Page 137 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

SOLID-LIQUID SEPARATION AND INTERCEPTORS

SOLID-LIQUID SEPARATION AND INTERCEPTORS 3.5

It is not necessary to determine the exact pressure of a given filter to prove its integrity.
If the pressure exceeds the minimum point determined by the manufacturer of the filter,
its integrity is assured. The bubble point test is also used to test the integrity of the filter
cartridge.

Diffusion Test
In a high-volume system where a large volume of water must be displaced before bubbles
can be detected, a diffusion test should be performed instead of the bubble point test. This
test is based on the fact that in a wetted membrane filter under pressure, air flows through
the water-filled pores of the filter at a differential pressure below the bubble point pressure
by a diffusion process following Fick’s law. In small filters, the flow of air is very slow. But
in a large filter it is significant and can be measured to perform a sensitive filter integrity
test. In a wetted filter, a constant air pressure is applied at approximately 80 percent of the
bubble point pressure established for that particular filter.
There are two widely used variations of the diffusion test. The first is the forward flow
test, which relies on direct measurement of the diffusive gas flow rate. This flow rate is
measured either by instruments placed in the gas flow upstream of the filter or by calculat-
ing the volume of airflow according to the rate of flow of displaced water downstream of the
filter. The second method, called the pressure decay method, calculates the loss of diffusion
gas pressure from a known volume of gas over a period of time.

Water Breakthrough Test
This is similar to the bubble test, except that water is used instead of air. Water pressure is
increased on the upstream side of the filter, and the pressure that results in a steady stream of
water downstream of the filter is recorded. The breakthrough pressure must be correlated to
empirical data on contaminant retention from the manufacturer in order to be a valid test.

Water Intrusion Test
Also called the water pressure integrity test, this is often used for hydrophobic filters (which
resist wetting by water). This test requires that the filter be wetted by an alcohol/water mix-
ture. Water pressure is applied upstream of the filter and the pressure decay is measured. Care
must be taken to test with water from the same source because of variations in surface tension.
When used as a vent filter, the membrane must be dried before being placed back in service.

STRAINERS

GENERAL

A strainer is a closed vessel with a cleanable screen or mesh generally used to remove and
retain foreign particles larger than 45 μm (325 mesh) from flowing fluids while allowing
the remainder of the fluid to pass through. If a particle is visible to the naked eye, a strainer
should be chosen to remove it from the liquid stream. If the device retains particles finer
than 45 μm, it is generally considered a filter. The relationship between mesh and opening
size is given in Table 3.2. The difference between filters and strainers is one of semantics;
a strainer could be considered a coarse filter.

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