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Recommendations for proper flow from rectangular or pyramid shaped hoppers, in view of the corners of the hopper that are less steep. Rectangular hoppers are widely used to store bulk solids. Although convenient to fabricate they have process drawbacks in that large flat sides are prone to deflection, so often require stiffening, and the converging faces form gully angles that offer poor flow and self-clearing properties. Some small compensation is given by the rectangular outlets being somewhat easier to fit with slide valves. ​​ Gully angle The gully angle of a symmetrical, pyramid shaped hopper is roughly 10 degrees less steep than sides and is prone to retention of product unless it is radiused. Flow pattern For mass flow design purposes, the inclination of the gullies should be considered as a cone. For funnel flow hoppers the gully angle should exceed the wall friction angle by at least 10 degrees. Some product retention in the corners is almost inevitable unless the corner is radiused or significantly increased in inclination. Mass flow in two steps Mass flow is much more practical