Thermal expansion of liquid. Thermal expansion coefficient

Similar to the temperature coefficient of linear expansion, you can enter and apply the temperature coefficient of volumetric expansion, which is a characteristic of the change in the volume of a body when its temperature changes. It has been empirically established that the increase in volume in this case can be considered proportional to the change in temperature, if it does not change by a very large amount. The coefficient of volumetric expansion can be designated in different ways; there is no one designation. A common designation is:

DEFINITION

Let us denote the volume of the body at the initial temperature (t) as V, the volume of the body at the final temperature as , the volume of the body at temperature as , then volumetric expansion coefficient define it as a formula:

Solids and liquids increase their volume slightly with increasing temperature; therefore, the so-called “normal volume” () at a temperature does not differ significantly from the volume at another temperature. Therefore, in expression (1) is replaced by V, which results in:

It should be noted that for gases the thermal expansion is different and replacing the “normal” volume with V is possible only for small temperature ranges.

Volume expansion coefficient and body volume

Using the coefficient of volumetric expansion, you can write a formula that allows you to calculate the volume of a body if the initial volume and temperature increment are known:

Where . Expression () is called the volumetric expansion binomial.

The thermal expansion of a solid body is associated with the anharmonicity of thermal vibrations of the particles that make up crystal lattice bodies. As a result of these oscillations, as the temperature of a body increases, the equilibrium distance between neighboring particles of this body increases.

Volume expansion coefficient and density of matter

If, with a constant mass, a change in the volume of a body occurs, this leads to a change in the density of its substance:

where is the initial density, is the density of the substance at new temperature. Since the quantity is that expression (4) is sometimes written as:

Formulas (3)-(5) can be used when heating a body and when cooling it.

Relationship between volumetric and linear coefficients of thermal expansion

Units

The basic SI unit for measuring the coefficient of thermal expansion is:

Examples of problem solving

EXAMPLE 1

Exercise	What pressure does the mercury barometer, which is located in the room, show if the temperature in the room is constant and equal to t = 37 o C. The coefficient of volumetric expansion of mercury is equal to The expansion of glass can be neglected.
Solution	The actual volume of mercury in the barometer will be the value V, which can be found according to the expression: where is the volume of mercury at normal atmospheric pressure and temperature. Since the temperature in the room does not change, we can use the Boyle-Mariotte law and write that: Let's go through the calculations:
Answer	Pa

EXAMPLE 2

The tensile strength of a liquid is not taken into account when solving practical problems. The thermal expansion of droplet liquids is characterized by coefficient of thermal expansion β t, expressing the relative increase in the volume of liquid with an increase in temperature by 1 degree, i.e.:

Where W - initial volume of liquid; Δ W - change in this volume when the temperature increases by an amount ΔT . The coefficient of thermal expansion of droplet liquids, as can be seen from table. 5, insignificant.

Table 5

The coefficient values ​​in (10) are found from tables within a given temperature range (see, for example, Table 5). The ability of liquids to change density (specific gravity) with temperature changes is widely used to create natural circulation in boilers, heating systems, to remove combustion products, etc. Table. 6 shows the density of water at different temperatures.

Table 6

(11)

Where R - absolute pressure; R - specific gas constant, different for different gases, but independent of temperature and pressure [for air R=287 J/(kg∙K)]; T - absolute temperature. The behavior of real gases under conditions far from liquefaction differs only slightly from the behavior of perfect gases, and for them the equations of state of perfect gases can be used within wide limits. In technical calculations, gas density is usually given by normal physical conditions(t=0°; р=101 325 Pa) or to standard conditions (t=20°C; p=101325 Pa). Air density at R=287 J/ (kg∙K) in standard conditions according to formula (11) it will be equal to ρ 0 =101325/287/(273+20)=1.2 kg/m3. Air density under other conditions is determined by the formula:

(12)

In Fig. Figure 1 shows graphs of the dependence of air density on temperature at different pressures.

Rice. 1 Dependence of air density on barometric pressure and temperature

For an isothermal process (T=const) from formula (12) we have:

(13)

(14)

Where k=с p /с ν - adiabatic gas constant; c p is the heat capacity of gas at constant pressure; With ν - the same, at constant volume. The compressibility of gases depends on the nature of the state change process. For an isothermal process:

(15)

For an adiabatic process:

