Filter Selection

Filter Media

The correct filter media must be selected considering many parameters most importantly the Contamination Class to be maintained in the reservoir dependent on system components.

Typical Applications	Aeronatic, test rings	Aeronatics, ind. robots	Ind. robotics, precision machine tools	High reliability, ind. machines, Hydrostatic transmissions
Pumps and / or Motors		Piston, variab. > 3,000 psi	Piston, variab. < 3,000 psi, Vane, variab. > 2,000 psi	Pist./Vane, variab. < 2,000 psi Pist./vane fixed > 2,000 psi
Valves	Servovalves > 3,000 psi	Servovalves < 3,000 psi Proportional > 3,000 psi	Proportional < 3,000 psi Cartridge > 2,000 psi	Cartridge < 2,000 psi
Contamination Class NAS 1638	4	5	6	7
Contamination class ISO 4406 - 1999	15/13/10	16/14/11	17/15/12	18/16/13
Recommended UFI filter media	FA-HA β3 > 200	FA-HA FB-HB β3 > 200 β6 > 200	FB-HB β6 > 200	FB-HB FC-HC β6 > 200 β12 > 200

Typical Applications	Industrial Machines, earth moving machines	Mobile machines	Machines for heavy industry	Machines for agriculture, systems not continuous service
Pumps and / or Motors	Pistons, fixed < 2,000 psi Vane, fixed > 2,000 psi	Vane, fixed gear > 2,000 psi	Vane, fixed gear < 2,000 psi	Vane, fixed gear < 2,000 psi
Valves	Solenoid > 3,000 psi	Solenoid < 3,000 psi	Solenoid > 2,000 psi	Solenoid > 2,000 psi
Contamination Class NAS 1638	8	9	10	11
Contamination class ISO 4406 - 1999	19/17/14	20/18/15	21/19/16	22/20/17
Recommended UFI filter media	FC-HC FD-HD β12 > 200 β25 > 200	FD-HD β25 > 200	FD-HD CC β25 > 200 β10 > 2	CC β10 > 2

The medias starting with the letter "F" are made of inorganic microfibers, strated and reinforced with binders; the medias starting with the letter "C" are made by cullulose fibers strated and reinforced with binders. For special applications, medias with wire mesh are also available.

Flow Rate

When sizing a pressure filter it is essential to calculate the real maximum flow rate passing through it. Usually it is given by the maximum pump flow rate, unless the system includes any accumulator acting on the line of the filter itself.

Filter element life is significantly effected by the pollution level at the machine location and by the maintenance level of the machine.

From these parameters, the actual flow rate should be multiplied by the following "Environmental Factor":

ENVIRONMENTAL FACTOR
System maintence level					Environmental contamination level
					LOW	MEDIUM	HIGH
tank with good protection, efficient air breathers few actuators, with very good protection from contaminant ingression frequent monitoring of filter conditions					1.0	1.0	1.3
tank with protection, good air breathers many actuators, with good protection from contaminant ingression scheduled monitoring of filter conditions					1.0	1.5	1.7
tank with poor protection many actuators, with low protection from contaminant ingressions random monitoring of filter conditions					1.3	2.0	2.3
					e.g. system located in climatized room	e.g. system located in industrial building	e.g. system located in hostile environment (foundry, wood working machings, mobile machines)

Clogging Indicators

A clogging indicator will recognize that a filter element needs to be replaced. As the filter retains contaminant it becomes increasingly clogged which causes the operating pressure drop (Δp) to increase. A clogging indicator helps keep your system pressure high and operating efficiently.

There are five main types of filter clogging indicators.

Pressure Gauge (visual indicator)
A pressure gauge will give a visual indication of the upstream filter pressure. Typical systems will indicate that the filter element needs replacement when the pressure value exceeds 22 psi.

Pressure Switch (electrical indicator)
A pressure switch works similar to a pressure gauge, but activates an electrical contact when the set upstream pressure is reached. This type of indicator is recommended only for applications where the filter outlet is directly connected to the reservoir.

Differential Indicator (visual indicator)
This type of an indicator measures the pressure drop across the filter and provides a visual indicator (a green bar switches to red) when the differential pressure reaches the set value, signaling the need for filter element replacement.

Differential Indicator (visual and electrical indicator)
The visual and electrical indicators measure the pressure drop across the filter and provide a visual indicator (a green bar switches to red) and active an electrical contact when the differential pressure reaches the set value, signaling the need for filter element replacement.

Differential Indicator (electrical)
Electrical differential indicators measure the pressure drop across the filter and provide activate an electrical contact when the drop reaches the set value, signaling the need for element replacement. Some electrical differential indicators also include thermostats, allowing the signal to be activated only when a temperature is reached, which avoids false alarms in case of a cold start.

Pressure Drop

Having determined the real flow rate, the filter size can be selected using the pressure drop (Δp) curves. The pressure drop curves are unique for each filter type, and the result is based upon the flow rate. This results in the "Assembly Δp" = (Housing Δp + Element Δp ) which, with clean element, must be 12 psi (0.8 bar) max for a "protection filter" or 7 psi (0.5 bar) max for a "system filter".

VISCOSITY FACTOR: it is different for the "Housing Δp" and the "Element Δp"; the pressure drop through the housing is directly proportional to the specific gravity:

i.e. : New Δp = Curve Δp x New s.g. / 0.9

the pressure drop through the element is directly proportional to the absolute viscosity:

i.e. : New Δp = Curve ΔP x [New V(cSt) / 30 x New s.g. / 0.9]

FILTERABILITY FACTOR: Fluid of the types HFA, HFB, HFC, HFD, HLPD and also some "environmentally friendly" fluids have a difficult filterability and a multiplication factor must be considered.