Page 135 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

SOLID-LIQUID SEPARATION AND INTERCEPTORS

SOLID-LIQUID SEPARATION AND INTERCEPTORS 3.3

Some filter materials may leach substances into the fluid as it is processed, thereby affecting
its purity. Such substances, called extractables, can be minimized by preflushing. There is
a test for plastics conforming to USP class VI that is used to ensure that there will be no
adverse reaction of body fluids to extractables from filter housing or media materials.
The molecular weight of any compound is measured in daltons. Some filter media mea-
sure passage through the filter by molecular weight for separation of one compound from
another.

FILTER RATINGS

Filters and strainers are rated in several ways. Absolute and nominal ratings are based on the
size particle the filter is expected to capture and retain. Particles are measured in micrometers
(microns), which is 1/1,000,000 m (1/25,000 in) and abbreviated μm. This rating is a single
number called the micrometer (micron) rating. The micron rating of a filter or strainer can be
absolute or nominal. These ratings are often misunderstood and this is an area of confusion
in the filtration industry. Another method is called the beta rating, which is based on actual
particle counts of different particle sizes of both the influent and effluent liquid stream. The
beta rating is considered the most accurate rating measurement of a filter. Refer to Table 3.1
for the relationship between beta value and percent removal efficiency.
Efficiency is a measure of particle removal. It indicates what percent of particles above a
certain size will be retained. For absolute rated filters, the rated pore size indicates 100 percent
removal and is based on the log reduction values associated with bacterial retention testing.
Because the pore size of some filters is not well defined, it is not possible to assign those filters
an absolute rating. Instead they are given a nominal pore rating, which indicates the particle size
above which a predictable percentage of particulates will be retained. As an example, a nomi-
nally rated 1.0-μm depth filter will remove 90 to 95 percent of all particles 1.0 μm or larger. For
a surface filter of the same rating, the efficiency would be 99.99 percent.
An absolute micron rating indicates the smallest size particle that the filter will capture;
no particles of that diameter or larger will pass through the filter. The absolute rating generally
depends on sieving, since the capture of particles by adsorption is never ensured. Since
absolute ratings are generally unrealistic for most services, nominal ratings are the most
common method used to rate filters. One exception is in pharmaceutical service, where
absolute ratings are required to ensure that all particulates of a certain size are removed.
Nominal ratings allow the filter rating to consider particles retained by adsorption.
The nominal rating has no generally accepted definition in the industry, and there are no
industry standards. As defined by ANSI, the nominal rating is an arbitrary micrometer
value indicated by the filter manufacturer. Due to its lack of reproducibility, this rating

TABLE 3.1 Relationship Between
Beta Value and Removal Efficiency

Beta ratio Removal efficiency %
1 0
2 50
10 90
100 99
1,000 99.9
5,000 99.98
10,000 99.99

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