USE OF THE CAPTURE EQUATION TO EVALUATE THE PERFORMANCE OF MECHANICAL SEPARATION EQUIPMENT USED TO PROCESS DRILLING FLUIDS

When mechanical separation equipment is used to remove suspended solids from liquids, capture determinations provide a simple means of evaluating its performance. Capture is defined as the fraction of incoming suspended solids that report to the discarded stream.

Capture is usually expressed as a percentage (%C) and is easily calculated if the concentration, by weight, of suspended solids is known for the process streams entering and leaving the separator. If the samples of the three process streams that are collected are representative of steady-state operation of the separator, then calculated capture is a good measure of the effectiveness of the separation process. The capture equation is written:

%C =u×(f-0)÷f×(u-o)×100

where
f=weight percentage of suspended solids in the feed
u=weight percentage of suspended solids in the underflow
o=weight percentage of suspended solids in the feed

Capture analysis has not been widely used in the drilling-fluids solids control industry for two reasons: first, the need for representative samples precludes successful use of capture to evaluate shale shaker performance; and second, solids concentration in drilling fluids has traditionally been reported in terms of volume rather than weight; volumetrically rather than gravimetrically. Capture analysis is a useful tool recommended for the evaluation of the performance of solids control equipment used on drilling-fluid systems. It must be understood that the data generated apply only to the moment in time at which the samples are collected. Capture data can be extrapolated to predict the solids removed by the separator over longer time periods only if (1) the separator is operating under steady-state conditions with consistent and homogeneous feed, and (2) sufficient data are collected to establish average performance for the time period studied.

This discussion will be limited to the application of the capture equation to centrifuges and hydrocyclones. The process stream terminology and abbreviations used are illustrated in Figure 1 and defined in the derivation that follows.

The calculation of capture is based on gravimetric analysis of the three process streams common to solid/liquid separators (i.e., feed, underflow, and overflow). The procedure yields good results when homogeneous and representative samples of the process streams are collected, as is usually the case with hydrocyclone units and decanter centrifuges. The procedure cannot be applied to shale shakers because of the difficulty in obtaining representative samples of the three process streams and the inherent inconsistency of shale shaker feed conditions.

The capture calculation, which is derived in the following equation, is based on a material balance of solids entering and leaving the separator. Analysis of small samples of the feed, underflow, and overflow streams permits the calculation of the percentage of the feed solids that are separated—the percent capture.

The derivation of the capture equation follows:

where
F=feed mass flow rate
f=weight percentage of suspended solids in the feed
U=underflow mass flow rate
u=weight percentage of suspended solids in the underflow
O=overflow mass flow rate
o=weight percentage of suspended solids in the feed