Standard cake formation time

No serious attempt has yet been made to standardize filtration tests and to categorize filtration behavior in generally accepted terms. A possibly useful measure of filterability, however, has been proposed by Purchas (1977 1981). The time in minutes required to form a cake 1 cm thick when the cell is operated with a differential of 500Torr (0.67 bar) is called the Standard Cake Formation Time (SCFT), tF. The pressure of 500Torr is selected because it is obtained easily with common laboratory equipment. The procedure suggested is to make a series of tests at several cake thicknesses and to obtain the SCFT by interpolation, rather than to interrupt a single test to make observations of cake thickness. A direct relation exists, of course, between the SCFT and resistivity a some examples are... 

THE STANDARD CAKE FORMATION TIME CONCEPT (SCFT)... 

Various rules of thumb exist for standard water filtration rates and cycle time before backwashing. Higher filtration rates may appear to be economically justified, however, when the filter loading is within conventional limits. In this example, we examine the issues involved for constant-rate filtration for a dual-media bed. Dual- and mixed-media beds result in increased production of water in a filter for two reasons. First, the larger grains (say charcoal approximately 1-mm size) as a top layer help reduce cake formation and deposition within the small (150-mm) top layer of the bed. Second, the head loss in the region of significant filtration is reduced. 

Fluid Loss. The ability of a cement slurry to prevent its dehydration through the formation of a filter cake after placement in the annulus is evaluated using a standard testing procedure (Appendix F of reference 6). The filtrate loss of around 300 cm3 of slurry is measured at 1000 psi differential pressure across a standard filtration medium (325 mesh, 45 /an, screen supported by 60 mesh, 250 /an, screen) in a 2.13-in. diameter cylindrical cell. The duration of the test is 30 min. If all of the filtrate passes through the screen in less than 30 min, the filtrate volume is extrapolated to 30 min, assuming it varies as the square root of time. 

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