HYDRAULIC FLUIDS ( Mineral oil based)
Hydraulic fluid is a complex component of every hydraulic system must be selected very carefully. Apart from its primary function of transferring energy, it also has to provide a viscous seal, maintain pressure, transfer heat, prevent corrosion, & lubricate critical components.
Hydraulic fluids generally comprise of a base fluid that has been doped with additives to produce a ready-to-use formulation. Approximately 85% of hydraulic fluids are mineral oil based. They are capable of meeting a broad spectrum of requirements cost-effectively. Hydraulic fluids used in mining, aircraft, die-casting, rolling mill applications often require fire resistant properties and regulations in some such industries specifically forbid use of mineral oil. Several aqueous & nonaqueous fire resistant hydraulic fluids have been developed for such applications. A range of environmentally friendly hydraulic fluids , mainly from bio-bases are also available for environmentally sensitive applications.
Quality and cleanliness of the hydraulic fluid are decisive factors for the operational reliability, efficiency and service life of a system. Hydraulic fluids must conform, be selected and used in accordance with the generally acknowledged rules of technology and safety provisions. Reference should be made to the country-specific standards and directives.
Solid particle contamination and cleanliness levels
Solid particle contamination is the major reason for faults occurring in hydraulic systems. It may lead to a number of effects in the hydraulic system. Firstly, single large solid particles may lead directly to a system malfunction, and secondly small particles cause continuous elevated wear.
For hydraulic fluids, the cleanliness level is given as a three-digit numerical code in accordance with ISO 4406. This numerical code denotes the number of particles present in a hydraulic fluid for a defined quantity. Moreover, foreign solid matter is not to exceed a mass of 50mg/kg (gravimetric examination according to ISO 4405).
In general, compliance with a minimum cleanliness level of 20/18/15 in accordance with ISO 4406 or better is to be maintained in operation. Special servo valves demand improved cleanliness levels of at least 18/16/13. A reduction in cleanliness level by one level means half of the quantity of particles and thus greater cleanliness. Lower numbers in cleanliness levels should always be striven for and extend the service life of hydraulic components. The component with the highest cleanliness requirements determines the required cleanliness of the overall system. Reference should be made to the respective data sheets of the various hydraulic components.
Hydraulic fluids frequently fail to meet these cleanliness requirements on delivery. Careful filtering is therefore required during operation and in particular, during filling in order to ensure the required cleanliness levels.
The lubricant manufacturer can tell you the cleanliness level of hydraulic fluids as delivered. To maintain the required cleanliness level over the operating period, you must use a reservoir breather filter. If the environment is humid, take appropriate measures, such as a breather filter with air drying or permanent off-line water separation.
Note: the specifications of the lubricant manufacturer relating to cleanliness levels are based on the time at which the container concerned is filled and not on the conditions during transport and storage.
Selection of the hydraulic fluid
The use of hydraulic fluids based on mineral oils for is often based on compliance with the minimum requirements of DIN 51524.
Selection criteria for the hydraulic fluid
The specified limit values for all components employed in the hydraulic system, for example viscosity and cleanliness level, must be observed with the hydraulic fluid used, taking into account the specified operating conditions.
Hydraulic fluid suitability depends, amongst others, on the following factors:
Viscosity is a basic property of hydraulic fluids. The permissible viscosity range of complete systems needs to be determined taking account of the permissible viscosity of all components and it is to be observed for each individual component.
The viscosity at operating temperature determines the response characteristics of closed control loops, stability and damping of system, the efficiency factor and the degree of wear.
The optimum operating viscosity range of each component should be kept within the permissible temperature range. This usually requires either cooling or heating, or both. The permissible viscosity range and the necessary cleanliness level can be found in the product data sheet for the component concerned.
If the viscosity of a hydraulic fluid used is above the permitted operating viscosity, this will result in increased hydraulic-mechanical losses. In return, there will be lower internal leakage losses. If the pressure level is lower, lubrication gaps may not be filled up, which can lead to increased wear. For hydraulic pumps, the permitted suction pressure may not be reached, which may lead to cavitation damage.
