Orbital Welding Passes Inspection Tests in Powdered Milk Plant
Orbital welding passes inspection tests in powdered milk plant
Major plant installations can require the combined efforts of experts from more than one country. Such was the case of the powdered milk plant installation in Conaprole, Uruguay, now the largest in Latin America. This project involved a joint venture between Tricom from Holland who designed the plant, MBL from New Zealand who supplied the process equipment, and Lincomax, from Uruguay, the installing contractor.
As part of the comprehensive plan to design and install a state-of-the-art facility, the engineers specified orbital welding equipment. Selecting orbital welding reflects the increasing concern for public health in the dairy industry, especially for products distributed world-wide. Engineers typically specify orbital welding for joining tubing in industries where cleanliness is essential for the integrity of the product, such as in the production of pharmaceuticals. This is because orbital welds are generally smoother than comparable manual welds on the inside of the tube. The smoothness eliminates crevices that could entrap the product and make cleaning and sanitation problematic.
From Liquid to Powder
The process of making powdered milk is complex, requiring tight control of temperatures and pressures for every part of the process. The plant is arranged vertically with milk stored in large tanks at the top of a 65 meter tower. First the milk is condensed into a slurry. Then the slurry is pumped under pressure and released in a spray. The spray dries as it falls through the center of a series of large coils filled with hot oil. The drying droplets convert into flakes. As the flakes continue to fall, cooling coils bring the flakes to room temperature. Workers collect and package the powdered milk at ground level.
Orbital Welding Equipment
At the Conaprole plant, operators used the gas-tungsten-arc welding (GTAW) process with orbital welding equipment from Arc Machines, Inc., Pacoima, CA. They welded process piping that handles the product, CIP (clean-in-place) piping that distributes cleaning solutions, a CIP transfer panel, and a 6 in. diameter process water supply loop.
Orbital welding using the GTAW process works either as a fusion (autogenous) process to fuse the ends of the joint together without the addition of filler metal, or by adding a wire filler metal. Typically welders join sanitary process piping by autogenous welding in a singlepass butt weld. Some applications on heavy-wall pipe, such as the 6 in. diameter main water supply, may require filler metal.
The Arc Machines Model 227 power supply has the capability for autogenous welding and the controls to add filler wire. It powered an Arc Machines Model 9-4500 fusion weld head and an Arc Machines Model 95-6625 weld head with filler wire capabilities. The microprocessor-based power supply stores weld programs or schedules for various tube or pipe sizes. It offers precise control of weld parameters, such as heat input control, and the process offers repeatability from weld joint to weld joint.
Welding of pipe or heavy-walled tubing typically requires water cooling of the weld heads during welding for a higher duty cycle. The Arc Machines water cooling unit, mounted under the power supply, circulates water through the cables as well as the weld heads.
Operators used the Model 9-4500 head to weld the majority of the welds - the process piping in-place at the job site and the CIP piping. Operators welded the CIP piping, which extends vertically to the top of the plant, as an assembly and then installed it. They also used the weld head to weld a transfer panel that directs the flow of product or cleaning solutions for CIP through the system.
The Model 9-4500 is an
enclosed type weld head providing shielding gas coverage to protect the outside
of the weld from oxidation. Although the outside weld is not typically a
product contact surface, a clean weld surface helps to maintain corrosion resistance
and general cleanability.
Engineers specified 6 in. diameter 316L stainless-steel pipe with 4 mm. wall thickness for the process water loop. Operators prepped tube-to-flange welds with a beveled joint configuration. It required two passes with the addition of wire using the Model 95-6625 weld head.
The two welding operators, who received training from Arc Machines, completed about 1,500 orbital welds.
Orbital tube welding is automatic in the sense that the operator only needs to start the weld sequence.
Orbital pipe welding is only semi-automatic. For pipe welding the operator may make fine adjustments during the weld to torch steering, AVC (if used) that controls the arc gap, amperage, or the wire-feed rate. All of this takes skill. Orbital pipe welder training is usually a week.
The inside of a 6 in. diameter tube-to-flange weld shows minimal oxidation after orbital welding.
Smooth Welds, Clean
Orbital welding is a major contributor to the development of modern CIP technology.
In order to clean equipment and lines effectively without disassembly, the product contact surfaces on the inside of the pipe, including all the weld surfaces, must be smooth enough to avoid entrapment of the product or cleaning residue.
While outside welds on fittings and other components may be smoothed and polished after welding, welds in piping systems are put into service in the as-welded condition. The lines may be passivated, which improves the corrosion resistance but does not affect the smoothness of the weld beads.
Manual welds, which are still permitted in the industry, are much more likely to have voids that can harbor micro-organisms.
The use of orbital welding is increasing in the dairy industry in the United States, Latin America, Europe, and New Zealand as the global distribution of dairy products increases.
Food-borne illnesses traced to outbreaks resulting from improper sanitation in the production of dairy products are still prevalent in many parts of the world. These illnesses might be prevented with better fabrication practices including the more wide-spread use of orbital welding technology.
Globalization of the dairy industry means that products produced in one part of the world must meet the standards of the importing countries.
The American Welding Society recently produced a new standard, AWS D18.1/D18.2, for welding austenitic stainless steel tubing and pipe for the dairy and food industries in the United States. This was done in cooperation with the 3A Sanitary Standards group, which has been writing standards for the dairy industry in the United States for more than 50 years.
The AWS D18 Standard, published in 1999, provides acceptance criteria for welds in sanitary piping systems. It includes a color chart showing the inside of a series of ten orbital welds on a 316L stainless steel tube. Each weld was done with a different amount of oxygen in the argon purge ranging from 10 parts per million oxygen up to 25,000 ppm (2.5 percent) oxygen. This chart allows owners, installation contractors and inspection contractors to agree upon an acceptance level for weld discoloration, which is proportional to the amount of oxygen in the purge, before the start of an installation.
At the Conaprole plant quality assurance personnel accepted the orbital welding procedures and documentation of weld results as a key element in approving the plant for validation. This protocol and the requirements for documentation represent a new level of quality control by Conaprole and sets a new standard for the dairy industry in Latin America.
AWS D18.1:1999 Specification for Welding of Austenitic Stainless Steel Tube and Pipe Systems in Sanitary (Hygienic) Applications
AWS D18.2 Weld Discoloration Levels on Inside of Austenitic Stainless Steel Tube
Barbara K. Henon, Ph.D., Manager of Technical Publications and Eng. Angel Brond, Director of Latin American Sales for Arc Machines, Inc., Pacoima, CA 91331. Phone (818) 896-9556. www.arcmachines.com