International Conference on Shot Peening and Blast Cleaning

ABRASIVE BLASTING AND BLAST RQ INSTALLATIONS - A GENERAL OVERVBE

R. G. Bosshard Development Engineer

ANVIL DEVELOPMENTS, CH-8604 Volketswil-Switzerland

ABSTRACT

Abrasive blast cleaning can be done in various ways. Quality, costs and environmental aspects will be the major points t o arrange or design such equipment. Blasting in fact can be done in open air w i th a simple pressure pot and an air compressor. A weatherproof shed, some media recycling facilities can give better service, finally ending wi th all the necessary items t o have a fully equipped installation. This paper lists the full range of such equipment and will give some key figures for costs and energy investigations. Additionally economical aspects between e.g. a low industrialized country (India) and a high industrialized country (Switzerland) are mentioned.

KEYWORDS

Sandblasting, air abrasive cleaning, abrasive blasting, blast room.

INTRODUCTION

First class surface protection bases on first class coating material and this again requires adequate surface p r e p a r a t i o n h most cases this will be abrasive sand blasting. This is one of the rare industrially common and universal preparation procedures that can be done anywhere. But in most cases this process asks for quite a b i t o f equipment and installations. We have steel constructions, marine, railway and aircraft components, pipelines, tanks and buildings down t o smallest parts of any kind in the mm-range.

Air operated blasting and wheel blasting are the t w o most common systems. Simple hand manipulated, up t o highly

mechanized and automated equipment is possible w i th the air operated technique. Treatments will take place indoor, outdoor, even under water. Areas will be less then a few square millimeters up to thousands of square meters.

The unique advantage of a blasted surface is, that this surface then has a defined surface structure and the contamination is nearly nil. These facts are the compulsory requirements for any coating t o be applied on such surfaces.

SURFACE PREPARATION REQUIREMENTS FOR COATINGS

.The blasting process affects the surface in t w o ways:

a) Roughness:

Depending on the blasting media used and also partly depending on the blast pressure, a certain roughness will be produced. For the work progress and production costs, it is more economical t o work wi th course blast media at media typical pressure. For the coating t o be applied on the surface, the roughness is very important. A higher roughness increases the bonding of the coating. But on the other hand, it has to be kept in mind that the coating first of all wil l fill out the grooves leaving only a very thin skin across the buckles. But the quality and durability of a coating is given by the thickness of the coating fi lm wi th the least thickness:

I

Example:

A coating system exists of apr imer 30my thickness, a I st and a 2nd coating of 40my each, all toge tha 1 lOmy. The roughness of the blasted surface should not exceed 40my. So an effective protective local layer thickness of e.g. 70 my can be expected. This indications are very approximative, only the coating expert can give details t o the specific product.

b) Cleanness: For economical reason it is important t o determine the necessary cleanness for the specific job as it affects the costs of the blasting process. A practical thing is the standardization according the "European Standard (Swedish)" that works w i th the figures 1 to 3. It bases on the verification of official standard photographs w i th the actual surface. Also a description in words helps, e.g. Sa3 (SSPCISP 5) asks for a perfectly metallic full clean surface (visually clean), whereas Sa3 % (SSPC?SP 10) permits some partial irritation in the appearance (near white blast cleaning).

Not absolutely defined is the contamination produced by the media applied. Following the instructions given for a specified coating system, this will be not a real problem, more a matter of cost. But it is very delicate for blasting processes prior t o e.g. plasma sprayed coatings.

Just after a process or even during a blasting process, the surrounding humidity. Some times it needs quite an effort t o fulfill the requirements, it might be necessary t o work together w i t h air condi t ioning instal lat ions. In many cases th is is compulsory when ordered by the coating engineering.

MEDIA SELECTION FOR INDUSTRIAL APPLICATION

Numerous media of various particle sizes and shapes can be used for blasting. The decision depends on process effect, chemical acceptability, availability, costs, waste disposal criteria, technical processing requirements (Fig.1) etc. For common industrial application the most important media are steel grit and corundum. Table. I

Table 1 Blast Media Selection

Exotic

Various

Common abrasives

QUARTZ

One way utilization

Health crkical !

Natural Exotic I Artificial

Choice (glass, Si-oxide)

Specific

Choice (ice, nut-shells,. . )

Specific

STEEL GRIT Fe. .

