Blasting Method in Modern Ship Building Industries

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Objective:

To study and identify advanced blasting method that is technically and economically beneficial for Ship Building industry

Introduction

Ocean going Vessels generally have steel hull plating protected by anti – corrosive anti fouling coating .Coating system on a ship hull must be renewed either partially or totally over time because of prolonged exposure of ship hull with salt water will gradually results in corrosion and fouling. This process of cleaning the surface before coating is called as surface preparation and blasting is one of the methods used in surface preparation. Blasting can be defined as process of removing and cleaning substances that have deleterious effect on coating effectiveness.(Piero Cardis,2001)

Literature Review

This can be of two parts one is to find the previous study related to this research and other part is literature and background study on present and advanced blasting cleaning methods.

Literature Findings

Two related previously done research work will be reviewed

Water jetting research

Research was conducted by National Defense center for environmental excellence.Aim was to find an alternative coating removal process that have the ability to perform as effectively and efficiently as conventional abrasive blasting. The task focused on two removal process alternatives high pressure water jet and abrasive blasting with alumaglass.Activities which were investigated were of depot level and field level,three depot vehicles and two field level vehicles were used. High pressure water jetting and abrasive blasting with alumaglass were found to be economical failure compared to traditional abrasive blasting for depot level but field level was found be financially beneficial (NDCEE ,2002)

NELP High pressure water jet system test plan research

The aim was to reduce or eliminate the volume of hazardous waste, air emission and worker health hazards and to minimize cost by reducing processing time A prototype was built and placed under water for the natural corrosion and fouling on metal surfaceThe system used ultra high pressure (UHP) water jet technology instead of dry media blastingThe experiment was found to be success and a prototype of high pressure steam jet was designed and build.The result involve technical specification, cost analysis and comparison between high jet water blasting and dry media air blasting method(NELP,2002)

Blast Cleaning Process

Degreasing is done before blasting which involves removal of oil, grease, mill scale, welding spatters and other loose dirt. Followed by degreasing blast cleaning is done to remove dust from surface ,this is done by impact of small metal or mineral particle injected at high speed on ship hull and in hydro blasting moisture is removed before painting. The prepared surface should be smooth but rough enough to hold the paint.(Cardis,2001)

Factors for effective blast cleaning

Appropriate pressure

occurrence of dry air

Apt removal of dust and dirt

successful parting of oil and water

Selection of blast material(material,size,possible pollution)

Suitable mix nlast material and transport medium(air,water)

Blast cleaning method

The bulk of work can be defined in terms of kinetic energy delivered by impacting media

Blasting method can be adjusted using this equation to give different results and this is required due to material sensitivity or the requirement of more energy.

Most important metal surface cleaning method used in marine industry are as follows

Abrasive Blasting

It uses mechanical energy to hurl particles at high speed for removing paint and other organic coating from metallic and non metallic.Sand is the most common blasting abrasive and other materials include coal slag,smelter slag,mineral, metal and synthetic abrasives .Abrasive blasting is considered to be the most economic and mainly used in cleaning large surfaces .Major problems faced by these method is it is hazard to worker’s health and it causes noise and air pollution.In recent developments efforts have made to limit production of dust (Cavallo.J,2001)

Vacuum Blasting

Air and abrasive are projected within rubber enclosure and abrasive and dust are drawn behind the blast until they are separated and the abrasive is re-circulated this can cause environmental pollution .The process time is higher than other methods and other disadvantages include cleaning difficulty in corners and extra work required for portable equipment .Due to these ,Vacuum blasting is used in repairs and for cleaning welded joints(Caridis,2001)

Water Jet Blasting

This uses high –pressure water jet of 150-300 bar. Blast cleaning is extensively used in ship hull maintenance and it ensures that marine organisms along with old paints and rust attached to the hull surface are removed .It is considered to be the most effective way for the removal of salt from corroded surface. High quality finish ,less abrasive usage, ability to be used in humid condition, less pollution and more safety are major advantages of water jet blasting. The main disadvantage of water jet blasting is its expensive and not suitable for direct application of coating as the surface will be wet.

Ultra High Pressure (UHP) Water Jetting

It is one the fastest growing surface preparation method used in ship repair yard and growing number of yards in both U.S.A and Europe are employing the process.(Anon,2002). In water jetting high pressure is obtained by pressurizing water into a ultra high pressure then passing it through a small precious stone orifice

UHP pump pressurize normal tap water at pressure levels above 2760 bar for the required blasting energy

Water then forced through a small precious stone to form an intensive blasting steam ,the velocity of stream is approximately equals to 2.5 times the speed of sound

Major tools used in UHP water jetting are Handheld tools,semi and fully automated systems (Dupuy.R,2007)

Closed Loop System

Majority of Water jetting in ship yards uses “men on guns “type of water jetting and closed loop system is the ability of water jetting equipment to be able to treat effluent on site and re introduce the filtered pure water to the ultra high pressure pumps. This system is well explained in article “Ultra-High Pressure Modelling and Experiments Review”(Costantino.M, Darnell.L,2005) .The water stream is vacuumed from the surface and solids were separated from water and the stream is separated to a holding tank for treatment or disposal. This process of reusing the water by using a filtration process is of enormous benefit from an economic as well as an environmental point of view. Steps involved are stated below

Separation of water from paint fragments
Removal of particulates down 5 micron size
Removal of metals(including tributyltin compounds) and heavy metals
Removal of Particulate from .003 micron to .02 micron in size
Reverse Osmosis i.e. Removal of chlorides and other molecular species and thus increases the conductivity of jetting water .

Blasting Method in Modern Ship Building Industries