A. Talukdar

History of Blast Cleaning dates back to 1870 when Mr.B.C.Tilghmaan of Philadelphia obtained the first patent for sand blasting. In this process a jet of sand propelled at high velocity by a steam or air blast, is employed as a tool for cutting stone and other materials, and at lower velocity of jet it is employed for grinding and ornamenting the surface of the glass. In the same year Mr.B.CTilghman obtained the patent for abrasive throwing wheels but it took almost 60 more years for the first operational machine to be produced.

Blast cleaning is mainly of two types depending on the method of propelling of abrasives:

  • Airless Blast Cleaning uses motor driven bladed wheel rotating at high speed. The abrasive is propelled by this bladed wheel, which employs the combination of radial and tangential forces to impart the necessary velocity to the abrasive. The position of the wheel from which the shot is projected is controlled to concentrate the blast in the desired direction. Among the advantages of the wheel method of propulsion are easy control of shot velocity, high production capacity, and freedom from the moisture problem encountered with compressed air.
  • Air blasting systems introduces the shot either by gravity or by direct pressure, into a stream of compressed air directed through a nozzle onto the work. Apart from being more economical for limited production quantities, develop higher intensities with small abrasive sizes, permits the blast cleaning of deep holes and cavities and has lower initial cost when a source of compressed air is also available.

Over the years with the advancement in technology both the types have increased in area of operation. Except the propelling of a abrasives which is different in both types the following things are in common:

  1. Shot cycling arrangements
  2. Work handling system
  3. Shot separation system
  4. Dust & fine Suction system

As with many other processes the two forms of blast cleaning are not competitive, but are complimentary and the comparison may be summed up as follows:


Air-Blast advantages

Centrifugal – blast advantages

1.Extremly flexible

1.High production rates

2.Simple to operate

2.Low cleaning cost

3.Unlimited scope

3.Compact plants

4.May be used in & out of works

4.Self sufficient (no auxiliary plant required)

5.Cost of simple plants relatively low

5.Compressed air eliminated

6.No moisture problem

7.High efficiency

Air-Blast disadvantages

Centrifugal-blast disadvantages

1.costly compressor air installation

1.High initial cost

2.Hold up due to choking or damp abrasive

2.Lack of flexibility

3.Low efficiency

3.High maintenance


  • Distortion of material
  • Unable to treat hidden area-pipes, tubes etc.
  • Immobile

High maintenance (3) is a disadvantage with centrifugal blast machines, because the entire effectiveness of the plant depends upon keeping the machine to the requisite standard of efficiency.

We will discuss here about the fundamentals of Blast Cleaning of a Centrifugal machine.

To increase quality and productivity at a competitive cost represents a challenge for today’s successful blast cleaning operation. To meet this challenge the blast cleaning function necessitates a return to the basics of the process.

The blast cleaning operation should be aimed at producing world-class quality of finish and achieving optimum productivity. To attain these goals it is necessary to return to “BACK TO THE BASICS” theme being applied in the industries everywhere because application of the basics in blast cleaning as well as other areas, directly affects bottom-line profitability.

To understand the basics we have to go back 131 years when the first patent for sand blasting was registered by Mr.B.C.Tilghman and he described the process of sand blasting as under (the principals are the same today except that steel shot and grit are substituted for sand):

       An impact cleaning operation that is neither cutting, grinding nor abrading. It is essentially a pounding, battering or bombarding of the work surfaces by successive impact of the flying abrasives.

Impact is the key to blast cleaning and in Centrifugal blast cleaning the blasting wheel is the one, which fires abrasives. Centrifugal blast cleaning machines are capable of high outputs, and whether this is achieved with the necessary degree of efficiency will depend upon the care and control of the machine. This is the sum of number of factors all equally important, and which must not be overlooked if the best result is to be achieved. The points to be kept under constant supervision:

  • The abrasive feed: It is necessary to keep the abrasive hopper 2/3rd  full at all time .It takes about 3 minute for the abrasive to return to the abrasive hopper from the times it leaves the abrasive hopper. So care must be taken to ensure that the hopper is always full. Low level of abrasive in the hopper leads to dry running of the wheel, which ultimately leads to increase cost of production. It is however wise to use shot replinisher in conjunction with shot level indicators.
  • Control cage: This is a vital component for it ensures that the abrasive is thrown in the correct direction. The adjustment that is available is very fine and provides precise settings.
  • The Abrasive: The choice of abrasive for a specific purpose must be carried out with great care. Once the correct abrasive is found the importance of maintaining the “ mix” cannot be over emphasized. It is equally important to ensure that the quality and the grading are kept to the uniform standard.


Fig. Format of Sieve Analysis report

  • The Separator: The function of this element is to remove contaminants and debris from the abrasive to enable the correct mix in the machine to remain stable. This should be inspected regularly since a malfunctioning separator can seriously increase abrasive consumption. Good abrasive should not be found in the separator refuse pipe nor in the Dust Collector.
  • The Ammeter: The amperage reading of the motor indicates the volume of abrasive thrown. The correct amperage should be maintained and in case is there is drop in the ampere reading than specified, then it indicates there is low volume of abrasives and the components have to be reshot blasted or the cycle time has to be increased. This is inefficient and in consequence expensive.
  • Wheel Efficiency: The efficiency of a wheel is measured by dividing the wheel amperage actually used for throwing abrasive by the total energy available for that purpose.