CORUNDUM

AI,O, Continous recycling wi th

separation

Widely used medias !

a) STEEL GRIT

This is specially made for blasting application in a wide varitey of chemical analyse, hardness, shape, size and price. It is very good for descaling but needs high speed, that means a comparably high effective blast pressure closed to 5 bar, equivalent t o a pressure pot pressure of 7 bar and more depending on length of hoses. It is quite suitable t o reach a Sa2, eventually a Sa2% quality, but scarcely Sa3. This is because the shot particles tend t o get rounded therefore more hammering the surface, rather then grinding. On the other hand, the actual media consumption is very little.

b) CORUNDUM Here also a wide range of quality and prices exists. From dark t o white it can be something wi th a lot of other elements and adds compared to the pure aluminiumoxide. It has a good grinding effect, but the blast pressure has carefully t o be watched to keep media consumption within economical limits. Total costs m u s t be ind iv idua l l y eva lua ted depend ing o n va r i ous circumstances such as labour costs, power costs, work progress, waste management.

MEDIA SIZE

A supply of new media for a common blasting installation will have an media range of e.g. 1 to 3 m m grain size. Most important is the result of the sieve fraction when running an installation. Special attention has to be paid on that. Refill must happen periodically as an addition wi th fresh media brings up the average coarseness, therefore affecting the job quality. Small size particles, e.g. below 0.2 mm should get extracted within the reconditioning process and work area ventilation. Such small particles will only little contribute t o surface treatment, but will bring more dust and reduce the operators visibility.

6. CONCEPT OF BLASTING INSTALLATIONS

Blasting installations are existing all over the world and can be something between scrap and high-tech machinery. But there is

a kind of standard known as cabin or blast room, again something between a simple shelter wi th some mobile machinery or a fully equipped high performance installation. As blasting normally is followed by coating, a blasting cabin is somehow connected t o a painting shop or eventually the blasting cabin can be used for painting and drying. The layout and concept for all this installations must be worked out individually and customer tailored for each situation. Some installations are for temporary use in remote areas, others are a section within an e.g. a construction company. Also many old installations will need an update specially in respect of environmental requirements.

PARTS AND NECESSARY INSTALLATIONS FOR ABRASIVE BLASTING PREPARATION

Table. 2 Overview: From Demand to Investment. t

Field of application

Aircraft Industry . Corrosion Supplementary Automotive Protection possibilities

Stripping Industry Roughening

Shop On site Micro General Cleaning

Wheal Forged parts Structures Structures Glass marking d c h i n e s Csgt Oomestic Parts i Pipes

Stritctures Vessels Big tanks Frames SmaP tanks Vesseb,Sktips

Bclild.corng. i

Hot-galvanized +

Type of installation

Customer tailored

machinery

Small cabins '

manual

Corrosion protection

install.

Occasionally used Blast

Rooms

\ i

Ordinary Blast

rooms

/ I

1 :

1 1p

3( ~raduction mode \ \y 4

Big sized cabins

Frull production full-time

1 or more shifts per day Corrosion protect. m p . Ship buiMiny comp. Steel mill

Automatic media recovery and separation Category mstaikition

Category 1!1 @stall. (big) ;

Temp,impravisations

Problematic or exotic

Any

Manual media handling, no active

separation

Occasional

sideline, seasonal 1. .2 days I week

Metal structure b~iM.comp. Public Work Oept,

Semi-automatic media handling ~ i t h automatic

dust separation Category I installation

There is a tremendous variation possible in layout and design of blasting installations. But other then in modern industr ial machinery concepts, cont ractors today can w o r k w i t h technologies 70 years old, at the same time there are up t o date remote controlled, computerized, semi and ful ly automatic installations active.

BLAST ROOM KEY FIGURES Approximative figures that give an idea for budgeting and conceptional studies. These figures can vary for many reasons such as air filtration principle, ventilation technique, process variations, general dimensions, etc. Table 3

Table. 3 Blast Room Key Figures

Floor area m2

Ventilation kW/m2

Media recovery

No. of pressure pots

Pressure pot capacity

Nozzle dia

Room air renewal rate

Air consumption m,/min

I steel grit 6bar I m3/min

dm3

mm

I/min

ccrrundum 4bar

1 nozzle 6 (4) bar

0.2-0.35

man+auto auto comb.systems

1- 2 2 - 4

120 each 120 to 240 each

8- lO(new) 10-12(new)

1.5-2.5 1.7

COMPONENTS OF AN INDUSTRIAL MIDDLE CLASS BLAST ROOM.

