26 Apr 2023

Fluid viscosity in hydraulic filter sizing

Fluid viscosity in hydraulics

In our March post, we gave an overview of the types of fluid used in hydraulic applications: the oil used and its characteristics influence the choice and sizing of the filter, as well as the materials to be used. However, our analysis doesn’t stop there.

The design of the most suitable filtration system for an hydraulic circuit is dependent on another characteristic of the fluid: the viscosity.
In fact, fluid viscosity is one of the most important quantities for calculating the pressure drop of a hydraulic system.

What is the viscosity of a fluid?

Viscosity is a measure of the resistance of a fluid to flow

In the hydraulics sector, it is usually defined according to the ISO 3448 Viscosity Classification standard in terms of an ISO-VG category, which indicates the degree of viscosity of the oil.

Fluid viscosity has an important impact on system pressure losses and must be carefully considered when designing a filtration system.
Correct assessment of viscosity at the design stage ensures the correct circulation of oil within the system and prevents problems with other components.

Pressure drops are in fact directly proportional to the viscosity of the fluid; therefore, higher oil viscosity will correspond to greater pressure drops inside the filter, and vice versa (as the viscosity of the oil decreases, the pressure drops become smaller).

What are pressure drops?

Pressure drop is the energy dissipated to overcome the resistance encountered by the fluid as it flows through the system. The dissipated energy is lost in the form of heat and is no longer recoverable. In systems, this results in a pressure drop along the circuit.
The pressure drop can be:

  • distributed, when it is generated by the internal friction of the liquid between one particle and another (viscosity) or by the friction of the liquid against the walls along the line
  • localised, when generated at a specific point due to the presence of specific elements or components of the system, such as, for example, valves, filters or bends

The pressure drop is the pressure difference between two points in a hydraulic system.

How are pressure losses calculated?

When building a hydraulic system, it is essential to consider pressure drops because they significantly affect the performance of the system as well as the consumption of the machinery.

There are two ways to calculate the pressure drops of a hydraulic circuit:

  • the mathematical method
  • the graphical method

The mathematical method

For a more precise calculation of the pressure drop of a filtration system, it is necessary to do one of the following:

  • rely on a professional, contacting the sales team of the component manufacturer directly to request a customised consultation
  • use a software tool, such as the CAF Computer Aided Filter Selector by UFI Filters Hydraulics, to obtain a quick and accurate indication of the pressure drop of your application, selecting the specific characteristics directly from the configurator
  • perform the calculation yourself using a formula

The graphical method

This rather approximate method allows a preliminary calculation to be made using available graphs and tables for a component, given certain characteristics of the system.

In fact, the most experienced manufacturers of hydraulic components include curves or tables with pressure drops in their catalogues. In this way, the system designer can independently assess the best solution for a hydraulic circuit.

For example, UFI Filters Hydraulics provides pressure drop curves for each series in the catalogue under standard conditions, i.e. with mineral oil having a kinematic viscosity of 30 cSt and density (specific weight) of 0.86 kg/dm3.
All curves are obtained from tests carried out at the UFI Filters Group laboratories, according to the ISO 3968 specification.

For fluids with different characteristics, it is, however, possible to calculate approximate values based on the considerations outlined in the following paragraph.

Filter sizing and pressure drop (Δp)

The filter size must be calculated based on the total pressure drop, which depends on the application, flow rate, fluid viscosity and filter media used, in order to obtain the level of oil cleanliness required by the system manufacturer.

The total pressure drop (Δp) is calculated by adding the pressure drop value for the filter housing (Δpfilter housing) to the pressure drop value for the filter element (Δpfilter element), and it must not exceed the following values, which differ depending on the application:

  • Suction: 3 kPa (0.03 bar) max
  • Return: 35 to 50 kPa (0.35 to 0.5 bar) max
  • Pressure up to 11 MPa (110 bar): 35 to 50 kPa (0.35 to 0.5 bar) max
  • Pressure above 11 MPa (110 bar): 80 to 120 kPa (0.80 to 1.2 bar) max

A lower initial pressure drop ( Δp0) guarantees better filter efficiency and a longer life for the filter element.

To calculate the Δp0 value, for a mineral oil with a kinematic viscosity (V) of 30 cSt and a density (ρ) of 0.86 Kg/dm3, the values identified in the tables in the catalogue are added to the reference flow rate.

If fluids with different characteristics are used, the following correction factors must be applied to the Δp0 values obtained from the curves as follows:

  • For the filter body, the pressure drop is directly proportional to the oil density (ρ), so in the case of ρ1 ≠ 0.86 Kg/dm3
    Δp1= (Δp0 × ρ1)/0.86
  • For the filter element, the pressure drop varies according to the kinematic viscosity of the oil, so in the case of the kinematic viscosity V1 (cSt) ≠ 30 cSt
    If the kinematic oil viscosity is ≤150 cSt, Δp1 = Δp0 × (V1/30)
    If the kinematic oil viscosity is >150 cSt, Δp1 = Δp0 × [V1/30 + √(V1/30)]/2

If you have any questions about calculating pressure drops or selecting the most suitable filter to guarantee the pressure drop required by a specific application, don’t hesitate to contact an expert, such as a member of the UFI Filters Hydraulics sales team.

Careful evaluation of pressure drops and application performance requirements guarantees reduced pressure drops, significantly increasing the energy savings of the system and extending operating times before maintenance is required.