If the viscosity of a hydraulic fluid is below the permitted operating viscosity, increased leakage, wear, susceptibility to contamination and a shorter component life cycle will result.
Hydraulic fluids are divided into different grades according to their respective kinematic viscosity. This classification is based on a mean viscosity at 40°C according to DIN 51519. This standard lays down a specified series of numbers for viscosity values. This series is 10,15,22,32,46,68,100,150 ….cSt. A deviation of ±10% is permitted around the mean value.
Increase in viscosity as the pressure rises exerts a positive influence under high bearing loads as the lubricating film undergoes a self-reinforcing effect, and is essentially responsible for the load-bearing capability of the lubricating film.
Wear protection capability
Wear protection capability describes the property of hydraulic fluids to prevent or minimize wear within the components. The wear protection capability is described in DIN 51524-2,-3 via test procedures “FZG gear test rig” (ISO 14635-1) and “Mechanical test in the vane pump” (ISO 20763). From ISO VG 32 DIN 51524-2,-3 prescribes a rating of at least 10 (FZG test). At present, the FZG test cannot be applied to viscosity classes ISO VG 32.
The losses in pipelines and flow channels of components are directly proportional to the density of the hydraulic medium.
The density of a hydraulic fluid is determined by the pressure. This compressibility is very important for the dynamic behavior of hydraulic systems. Undissolved air components of 5 to 10% are often found , particularly in systems with short circulation & residence times and unfavorable tank designs. These gas bubbles exert a very strong influence on the compressibility of the fluid and therefore on the stiffness of the system under load and its dynamic performance. Another effect can also occur; a sudden rise in fluid pressure, e.g. when it is being pumped, can cause adiabatic compression of the gas bebbles, which suddenly heat up to a high temperature. In extreme cases the temperature reaches inflammation temperature of the fluid and the fluid is damaged by “micro-dieseling”.
The hydraulic fluid must not negatively affect the materials used in the components. Compatibility with coatings, seals, hoses, metals and plastics is to be observed in particular. The fluid classifications specified in the respective component data sheets are tested by the manufacturer with regard to material compatibility.
The above table is an indicator for hydraulic fluids with water content < 0.1%.
Hydraulic fluids based on mineral oils and related hydrocarbons are tested with 20% water additive during testing of aging resistance according to ISO 4263-1
The calculated fluid service life is derived from the results of tests in which the long-term characteristics are simulated in a short period of time by applying more arduous conditions (condensed testing). The calculated fluid service life is not to be equated to the fluid service life in real-life applications.
Air separation ability (ASA)
The air separation ability (ASA) describes the property of a hydraulic fluid to separate undissolved air. Hydraulic fluids contain approx 7 to 13 percent by volume of dissolved air (with atmospheric pressure and 500C). Hydraulic fluids always contain dissolved air. During operation, dissolved air may be transformed into undissolved air, leading to cavitation damages. Fluid classification, fluid product, reservoir size and design must be coordinated to take into account the dwell time and ASA value of the hydraulic fluid. The air separation capacity depends on the viscosity, temperature, basic fluid and aging. It cannot be improved by additives.
According to DIN 51524 for instance, an ASA value < 10 minutes is required for viscosity class ISO VG 46, 6 minutes are typical, lower value are preferable.
Demulsifying ability and water solubility
The capacity of a hydraulic fluid to separate water at a defined temperature is known as demulsifying ability. ISO 6614 defines the demulsifying properties of hydraulic fluids.
For larger systems with permanent monitoring, a demulsifying fluid with good water separation capability (WSC) is recommended. The water can be drained from the bottom of the reservoir. In smaller systems (e.g. in mobile machines), whose fluid is less closely monitored and where water contamination into the hydraulic fluid, for instance through air condensation, cannot be ruled out completely, an HLPD fluid is recommended.
The demulsifying ability upto ISO-VG 100 is given at 540C, and at 820C for fluid with higher viscosity.
Water emulsifying HLPD hydraulic fluid have no, or a very poor, demulsifying ability.