From expression (15) it follows that the isothermal compressibility for atmospheric air is ~9.8∙10 4 Pa ​​(about 1 at), which is approximately 20 thousand times the compressibility of water. Since the volume of a gas largely depends on temperature and pressure, the conclusions obtained from the study of droplet liquids can be extended to gases only if, within the limits of the phenomenon under consideration, changes in pressure and temperature are insignificant. Significant pressure differences, causing a significant change in the density of gases, can occur when they move at high speeds. The relationship between the speed of fluid movement and the speed of sound in it allows one to judge the need to take compressibility into account in each specific case. In practice, gas can be assumed to be incompressible at speeds not exceeding 100 m/s. Viscosity of liquids. Viscosity is the property of liquids to resist shear. All real liquids have a certain viscosity, which manifests itself in the form of internal friction during the relative movement of adjacent particles of the liquid. Along with easily mobile liquids (for example, water, air), there are very viscous liquids whose shear resistance is very significant (glycerin, heavy oils, etc.). Thus, viscosity characterizes the degree of fluidity of a liquid or the mobility of its particles. Let the liquid flow along a flat wall in layers parallel to it (Fig. 2), as is observed with laminar motion. Due to the braking effect of the wall, the layers of liquid will move c at different speeds, the values ​​of which increase with distance from the wall.

Rice. 2 Velocity distribution when fluid flows along a solid wall

Consider two layers of liquid moving at a distance Δу from each other. Layer A moves at speed u , a layer IN - with speed u + Δu . Due to the difference in speed per unit time, the layer IN shifts relative to layer A by an amount Δ u . Magnitude Δ u is the absolute shift of layer A over layer B, and Δ u /Δ y there is a velocity gradient (relative shift). The tangential stress (friction force per unit area) that appears during this movement will be denoted by . Then, similar to the phenomenon of shear in solids, we obtain the following relationship between stress and strain:

(17)

Or, if the layers are infinitely close to each other,

(18)

Magnitude µ , similar to the shear coefficient in solids and characterizing the shear resistance of a liquid, is called dynamic or absolute viscosity. The existence of relation (18) was first indicated by Newton, and therefore it is called Newton’s law of friction. IN international system units, dynamic viscosity is expressed in H∙s/m 2 or Pa∙c. IN technical system units, dynamic viscosity has the dimension kgf∙s∙m -2. In the CGS system, the unit of dynamic viscosity is taken into memory as poise (P) French doctor Poiseuille, who studied the laws of blood movement in the vessels of the human body, equal to 1 g∙cm -1 ∙s -1 ; 1 Pa∙s=0.102 kgf∙s/m 2 =10 P. The viscosity of liquids is highly dependent on temperature; In this case, the viscosity of droplet liquids decreases with increasing temperature, and the viscosity of gases increases. This is explained by the fact that the nature of the viscosity of droplet liquids and gases is different. In gases average speed(intensity) thermal movement molecules increases with increasing temperature, therefore, the viscosity increases. In droplet liquids, molecules cannot move, as in a gas, in all directions; they can only oscillate around their average position. With increasing temperature, the average speeds of vibrational movements of molecules increase, due to which the bonds holding them are more easily overcome, and the liquid acquires greater mobility (its viscosity decreases). Thus, for pure fresh water, the dependence of dynamic viscosity on temperature is determined by the Poiseuille formula:

(19)

Where µ - absolute (dynamic) viscosity of the liquid in P; t - temperature in ° C. With an increase in temperature from 0 to 100 ° C, the viscosity of water decreases almost 7 times (see Table 6). At a temperature of 20°C, the dynamic viscosity of water is 0.001 Pa∙s=0.01 P. Water belongs to the least viscous liquids. Only a few of the practically used liquids (for example, ether and alcohol) have a slightly lower viscosity than water. Liquid carbon dioxide has the lowest viscosity (50 times less than the viscosity of water). All liquid oils have a significantly higher viscosity than water (castor oil at a temperature of 20 ° C has a viscosity 1000 times greater than water at the same temperature). B table 1.7 shows the viscosity values ​​of some liquids.

Table 7

Which gives at t=15° C =1.82∙10 -6 kgf∙s/m 2 (~1.82∙10 -5 Pa∙s). The dynamic viscosity of other gases is approximately the same order of magnitude. Along with the concept of absolute or dynamic viscosity, the concept is used in hydraulics kinematic viscosity; which is the ratio of absolute viscosity to liquid density:

(21)

This viscosity is called kinematic, since its dimension does not contain units of force. In fact, substituting the dimension µ And ρ , we get [ v]=[L 2 /T]. In the international system of units, kinematic viscosity is measured in m 2 /s; The unit for measuring kinematic viscosity in the CGS system is Stokes (in honor of the English physicist Stokes): 1 Stoke = 1 cm 2 / s = 10 -4 m 2 / s. The hundredth part of Stokes is called centistokes (cSt): 1 m 2 /s = 1∙10 4 St = 1∙10 6 cSt. In table Figure 7 shows the numerical values ​​of the kinematic viscosity of droplet liquids; Fig. 3 - dependence of the kinematic viscosity of water and industrial oil on temperature. For preliminary calculations, the value of the kinematic viscosity of water v can be taken equal to 0.01 cm 2 / s = 1.10 –6 m 2 / s, which corresponds to a temperature of 20 ° C.

