Optimising hydraulic tanks: removing air from oil

The ongoing pressure to reduce emissions, driven by international regulations such as the European emission standards, is forcing mobile machine manufacturers to integrate complex exhaust gas aftertreatment systems. The significantly reduced installation space means that, in practice, the hydraulic tank must take on increasingly complex and smaller structural forms. The physical consequence is a significant reduction in natural air separation within the fluid, which dangerously increases the amount of free, undissolved air in the entire system.

Designers and development engineers must therefore redefine the physical laws governing the hydraulic tank to proactively prevent serious damage to components.

Sustainability potential and CO₂ savings at a glance

The following matrix provides a direct comparison between conventional system designs and RT-Filtertechnik’s flow-optimised fluid technology, highlighting key technical metrics for quick analysis:

Technical specifications / parameters	Conventional system (standard)	Optimised RT filter tank system
System behaviour at the fluid surface	Vigorously moving, turbulent flow with constant splashing	A completely calm surface with reduced flow velocity
Required tank size	Conventional, fixed tank size limits design flexibility	Significantly reduced tank size with increased degassing performance
Flow characteristics & separation	High risk of entrainment of micro air bubbles	Forced coalescence (large bubble formation) at the inlet tube
Resource efficiency	A significant amount of fluid and tank volume is required	Reduced material usage and minimised oil fill volume
Environmental impact	Higher system weight results in increased fuel consumption	Direct CO₂ savings through significant weight reduction

System degassing as a technological foundation

Free, undissolved air carriers in hydraulic systems are a primary cause of serious operational issues and accelerated wear processes. While dissolved air has no effect on fluid properties up to specific pressure limits (around 300 bar), dispersed air bubbles can cause significant damage to the system:

Cavitation and component wear: local collapsing air bubbles cause severe corrosion in pumps and control systems.
Efficiency losses: the increased compressibility of air-saturated media reduces the efficiency of pumps and hydraulic motors.
Fluid degradation: the micro-diesel effect (localised thermal overheating of the air-gas mixture) accelerates oil degradation, varnish formation, and noticeable viscosity changes.
Disruptions to operation: the output component exhibits jerky movements, uncontrolled noise, and an extreme temperature increase within the system.
Safety risks: during periods of inactivity, micro bubbles accumulate into larger bubbles at exposed cylinder points or at brake pistons. This results in elastic, "hydraulically soft" system responses, which can have fatal consequences, especially in safety-critical drive and braking systems in autonomous machines. In safety-critical systems like these, degassing the fluid is absolutely essential.

RT-Filtertechnik's engineering approach leverages this physical causality to ensure the system's functional safety and achieve measurable CO₂ savings through weight reduction in the field.

Cost-effectiveness and sustainability – measurable CO₂ savings

In an era of ambitious sustainability goals, the economic and environmental benefits of compact tank systems are a crucial competitive advantage. A reduced tank volume directly leads to a series of cost-saving effects throughout the entire product lifecycle:

Resource efficiency in production: reducing the tank volume means that less material (steel or aluminium) is required to manufacture the tank structures.
Weight reduction in the field: every kilogram of system weight saved in the challenging field of mobile hydraulics directly translates into a lasting reduction in CO₂ emissions in the machine's daily operations.
Reducing the total cost of ownership (TCO): Significantly reducing oil fill volumes doesn't just lower the initial filling costs for the operator. It also minimises ongoing expenses for oil changes and the disposal of used materials throughout the system's service life.

Engineering

To achieve the best possible degassing effects, a general standard approach must be avoided. Every system must be examined in detail and on a case-by-case basis to identify existing air intake sources and develop specific degassing methods. RT-Filtertechnik supports mobile machine manufacturers from the early design phase, following a validated, five-step engineering process:

Initiating optimisation measures (peripheral equipment): checking whether air ingress sources can be mechanically prevented, for example by specifically replacing unsuitable seals.
System analysis: initial mathematical and physics-based analysis, along with a comprehensive evaluation of the overall system's current state of air separation.
Implementing optimisation measures (structure): depending on the specific system, the hydraulic tank, component and available installation space are targeted when it comes to enhancements.
Testing optimisation strategies: the changes made to the system are tested using measurement technology. They are also holistically evaluated and supplemented with additional modifications as needed.
Implementation: the measures validated in the test environment are ultimately implemented on the actual machine in the field.

