Basic physical principles of types of hydraulic fluids

Basic physical principles of types of hydraulic fluids. SPEO 02/20

Introduction In this unit you will be expected to: • identify types of fluids used in hydraulic systems • describe hydraulic fluid properties • describe environmental effects on hydraulic fluids

Hydraulic Fluids To do its job well, a hydraulic fluid must do at least six things: transfer fluid power efficiently lubricate the moving parts provide bearings clearances between parts absorb, carry and transfer heat generated within the system be compatible with hydraulic components and fluid requirements • remain stable against a wide range of possible physical and chemical changes, both in storage and in use • • •

Mineral Fluids • The widespread use of the term “hydraulic oil” reflects the dominance of petroleum based hydraulic fluids. Hydrocarbon oils protect well against rust, have excellent lubricity, seal well, dissipate heat readily and are easy to keep clean by filtration or gravity separation of contaminants. • Petroleum oil is a common, serviceable industrial fluid when specifically refined and formulated with various additives to prevent rust, oxidation, foaming, etc, however heat and fire hazards must be specifically considered.

Water Based Fluids • Water is a long established hydraulic fluid, however, specific measures have to be taken to cope with evapouration, corrosion, lubricity and freezing. • There are two main types: • Oil-in-water emulsions or “soluble oils” where the oil and water are mixed by using a specific chemical additive. • Water – glycol blends. • Both types have considerable benefits, such as fire resistance, lower toxicity and cheaper to produce.

Synthetic fluids • Synthetic fluids are man-made lubricants and many offer excellent lubrication characteristics in highpressure and high- temperature systems. • Some of the advantages of synthetic fluids may include fire-resistance (phosphate esters), lower friction, natural detergency and thermal stability. • The disadvantage to these types of fluids is that they are usually more expensive than conventional fluids, they may be slightly toxic and require special disposal, and they are often not compatible with standard seal materials.

Fire Resistant Fluids • Phosphate Esters • Sometimes called the straight synthetic fluids, the phosphate esters are excellent lubricants. In fact, they are the best among F-R types. They have good fire-resistance and are better at higher temperature ranges and at higher pressures than many other F-R fluids. • However, they are less useful at lower temperatures, their high specific gravity requires care in selecting pumps and they are the more costly of all the industrial fire-resistant hydraulic fluids.

Fire Resistant Fluids • Water Glycols • These fluids are true solutions, not emulsions, containing a three-component mixture of water (35 to 40%), a glycol and a high molecular weight water-soluble polyglycol. They have excellent fire-resistance, good lubricating properties, and are available in a range of viscosities.

Fire Resistant Fluids • However, they should not be used at temperatures higher than 48 o. C and require periodic checks on water content and additive levels because of evaporation, topping up with distilled water being a common service requirement.

Fire Resistant Fluids • Water-in-Oil Emulsions • These fluids, sometimes termed emulsions, are intended for moderate-duty F-R applications. They consist of 35 to 40% water dispersed in petroleum oil by means of an emulsifying additive package.

Fire Resistant Fluids • They have adequate viscosity for hydraulic service; however, although superior to petroleum oil, water-in-oil emulsions do not have the inherent fire-resistance of either phosphate esters or water-glycol fluids. They require greater care to avoid contamination and should not be repeatedly frozen and thawed, which will cause the two fluid phases to separate.

Fire Resistant Fluids • Oil/Synthetic Blends • Where fire hazards are moderate, blends of phosphate esters and refined petroleum stocks are increasingly used, together with a coupling agent to stabilise the solution. They have good lubricating properties. Their fireresistance reflects their composition and depends largely on the ratio of phosphate ester to petroleum oil.

Hydraulic Fluid Properties • Low compressibility (high bulk modulus) - a measure of the amount of volume reduction due to pressure. Compressibility is sometimes expressed by the bulk modulus, which is the reciprocal of compressibility. • Viscosity - a fluid's internal resistance to flow and may be thought of as a measure of fluid friction.

Hydraulic Fluid Properties • Viscosity Index - an arbitrary numbering scale that indicates the changes in oil viscosity with changes in temperature. A high VI indicating an oil which does not change viscosity readily. • Pour Point - the minimum temperature at which a liquid, particularly a lubricant, will flow.

Hydraulic Fluid Properties • Lubricating ability (lubricity) - the measure of the reduction in friction of a lubricant. • Oxidation resistance - the ability of an oil to resist the direct and indirect attack of oxygen during operation.

Hydraulic Fluid Properties • Corrosion resistance – the ability to resist internal corrosion, either by the natural resistance of a mineral oil or by chemical additive. • Demulsibility - the resistance of a hydraulic fluid to emulsification, or how well a hydraulic fluid resists mixing with water.

Environmental Effects (Contamination) • It has been estimated that up to 75% of all hydraulic system failures are due to contamination of the working fluid. • Due to the extremely close working tolerances, microscopically fine particles can score bearing surfaces, tear seals and increase wear by causing the breakdown of lubricant films.

Environmental Effects (Contamination) • There are 3 main sources of contamination: • Built-in contamination, Ingressed contamination and Self-generated contamination.

Built in contamination • Manufacturers are generally careful in providing internally clean products. However, some systems may be supplied incomplete, or require final assembly on site, potentially leading to ingress of contaminants. • In spite of these efforts, new equipment usually contains some built-in contamination. These contaminants may include burrs, chips, flash, dirt, dust, fibers, sand, moisture, pipe sealant, weld splatter, paints and flushing solution.

Built in contamination • Of these potential contaminants, those of chemical nature are possibly the most important, as their reaction with the hydraulic fluid could be catastrophic.

Ingressed contamination • Ingressed or environmental contamination gets added into the system during servicing or maintenance (or the lack of maintenance) or is introduced into the system from the surrounding environment. • Water is possibly the most common ingressed contaminant, not only through poor maintenance practice and poor design, but through condensation.

Ingressed contamination • Special consideration must be given to equipment used in cold environments, as condensation will form inside cold containers, potentially contaminating the working fluid. Adequate ventillation must always be provided.

Self generated contamination • Self-generated contamination is created internally within the system by the moving parts of Hydraulic systems. These contaminants are produced by wear, corrosion, cavitations, decomposition and oxidation of the fluid in the system. • The most obvious self generated contaminants are metal particles from surface wear, however, chemical contaminants such as tars and varnishes should also be considered.

Types of Contaminant • Magnetic – Worn metal fragments from gears and pistons etc can be very harmful to seals and close tolerance surfaces. However, if they are iron based, they can easily be removed by magnetic filtration. • Liquid – Liquid contamination can easily be removed if the contaminant does not easily mix with water, separators can be used to skim off liquid contaminants which would effect viscosity, lubricity and chemical resistance.

Types of Contaminant • Particulate – Particles from seals, hoses and ingress, tend to be much softer than metal fragments, however, still have the same effect on thin lubricant layers. Removal is by standard filtration methods.

Types of Failure • Catastrophic – A sudden, unexpected complete failure. Normally caused by very large sized foreign bodies (loose fasteners etc) which cause machine seizure or destruction. • Intermittent – A short lived failure which often causes no immediate failure. For example, the effect of moderate sized metal particles on a bearing clearance may cause surface damage, which will not result in bearing failure for some time.

Types of Failure • Degradation – A slow or gradual deterioration in output performance, if not properly monitored and repaired this can lead to catastrophic failure.