MIG Welding Gas

Gas Compressiblity

The Compressibility experiment was based on an R&D team's findings that a welding process would provide better results in the absence of oxygen. We were asked to analyze the PVT behavior of pure CO2 and a 75/25 CO2/Ar mixture to recommend which gas to use. The resulting gas would eventually be pressurized to 800 psia at room temperature prior to purging the MIG site of oxygen.

The apparatus (pictured right) was avaible for PVT testing. Using the Burnnett method, the plan was to fill cell A to ~800 psia, close the inlet flow, open the path to cell B, and allow it to equilibrate. Then we closed B, evacuated it, and again allowed A to equilibrate with B upon open of the middle valve. We recorded the pressure at each step till atmospheric pressures were reached. We did this with pure Helium, CO2, and a 50/50 CO2/Argon mix.

From the helium trials we found N, the apparatus constant, which then allowed us to find Z for our CO2 and gas mixture trials.

At 800 psi, we couldn't assume any gas would behave ideally. Additionally, gas mixtures have binary interactions that need to be accounted for with mixting rules. Without knowing the binary interaction coefficient, we had to test both the Virial Equation of State and the Peng Robinson EOS and compare our data with both models. A regression of the data to the two would reveal which model best suits the mixture, as the model which minimizes data residuals best models the real system.

The data shows that for both the pure CO2 system and a 50/50 CO2/Ar system, the Peng Robinson EOS better predicts the real PVT behavior of the gases in consideration. Given this, we recommended the Peng Robinson EOS and used this model to predict the flow rates of each gas needed to implement the 75/25 CO2/Ar mixture were that the decision of managament to pursue. This graph is shown below.

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