                                              Full load current – No load Current

           Wheel efficiency % = -------------------------------------------------- X 100

                                               Rated Current – No load current

No load current is the power required to absorb friction, particularly through a Vee belt drive, The type of bearing housing and also the type & weight of bare wheel. It is advisable to run the wheel at  +90% efficiency.



Machine No……………………………..                        Date…………….






















  • It is necessary to accept the fact that spillage of abrasive around machines of many types is inevitable. Overhead monorail conveyor machine, batch type tumblast machine, Roll conveyor machine Table type machine are prone to this. The abrasives are caught in the pockets and recesses of the components and are spilled during unloading. Abrasive spilled through” carry out” should be returned to the machine at regular intervals.

The abrasive may escape from the machines due to holes in the fabric of the machine, through gaps or by means of torn vestibule curtains. These holes are not only risk to the personnel’s but can also cause damage to other machineries. So care should be taken for immediate corrective rather than delaying it.

  • Wheel hour Meter: This indicates the blasting time of the machine. If the ampere meter has been constantly maintained at the desired level, then the hour meter will indicate the number of hours of efficient running. In addition to this it gives the life of wheel parts and an efficient tool for spare part planning.
  • Full process Control: The blast cleaning and finishing method has grown to such dimensions that is integral part of many important processes like blackening, coatings for protection, paintings etc. Shot peening also is a process where strict process control is essential. In some instances the poor or incorrectly carried out blasting has resulted in very expensive losses.

In many instances users have complained that there hopper if full, full amperage is maintained but the finish even after extended blasting time is not as desired. The possible reason could be:

  1. Improper operating mix.
  2. Separator not functioning properly
  3. May be starting size shot does not meet the specs.

            To avoid this situation the obvious choice would be take all possible steps 

            to eliminate the possible faults. In order to look at the problem and to

            place it in perspective; it is necessary to go back to the point of the first

            main essential; that of the abrasive. The consistency of size, grading,

            particle hardness and shape are all, extremely important when

 considering the preparation of surfaces for subsequent  treatment.



Figure shows a suggested daily report sheet.

The all-important question is how many times do we inspect the incoming

          abrasives as per laid down specifications. The answer would be we go as

          per the suppliers test certificate. The problem starts from here.

Similarity is there any routine for inspecting the hot spot again the answer would be whenever it is replaced, the hot spot is checked.

The above clearly signifies that if economy and efficiency are to be achieved then there has to be process control.

  • Routine Inspection & Maintenance: The life of centrifugal blast machine will be lengthened or shortened by the manner in which it is used. The machines are self-destructive and in consequence require constant attention. this we will discuss in detail separately


Care & Caution:

Blast cleaning operation has many dangerous areas, which everybody is familiar with but have become obvious and at times become highly risky.

  • Flying abrasive-It is very obvious that perfectly abrasive tight continuous machine is very difficult to achieve though batch type machine are abrasive tight to a very large extent. This so because abrasive particles are so relatively small and velocities are so high that only tiny outlet is necessary for the escape of blast media. The principal loss of abrasive media is through the hole in the fabric of the machine because of wear and tear. All operators must be asked to wear safety glasses and corrective action taken for the wear and tear on the fabric of the machine.
  • Round shot underfoot- This is extremely hazardous and often-insufficient care is given to this matter. It is advisable to use steel grading around the base of the machine, which will not only reduce the danger of personnel slipping but will also act as collection areas for the abrasives. Instead of brooming the area it is advisable to constantly use magnetic sweepers.
  • Dirt & Dust- These are bugbears of any machinery and blast cleaning is in no way in a different category. You should be ware that blast cleaning is a process where a high volume of metallic dust is generated, which is taken care of by the separator and ventilation system. Any drop in efficiency of this will lead to dust leaking out of the system and find its way into the electrical switchgears.
  • Dust Collectors-Mostly dry type dust collectors are used and the biggest problem is the disposal of these dusts. Secondly since most of the dust collectors are in the open care should be taken that the dust not get scattered with the wind. Cases have been reported that welding work had been carried out at a considerable distance but after 2 –3 hours smoke was found to be emitting around the dust collector area. One note of caution no welding work should be carried out in the ducting or dust collector. For any welding work to be carried out in the dust collector the bags to be removed, the inside thoroughly cleaned and then welding work to be carried out.
  • Abrasive Elevators-For blast cleaning equipment, because of the arduous duty, should be of heavy-duty type. Many a times the elevator belt gets jammed because of the choking of the abrasive in the boot section. Elevators should be provided with proper inspection doors in the bottom for removal of abrasive. Many a times during the removal of belts accidents have occurred because of the reversing of the belts. It is advisable to equip the machine with anti reversing arrangement fitted on the head section of the elevator.

Fire hazards-As explained earlier regarding dust collector, also care must be taken to ensure the dust collector fan is not in the running mode when welding work is carried out in the machine fabric. More than one fire has been caused by permitting work pieces to be conveyed into a machine covered with oil. This in itself is bad practice for the abrasive may become saturated to a point when oil is thrown back upon the work pieces. The condition of the abrasive will destroy the separation of the scale and rust from the abrasive thereby dropping the efficiency of the plant. More seriously the oil may be drawn into the dust collector thereby impairing the efficiency of the dust collector and there are innumerable instances when the dust collector has caught fire and the dust collector ultimately is beyond repair. This has been found all the more in Heat treatment shops. Hence it is mandatory that oil free components to be taken up for blast cleaning.