1) get representative sample

2) do shear cell tests

3) use results in the methods of Jenike et. al.

That will enable you to answer your questions. ■

If the material already exists then do as advised.

If it flows out of existing containers OK then you simply copy them. If not then you should know what you have to do.

If it is a new application then you simply select a density from the numerous material property tables available, do some sums and hope it works along the lines previously mentioned.

Ifs apart: if the material cannot be tested and guaranteed then all the theory is out of the window and we rely on expereience which is not certifiable. In such cases you should use the steepest angles which will allow the required storage and extraction within the available height. That is the convenient due dilligence, ethical and legal escape provided the client has been informed of the risk.

Look at it this way, property unknown; steepest possible valley angle, largest practical outlet, suitable liners and stil poor flow, then you've done a good job and the punter will have to dig in and do some retrofitting. This business carries lots of flow promoting operations who correctly rely on the unkown always appearing. Despite acres of printed theory they are still in business.

PS. Dont forget to cater for air cannons etc in the utilties consumption list. That way you are doubly covered.■

Re - Bulk-online query re PFA density and gulley angle.

The density of pulverized fuel boiler ash is a variable property that depends on the conditions of its storage. The material is easily aerated and, because the constituent particles are in the region of 10 microns, it is slow to de-aerate, particularly if the storage bed is deep and at an elevated temperature where air has a higher viscosity and is slow to escape from the narrow intensities of the void space. Account must therefore be taken of the size of the storage unit and the filling conditions to establish the effect of consolidating loads and time consolidation. As a rough guide however, the range of density condition for PFA varies from around 1.2 g/cc when aerated to 1.7 g/cc in a fully settled and compacted condition of storage; which should be adequate for a first approximation of the design requirements.

As the product is inert, there is no fundamental requirement for the totality of a storage hopper to be of mass flow design. However, well-settled fly ash has very poor flow characteristics, so serious consideration should be given to the size and form of the outlet region and the way that reliable discharge will be guaranteed under all operating conditions and a local mass flow construction of an 'expanded flow' design offers clear benefits. Self clearing of the contents then depends on creating a flow channel larger than the ‘critical rathole dimension’ and material moving from the walls in a funnel fl ow construction or sliding down the walls in mass flow. The first step of designing the hopper is to establish the form of flow regime to be adopted and how this is to be secured. The demands of pure gravity flow for PFA in a simple form of pyramid shape hopper is likely to raise some awkward problems of orifice size and wall angle for mass flow, or ways of otherwise exceeding the ‘critical rathole diameter’.

These measurements may be determined by conventional powder testing procedure, but could turn out to be impractiable. The most common ‘brute force’ method to stimulate the flow of PFA is to fluidise the outlet region, but this can lead to awkward control problems of ‘flushing’ and dust generation. A more appropriate way is to supply continuous, limited volume, direct dry air injection to stabalise the density from its initial loose fill condition to a consistent flow condition, without seriously affecting its settlement rate or fluidising the bulk. Air cannons risk uncontrolled discharge.

A pyramid shape hopper may be convenient for fabrication and nesting of multiple units, but is jus about the worst form of geometry for flow. A hopper of this form of practical proportions will exhibit strong restraint to flow in the gullies, in fact square cornered gullies at any inclination are unlikely to completely clear because of the 'corner effect' on cohesive powders. This rarely matters, so long as an excessive amount is not left to unacceptably reduce the retrievable inventory of the hopper. The valley angles are determined by the slope of the side walls, which must at least self-clear by ensuring that slip of the material is generated against the wall friction resistance, as measured by testing. A useful way to counter the adverse combination of a square corner and poor gully angle is to fit taper filler plates that are narrow at the bottom and wider at the top of the gully. These ‘open’ the corners to obtuse angles, increase the slope of the corners and stiffen the side walls by reducing the effective span of the flat plate, to give both improved flow behaviour and greater structural strength.

It is important to recognise that solids flow is not a simple phenomenon and there is much more to hopper design than selecting gulley angles and welding sheet metal to hold a given volume. Both structural and operational integrity are vital for successful operation and the cost of failure can far exceed the capital cost of the equipment. A specialist should be consulted if there is any doubt of the competence to prepare a properly calculated design. ■