Page 150 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

SOLID-LIQUID SEPARATION AND INTERCEPTORS

3.18 CHAPTER THREE

system uses a double open-ended cartridge with a top compression spring on the cartridge.
This provides a knife-edge seal on a seat at the top of the cartridge. Single open-ended
cartridges, most often used in single cartridge housings, use a piston type O-ring seal con-
tained within the cartridge itself. The material of the seal must be compatible with the type
of fluid being processed, the fluid temperature, and the proposed use.

Filter Selection Criteria
There is a wide variety of filter options available to accomplish separation, and many of
these options have overlapping effectiveness. Choosing the appropriate method will often
involve tradeoffs between performance and value. The following factors should be taken
into consideration when selecting a filter based on the intended service and the quality of
the feedwater.

1. The first step to decide the suitability of any filter is to obtain an accurate analysis of the
feedwater and to determine the size and quantity of the particulates to be removed.
2. It should be decided whether absolute or nominal filter ratings will be used. For filter-
ing service, most filters are selected by using nominal ratings, with the intended use of
the water dictating the various degrees of removal.
3. If the particles are deformable, the wound depth filter is more efficient in removing
them. They tend to be extruded through a thin, solid, pleated filter. For hard particles,
either wound or pleated can be used.
The selection of a pleated or wound filter should be based on the type and quantity
of solids to be removed. The pleated filter has about 16 times the effective filtration
area of a similar wound filter and so is much more effective in removing smaller par-
ticulates captured on the surface. For larger sized particulates or for a large amount
of dirt to be held in the filter, the depth-type filter is more effective. Pleated filters are
usually more costly than wound filters.
Depth filters have high dirt-holding capacity and are used most often as prefilters
for clarification of water prior to treatment by water purification equipment. They are
less costly than other types of filters.
4. The removal efficiency of the filter must be known. This value is typically specified by
the end user and often based on past experience. Since the micrometer rating alone is
not a fair representation of filter efficiency, the degree of removal must also be speci-
fied. A rating of 90 percent efficiency at 5 μm means that 90 percent of the particles
of that size will be retained on the filter medium.
5. The flow rate must be determined. This is also called the flow density, which relates
flow rate to unit of filter area. As the flow rate increases, the contaminant capacity
and service life of the filter decrease. Manufacturers have established flow rates for
filters. Housings containing multiple cartridges can be selected to lower the flow rate
of individual cartridges.
6. The pressure drop across the filter is another consideration. The smaller the pore diam-
eter, the higher the pressure drop across the filter. If the ratio of the initial clean pres-
sure drop through the filter to the total available pressure is high, unacceptably low
flow will quickly result even when the particulate-holding capacity is not fully real-
ized. Pressure requirements across the filter from new to plugged condition must also
be considered, with the filter selected based on the highest allowable pressure drop.
Accepted practice is to have a maximum of 80 percent filter plugging. If the pressure
drop is too high or if the amount of plugging is not acceptable, the size of the filter
can be increased or multiple filters used. Housings that hold multiple filters are used
to reduce the pressure drop when available water pressure is low.

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