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.
Two 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
Surfaces
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.
Food-Borne Illness
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.
References:
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
View our line of orbital welding equipment from Exel Systems