Rice. 3 Dependence of kinematic viscosity of water and oil on temperature

The kinematic viscosity of droplet liquids at pressures encountered in most cases in practice (up to 200 atm) depends very little on pressure, and this change is neglected in conventional hydraulic calculations. The kinematic viscosity of gases depends on both temperature and pressure, increasing with increasing temperature and decreasing with increasing pressure (Table 8). Kinematic viscosity of air for normal conditions (temperature 20° C, pressure ~1 at) v= µ/ ρ =1.57∙10 -5 m 2 /s, i.e. approximately 15 times more than for water at the same temperature. This is explained by the fact that the denominator of the expression for kinematic viscosity (21) includes density, which for gases is significantly less than for droplet liquids. To calculate the kinematic viscosity of air at different temperatures and pressures, you can use the graph (Fig. 4).

Table 1.8

When the temperature changes, the size of the solid changes, which is called thermal expansion. There are linear and volumetric thermal expansion. These processes are characterized by coefficients of thermal (temperature) expansion: - average coefficient of linear thermal expansion, average coefficient of volumetric expansion thermal expansion.

DEFINITION

Thermal expansion coefficient is a physical quantity that characterizes the change in the linear dimensions of a solid body when its temperature changes.

The average coefficient of linear expansion is usually used. This is a characteristic of the thermal expansion of a material.

If the initial length of the body is equal to , its elongation with an increase in body temperature by , then is determined by the formula:

The linear elongation coefficient is a characteristic of relative elongation (), which occurs when body temperature increases by 1 K.

As the temperature increases, the volume of the solid increases. As a first approximation, we can assume that:

where is the initial volume of the body, is the change in body temperature. Then the coefficient of volumetric expansion of the body is physical quantity, which characterizes the relative change in the volume of a body (), which occurs when the body is heated by 1 K and the pressure remains constant. The mathematical definition of the coefficient of volumetric expansion is the formula:

The thermal expansion of a solid body is associated with the anharmonicity of thermal vibrations of the particles that make up the crystal lattice of the body. As a result of these oscillations, as the temperature of a body increases, the equilibrium distance between neighboring particles of this body increases.

When the volume of a body changes, its density changes:

where is the initial density, is the density of the substance at the new temperature. Since the quantity is that expression (4) is sometimes written as:

Thermal expansion coefficients depend on the substance. In general, they will depend on temperature. Thermal expansion coefficients are considered independent of temperature over a small temperature range.

There are a number of substances that have a negative coefficient of thermal expansion. So, as the temperature increases, such materials shrink. This usually occurs within a narrow temperature range. There are substances whose coefficient of thermal expansion is almost zero around a certain temperature range.

Expression (3) is used not only for solids, but also for liquids. It is believed that the coefficient of thermal expansion for droplet liquids does not change significantly with temperature changes. However, when calculating heating systems it is taken into account.

Relationship between coefficients of thermal expansion

Units

The basic SI unit for measuring coefficients of thermal expansion is:

Examples of problem solving

EXAMPLE 1

EXAMPLE 2

Thermal expansion of a liquid means that it can change its volume when the temperature changes. This property is characterized by temperature coefficient of volumetric expansion , representing the relative change in the volume of liquid when the temperature changes by one unit (by 1 o C) and at constant pressure:

By analogy with the compressibility property of a liquid, we can write

or through density

A change in volume with a change in temperature occurs due to a change in density.

For most liquids the coefficient  t decreases with increasing pressure. Coefficient  t with a decrease in the density of petroleum products from 920 before 700 kg/m 3 increases from 0,0006 before 0,0008 ; for hydraulic fluids  t usually taken to be independent of temperature. For these liquids, increasing pressure from atmospheric to 60 MPa leads to growth  t by about 10 – 20 % . Moreover, the higher the temperature of the working fluid, the greater the increase  t . For water with increasing pressure at temperatures up to 50 O C  t grows, and at higher temperatures 50 O C decreases.

Dissolution of gases

Dissolution of gases - the ability of a liquid to absorb (dissolve) gases in contact with it. All liquids absorb and dissolve gases to one degree or another. This property is characterized solubility coefficient k R .

