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
Probably the commonest fire-resistant fluid is water glycol which, as its name suggests, is a combination of water and polyglycol (a fluid similar to car anti-freeze).
The proportions of the two fluids will vary from one manufacturer to another, but are typically in the ratio 40% water to 60% glycol. The water component provides the fire resistance while the glycol thickener provides the lubrication and other properties required by the hydraulic components. As with mineral oils, additives help to improve the lubrication properties of the fluid and help with corrosion resistance, air release, etc.
Inevitably, however, the lubrication properties of any water-based fluid will never be as good as those of mineral oil, so the pressure (and sometimes speed) rating of hydraulic components may have to be lower than if mineral oil is used. This is especially true for pumps where inlet conditions and outlet pressures are very important to the life expectancy of the pump. In some cases, pumps can be modified (with surface coatings) to further improve their compatibility with water glycol fluids.
A second main type of water-based fire-resistant fluids are known as water–oil emulsions. Unlike water glycol, these are not true solutions but mixtures of very small droplets of either water in oil or oil (or synthetic chemicals) in water. In some cases the water content may be as high as 90–95%, in which case the fluid is referred to as a ‘high water content fluid’ (HWCF) or an oil-in-water emulsion. In other types the oil/water ratio is similar to that of water glycol (i.e. approximately 60%
oil and 40% water) (Fig. 3.6), in which case they are then known as water-in-oil or, more commonly, invert emulsions.
OIL WATER
WATER-IN-OIL EMULSION
OIL-IN-WATER EMULSION OIL WATER
fig. 3.6 Water and oil emulsions
As with water glycols, the water content of water–oil emulsions provides the fire resistance, and the oil lubricates the components of the system. However, the oil or chemical content of a HWCF fluid is relatively low, so components often have to be DEFINITION
High water content fluid (HWcf) is sometimes referred to as high water based fluid (HWBf).
DEFINITION An invert emulsion is a fluid where the oil content is greater than the water content.
de-rated in terms of pressure, speed or both when this type of fluid is used. The use of HWCFs is normally confined to applications where a high level of fire resistance is required and the hydraulic components in the system are not heavily loaded or do not have continuous duty cycles. This type of fluid was originally developed for use in the coal-mining industry.
All water-based fluids will require a relatively high level of maintenance to keep them in good condition. Inevitably the water in the fluid will tend to evaporate over time, resulting in a more viscous fluid with lower fire-resistance properties. In such situations the water content must be restored to its correct level, usually by the addition of distilled or de-ionised water. Bacterial growth can also be a problem with water-based fluids, causing clogging of the system, especially filter elements.
Bactericides can be added to the fluid to limit this problem, but compromises may then have to be made with regard to environmental concerns.
Maximum temperatures for water-based fluids will be lower than for mineral oils, with 50°C (120°F) being the normal recommended level to avoid excessive water evaporation. Fluid cleanliness and pump inlet conditions may also require special attention. The relatively high vapour pressure and higher density of water means that pump cavitation can be more of an issue, so positive-head reservoirs are generally recommended. As for mineral oils, a systematic fluid analysis programme is recommended when water-based fluids are used, to ensure that the fluid and additives remain in ideal condition. However, fluid sampling will inevitably have to be carried out more frequently than when mineral oil is used.
Where the demands of the hydraulic system components are beyond the capabilities of a water-based fire-resistant fluid, fully synthetic fire-resistant oils can be specified.
These generally have superior lubrication properties. The two common fluids in this category are phosphate esters and polyol esters. The lubrication properties of these synthetic fluids are very good, as is their fire resistance, but they do have some disadvantages, which can be summarised as follows:
Phosphate esters
• are aggressive to standard paints and seal materials, and normally
fluorocarbon (Viton) or ethylene propylene diene monomer (EPDM) seal materials have to be used
• have a high specific gravity (heavier than water), which tends to increase system pressure drops and may affect pump inlet conditions
• are very expensive to purchase and to dispose of
• have a low VI, so system temperatures have to be kept as stable as possible
• are a potential health hazard if they come into contact with skin or eyes, and may produce toxic fumes when heated to high temperatures.
Polyol esters
• are aggressive to some metals and require fluorocarbon seal materials
• have a high specific gravity (but less so than phosphate esters)
• are expensive (but cheaper than phosphate esters).