6 - . . . 320 each

14(new)

0.3

(Example)

a) For general corrosion protection work a cabin of 5 by 10 meter area, 4.5 meters free height can be normal. Door cutout 4x4 m.

,I For,occasional use only

1

€3 Approved, no maintenance

C Vanatron. Vacuum

1 Ventilation air IN 2 Air flow inside room 3 Air OUT after filter 4 Media reconditioning 5 Media silo 6 Pressure pot 7 Reclaiming system 8 Work level 9 Door

D Variation: Screw A

E Variation; Scrapper t + elevator

!--- - - I . ,

I

Fig. 2 Examples of Blast Room Designs. Floor areas 2x2m to 5x1 m.

Walls are brickwork, metal structure or concrete. Soft rubber lining required for inner cabin walls. Doors in very rugged design, preferably swinging type.

A basement, even extensive, should be taken i n to consideration. This can be a key point for future overall economy. This section is responsible for the media recovery. If a comparably deep basement of little more then 3 m can be afforded, a most simple and inexpensive pneumatic technique can be installed. If not possible, there are various quite different systems but wi th lot of mechanical devices involved. There are even mechanical slider systems or vacuum systems that build within 15 cm. But such installations are n o t a lways ve ry rel iable and m i g h t need ex tens i ve maintenance. A strict rule is, specially for production use, that the floor has full recovery possibilities, so that there is no media deposit anywhere in the room at any time.

Ceiling preferably perforated in special design, resistant against blasting abrasion and acting as fresh air inlet. Preferably over full area or eventually partly.

Floor shall be grating like, w i th rails for trolley. Floor must be able t o carry fork lift. Cranes or similar such equipment is not recommended for safety reasons and reliabil ity o f the equipment.

Pressure pot (s) below media storage container. This unit, together wi th recovery devices and reconditioning device preferably inside a recess of the cabin.

Filter equipment and ventilator (s) closed outside the cabin or above the cabin. Waste storage and disposal must be easily accessible.

Compressor unit for blasting only, supply pressure set t o blast pressure.

Operator related air supply and control gear easily accessible.

ENVIRONMENTAL CONSIDERATIONS

Sandblasting is a process w i th heavy energy consumption and serious pollution problems. However, in long term thinking, it is positively environmental friendly. In Europe, specially in highly mechanized countries, any commercial sandblasting activities will be somehow connected t o governmental regulations in respect of pollution and safety.There are 3 different subjects:

a) Operator

In common sandblasting there is normally an operator w i th his tool -the blasting gun with hose- directed to the workpiece. He conducts e.g. 30 HP energy conversion w i th e.g. 20 kg blast media and 6 kg of air per minute. Extreme noise, lot of dust and intensive exposure t o ricocheted blast media are accompaigning th is hard job. Adequate p ro tec t ion is compulsory.

Helmet w i th breathing air and climatisation feed from a ventilator/cooler/heater via e.g. a 15 rnm dia hose or from a compressor/particle filtedcarbon filter via a 7 m m hose is common. Also special hotlcold controllable man packages are available (known from fire brigades).The helmet must have a simple t o change full sight protection glass wi th the security glass behind. Depending, a glass will last hardly t w o hours of work. Helmets equipped w i th radio and communication facilities could increase operators's performance.

To simplify working, and in many cases essential, are remote controlled on/off facilities for air and media or combined. Also a self activating emergency shut off system is required by many governmental regulations.

b) Pollution control / Ventilation

Here t w o different requirements must be obseived. This affects mainly stationary blasting installations but some field applications as well. We have the actual blasting cabin or enclosure as an internal system 1) and w e have an air discharge wi th polluted air from the complete installation 2 ) .

1 ) The dust from the process n~ rma l l y is of such an intensity, that the visibility for the operator is insufficient and e.g. he will not be able t o decide between a Sa2 and a Sa2% quality. But the official cost calculating figure shows something like a 20% difference. Dust can also be reduced by an efficient media reconditioning when media gets recycled. When working wi th corundum, in medium sized blasting cabins (e.g. 4x1 0 m area), a vertical air downward component of 0.1 misec can be sufficient, a 0.13 mlsec is comfortable. For big installations and due to costs, a room exchange rate of 0.5lminute can be a figure for further investigations. Also in big installations a punctual supply of fresh air t o the point of activities can be taken 'into consideration.

