Mineral oil is a good fluid in terms of its lubrication properties and that it can be made as free-flowing as necessary; however, it can be improved further by incorporating additives. Additives are chemicals dissolved or suspended in the base oil in order to enhance its properties specifically for use in hydraulic systems. For example, additives may be incorporated in the base fluid in order to:
• make the fluid more easy-flowing at low temperatures (pour-point depressants)
• increase the VI of the fluid (VI improvers)
• reduce the effects of high temperatures on the fluid (anti-oxidants)
• further increase the lubrication properties of the fluid (extreme-pressure additives)
• reduce the corrosive effects of water vapour (rust inhibitors)
• increase the rate at which water and air can separate from the fluid and thus reduce ‘foaming’ (demulsifiers and anti-foam agents).
ADDITIVES
• Anti-wear agents
• Anti-oxidants
• Rust inhibitors
• Anti-foam agents
• Extreme-pressure additives
• Viscosity index improvers
• Emulsifiers
• Pour-point depressants
• Dyes
fig. 3.2 Fluid additives
What causes a fluid to ‘wear out’?
Heat
In most cases, excessive heat in a hydraulic system will result in a reduction in the fluid’s viscosity. This in itself may cause wear in heavily loaded components such as pumps and motors if the fluid is no longer able to perform its lubrication function. Heat will also accelerate the rate at which the fluid oxidises, which is often characterised by a darkening of the fluid’s colour (Fig. 3.3) and sometimes a noticeable smell. As the fluid oxidises it becomes more acidic and causes waxes and varnishes to form, which can coat the surface of components or block up small orifices and clearances.
The question arises then of how to define ‘excessive’ heat. There is no hard and fast boundary between an acceptable operating temperature and an unacceptable one, so normally it is necessary to follow the supplier’s guidelines. Inevitably there will be variations from one fluid to another, but around 65°C (150°F) is normally regarded as the maximum operating temperature of a mineral oil based fluid. This does not mean that at 66°C the oil will instantly break down, but it does mean that the oxidation rate of the fluid (which often determines its useful life) starts to noticeably accelerate above this temperature.
Certainly by the time fluid temperatures reach around 80°C (180°F) permanent damage to standard seals and hoses is likely to occur, unless the system has been specially designed to operate at higher temperatures. The recommended maximum temperature for a hydraulic fluid should not be regarded as the normal operating temperature, as then there would be no factor of safety should something go wrong.
A stall in the machine operation causing a relief valve to blow for even a short period of time could cause a rapid increase in fluid temperature. A blowing main relief valve in a closed-loop hydrostatic drive system, for example, could easily heat up the fluid by 20°C (36°F) per minute or more.
The effect of excessive heat on both the base oil and the additives is often the most important factor in determining the useful life of the fluid in a hydraulic system.
Indicators such as colour and smell may provide clues about the condition of the fig. 3.3 Fluid degradation with excessive heat – darkening due to oxidation (Image courtesy of GPM Hydraulic Consulting Inc.)
HEAT DEGRADATION DEFINITION
The acidity of a fluid is normally quantified by determining its total acid number (TaN). The TAN of a hydraulic fluid can be used as a measure of when it has reached the end of its useful life. Typically, for mineral oils the TAN value should not exceed 2.0.
fluid. However, a proper fluid analysis (e.g. to measure the acidity level of the fluid) is really the only way to determine for sure the true condition of the fluid and whether or not it has reached the end of its useful life. Such analyses are generally readily available from fluid or component suppliers and third-party test laboratories.
The way to avoid excessive heat build-up in a hydraulic system is to design the system to be as efficient as possible under all working conditions so that waste heat is not generated in the first place. Sometimes this is easier said than done, and heat exchangers or coolers have to be fitted into the system to remove the excess heat. In stationary applications, water coolers are normally the preferred component.
However, on mobile machinery, air-blast radiators will normally be the only practical option. In this latter case either an electric or hydraulic motor drives a fan to blow air through a cooling matrix, with the system normally being thermostatically controlled.
Water
Water in an oil-based hydraulic fluid is probably second only to heat in its destructive effect on the fluid itself. Water not only causes corrosion (rust) of components and reduces the lubrication properties of the fluid but can also destroy the effectiveness of the additives in a hydraulic fluid. Again, it is not always easy to define how much water is too much, but it is of the order of a few hundred parts per million.
