Ratholing

In order to avoid the formation of a stable rathole, it is necessary that the size of the flow channel exceed the critical rathole diameter, DF, which is calculated as follows  

The two key parameters in this equation, flow function and angle of internal friction, can be strongly affected by the bulk solid s temperature, time of storage at rest, moisture content and particle size distribution. 

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Until downhole motors became available a whipstock (Fig. 3.15) which is a slightly asymmetric steel joint, was inserted in the drill string. The assembly is oriented downhole and a rathole is drilled which is then enlarged to full bore hole size. The technique is still used in wells where hole conditions e.g. high temperatures, are unsuitable for downhole motors (see below). 

Bulk sohds do not always discharge rehably. Unrehable flow, which can occur with some frequency, can be expensive in terms of inefficient processes, wasted product, and operational comphcations. Predictable flow is often impeded by the formation of an arch or rathole, or fine powders may flood uncontroUably. 

Flooding. When a stable rathole forms in a bin and fresh material is added, or when material falls into the channel from above, a flood can occur if the bulk sohd is a fine powder. As the powder falls into the channel, it becomes entrained in the air in the channel and becomes fluidized (aerated). When this fluidized material reaches the outlet, it is likely to flood from the bin, because most feeders are designed to handle sohds, not fluids (see Eluidization). Fimited Discharge Kate. Bulk sohds, especially fine powders, sometimes flow at a rate lower than required for a process. This flow rate limitation is often a function of the material s air or gas permeabihty. Simply increasing the speed of the feeder does not solve the problem. There is a limit to how fast material... 

A funnel flow bin typically exhibits a first-in/last-out type of flow sequence. If the material has sufficient cohesive strength, it may bridge over the outlet. Also, if the narrow flow channel empties out, a stable rathole may form. This stable rathole decreases the bin s five or usable capacity, causes materials to cake or spoil, and/or enhances segregation problems. Collapsing ratholes may impose loads on the stmcture that it was not designed to withstand. 

Expanded Flow. Expanded dow uses the best aspects of fuimel dow and mass dow by attaching a mass dow hopper section below one that exhibits fuimel dow. The dow pattern expands sufficiendy at the top of the mass dow hopper to prevent a stable rathole from forming in the funnel dow hopper above it. In this way, the dow channel is expanded, material dow is uniform, and the bin height is limited. 

In order to characterize this bonding tendency, the flow function of a material must be deterrnined. Data on flow function can be generated in a testing laboratory by measuring the cohesive strength of the bulk soHd as a function of consoHdation pressure appHed to it. Such strength is directly related to the abihty of the material to form arches and ratholes in bins and hoppers. 

Sizing the outlet of a fuimel flow bin involves consideration of both arching and ratholing. Minimum dimensions to overcome both can be calculated from the material s flow function. 

The area of influence of a vibrating discharger is limited to a cylinder, the diameter of which is roughly equal to the top diameter of the discharger. Hence, if a vibrating discharger is mounted onto a conical hopper section, flow is confined predominantly to a central flow channel located directly above the discharger. This is tme unless the slope and smoothness of the static cone meet requirements for mass flow, or the diameter of the flow channel exceeds the critical rathole diameter for the material. 

Material-Flow Cbaracteristics Two important definitions of the flow characteristics of a storage vessel are mass flow, which means that all the material in the vessel moves whenever any is withdrawn (Fig. 21-17), and funnel flow, which occurs when only a portion of the material flows (usually in a channel or rathole in the center of the system) when any material is withdrawn (Fig. 21-18). Some typical mass-flow designs are shown in Fig. 21-19. 

FIG. 21-18 Funnel-flow hin. The material segregates and develops ratholes. Coutiesy of Chemical Engineering.)... 

In the event of a dryhole, the reserve pit water usage should be maximized to prepare the mud spacers between plugs. Water in excess of this may be pumped into the hole, including solids. All USDW s must be protected in this event. Once the hole has been properly plugged and the drilling rig removed, the mousehole and rathole should be backed filled immediately to preclude any accidents. Trash is removed from the location and adjacent areas and is hauled to permitted facilities. 

Two flow patterns can develop in a bin or hopper funnel flow and mass flow. In funnel flow (Fig. 1), an active flow channel forms above the outlet, which is surrounded by stagnant material. This is a flrst-in, last-out flow sequence. As the level of powder decreases, stagnant powder may slough into the flow channel if the material is sufficiently free flowing. If the powder is cohesive, a stable rathole may remain. 

Cohesive strength The consolidation of powder may result in arching and ratholing within transfer equipment. These behaviors are related to the cohesive strength of the powder, which is a function of the applied consolidation pressure. Cohesive strength of a powder can be measured accurately by a direct shear method. The most universally accepted method is described in ASTM standard D 6128-00 (3). 

By measuring the force required to shear a bed of powder that is under various vertical loads, a relationship describing the cohesive strength of the powder as a function of the consolidating pressure can be developed (4). This relationship, known as a flow function, FF, can be analyzed to determine the minimum outlet diameters for bins to prevent arching and ratholing. 

If funnel flow develops instead of mass flow, the minimum outlet diameter is given by the tendency for a stable rathole to occur, because this diameter is usually larger than that required to overcome arching. In this case, the minimum outlet diameter is... 

Figure 9 Plot showing derived function used in calculating ratholing potential in funnel flow bins.

A number of problems can develop as powder flows through equipment such as bins, chutes, and press hoppers. If the powder has cohesive strength, an arch or rathole may form. An arch is a stable obstruction that usually forms within the... 

Erratic Flow. A combination of the two no-flow conditions can lead to erratic flow. If flow has been initiated but a stable rathole develops, then when the rathole collapses using vibration, the material may arch as it impacts the outlet. Flow may be restarted by vibration and maintained for a short time until the rathole forms again. Erratic flow can be a serious problem when handling bulk solids owing to fluctuating flow rates and bulk densities, and unreliable discharge. It can also jeopardize the structural integrity of the bin when a rathole collapses. 

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