Eco-Coating Boosts Green Ship Repair

Liu Xixian

China ship repair industry has forged ahead in recent decade and has for years in a row become world largest ship repair country. In 2017, there were more than 200 ship repairing yards of above the designated scale, undertaking global almost 70% of ship repair workload. Meanwhile, environmental pollution caused by ship repair coating work has become a problem which can’t be evaded, pollutant emissions were far higher than the international average, given no effective control and no implementation of technical equipment upgrade, ship repair coating operation mainly including derusting by sandblast and paint spraying will be severely affected. Due to site and technology restrictions, most coating operations are done in out-of-door environment, often causing dust rise and VOC emission outof-limits.

Green Process Imperative

“Green technology” also called environment-friendly technology and eco-technology first appeared in the mid-1990s, core content was protection of human health, reduction of environmental pollution and energy consumption and promotion of sustainable development of economy. Green ship repair coating process refers to the technological process which economically meets customer need,saves resources and reduces or removes environmental protection by applying advanced technology and pollution-free coating material in the process of ship repair coating.

To adapt to the State policies,regulations and social requirements for energy saving, environmental protection and clean production, advanced ship repair yards have in recent years developed and applied green coating technologies, such as ultrahigh pressure derusting technology, water mist sand blasting technology. As a whole, however,Chinese energy saving, environmental protection indexes of ship repair coating are still far behind the international average. Most ship repair coating processes involve severe pollution, large energy consumption and low resource utilization rate. This shows ship repair yards generally lack green coating idea, a top priority should be to further increase energy saving and environmental protection consciousness of government,enterprises and professionals and by legislation and fiscal subsidies among other administrative means boost ship repair coating greening and transformation and upgrade.

To better response to increasingly rigorous State environmental protection requirements, national ship research institutions, leading ship repair yards and relevant ancillary companies are stepping up R&D and launching new technologies and new processes meeting green coating requirements, primarily including two major parts -- flying dust control technology and VOC treatment technology.

I. Flying Dust Control Technologies

1. Ultrahigh Pressure Water Jet Cleaning Technology

Ultrahigh Pressure Water Jet Cleaning Technology sprang up in the US and European countries in the 1960s, its working principle is use of ultrahigh pressure pump to pressurize tap water to hundreds or even thousands of barometric pressures, and then through a special nozzle (about 1-2mm aperture)a highly energy concentrated flow is ejected at a very high rate (200-500m/s). With huge impact energy, the flow,like pellet, can be used for steel plate cutting, metal surface derusting, casting sand removal as well as removal of salt,alkali, limescale and other attachments on pipe inner surface, and the technology utilizing high-speed flow for cleaning is called Ultrahigh Pressure Water Jet Cleaning Technology.

Compared with traditional mechanical abrasive blasting operation,Ultrahigh Pressure Water Jet Cleaning will not generate a lot of flying dust,polluting atmospheric environment and damaging staff health. The process uses tasteless, odorless and nontoxic water as medium, ejected water mist can effectively reduce dust concentration in the operation area and eradicate sources of pneumoconiosis, being an important move for the Chinese environmental protection cause. Everything has two sides, with no exception of Ultrahigh Pressure Water Jet Cleaning. In spite of environment-friendly and efficient advantages, the process involves high investment cost, prone to second re-rust problem.

2. Water Mist Abrasive Blasting Technology

The technology is based on traditional dry-type abrasive blasting process, through technical innovation,water mist spray equipment is added around the sand gun, during sandblasting operation, water is ejected from the mist nozzles around the sand gun, forming funnel-shaped water mist, which, after integrating with flying dust, settles onto the ground, reducing dust without affecting sandblasting effect. The technology has features of simple design,low cost and good dust reduction.

