The Pulp and Paper Industry Continues to experience rapid metal wear in their pressure vessels from abrasion, erosion, fretting, cavitation, and oxidation/corrosion mechanisms as a result from various acid and caustic cooking environments. These types of service damage decrease component life, lower equipment reliability and has a negative impact on maintenance budgets and outage durations. If unprotected, excessive reductions in wall thickness can eventually lead to cracking and component failure. Use of electric wire arc spray coatings to prevent corrosion/erosion damage in cooking digester environments can be a cost effective alternative to component replacement or extensive weld overlay, which has proven to have a real potential for thermal distortion of the base metal.
The arc spray process described by this paper uses a proven Twin Wire Electric Arc spray process to melt the selected wire alloy and apply it to the prepared surface. Recent advances in equipment design have resulted in an automated delivery system for spray coating applications in large I.D. vessels. A 480-volt hydraulic powered manipulator tool can be easily assembled in the vessel through existing access ports. The system consists of a drive shaft, drive motors, bushing and/or spider leg supports and an adjustable mechanical slide arm which is used to articulate the blast profiling head or the arc spray gun, at the prescribed stand-off distance from the base metal wall during the blast and coating process.
After the robotic equipment is assembled and interfaced with the control panel located directly outside the vessel, the profiling head or wire arc spray gun can be easily installed on the manipulator slide arm. A two-axis motor controller oscillates the manipulator arm around the vessel I.S. at the pre-selected rotational speed while it indexes down the vessel I.D. to provide an overlapping, seamless arc spray pattern. Operating parameters such as arc spray head and index travel speeds, atomizing pressure, amperage and voltage current are monitored and controlled remotely form a console located outside the digester. Remember arc spray is consistent, had no heat effected zone, requires no pre-heat, and unlike weld overlay has no pinholes, crater cracks, or cold lap, and there is no additional maintenance required, unlike weld overlay.