Outlet size

Outlet Size Determination. The second consideration for proper design of a mass flow bin is the size of the outlet required to prevent arching and to achieve the required discharge rate. 

There are two mechanisms by which arching can occur particle interlocking and cohesive strength. The minimum outlet size required to prevent mechanical interlocking of particles is directly related to the size of the particles. The diameter of a circular outlet must be at least six to eight times the particle size, and the width of a slotted outlet must be at least three to four times the particle size. These ratios normally only govern the outlet size of mass flow hoppers if the particles are at least 0.6 cm or larger. 

B = outlet diameter or width, g = acceleration owing to gravity, m = 1 for circular opening and 0 for slotted opening, and 0 = hopper angle (measured from vertical) in degrees. A modification of this equation takes particle size into account. This modification is only important if the particle size is a significant fraction of the outlet size (8). 

Testers are available to measure the permeabihty and compressibiUty of powders and other bulk soflds (6). Erom such tests critical, steady-state flow rates through various outlet sizes in mass flow bins can be calculated. With this information, an engineer can determine the need for changing the outlet size and/or installing an air permeation system to increase the flow rate. Furthermore, the optimum number and location of air permeation levels can be deterrnined, along with an estimate of air flow requirements. 

Some researchers (e.g., Abramovich,Baturin,Rajaratnam,- and Nielsen and Moller ) consider x to be the distance from a point located at some distance Xq upstream from the diffuser face. Equations for the jet boundaries and velocity profile used in the centerline velocity derivation assume that the jet is supplied from the point source. Addition of the distance Xq to the distance from the outlet corrects for the influence of the outlet size on the jet geometry. For practical reasons some researchers neglect Xq. 

The orifice area of these devices (see illustrations) is at the outlet end of the SRV nozzle through which the discharging vapor/gases/liquids must pass. These values are identified in industry as (valve body inlet size in.) X (orifice letter) X (valve body outlet size, in.). For example, a valve would be designated 3E4. 

If gravity discharge is used, the minimum outlet size required to prevent arching is dependent upon the flow pattern that occurs. Regardless of the flow pattern, though, the outlet size is determined with the powder s flow function, which is measured by way of the cohesive strength tests described earlier. 

The outlet size required to overcome no-flow conditions depends highly on the flow pattern that develops. If mass flow develops, the minimum outlet diameter. Be, to overcome arching is (4) ... 

When considering scale effects, the implication of the foregoing analysis is that the hopper angle required for mass flow is principally dependent on the outlet size selected for the hopper under consideration. Note that the... 

Outlet size The nominal pipe size of the oudet of an SRV, unless otherwise designated. 

On a positive note, here the oudet tail pipe is the same size as the valve outlet size and is equipped with a drain system. ... 

Let s consider a 4P6 valve ( P orifice, 4 in. inlet size and 6 in. outlet size) ... 

Outlet piping smaller than outlet size of the valve. This will cause violent turbulences when valve opens and the valve will start to chatter Make sure outlet piping is at minimum the same size as the valve outlet size. 

If the hopper-flow factor lies above the powder-flow function, cohesive arching is not possible with the material in that hopper. If the hopper factor lies below the powder-flow function, arching of material in the hopper is possible even for large-outlet sizes. 

One of the primary flow problems that can develop is a no-flow obstruction due to the formation of a cohesive arch or rathole. The required outlet size to prevent a stable cohesive arch or rathole from forming is determined from the results of a cohesive strength test by applying the flow-no-flow criterion. In order to apply the flow-noflow criterion we need to determine ... 

If the bin discharges in funnel flow, the bin outlet diameter should be sized to be larger than the critical rathole diameter (Of) to prevent a stable rathole from forming over the outlet. For a funnel flow bin with a circular outlet, sizing the outlet diameter to exceed the Of will also ensure that a stable arch will not form (.since a rathole is inherently stronger than an arch). The Of value is calculated in Equation (2), and additional details of the calculation are provided in Ref. 1. 

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