Pressure drop in hydraulic filters

The basic parameter informing us about the need to replace the filtration cartridge is to measure the pressure difference on the filter.

 

The choice of a hydraulic filters depends on the basic function that the filters will perform !!!

 

In easiest way, hydraulic filters are divided into: 

  • Pressure Filters (pilot, in line)
  • Return Filters, usually mounted on a hydraulic oil tank
  • Suction Filters, mounted before the hydraulic pump, in the tank, on the tank or on the line
  • Off-line Filters, completely different group of hydraulic filters, working in bypass system, used to thoroughly clean oil or to capture water from oil oil

 

Each filter group operates at a different standard pressure difference.

Maximum differential pressure (bar) With By-pass Without By-pass
Pressure Filters 5-6 8
Return Filters 1.25-1.75 2
Suction Filters 0.2-0.3 ( underpressure ) 0,3 ( underpressure )

 

The overriding principle in the design of filters concerns the necessary consideration of the aging aspect of the filter.

Decreasing flow at constant differential pressure (or increasing the differential pressure at constant flow), associated primarily with the build-up of the contamination layer on the filter cartridge.

It is assumed that the calculation calculation should be performed for the differential pressure on the filter and the cartridge with a new 10-15% of the target value.

 

Hence, we get the suggested calculation parameters necessary to design / filter the filter:

The maximum differential pressure for the new filter (bar)
Pressure filters 0,5-0,8
Return filters 0,15-0,2
Suction filters 0,02-0,03 (underpressure)

We refer these parameters to the calculated maximum filter throughput at a given maximum oil viscosity.

 

The calculation of the pressure drop on the hydraulic filter can be separated into the analysis:

  • pressure drop on the filter housing
  • pressure drop on the cartridge.

The effect of aging (dirt) on the pressure drop on:

  • Filter housing    ————>   virtually irrelevant
  • Contribution      ————>    very important

 

Determination of the pressure drop characteristics on the filter housing.

 

In order to determine the real characteristic of the pressure drop on the filter housing, it is absolutely necessary to build a measuring stand.

When designing the filter housing, it is worth remembering the important calculation of the minimum inlet and outlet diameter in the filter housing. Underflow on the connection diameter calculation has a significant effect on flow suppression.

The typical pressure drops on the housings depend on the size of the input.

 

Pressure filters 150-250 bar. Test oil 30 cSt.

input / output type flow (l / min) at a pressure drop of 0.5 bar
1/2″ BSP 60-80
3/4″ BSP 70-110
1″ BSP 80-180
1 1/4″ BSP 200-280
1 1/2″ BSP 280-360

 

Return filters. Test oil 30 cSt.

input / output type flow (l / min) at a pressure drop of 0.15 bar
3/8″ BSP 25-35
1/2″ BSP 35-45
3/4″ BSP 50-70
1″ BSP 70-100
1 1/4″ BSP 90-120
1 1/2″ BSP 130-160

 

Suction filters. Test oil 30 cSt.

input / output type flow (+/- 20%) (l / min) at a pressure drop of 0.03bar
3/8″ BSP 20
1/2″ BSP 30
3/4″ BSP 55
1″ BSP 85
1 1/4″ BSP 130
1 1/2″ BSP 180
2″ BSP 300
2 1/2″ BSP 450

 

Determination of the pressure drop characteristics on the filter cartridge.

In order to determine the real characteristic of the pressure drop on the filter cartridge, it is absolutely necessary to build a measuring stand.

The value of the measurement is influenced by many factors, such as the type of filter material, its placement, flow direction, core type, shape of the filter cartridge.

 

Selection of a filter cartridge for use in new applications.

The designer can choose the size of the filter cartridge by two methods:

  • the designer designs his own filter cartridge
  • the designer selects one contribution from the pool of existing producer’s contributions.

 

Designing your own filter cartridge, in particular with regard to the spatial requirements of the target customer.

 

A large flow estimate, one-time measurement is made. Based on the results of the initial design project. It should be remembered that they are valuable, performances with laboratory factors.

Resistance table for 1m2 of a ply of filtered filter material, test oil 30cSt:

The type of filter material flow l / min at 0.1 bar resistance flow l / min at 0.2 bar resistance flow l / min at a resistance of 0.5 bar
glass fiber 3μm 55 110 275
glass fiber 6μm 91 182 455
glass fiber 10μm 149 298 745
glass fiber 15μm 171 342 855
glass fiber 20μm 208 416 1040
chromium-nickel mesh 10μm 1200 2400 6000
chromium-nickel mesh 20μm 2800 5600 14000
chromium-nickel mesh 40μm 6200 12400 31000
chromium-nickel mesh 63μm 11000 22000 55000

 

Selection of the contribution from the pool of existing producer’s contributions.

By applying irrelevant simplifications to the calculation, in particular by linearizing the pressure drop curve depending on the flow, each typical filter cartridge can be described by the following formula:

Δp = (Q / 1000) • K • (current_viscosity / 30)
where:

Δp – pressure drop (bar)

Q – flow (l / min)

current_viscosity – current viscosity of oil during operation

K – coefficient obtained during laboratory tests, depending in particular on the size of the cartridge and the filter material used.

       K factor:

          – decreases with increasing size of the contribution

          – increases with increasing accuracy of the filter material

          – increases with the increase of the cartridge shape factor (ratio of height to the diameter of the cartridge)

For the operation of the hydraulic system in the 30cSt reference oil, the model is simplified: Δp = (Q / 1000) • K.

 

Where to get the K factor for the selected filter cartridge?
– The K factor is available from the manufacturer of cartridges for selected distributors of its products.

 

The values of K coefficient for typical cartridges are presented below:

Glass fiber 3μm

WFZD code height (mm) outer diameter (mm) filtration area (m2) K
ZD03800 94 35 0,035 92
ZD70655 72 56 0,04 48
ZD75501 118 55 0,08 25
ZD07432 84 47 0,04 59
ZD25943 152 47 0,08 26
ZD50430 193 47 0,1 20
ZD10321 120 70 0,12 17
ZD89356 173 69 0,15 10,8
ZD92928 355 69 0,34 5,8
ZD61131 162 90 0,25 7,8
ZD38627 253 90 0,42 4,3
ZD75287 328 90 0.52 3.4
ZD65003 487 90 0,83 2.2
ZD52447 651 90 1.11 1.6
ZD55389 100 52 0.075 15
ZD45548 160 52 0.12 9.5
ZD82897 250 58 0,2 7
ZD43410 160 99 0.4 3,7
ZD86158 250 99 0,55 2,9
ZD03504 250 139 0,85 2,3

 

Glass fiber 6μm

WFZD code height (mm) outer diameter (mm) filtration area (m2) K
ZD06519 94 35 0,035 52
ZD40926 72 56 0,04 29
ZD87006 118 55 0,08 15
ZD91349 84 47 0,04 33
ZD27767 152 47 0,08 15
ZD77007 193 47 0,1 11,5
ZD95105 120 70 0,12 10,5
ZD03151 173 69 0,15 7
ZD80754 355 69 0,34 3,5
ZD32370 162 90 0,25 4,5
ZD25591 253 90 0,42 2,7
ZD27475 328 9 0,55 2
ZD84757 487 90 0,83 1,3
ZD97659 651 90 1,11 1
ZD45216 100 52 0,075 8
ZD95257 160 52 0,12 5,3
ZD28339 250 58 0,2 3,4
ZD05399 160 99 0,4 1,9
ZD39325 250 99 0,55 1,5
ZD42034 250 129 0,85 1,3
ZD05548 400 129 1,4 1,2
ZD49470 400 154 1,65 0,5