2) Certainly a ventilation system can work w i th circulating air, passing a filter, especially when heating is required. Even then e.g. 20% should be renewed to keep the cabin climate within a certain standard. If the outdoor climate is relatively warm, the aspirated fresh air normally gets blown back to the atmosphere after passing a filter. In all this cases the pollution may not be higher then 2.5 or 5.0 rngIm3 depending on government regulations. Under normal condition the exhausted air can be regarded as non toxic.

C) Waste management and disposal

The waste from the filter will be a mixture of undersize mineral or metallic blast media together w i th all particles abraised from the workpiece. Those can be rust, any kind of oxides, any kind of old or new coatings, sometimes non toxic but also toxic. Disposal of such material can be fairly tricky as it depends on governmental regulations. Samples have t o be taken and laboratory analyses have to be made. According the result the waste type is defined and disposal is officially strictly guided and costs can be taken in account and can have effect to the blasting process economy.

BUSINESS WITH SANDBLASTING INSTALLATIONS Sandblasting normally is not a stand alone job because the so treated surface is very delicate and, depending on the final purpose, the next operation has to follow within a short time. For critical parts, also the surrounding humidity and temperature is important, there is a small "window" only that gets permitted by the coating specifications. So normally coating facilities are connected to blasting installations. Commonly there is a painting shop of similar size of the blast room, only for very big plants, both jobs can be done in the same, therefore specially equipped room. Following are mentioned the special branches included in a commercial blasting shop together wi th coating activities, finally representing a corrosion protection company. Table 4

Table 4 BlastingICorrosion Protection Company Resorts

CAREFULLY MANAGED RESORTS TO GUARANTEE PROSPEROUS ACTIVITIES

Qualified management Financial criteria Sustainable Qualified supervision Management- development

Social services Workers-Shareholders

Location Purchase Processs chedule

Plantlayout Spares Logistics

Power Media Part supply -- purchase Handling

Blast room Main- Mensuration Protection+ Concept tenance safety Media Store Quality Insurances

recovery control

Adequate cost policyTop quality

I Controlling

relation

Media Transport Techn. Correct New separation vehicles Laboratory Salaries markets Ventilation Wastema Wrapping Staff Publicity

nagem. organizat. I

Air Environ- Staff Competitor com~ressor ment welfare observation

In highly industrialized countries such as Germany, Switzerland, etc. it figures out that one of such companies w i th an average staff of 5 t o 30, wil l serve the needs of around 1 0 0 ' 0 0 0 inhabitants.

COST COMPARISON

lndia -Switzerland -Slovenia-Germany (approximative figures, summer 2001)

given in local currency and also in EUR for comparison (1 EUR =

Table 5 Cost Comparison between Countries Action Reference India Switzerl.

IRU CHF EUR EUR

Blast cleaning Sa2.5, manualflat plate, 13.20 1 side, 1 Om2

Blast cleaning Sa3, manualflat plate 0.95 16.50 1 side, 10m2

1 Om2 Zn-primer2 epoxy-coatings, 100my tot.

Corundum common (1 ton purchase) 0.53 0.72

Steel grit common (1 ton purchase) 0.58 0.59

Compressed air, 100 m3 58 ? 3.0 6bar ? 1.40 2.00

Electric energy 100kWh 500 11 12.10 7.20

Sandblaster, 1 hr 56 56 overhead costs 1.60 37.00

Sandblaster, net salary I l h r /25:0.60 /2818.00

EUR EUR

CONCLUSION

Blast cleaning and also cleaning by wheel technique, is a most important tool in the field of corrosion protection. Together wi th a good coating, e.g. the life of structures are not limited by the condition defined by its surface, only the coating has to be kept in good order. A coating film may never get below a certain minimum and then i t is not a complicated job just t o repaint it. As soon as corrosion starts, there is hardly any other possibility then to get active wi th a sandblasting procedure. Today, industry can offer any kind of machinery necessary and an increasing number of specialized companies all over the world participate in this business. I t can be a small shop wi th a f ew employees up to a big company wi th various shop facilitieb and equipment for field work. Modern equipment is positLvely environmental friendly and economical. Sandblasting together wi th the correct coating helps t o extend life of products and to reduce the wastage of resources.

RECOMMENDED INFO SOURCES

1 . Newsletter: "Abrasive Blasting Cleaning News" (monthly) through: www.shotpeener.com/EI

2. Journal " IPCL - Journal of Protective Coatings & Linings" (quarterly) through: www.sspc.org