Depending on your conveying systems; high pressure low velocity conveying of fly ash is possible. Since not all fly ash is same “It will depend mostly on the conveying properties your ash”. Get your ash tested and you will know for sure. ■

The minimum conveying air velocity required for fly ash depends upon the grade of ash that you have to convey and the poressure gradient available for conveying the ash (pressure drop available divided by conveying distance).

Ash that naturally drops out of the gas stream into economiser hoppers, air pre-heater hoppers, duct hoppers, etc, is generally 'coarse' ash and has no natural dense phase conveying capability as it lacks the required bulk property of air retention capability. For this grade of ash you will always require a minimum conveying air velocity of about 12 m/s (2400 ft/min) if you use a conventional pneumatic conveying system.

For ash that has to be physically removed from the gas stream in bag filters and electrostatic precipitators, which is 'fine' and has good air retention properties then 3 m/s and below is quite possible. This can be conveyed in dense phase in sliding bed flow (fluidised dense phase) in a conventional pneumatic conveying system.

High concentrations of fly ash, however, can only be conveyed if a high pressure gradient is available. Fly ash can be conveyed at a solids loading ratio of 100:! (fly ash to air by mass) if the pressure gradient is about 20 mbar/m (10 lbf/in2/100 ft) with distance based on equivalent length to take account of vertical lift and bends in the pipeline. Dense phase conveying with this grade of ash, therefore, is quite possible in vacuum conveying if the conveying distance is 'short' and by the same token dense phase conveying will not be possible for very long distance conveying unless are prepared to use a very high pressure.

I would not, however, recommend using as low a velocity of 3 m/s, particularly with a high pressure and a stepped pipeline, because you will have great difficulty in purging the pipeline clear of material. ■

The term solids loading ratio is in terms of mass for both consituents (material and air) and so under steady flow conditions this will not change along the length of the pipeline. Concentration in terms of mass of material per unit volume of air (lb/ft3 or kg/m3) will change since air is compressible and consequently most people use solids loading ratio for convenience.

In a stepped pipeline the bore is increased to a larger diameter along its length in order to take up this expansion and so provide a more uniform conveying air velocity. This, in turn, will reduce pressure drop and allow more material to be conveyed. It is the very last section of stepped pipeline that you will have problems in purging if you employ a very low conveying line inlet air velocity (such as 3 m/s) because all the expansion has been taken up by then. Do a calculation to check this out. ■

Low velocity systems very common for flyash handling and normally they are tandem blow tanks and the system is designed so that there is constant conveying without blow tank purging in the conveying line. Blow tank outlet valve is closed at conveying pressure and air purging is done through the vent valve which is considered a consumable item.

The other breed of low velocity conveying are the conveying line air injection system they come in wide names but principle is the same. These system can start with pipe full and don’t need purging either!

Fly ash conveying is a tricky business if you don’t know what you are doing you will get your figures burnt !! Get some specialist help. ■

Originally Posted by Dr David Mills

The minimum conveying air velocity required for fly ash depends upon the grade of ash that you have to convey and the poressure gradient available for conveying the ash (pressure drop available divided by conveying distance).

Ash that naturally drops out of the gas stream into economiser hoppers, air pre-heater hoppers, duct hoppers, etc, is generally 'coarse' ash and has no natural dense phase conveying capability as it lacks the required bulk property of air retention capability. For this grade of ash you will always require a minimum conveying air velocity of about 12 m/s (2400 ft/min) if you use a conventional pneumatic conveying system.

For ash that has to be physically removed from the gas stream in bag filters and electrostatic precipitators, which is 'fine' and has good air retention properties then 3 m/s and below is quite possible. This can be conveyed in dense phase in sliding bed flow (fluidised dense phase) in a conventional pneumatic conveying system.

High concentrations of fly ash, however, can only be conveyed if a high pressure gradient is available. Fly ash can be conveyed at a solids loading ratio of 100:! (fly ash to air by mass) if the pressure gradient is about 20 mbar/m (10 lbf/in2/100 ft) with distance based on equivalent length to take account of vertical lift and bends in the pipeline. Dense phase conveying with this grade of ash, therefore, is quite possible in vacuum conveying if the conveying distance is 'short' and by the same token dense phase conveying will not be possible for very long distance conveying unless are prepared to use a very high pressure.

I would not, however, recommend using as low a velocity of 3 m/s, particularly with a high pressure and a stepped pipeline, because you will have great difficulty in purging the pipeline clear of material.

KA Dr David mills

Dear sir

What is the reason , vaccum conveying system is not feasible for long conveying system ■

Originally Posted by Dr David Mills

The term solids loading ratio is in terms of mass for both consituents (material and air) and so under steady flow conditions this will not change along the length of the pipeline. Concentration in terms of mass of material per unit volume of air (lb/ft3 or kg/m3) will change since air is compressible and consequently most people use solids loading ratio for convenience.

In a stepped pipeline the bore is increased to a larger diameter along its length in order to take up this expansion and so provide a more uniform conveying air velocity. This, in turn, will reduce pressure drop and allow more material to be conveyed. It is the very last section of stepped pipeline that you will have problems in purging if you employ a very low conveying line inlet air velocity (such as 3 m/s) because all the expansion has been taken up by then. Do a calculation to check this out.

Dear sir

Great to have your perspective in this context

Emphatically you stated

would you elaborate in what ways the last seection of pipe in stepped pipeline will have a problem of purgin , in case of loaw velocity ■