the amount of gas mixture in the con¬ tainer. The container was fitted with a pressure safety valve which opened automatically when the pressure in the container had risen to 17.5 bars overpressure. The pressure varied between about 3 bars and 17.5 bars during the experimental period. Thus, when producing a protective gas consisting solely of argon and nitrogen monoxide, the predetermined quantity of nitrogen monoxide can be dissolved in the liquid argon and then taken out either in vapor phase or liquid phase, it being necessary in this latter case to vaporize the mixture in liquid phase. When producing a protective gas consisting of 300 ppm nitrogen monoxide, 20 percent by volume carbon dioxide, 30 percent by volume helium and the remainder argon for in¬ stance, there is prepared a master gas which consists of 600 ppm nitrogen monoxide in liquid argon. This gas mixture, which constitutes 50 percent by volume of the protective gas, is then mixed with 20 percent by volume carbon dioxide and 30 percent by volume helium. This requires the use of a gas mixer which has three inputs. On the other hand, when working on the basis of a known gas mixture of nitrogen monoxide in argon in gas form with a nitrogen monoxide concentration of 1.8 percent, it will be necessary to admix pure argon in order to obtain the desired protective gas. This would require the use of a gas mixer equipped with four inputs. The present invention enables the use of a cryogenic liquid container which is filled with condensed argon in which at most 0.1 percent by volume nitrogen monoxide is dissolved. Available commercially are transport containers for cryogenic liquids which accommodate a gas quantity corresponding to more than 40 large steel bottles (40 liters) at a pressure of 200 atm. In this case, the large consumer does not require a tank for pure argon, but need only have access to those gases which shall be included in the protec¬ tive gas. One advantage with the inventive master gas in con- densed form is that no gas mixer is required when the protective gas shall only contain argon in addition to nitrogen monoxide. In other cases a simple, and therewith less expensive gas mixer can be used, since it is no longer necessary to add argon in order to obtain a protective gas of the predetermined composition. According to the present invention an advantage is afforded when the liquid phase is taken from the argon- nitrogen monoxide mixture in the container and vaporized before use as a protective gas, or admixed with other protective gas constituents. According to another preferred embodiment, and gas and liquid phases are taken from the container, and the latter phase vaporized prior to combining it with the gas phase. An advantage is also afforded when the cryogenic gas container is a pressure vessel fitted with a pressure valve, such as to prevent the build-up of non-permitted high pressures in the container. The valve should be set to a relatively high pressure, e.g. a pressure of 20 bars. It has also been found that the gas pressure above the liquid in the container should be 3 bars or more, in order to obtain a gas phase of approximately the same composition as the liquid phase. Since the cryogenic gas container is not normally constructed for gas pressures higher than 25 bars, the preferred gas pressure in the container is from 3-25 bars, particularly 5-18 bars. An overpressure valve which opens at a lower pressure than 3 bars is not recommended, since when the master gas is not used, for instance over the weekend, the pressure will quickly rise to 3 bars and all gas formed thereafter and resulting in a higher pressure than 3 bars will be lost to the surroundings.

Claims: CLAIMS 1. A method of producing a protective gas comprising 0.0010-0.100 percent by volume nitrogen monoxide, 0-90 percent by volume helium, 0-5 percent by volume hydrogen, 0- 50 percent by volume carbon dioxide, and the remainder argon, characterized by dissolving nitrogen monoxide in condensed argon to a concentration of 0.0010-0.100 percent by volume, calculated on gas phase, vaporizing the resultant solution and admixing remaining constituents in the vaporized solution in such proportions as to obtain a protective gas of prede¬ termined composition. 2. A gas mixture for producing a protective gas consisting of 0.0010-0.100 percent by volume nitrogen monoxide, 0-90 percent by volume helium, 0-5 percent by volume hydrogen, 0- 50 percent by volume carbon dioxide, and the remainder argon, characterized in that it consists of liquid argon in which 0.0010-0.100 percent by volume nitrogen monoxide, calculated on gas phase, is dissolved. 3. A gas mixture according to Claim 2, characterized in that the gas mixture is held under an overpressure. 4. A gas mixture according to Claim 3, characterized in that the overpressure is 3-25 bars, particularly 7-18 bars.

Inventors: HAMMARLUND NILS, PERSSON KJELL-ARNE, SIPEK LADISLAV, HAMMARLUND, NILS, PERSSON, KJELL-ARNE, SIPEK, LADISLAV
Applicants: AGA AB, AGA AKTIEBOLAG
Application year: 1991
Main classification: B23K35
IPC codes: B01F3, B01J19, B23K9, B23K35