3. Automatic Recycling Environmental Protection Sand Ejector

Automatic Recycling Environmental Protection Sand Ejector uses compressed air as power to eject abrasive to the workpiece surface at a very high speed,huge impact removes rust stain on the workpiece surface, so that the surface meets paint spraying cleanliness and roughness. The sand ejector is equipped with a set of recycling system, which first uses negative pressure generated from vacuum pump or vacuum generator to recycle abrasive and dust, and then separate complete dull polish for recycling via the sand and dust separation system, while dust stain and dust enter the dust box. The equipment has characteristics of energy conservation,environmental protection, extensive application and convenient move, with fairly low operation efficiency, though.

4. Dust-Proof Water Curtain System

Dust-Proof Water Curtain System is mainly composed of sprinkler pipe, water pump, outlet hole, sprinkler curtain, etc.,and by building a complete anti-flying dust water curtain on the production site,meets dust treatment need. Compared with traditional dust reduction measures such as sprinkling and setting fence,the Dust-Proof Water Curtain System has characteristics of simple structure,less material consumption and good prevention and control effect, with strong application value, applicable for flying dust treatment on construction site, coal storage yard, mine field and ship repair floating dry dock.

I I. VOCTreatment Technologies

1. Spreading Use of Water-Based Coatings

Water-Based Coatings use water as diluent and does not contain VOC(e.g., formaldehyde, benzene, toluene and xylene) and toxic heavy metals.Compared with traditional solvent-based coating, Water-Based Coating neither gives rise to environmental pollution nor harms human health, marked by resistance to aging and yellowing, good gliding property and easy use, dubbed“Third Coating Revolution” in the industry, applicable to metal, woodware,glass and building surface and mother materials. In the ship coating field,frequently-used water-based coatings include waterborne inorganic silicic acid coatings, waterborne epoxy coatings and waterborne acrylic coatings, etc., mainly applied in engine room, residential districts, void compartments and inner cabins with not high anti-corrosion requirements, availability makes up 10%-20% of single ship total coating.However, Water-Based Coatings have obvious shortcomings, such as high cost, poor water resistance and rigorous construction conditions (temperature,humidity), unable for large-scale promotion at present.

2. High Solid Coat

As environmental protection is uplifted to the State strategy height and coating production technology is improved, High Solid Coat has gradually entered people's vision and extensively applied in the automobile production and metal processing industries. High Solid Coat (HSC) stands for coatings of 65%-85% general solid content and 15%-35% organic solvent content. The development of high-solid coatings to the extreme is active solvent coatings (also called solventless coatings), with typical representative of Polyurea Elastomer Coatings that suddenly sprang up after 2012.

Compared with other low-pollution coatings, HSC has obvious advantages.With production equipment, coating process and detection and evaluation instruments same as traditional solventbased coatings, HSC can be directly used in existing construction equipment without additional investment. One coating film thickness is 1-4-fold of traditional solvent-based coatings, it not only improves coating work efficiency and saves labor cost and solvent cost, but also effectively reduces VOC emissions.Its shortcoming is prone to crawling,shrinkage cavity and sagging problems during application.

Environment-Friendly Water Mist Sandblasting Derusting Technology

An environment-friendly water mist sand gun independently developed by Guangdong COSCO Shipping Heavy Industry Co Ltd consists of spear, sand gun, water mist gun holster, water inlet tube, water mist control head, O-ring seal and other parts. The water mist gun holster is covered on the sand gun head, water tube is connected to the holster side, water mist control head is connected to the sand tube outlet end,thus assembled into a water mist sand gun.

Through technical innovation,after copper ore is ejected from the sand gun, water is ejected from the water mist spray gun, forming funnel water mist encirclement to surround copper ore,acting as reducing dust without affecting sandblasting. The application of the new technology can improve sandblasting environment, improve the bad condition of dust flying in sandblasting, almost no dust flies, improving sandblasting environment and air quality, is of major importance to environmental protection.Moreover, water tube and sand tube are separated, water and sand can be ejected synchronously or separately,water tube can be removed when water spray is not needed. Single operation does not affect sandblasting, separate water spray is possible and can be used for fire-fighting. Sewage flow generated from sandblasting drains into the bilge well of the dock deck for copper ore PM storage and settling, which is discharged after sewage treatment to prevent sewage flowing into river and lake and avoiding pollution to water areas.