AMIAD - AUSTRALIA & ASIA PACIFIC NEWS - VOLUME 9 - APRIL 2017 MARCH 2014 Vol.3 | Page 8

The use of a pressure sustaining or downstream
valve in this situation will restrict flow to the field
and maintain flow and pressure to the filter
during flush.
Another, very important, consideration is the flow
and pressure in the system during start up as this
is also the most likely time a filter will flush.
In many cases, during start up the pump will
increase flow to fill the system and therefore
reduce pressure. This increased flow will put
more load on the filter system and in many cases
create a pressure differential across the filter
which in turn will activate a flush cycle. The
conditions created in this situation will mean that
not enough flow and pressure will be available for
effective flush.
It is therefore important to calculate flow and
pressure from your pump system to cater for
these circumstances and install a pressure
sustaining valve or downstream isolation valve if
needed.

PAGE 8 - March 2014

Step 5: Maximum flow rate & de-rating factors
Most manufacturers state maximum flow rates for
their filters based both on the largest available
screen orifice in the range and on good water
quality. As a result, these figures have to be derated to suit your application.
This de-rating calculation is determined by water
quality and micron size related to the screen area
of the filter screen. This screen area is expressed in
CM2 units but this figure does not give the whole
story. It should be noted that an important factor is
the open area of the screen (where the water can
flow) and not the total area of the screen as some
screens can be more support structure and not
open area mesh.
Ultimately the Total “Open Area” of the Filter
screen will determine the performance of the filter
if all other operating parameters are in place.
Based on this, the designer should always check
the open area of a filter and not just its inlet/outlet
diameter.