Flow optimisation – Minimised tank size for maximum space savings

Within this engineering process, design engineers find the answer to the following key question: "How does tank volume affect the dwell time of hydraulic oil?" In completely conventional conditions, the following applies: The smaller the tank volume, the shorter the fluid's dwell time in the tank. Tiny micro bubbles rise extremely slowly due to their low buoyancy. If the dwell time is insufficient, these bubbles will inevitably be sucked back into the pump and dispersed as mist throughout the system.
Traditional large tanks are designed based on rigid dwell time calculations that require a high volume. RT-Filtertechnik's engineering eliminates this dependency: by precisely controlling the fluid flow within the tank, the degassing process is mechanically accelerated, enabling a compact tank size without compromising on functionality. This, in turn, results in significant CO₂ savings.

Sensors

Thorough validation of optimisation measures requires highly advanced measurement tools and simulation tools. Anyone aiming to reduce the air content in hydraulic oil through filtration technology must view the filter and tank as an inseparable functional unit and support this with measurement data:

Comprehensive simulations

Before hardware is built, CFD calculations (computational fluid dynamics), multiphase simulations (to accurately determine sloshing behavior in the tank), and heat transfer simulations ensure that the theoretical foundation is sound.

Test facilities & field measurements

The verification of air separation performance is conducted in real-world conditions, either at our in-house test facility or right at the machine in field operations.

Highly efficient light intensity measurement

Using two photometer sensors and a stationary two-channel device with an integrated data logger, the light intensity is measured in lux (measuring rate: 10 measurement points per second). Since air bubbles in the oil block the light, changes in light intensity provide a precise indicator of the current air content, allowing the load profiles of real machines to be recorded and evaluated.

Visual determination of the size of the air bubbles

Since the size of the air bubbles is the key factor in determining their ascent speed, a special acrylic glass adapter is used to photographically measure the bubble sizes within the oil flow.

Mass flow rate measurement (Coriolis principle)

By harnessing inertial forces during the flow through a vibrating tube, precise measurement of emulsions and suspensions is achieved with an extremely low pressure drop and a simple, robust design.

Filter tank systems

Years of application experience have clearly demonstrated that the direct, physical interaction between the filter component and the tank structure plays a crucial role in air separation performance.

Technological implementation & validation in the overall system

A high-quality return line filter protects the entire hydraulic system from secondary contamination, prevents costly system failures, and directly addresses the issue of insufficient service life at its source. To successfully meet the requirement of optimising hydraulic tank structures to prevent air bubbles and to reliably avoid cavitation in hydraulic systems caused by insufficient degassing, RT-Filtertechnik relies on innovative design principles:

Patented inlet tube: small, dispersed air bubbles are physically forced to coalesce due to geometric constraints, forming larger bubbles that rise much faster and are more easily separated.
Increased outflow cross-section: the oil flows out significantly slower due to the filter's large design. The air bubbles are therefore given exactly the amount of time they need to rise to the surface (according to physical principles).
Calm outflow behaviour: The specialised filter design and housing tubes, featuring precise window hole patterns or integrated diffusors, ensure an absolutely uniform and calm fluid outflow at the tank oil level. Harmful splashing in the tank is eliminated, preventing any new, secondary air from entering the hydraulic tank.
Unobstructed flow: There are also highly innovative solutions that direct flow directly from below into the tank, resulting in virtually no deflection of the oil column – something that has been proven to maximise degassing processes.
Integrated magnetic core: Connected directly to the filter element via a robust bayonet lock, it ensures continuous magnetic pre-filtration of ferrite particles before the oil even reaches the actual filter material.
Pressure loss minimisation (Helios pleat): The optional Helios pleat in the element maximises the effective filter area, stabilises pulsation stability, and ensures a long filter service life even when handling highly viscous media or in the case of critical cold starts.

For flexible installation in limited installation spaces, our product range offers highly modular components with a standard nominal pressure of 10 bar. These include classic tank-mounted filters with a filter head, RME tank installation filters, and RMT submersible versions. The product line includes the RFB Compact and RFB models (with capacities up to 210 and 600 lits per minute, respectively), the RKB and RKT models (capable of up to 1,200 liters per minute, designed for space-saving installation in intermediate chambers), and the high-performance tank-mounted filter RFT (with a capacity of up to 2,400 litres per minute).

Using these highly advanced hydraulic filters maximises air separation efficiency. The optimised separation performance enables engineers to achieve the fundamental goal of reducing tank size in mobile hydraulics through the use of degassing filters. The tank size can be made significantly more compact while meeting the same performance requirements. Realistic numerical flow simulations and field measurements confirm that the air separation performance in a structurally optimised, smaller tank equipped with RT filter components far surpasses that of conventional large tanks.

Air from oil System degassing & fluid management