Page 136 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

SOLID-LIQUID SEPARATION AND INTERCEPTORS

3.4 CHAPTER THREE

is depreciated. The ambiguity of this rating method makes it difficult to achieve reliable
and consistent results. Many manufacturers use different methods to rate their filters, for
example, expressing the results gravimetrically, which does not represent the particle size
and number in the effluent stream. Some have specific test conditions that do not represent
the actual conditions for which the filters will be used. These test conditions may use fine or
coarse particles such as AC test dust, latex beads, carbon fines, or bacteria. The nominal rating
can be used as a guideline, provided that the micron rating includes the percent removal
efficiency rating of that micron size.
Void volume of preformed fibrous media is the ratio of pore area to the fiber diameter
of the filter media. If all other factors are equal, the medium with the greatest void volume
will have the longest life and lowest initial clean pressure drop per unit thickness. Factors
such as strength, compressibility of the fiber material under pressure (which reduces void
volume), cost, and compatibility of the media with the water contaminants being removed
should all be considered when selecting a filter for a specific application.

MEMBRANE FILTER TESTING

An important feature of a membrane filtration system is its ability to be tested before and
after filtration runs. Testing can detect a damaged membrane, ineffective seals, or a system
leak that may result in passage of contaminants that the filter is designed to trap. These tests
are commonly called integrity tests. Testing before and after a run will ensure that the
entire system is intact, thereby validating the process. Prior to testing, cleaning to remove
large-scale contamination (and sterilizing the filter and apparatus if necessary to ensure
elimination of microbial contamination) is required.
The type of test selected is dependent on the specific filter chosen. However, if the previous
history of a specific filter is not available, the only accurate method of testing the filter is to
place it in service and run an on-site fouling and compatibility test.

Air Permeability Test
An air permeability test is normally used to test wound cartridges. It is a simple, nonde-
structive test that correlates well with filter performance and it is considered more revealing
than micron rating.

Bubble Point Test
Membrane filters have discrete, uniform passages from one side of the membrane to the
other which, in effect, are fine uniform capillaries. The bubble point test is based on the fact
that a liquid is held in these capillary-like structures by surface tension and the minimum
pressure required to force this liquid out of the capillary space is a measure of the capillary
diameter. The pressure required is inversely proportional to the largest pore size. After the
filter is wetted, air pressure upstream of the filter is increased.
There are two widely used variations of the bubble point test. The first is the visual
test. For this variation, the downstream side is watched for the appearance of bubbles,
which indicate that the air is passing through the capillaries. The pressure that produces
a steady, continuous stream of bubbles is the bubble point pressure. The second varia-
tion is the monitored method, where a pressure drop will occur as the gas begins to flow
through the filter.

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