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Local pressure reduction and water removal in underwater plasma arc cutting using centrifugal pump impellers

 

Dr.-Ing. M. Creutz

Dr.-Ing. J. Bartzsch

Dr.-Ing. H. Steinkamp

 

Financed by the "Deutsche Forschungsgemeinschaft", SFB 264

 

In comparison to hyperbaric underwater welding in diving chambers, wet welding techniques promise higher flexibility and lower costs. One technique for creating a local dry and pressure reduced welding zone is the use of a centrifugal pump. Results of experimental investigations in combination with a plasma-MIG arc welding system are presented in this paper.

 

Special importance is attached to the local pressure reduction in view of the fact that low pressure, i.e. a high pressure difference between surrounding water and dry welding area, is a good condition for welding but is difficult to be obtained with other shielding systems than pressure chambers. Plasma-MIG welding has been done under water with a good result on the weld quality. Values of the hardness of the joint and the appearance of the weld structure are nearly comparable to atmospheric welds.

 

 

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Figure 1: Scheme of the welding process with local pressure reduction

Underwater welding is encountered mainly in the maintenance of underwater structures such as pipelines and platform legs in the offshore industry. Nowadays it is performed by divers using manual metal arc welding processes either in direct contact to the water or in the dry area inside a diving chamber under hyperbaric conditions.

 

Gas-shielded arc welding enables the use of higher energy fluxes and thus enlarges the speed of welding. Considering the process properties it is an ideal technique for automation. The presence of water and the static pressure growing with the water depth cause several problems concerning welding techniques that are originally developed for atmospheric application.

 

Water removes the heat from the workpiece rapidly, causing a large temperature drop inside the welded joint with an unpleasant effect on the material quality using low alloyed carbon steels: The high cooling rate leads to a martensite or bainite structure with high hardness values in the heat affected zone. Moreover, defects like porosity and slag enclosures occur in the weld metal. Due to the high temperature inside the plasma arc, the water dissociates to release hydrogen. This hydrogen diffuses into the molten metal. The decreasing hydrogen solubility with decreasing temperature leads to a forced condition and a so-called hydrogen embrittlement. To avoid these problems in underwater welding, water has to be removed from the welding pool by a special shielding tool.

 

A further complication is the fact that the hydrostatic pressure grows with the water depth. To create a non reacting atmosphere around the molten metal, inert gases are used to protect the welding pool. Due to the hydrostatic pressure, the density and thus the thermal conductivity of the shielding and the protection gas increases. This leads to a higher heat transfer from the arc as well as from the weld area towards the gas. As a result, the speed of cooling is increased and energy is taken away from the plasma arc. With increasing pressure the cross section of the plasma arc is reduced until only a small ionised channel can be maintained. This behaviour induced the authors to demand a pressure reduction going along with the removal of the water. The concept presented here is based on investigations of Corriatt and Roggen (1979). They carried out investigations using centrifugal forces to build up a local dry area. A MIG welding gun was equipped with a rotating bell that serves as a shielding nozzle. It creates a small radial pressure drop against the ambient pressure by the centrifugal forces of the rotating gas and liquid.

 

In order to enable a welding process corresponding to a dry process under atmospheric conditions, the authors suggest a method to reduce the pressure in a wide zone around a plasma arc by means of centrifugal forces. Fig. 1 schematically shows the concept of the proposed shielding technique. A rotating cylinder is placed around the welding torch. A disc is fixed to the underneath of the cylinder to rotate close above the work surface. Using blades to increase the momentum transfer towards the two-phase fluid, this disc is similar to the rotor of a radial pump, where the work surface corresponds to the stator.

 

This system is able to reduce the pressure inside the welding area. The pressure reduction is influenced by the rotational velocity and the geometry of the rotor, by the height of the gap between rotor and workpiece and by the mass flow rate of the shielding and the protection gas.

 

Beneath the ability to reduce the pressure, the system is also able to remove the water from the welding zone for simple applications. For welding especially coating on a plain work surface, no special shielding technique is required. Other shielding techniques can be added to the "centrifugal pump" as well. Since the pressure is controlled by the pump, the added shielding technique has to focus only the removal of the water. Pressure reduction and water removal are decoupled from each other.

 

 

EXPERIMENTAL INVESTIGATIONS

 

Experimental investigations have been carried out by Steinkamp (1994) on the fluid dynamics of the two phase gas liquid flow inside the impeller. The correlation between void fraction, bubble size and velocity and the efficiency of the pump have been investigated.

 

CFD-simulations of the single phase fluid flow have been performed and were in good agreement with the experimental results on the pressure field below the impeller. The heat transfer towards the welding material was investigated. In cooperation with Draugelates and Bartzsch (TU Clausthal), the shielding concept was realized in combination with a plasma-MIG welding system (Creutz et al., 1995). It was shown that a centrifugal pump is able to create a stable pressure reduced welding area around the welding system which can be of advantage especially using non-consumable welding techniques. Bead-on-plate welding was performed with a plasma-MIG welding system that was developed for under water applications. The quality of the weld was shown to be comparable to the quality of welds under atmospheric conditions. In figure 2a a macrophotograph of a bead-on-plate weld that was produced under water is illustrated. Figure 2b shows the microstructure of the weld material. No significant differences in the weld structure could be observed. Hardness tests have been performed for underwater welded seams and seams that were welded under atmospheric conditions. The hardness traverse of the welds were similar. The quality is mainly effected by the size of the area, that is free from water.

Figure 2a: Macrophotograph of an underwater welded seam and microstructure of the weld material

 

 

In cases of simple weld seam preparation like bead-on-plate welding or surfacing, the pump is able to remove the whole water from the welding area. For doing more complicated welds like joint-welding of thick plates, further investigations are due on the shielding technique inside the pressure reduced area.

 

The efficiency of the pump could be enlarged by not compressing the plasma gases to the high ambient pressure and thus avoiding a two-phase flow inside the impeller. This would require the development of a special separation technique inside the pressure reduced zone.

 

 

REFERENCES

 

Coriatt, G, and Roggen, R (1979). "Development of a New Welding Gun for Underwater Welding," 11. Offshore Technology Conference, Houston.

 

Creutz, M, Bartzsch, J, Mewes, D, Draugelates, U (1995) Underwater Plasma-MIG Arc Welding: Shielding Technique and Pressure Reduction by a Centrifugal Pump

 

Draugelates, U; Bouaifi, B; and Bartzsch, J (1993). "Underwater Welding using the Plasma MIG Method," Proceedings of the 12th International Conference on Offshore Mechanics and Arctic Engineering, Glasgow, pp. 175-181

 

Draugelates, U, Bouaifi, B, and Bartzsch, J (1994). "Investigations on Local Shielding Systems for Underwater Applications," Proc. of the 4th Intern. Offshore and Polar Engineering Conference, Osaka 1994

 

Steinkamp, H (1994). "Rotierende zweiphasige Strömung zur lokalen Druckreduktion beim Schweißen unter Wasser," Fortschritts-bericht VDI Reihe 7 Nr. 239, Düsseldorf, VDI-Verlag