To determine the coincidence between the filter fabric and impurities removed from the treated water, at power plants’ condensate it is necessary to compare the geometric parameters of pore channels and removed particles. Granulometric size distribution of mechanical impurities is investigated. The methodology of filter fabric selectivity calculation using results of fabric pores’ and suspended impurities’ distribution experimental study is exposed. The authors built an integral curve of particles’ size cumulative distribution, with a graphical differentiation of integral dependency serving to calculate the average-weighted size of impurities. The value of selectivity parameter representing the probability of simultaneous occurrence for events: impurities’ “particles size below…” and filter fabrics’ “pore size above…” is obtained. This will allow an evaluation of the feasibility and effectiveness of membrane-type filter fabrics use at standard operation of a powerful unit’ water treatment plants.