Filterability describes the ability of a hydraulic fluid to pass through a filter, removing solid contaminants. The hydraulic fluids used require a good filterability, not just when new, but also during the whole of their service life. Depending on the basic fluid used and additives (VI enhancers) there are great differences here.
The filterability is a basic prerequisite for cleanliness, servicing and filtration of hydraulic fluids. Filterability is tested with the new hydraulic fluid and after the addition of 0.2% water. The underlying standard (ISO 13357-1/2) stipulates that filterability must have no negative effects on the filters or the hydraulic fluid.
Hydraulic fluids should not just prevent corrosion formation on steel components, they must also be compatible with non-ferrous metals and alloys. Corrosion protection tests on different metals and metal alloys are described in DIN 51524. Hydraulic fluids that are not compatible with the materials listed above must not be used, even if they are compliant with ISO 51524.
The properties described above can be modified with the help of suitable additives. A general distinction is made for fluids between heavy metal-free and heavy metal-containing (generally zinc) additives systems. Both additive systems are most often incompatible with each other. The mixing of these fluids must be avoided even if the mixing ratio is very low.
Increasing additivation generally leads to deteriorated air separation ability (ASA) and water separation capability (WSC) of the hydraulic fluid. Most hydraulic fluids in current use, independently of the actual additivation, can be filtered using all filter materials with all known filtration ratings > 1µm without filtering out effective additives at the same time.
Hydraulic Fluids in Operation
The properties of hydraulic fluids can change continually during storage and operation.
Note that the fluid standard DIN 51524 merely describes minimum requirements for hydraulic fluids in new condition at the time of filling into the drums. The operator of a hydraulic system must ensure that the hydraulic fluid remains in a utilizable condition throughout its entire period of use.
Please note the following aspects in operation.
Storage and handling
Hydraulic fluids must be stored correctly in accordance with the instructions of the lubricant manufacturer. Avoid exposing the containers to lengthy periods of direct heat. Containers are to be stored in such a way that the risk of any foreign liquid or solid matter (e.g. water, foreign fluids of dust) ingression into the inside of the container can be ruled out. After taking hydraulic fluids from the containers, these are immediately to be properly resealed.
• Store containers in a dry, covered area
• Store barrels on their sides
• Clean reservoir systems and machine reservoirs regularly
Filling of new systems
Usually, the cleanliness levels of the hydraulic fluids as delivered do not meet the requirements of many hydraulic system components. Hydraulic fluids must be filtered using an appropriate filter system to minimize solid particle contamination and water in the system.
As early as possible during test operation, new systems should be filled with the selected hydraulic fluid so as to reduce the risk of accidentally mixing the fluids. Changing the hydraulic medium at a later point represents significant additional costs .
Hydraulic fluid changeover
Changeover, in particular between hydraulic fluids with heavy metal-free and heavy metal-containing (generally zinc) additives frequently lead to malfunctions.
In the case of changeovers of the fluid in hydraulic systems, it is important to ensure compatibility of the new hydraulic fluid with the remainder of the previous hydraulic fluid. A written performance guarantee from the manufacturer or supplier of the new hydraulic fluid may be obtained. The quantity of old fluid remaining should be minimized. Mixing hydraulic fluids should be avoided.
Mixing and compatibility of different hydraulic fluids
If hydraulic fluids from different manufacturers or different types from the same manufacturer are mixed, gelling, silting and deposits may occur. These, in turn, may cause foaming, impaired air separation ability, malfunctions and damage to the hydraulic system.
If the fluid contains more that 2% of another fluid then it is considered to be a mixture. Exceptions apply for water.
Mixing with other hydraulic fluids is not generally recommended by OEMs. If individual lubricant manufacturers advertise miscibility and/or compatibility, this is entirely the responsibility of the lubricant manufacturer.
Additives added at a later point in time such as colors, wear reducers, VI enhancers or anti-foam additives, may negatively affect the performance properties of the hydraulic fluid and the compatibility with our components and therefore are not recommended.
Foam is created by rising air bubbles at the surface of hydraulic fluids in the reservoir. Foam that develops should collapse as quickly as possible.
