Surface Preparation for Shipyards

7/28/2019 Surface Preparation for Shipyards

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Pre-heating, blasting, coating, drying

Ship building

The size of metal sections used in the shipbuilding industry presents great challenges forsurface preparation and treatment suppliers.

Huge quantities of sheet steel, profiles andfabrications have to be cleaned, treated andpainted before final assembly. Corrosionprotection is of paramount importance.

The first part of the treatment process occurs

on a preservation line, which incorporates thedifferent stages of blasting and priming.

On the preservation line, sheets and profilesare thoroughly blast cleaned and provided witha temporary corrosion protection coating.

Along with the external transport unit,a preservation line consists of a pre-heatingoven, a pass-through blasting machine, apainting unit and a drying tunnel with itsassociated slat conveyor.

Typical plate widths can vary from 800mm to5m with process line speeds between 1.0m/min to 6.0m/min.

Preservation line

Sections 1 & 2:Cross transfer conveyorand inlet roller conveyor

The first section of plant on this scale is thematerials handling section, the cross transferconveyor. The steel sheets and profiles aredelivered here, and then fed intothe treatment process by the Inlet RollerConveyor.

Section 3:Pre-heater

The plates and profiles first pass through thepre-heater, this raises the temperature ofthe metal ready for blasting and removes anysurface moisture.

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Wheel blasting

Preservation line

Section 4:Roller conveyor shot blast machineThe blast chamber removes rust and mill scale(fig 1) and provides a finish to internationallyrecognised preparation grades (fig 2) accordingto ISO 8501-1.

Fig 1. Rust gradesFour rust grades are specified. These aredefined by precise written descriptions

and photographic examples in ISO 8501-1documentation. They vary from A: mill scale,to D: where the mill scale has rusted away andgeneral pitting is visible.

Fig 2. Preparation gradesSurface preparation by blast cleaning isdesignated by the international standard ISO8501. Four grades are specified, ranging fromSa 1: light blast cleaning, to Sa 3: blast cleaningto visually clean steel. Fig. 2. Shows surfacepreparation grades as applied to steel of rustgrade B: a surface which has begun to rustand from which the millscale has begun to

flake.

A B Sa 1B

D C

B Sa 2

B Sa 3 B Sa 2

Fig 2. Preparation gradesFig 1. Rust grades

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Quality control

Quality control

The controlled removal of rust and scale,

and the generation of a consistent surfaceprofile are critical to the successful bondingof coatings that follow. Throughout theseprocesses the strict application of exhaustivequality controls is mandatory.

Priming

Sections 6-9:

Paint spray chamber, voc-system and filterThe various plate and profile widths areautomatically identified and are coated in acontinuous process with a weld-primer coatingthickness of approximately 15 - 25m.

Paint dust and solvents (if no water basedpaint is used) are treated according to the localrequirements in an automatic filter unit and aVOC-treatment plant.

Sections 10 & 11:Drying chamber & slat conveyor

This chamber can be heated by the exhaustedair from the pre-heater (section 3). Additionalcirculation of high quantities of air acceleratesthe drying process.

Lying on the support points of the conveyorcross slats (section 11), the wet primer remainsundamaged at the plates pass through thedrying chamber.

Marking

Each sheet has its own unique identification toallow subsequent allocation and control. Themark is spray painted by computer controllednozzles. The paint can be fed from reservoirsor from the paint suppliers drums.

Edge cleaning

To ensure optimum weld quality, the edges of

the profiles are blasted to remove paint fromthe weld area (sections 11-13). High throughspeeds are the rule.

Airblast units for this operation are suitablefor smaller profiles and often precede a gas-cutting machine.

Sections 12 & 13:

ConveyingThe plates are now conveyed to be fabricatedinto ship segments.

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Painting and drying

Painting and drying

Painting is a critical part of the corrosionprotection process and must meet strictquality criteria.

Large component dimensions dictate that thepainting process is performed in large paintinghalls, this requires great attention for botheconomic and ecological reasons.

Wheelabrator has developed a specific wide-

angle nozzle air distribution technique whichensures good ventilation of the working areaand a significant reduction of installed airmovement, heating energy consumption, andthus operating costs.

Similar to blasting, 50% to 70% of the paintingwork is carried out inside the welded sections.Ventilation within the sections is performed bygrounded hoses for removing dust and solvent

vapours. 1-C pumps as well as 2-C mixing anddosing units can be used. A unique fresh and

exhaust air system has been developed tominimise operational costs.After the painting process the facility can beswitched over to the drying process.

Mobile systems for marine blastingand painting processes

A full range of horizontal and vertical mobile

systems are also available. Mobile systems areused to blast and coat the welds of new shipsections that are too large to fit in the blastrooms, and are used extensively in ship repair.

Vacuum recovery units, dust collectors, specificapplication technology and other equipmentare also available. Ask your Wheelabratorrepresentative for more detail.

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Documents

Surface Preparation for Shipyards