Preservation line solves challenges in finishing, painting large-scale steel parts
Every steel fabricating company faces the challenge
of protecting its steel plates and beams from corrosion. It is critical for producing quality, high-end
General Dynamics NASSCO, which designs and
builds auxiliary and support ships for the U.S. Navy,
oil tankers, and dry cargo carriers for commercial
markets, was facing that challenge in the production
of steel plates, profiles, and tubes. The parts, up to
60 by 12 by 1.5 ft. and weighing up to 20 tons, were
covered in surface rust and scale before processing.
The preservation line in use at the company’s San
Diego facility was built more than 40 years ago. And
while the plant was equipped with a crane storage
system that could pull workpieces from storage and
deliver them to the preservation line, a delivery system did not exist.
The firm acquired a turnkey preservation line from
Rosler Metal Finishing USA that comprised a transport system for loading steel plates, profiles, and
tubes; a preheater; a roller conveyor shotblast machine; an automatic paint booth with overspray
separation; a paint dryer with slat conveyor; and a
transport system for the next downstream operation.
The most significant challenge faced when
planning this project was the coordination among
the three parties involved: Rosler USA, Rosler
Germany, and NASSCO. With each party located
in a different time zone, arranging meetings
required extra planning. The distance also meant
that NASSCO was able to visit Rosler’s world
headquarters in Germany only once for inspection
of the line before delivery, which is rare for a project
of this size. Weekly status reports and meetings
continued through the planning and building
phases of the project, ensuring that NASSCO was
kept in the loop regarding Rosler’s progress.
The line operates 30 percent faster than NASSCO’s previous equipment, has improved blast and
paint quality, and has eliminated nearly all sources
of emissions, creating a more efficient and environmentally friendly surface finishing process. The
components are interlinked for fully automatic operation with little or no operator involvement.
The shuttle delivery system, which runs on tracks
embedded in the ground, delivers workpieces directly to the intel of the roller conveyor from the
storage system. The transfer shuttle delivers workpieces weighing up to 74,000 lbs. to the blast machine at a speed of 12 to 50 feet per minute.
The blast chamber is made from manganese steel
and lined with overlapping, replaceable plates also
made from manganese steel to add durability. The
shotblast machine itself is equipped with Rosler’s
patented Gamma 400 G turbines, which produce a
15 to 20 percent higher blast performance than conventional blast wheels, according to the manufacturer, with at least 100 percent higher uptimes.
The turbines are placed in the upper and lower
sections of the blast chamber at an incline so that
the blast media hits the workpieces at a defined angle. Since the angle of the blast turbines varies from
project to project, Rosler makes a detailed check of
all parts that will pass through the blast chamber
using a CAD system, which allows for optimal blasting impact and minimal wear inside the chamber.
According to the company, this inclined turbine
arrangement results in a 15 to 20 percent improvement in descaling and rust removal, which reduces
required cycle times.
The integrated cleaning system for removing residual blast media from the workpieces includes
a rotating brush and a targeted, high-velocity air
stream. It is equipped with a workpiece recognition
system and automatic height adjustment, ensuring
workpieces leave the blast chamber free of blast
media and dust and ready for painting. It also eliminates the need for lengthy manual adjustment and
minimizes the wear of the rotating brush.
The automatic paint booth is equipped with sensors that measure workpiece dimensions. The system controls guide the paint gun movement, which
follows the workpiece contours to ensure the paint
is applied at the required thickness and only when a
part is under the spray nozzle.
The system works with almost any solvent- or
water-based paints with one, two, or three components. For applications requiring different types of
paint, the automatic paint booth can be equipped
with multiple independent paint delivery systems.
The cabin exhaust air system features a trapezoidal, flow-optimized arrangement that prevents
air turbulence and transfers the exhaust air mixed
with paint mist to the standard dry separation unit,
which includes a patented brush preseparator with
slowly rotating brush rollers. The paint particles
stick to the brush, dry, and fall to the bottom. This
precleaning stage is followed by a sinter-plate filter,
which reduces the residual dust load significantly
below the legal requirements. The brush preseparator reduces the filter load so much that uptimes of
about 15,000 hours for the sinter-plate filter can be
achieved, the company states.
Minimal paint particles are deposited on the
Teflon®-coated inner walls of the cabin, which reduces cleaning times. Steel-slat conveyors transfer
the freshly painted workpieces to the paint dryer.
They have only six contact points per 10 sq. ft. to
prevent disturbing the paint. The dryer is heated to
104 and 180 degrees F, and recirculating fans and air
guides help ensure even, all-around drying of the
A three-chamber regenerative oxidation system
processes the exhaust from the painting process,
removing harmful solvents before they are released
into the atmosphere. The polluted air is channeled
from the painting process into the first chamber,
where the air is warmed up and the volatile organic
compounds are oxidized. It then passes through the
second chamber, where the air is cooled by a ceramic bed before passing into the third chamber, where
the air is put through a washing process. The resulting clean air is then sent through the stack.
Rosler Metal Finishing USA
1551 Denso Road
Battle Creek, MI 49037
The system works with almost
any solvent- or water-based
paints with one, two, or three