Common hydraulic fluids in accordance with DIN 51524 are sufficiently inhibited against foam formation in new condition. On account of aging and adsorption onto surfaces, the defoamer concentration may decrease over time, leading to stable foam.
Defoamers may be re-dosed only after consultation with the lubricant manufacturer and with his written approval.
Defoamers may affect the air separation ability.
The hydraulic fluid is to guarantee sufficient corrosion protection of components under all operating conditions, even in the event of impermissible water contamination.
During storage and operation, hydraulic fluid based on mineral oils with ant-corrosion additives protect components against water and “acidic” oil degradation products.
Under atmospheric conditions, the hydraulic fluid contains dissolved air. In the negative pressure range, for instance in the suction pipe of the pump or downstream of control edges, this dissolved air may transform into undissolved air. The undissolved air content represents a risk of cavitation and of the “dieseling” effect. This results in material erosion of components and increased hydraulic fluid aging.
With the correct measures, such as suction pipe and reservoir design, and an appropriate hydraulic fluid, air intake and separation can be positively influenced.
Water contamination in hydraulic fluids can result from direct ingress or indirectly through condensation of water from the air due to temperature variations.
Water in the hydraulic fluid may result in wear or direct failure of hydraulic components. Furthermore, a high water content in the hydraulic fluid negatively affects ageing and filterability and increase susceptibility to cavitation.
Undissolved water can be drained from the bottom of the reservoir. Dissolved water can be removed only by using appropriate measures. If the hydraulic system is used in humid conditions, preventive measures need to be taken, such as an air dehumidifier at the reservoir vent. During operation, the water content in all hydraulic fluids, determined according to the “Karl Fischer method” for all hydraulic fluids must constantly be kept below 0.1% (1000 ppm). To ensure a long service life of both hydraulic fluids and components, most OEMs recommend that value well below 0.05% (500ppm) are permanently maintained.
Detergent and or dispersant hydraulic fluids (HLPD/HVLPD) are able to absorb (and keep suspended) more water. Prior to using these hydraulic fluids, please contact the lubricant manufacturer.
Fluid servicing, fluid analysis and filtration
Air, water, operating temperature influences and solid matter contamination will change the performance characteristics of hydraulic fluids and cause them to age.
To preserve the usage properties and ensure a long service life for hydraulic fluid and components, the monitoring of the fluid condition and a filtration adapted to the application requirements (draining and degassing if required) are indispensable.
The effort is higher in the case of unfavourable usage conditions, increased stress for the hydraulic system or high expectations as to availability and service life.
When commissioning a system, the required minimum cleanliness level can frequently be attained only by flushing the system. Due to severe start-up contamination, it may be possible that a fluid and/or filter replacement becomes necessary after a short operating period (< 50 operating hours).
The hydraulic fluid must be replaced in regular intervals and tested by the lubricant manufacturer or recognized, accredited test labs. A reference analysis after commissioning is highly
The minimum data to be tested for analyses are:
• Viscosity at 400C and 1000C
• Neutralization number NN (acid number AN)
• Water content (Karl-Fischer method)
• Particle measurement with evaluation according to ISO 4406 or mass of solid foreign substances with evaluation to EN 12662.
• Element analysis (RFA (EDX)/CP, specify test method)
• Comparison with new product or available trend analyses
• Assessment/evaluation for further use
• Also recommended: IR spectrum
Compared to the pure unused hydraulic fluid, the changed neutralization number NN (acid number AN) indicates how many aging products are contained in the hydraulic fluid. This value must be kept as low as possible. As soon as the trend analysis notes a significant increase in the acid number, the lubricant manufacturer should be contacted.
Disposal and environmental protection
Hydraulic fluids based on mineral oil and related hydrocarbons are hazardous for the environment. They are often subject to special disposal regulations. The respective lubricant manufacturers provide specifications on environmentally acceptable handling and storage. Ensure that spilt or splashed fluids are absorbed with appropriate adsorbents or by a technique that prevents it contaminating water courses, the ground or sewerage systems.
Comply with the national legal provisions concerning the disposal of the corresponding hydraulic fluid. Comply with the local safety data sheet of the lubricant manufacturer for the country concerned.