Depending on the temperature and the base oil itself, a certain amount of water can be dissolved in oil naturally. This is not normally visible to the naked eye but may still have detrimental effects on heavily loaded components such as shaft bearings in pumps and motors. The maximum amount of water that can be dissolved in an oil is referred to as the ‘saturation level’. Targeting an operational level of no more than approximately 50% of the saturation level provides a factor of safety and will have a positive effect on bearing life in particular. Levels of water above the saturation level start to become visible as cloudiness or a milky appearance of the fluid, and at this level of water damage to components and the fluid is almost certain to occur (Fig. 3.4).
1000 ppm (0.1%) water 300 ppm (0.03%) water
fig. 3.4 Water-contaminated mineral oil
DEFINITION ppm – parts per million
If a mineral oil fluid is cloudy or milky in appearance, there is definitely too much water present, but it does not follow that if there is no such visible cloudiness the water content is acceptable. Removing water from a hydraulic system fluid is not always an easy task, but the options include:
• Draining water from the tank – a shallow V-shaped profile at the bottom of the tank helps to collect any water, which can then be removed periodically via a drain connection situated at the bottom of the ‘V’.
• Water-removal filters – these combine the tiny droplets of water into larger drops, which then fall to the bottom of the filter bowl and can then be drained off via a drain connection.
• Centrifuges – these can be used to separate large volumes of water from the hydraulic oil. They are normally used on a temporary off-line basis to recover the fluid from a catastrophic failure such as a burst cooler.
• Vacuum dehydration – this effectively ‘sucks’ the water out of the fluid. It can be used either online or off-line.
Preventing water from getting into a hydraulic system is usually much easier than trying to remove it. Entry points for water into a hydraulic system are through ineffective sealing of the system, via the air breather or from a leaking water cooler (Fig. 3.5). All connections into a hydraulic reservoir or components mounted on it must be effectively sealed to prevent the ingress not only of dirt but also of water from rain, splashing or machine wash-down. When the level of fluid drops in a reservoir, due to the movement of cylinders or charging of accumulators, air is drawn into the
Moisture enters tank through unsealed pipe connections or air breather …
… condenses on tank walls when the temperature drops …
… then collects at the bottom of the tank
Oil
Water Damaged cooler leaking water into oil (or oil into water)
fig. 3.5 Water contamination
reservoir to replace the fluid volume. In warm humid environments the air can contain a significant amount of water vapour, which may condense on the walls of the tank when the temperature drops (e.g. overnight or when the machine is switched off).
Over a period of time this can build up to a significant amount of water collecting at the bottom of the reservoir (as water has a higher density than oil).
To reduce the amount of water vapour entering a reservoir, a desiccant material or water-impervious membrane can be included in the air breather to absorb or prevent moisture being drawn into the reservoir. A burst or leaking water cooler, however, has the potential to dump a large volume of water into the system in a short space of time, so effort should be made to protect the cooler from pressure peaks, etc., which could cause such damage.
For critical applications, online sensors can be used to monitor the water content of a fluid and thus provide a warning of when action needs to be taken. Such sensors are now available at moderate cost.
Dirt
As will be discussed in Chapter 4, dirt can have a very damaging mechanical effect on hydraulic components, causing wear, erosion, jamming of components, etc. What may not always be appreciated, however, is that dirt can also have a detrimental effect on the fluid itself. Fine particles of solid contamination can interfere with water-separation additives and can act as a catalyst for the oxidation process, thus reducing the useful life of the fluid.
Ageing
As mentioned previously, some of the additives used in hydraulic fluids are sacrificial (i.e. they are ‘used up’ during the natural operation of the system). Anti-wear additives, for example, which tend to form a low-friction coating on metal surfaces, have a finite life. VI improvers, which are composed of long-chain molecules that ‘tangle’
together to increase a fluid’s viscosity, tend to get ‘chopped up’ by the mechanical action of some hydraulic components, a phenomenon known as ‘shearing down’.
Prolonging the life of a hydraulic fluid
While some of the above processes may be obvious from a simple visual inspection of the fluid or comparison with a sample of new fluid, others will not be. A good maintenance procedure should therefore include a fluid analysis that is carried out at regular intervals depending on the type of fluid and the duty cycle of the system. In short, however, the key to prolonging the life of a hydraulic fluid can be summarised as:
Keep it cool – Keep it clean – Keep it dry