 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. The Twin Wire Arc Spray process is an automated system for depositing a molten, high performance metal coating on the base metal of pressure vessels, boiler firesides, fan blades, piping, and turbine parts subject to corrosion and erosion. The inconel materials, with high nickel, high chrome matrix content are selected not only for corrosion and erosion properties, but also for thermal expansion, matching it to carbon steel. The resulting coating provides and excellent barrier layer to protect the surface from further deterioration. Table 1 compares characteristics of the Twin Wire Electric Wire Arc spray process with other field repair technologies including flame spray coatings and weld overlay. The Twin Wire Arc spray process exhibits superior mechanical bond strength and coating density than flame spray and unlike weld overlay has no heat affected zone (HAZ).  The surface to be coated is thoroughly prepared by abrasive blasting to remove surface deposits, oxides, and minor pitting. Note that severe pitting and all sharp edges must be removed by grinding. All weld seams (circum. and longit.) will be ground and dye-penetrant performed. Thus insuring that no surface stress cracking is covered up by the coating process. Any base metal low areas (below minimum wall) that is discovered by third party N.D.E., will be welded and ground smooth. Thus restoring the vessel to a safe condition prior to the coating process. Prior to the coating process is abrasive blast cleaning using a special crushed steel grit to achieve a clean "white" metal condition and a (4-7 mil) anchor tooth profile, as shown in the figure below. Profiling the surface creates small finger-like projections in the base metal to which the molten particles can mechanically attach. So, in summary, once the surface has been properly prepared, the inconel alloy wire is then melted and arc sprayed against the substrate in this (3-5 mil) overlapping layers. Built-up in thin, multiple layers to the desired thickness which assures a coating with low internal stresses and a mechanical bond in excess of 5000 psi.  The Twin Wire Electric Arc 400 Process utilizes an arc spray head to melt the alloy feed wires. DC electric current is carried by two electrically conductive, consumable .0625 diameter wires, which are fed to the spray gun from a dual drive wire feeder. An electric arc forms between the wire tips in the gap where the two are continuously directed. A high velocity jet of nitrogen atomizes the molten metal as it forms continuously, while the wires are melted by the electric arc. The atomizing gas accelerates the fine metal droplets away from the electrode tips to the prepared substrate (base metal). The molten particles impact the substrate to form a coating in a series of thin (3-5 mil), overlapping layers to the desired thickness. 
North America 
15 Year Study Swinging Hardwood/Softwood Batch Digester .80" NI/Cr Coating applied in 1985 Date of last inspection Feb 00 Condition of coating: excellent, .40" thk remaining 2-year study Weld overlay, 24" Batch Digester charging nozzle Extensive repair required due to chemical erosive attack via, Porosity, slag inclusions and cracks of overlay Condition of Overlay 2 year inspection: poor, extensive ASME inspections and repairs required. Condition of Substrate behind overlay: Critical with catastrophic hazard potential. One of the most important part of the TWAI 400 Process is surface preparations. Every inch of substrate is profiled to receive a coating of uncontaminated Ni and Cr molecules. Mechanical bond strength of excess 5000 psi is obtained by using our Automated Blasting Equipment which allows for easy installation and shorter downtime. Homogeneous matrix of Nickel and Chromium Outlast all other processes and coatings in Continuous and Batch Digesters. 
Our automated system provide workers and technicians a clean working environment. No breathing toxic fumes. Our out of tank application proves our workers a safe working environment during the preparation and coating process. TwinAlloy Cast sealing system protects the coating by forming an electrolytic protective barrier after the initial cook of digester liquors. ISO 9002 approved quality systems manufacture our wires. Our products meet or exceed the specification requirements of AWS, ASME, TUV, MIL and DIN standards. Twinalloy 625 Designed, milled and spooled to our strict specifications and made exclusively for Twin Wire Arc, Inc. Our Thermal Spray Wire had proven to be impervious to the environments commonly found in Digesters. Tensile strength, cast and helix is controlled enabling continuous feed over long distance, as required in Digesters and our Stand Alone Automated Delivery System. Our Alloy is proven to be the most effective against corrosion, erosion and stress cracking in caustic mediums, such as digesters.  Twin Wire Arc, Inc. holds certification No. R-5685 of the NBIC to repair or alter ASME Section VIII. Div. 1, Pressure Vessels and Section 1, Power Boilers, B31.1 and B31.3 Piping. Inlet Nozzle of Batch Digester, Stainless Steel overlay was repaired along with the cavaities found in pressure retaining shell behind all the holes seen in the weld.  Batch Digester; These wasted sections of the cone were hidden by a layer of weld overlay. Twin Wire Arc, Inc. inspection team routinely identifies such problem areas for ASME repairs.  Erosion, Currosion, and Stress corrosion cracking are common in digesters. Twin Wire Arc, Inc. is a qualified agency that repairs and replaces wasted sections. Cones, heads and nozzle attachments are installed in accordance to the NBIC. Ask about our special alloy replacements. 
Separators for the Power Industry ASME Section VIII, Div. 1, Pressure Vessels and Pressure Retaining Parts are fabricated under Certificate of Authorization No. 32,117. 
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