Methods for weld purging

Abstract

A method for purging air from a structure to be joined by welding by feeding a liquid cryogen to the structure. The liquid cryogen will enter the structure, warm up and enter the gaseous phase very rapidly. The gaseous cryogen will displace the air that is present in the structure out of the structure and reduce the content of oxygen in the structure to about 10 parts per million when welding can begin.

Claims

1. A method for purging air from inside a structure that is capable of entraining air to be welded and prior to commencing welding operations comprising feeding a liquid cryogen selected from the group consisting of argon, helium, nitrogen, and mixtures thereof to the inside of said structure wherein said liquid cryogen will expand to a gaseous state and displace said air from the inside of said structure out of said structure wherein the concentration of oxygen in the inside of said structure where the oxygen is measured is about 10 parts per million after said air is displaced out of said structure.

2. The method as claimed in claim 1 wherein said structure to be welded is selected from the group consisting of a vessel and a pipe.

3. The method as claimed in claim 2 wherein said structure is made of a material selected from the group consisting of creep resistant materials, alloy steels, stainless steel, nickel, titanium, zirconium and their respective alloys.

4. The method as claimed in claim 1 wherein said liquid cryogen is added to a weld joint of said structure to be welded thereby permitting the liquid cryogen to enter the structure.

5. The method as claimed in claim 1 wherein said liquid cryogen is fed to said structure at ambient temperature.

6. The method as claimed in claim 1 wherein said structure to be welded is welded by a process selected from the group consisting of Gas Tungsten Arc Welding, Plasma Arc Welding, Gas Metal Arc Welding, and Laser Welding.

7. The method as claimed in claim 1 wherein oxygen concentration is measured with an oxygen meter or an oxygen concentration monitor.

8. The method as claimed in claim 1 wherein said displaced air exits through an open end of said structure.

9. The method as claimed in claim 1 wherein said liquid cryogen is fed to said structure in an amount ranging from about 0.25 liquid liters to about 5 liquid liters.

10. The method as claimed in claim 1 further comprising purging said structure with a purge gas selected from the group consisting of Argon, Nitrogen and Nitrogen and Hydrogen mixtures after said air is purged from said structure.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a representation of a structure to be welded using liquid cryogen to displace air from the structure.

(2) FIG. 2 is a graph showing oxygen amounts versus time for a traditional purging operation.

(3) FIG. 3. is a graph showing oxygen amounts versus time for the inventive purging operation.

(4) FIG. 4 is a graph showing oxygen concentration versus time for a regular gas purge and the inventive method.

(5) FIG. 5 is a graph showing the effects of oxygen levels versus pitting corrosion potential.

DETAILED DESCRIPTION OF THE INVENTION

(6) In FIG. 1, there is disclosed a structure to be welded according to the methods of the invention. This structure 10 can be a pipe or vessel that contains an empty space that is capable of entraining air. The structure 10 can be welded to an adjacent structure 40 which can be likewise in design (i.e., pipe to pipe fitting). The structure 10 to be welded can be any material that is capable of being welded, for example a material selected from the group consisting of creep resistant materials, alloy steels, stainless steel, nickel, titanium, zirconium and all their respective alloys.

(7) The structure 10 to be joined with adjacent structure 40 can be joined by a variety of welding and joining means selected from the group consisting of GTAW (Gas Tungsten Arc Welding), PAW (Plasma Arc welding), GMAW (Gas Metal Arc Welding), Laser Welding and other suitable welding processes.

(8) The weld joint 25 is the place where one end of structure 10 is joined with one end of structure 40 and is where the welding operation takes place. Liquid inlet 15 contacts the weld joint through a filling means 20 such as a funnel. The liquid inlet 15 will allow liquid cryogen selected from the group consisting of argon, helium, nitrogen and mixtures thereof to enter the structure 10. The liquid cryogen will be fed to the structure 10 at typical ambient temperatures. As the liquid cryogen warms up inside the structure, it will convert to the gaseous phase and begin to expand. As the liquid cryogen expands to a gas, it will force the air that is already present in the structure 10 and the adjoining structure 40 out their ends 30 and 35 respectively. Typically this expansion of gas will result in oxygen content inside the structure 10 being reduced from around 200,000 ppm to about 10 ppm.

(9) The amount of liquid cryogen employed depends upon the size and volume of the structure to be joined as well as the liquid cryogen itself. Typically this amount ranges from 0.25 of liquid liter to 5 liters depending on how large the pipe or vessel volume is and this amount is fed into the system for an amount of time necessary to allow the requisite amount of liquid cryogen to enter the structure.

(10) Once the appropriate oxygen level has been reached, welding of the structure can commence. In order to maintain this level of oxygen, purging with a purge gas selected from the group consisting of Argon, Nitrogen, and Nitrogen and Hydrogen mixtures, should be performed at about 10 to 20 liters per minute of purge gas during the length of the welding operation.

(11) FIG. 2 is a graph showing the concentration of oxygen versus time for a typical purging process. The structure to be purged was a 2205 Duplex Stainless Steel Vessel having a diameter of 460 mm and 1000 mm length. This normal gas purging utilized Argon as the purge gas and consisted of directing the purge gas through the pipe until the requisite oxygen concentration is reached. As noted earlier, the nature of the purging process follows a mathematical power curve of the form Y=AX.sup.−b. The nature of this curve is such that the tail of the curve is very long, leading to extended times for reducing the oxygen concentration from 200,000 ppm to about 10 ppm. As seen in FIG. 2, the normal gas purging had a formula Y=2863.4X.sup.−1.3548 and took 65 minutes at a flow rate of 45 liters per minute to reach a 10 ppm oxygen level inside the vessel.

(12) In FIG. 3, the same vessel as in FIG. 2 was purged of gas using the inventive method and liquid argon. The power curve formula was Y=20.523X.sup.−1.148 and it can be seen that with the liquid argon being fed into the stainless steel vessel that it took 1 minute and 50 seconds to reach a 10 ppm oxygen level inside the vessel. This is a significant time savings versus the 65 minutes it took using the traditional purging method.

(13) FIG. 4 is a graph showing the differences between the gas purging method as described in FIG. 2 and the inventive method using a liquid cryogen as described in FIG. 3. The normal gas purging process took 65 minutes to reach 10 ppm oxygen level while the inventive process was able to reach this oxygen concentration in 1 minute and 50 seconds.

(14) FIG. 5 is a graph showing the effect of purge gas oxygen levels on pitting corrosion potential in millivolts. As noted in FIG. 5, the less oxygen present in a system, the higher the pitting corrosion potential is. Consequently, the higher pitting corrosion potential equates to higher corrosion resistance, therefore corrosion resistance is improved by purging oxygen from the vessel to be joined,

(15) While this invention has been described with respect to particular embodiments thereof, it is apparent that numerous other forms and modifications of the invention will be obvious to those skilled in the art. The appended claims in this invention generally should be construed to cover all such obvious forms and modifications which are within the true spirit and scope of the invention.