E If in a closed container the liquid is in contact with a gas at pressure P 1 , then the gas will begin to dissolve in the liquid. After a while

the liquid will become saturated with gas and the pressure in the vessel will change. The solubility coefficient relates the change in pressure in a vessel with the volume of dissolved gas and the volume of liquid as follows:

Where V G – volume of dissolved gas under normal conditions,

V and – volume of liquid,

P 1 And P 2 – initial and final gas pressure.

The solubility coefficient depends on the type of liquid, gas and temperature.

At a temperature 20 ºС and atmospheric pressure, water contains about 1,6% dissolved air by volume ( k p = 0,016 ). With increasing temperature from 0 before 30 ºС the solubility coefficient of air in water decreases. Solubility coefficient of air in oils at temperature 20 ºС equal to approximately 0,08 – 0,1 . Oxygen has a higher solubility than air, so the oxygen content in air dissolved in a liquid is approximately 50% higher than in the atmosphere. When the pressure decreases, gas is released from the liquid. The process of gas evolution is more intense than dissolution.

Boiling

Boiling is the ability of a liquid to change into a gaseous state. Otherwise, this property of liquids is called volatility .

A liquid can be brought to a boil by raising the temperature to values ​​greater than the boiling point at a given pressure, or by decreasing the pressure to values ​​less than the pressure saturated vapors p np liquid at a given temperature. The formation of bubbles when the pressure is reduced to saturated vapor pressure is called cold boiling.

A liquid from which the gas dissolved in it has been removed is called degassed. In such a liquid, boiling does not occur even at a temperature higher than the boiling point at a given pressure.

15.07.2012
Physical properties of hydraulic oils and their impact on performance characteristics

1. Viscosity, viscosity-temperature characteristics
Viscosity is the most important criterion for assessing the load-bearing capabilities of hydraulic oil. Viscosity is differentiated by dynamic and kinematic indicators.
Industrial lubricating oils and hydraulic oils are classified according to ISO viscosity classes based on their kinematic viscosity, which in turn is described as the ratio of dynamic viscosity to density. The reference temperature is 40 °C. Official unit of measurement ( St) for kinematic viscosity is m 2 /s, and in the oil refining industry the unit of measurement for kinematic viscosity is cSt(centistokes) or mm 2 /s. Viscosity classification ISO, DIN 51519 for liquid industrial lubricants describes 18 grades (classes) of viscosity from 2 to 1500 mm 2 /s at a temperature of 40 °C. Each grade is determined by its average viscosity at 40 °C and with a permissible deviation of ±10% from this value. The viscosity-temperature dependence has great importance for hydraulic oils. Viscosity increases sharply as temperature decreases and decreases as temperature increases. In a practical sense, a threshold fluid viscosity (permissible viscosity at start-up, approx. 800–2000 mm 2 /s) is required for use in pumps various types. The minimum permissible viscosity at high temperatures is determined by the beginning of the boundary friction phase. The minimum viscosity should not be lower than 7-10 mm 2 /s to avoid unacceptable wear of pumps and motors. The curves on the viscosity-temperature graphs describe the dependence of the viscosity of hydraulic fluids on temperature. In linear conditions V—T- the curves are hyperbolic. By mathematical transformation these B—T- curves can be represented as straight lines. These lines allow accurate determination of viscosity over a wide temperature range. Viscosity index (VI) is a criterion B—T-dependencies, and V—T- curve - gradient on the graph. The higher the VI of the hydraulic fluid, the smaller the change in viscosity with temperature change, i.e., the more flat B—T- curve. Hydraulic oils based on mineral oils usually have a natural VI of 95-100. Synthetic hydraulic oils based on esters have a limiting VI of 140-180, and polyglycols have a natural VI of 180-200 (Fig. 1)

The viscosity index can also be increased using additives (polymer additives that must be shear stable) called VI improvers or viscosity additives. High VI hydraulic oils provide easy starting, reduced performance loss at low ambient temperatures, and improved sealing and wear protection at high operating temperatures. High index oils improve system efficiency and extend the life of components subject to wear (the higher the viscosity at operating temperatures, the better the volume ratio).

2. Dependence of viscosity on pressure
The pressure dependence of the viscosity of the lubricant is responsible for the load-bearing capacity of the lubricant film. The dynamic viscosity of liquid media increases with increasing pressure. Below is a method for regulating the dependence of dynamic viscosity on pressure at a constant temperature.
The dependence of viscosity on pressure, namely the increase in viscosity as pressure increases, has a positive effect on the specific load (for example, on bearings), because the viscosity of the lubricating film increases under the influence of high partial pressure from 0 to 2000 atm. Viscosity HFC liquid increases twice, mineral oil - 30 times, in HFD liquids - 60 times. This explains the comparative short term service of roller bearings, if they are lubricated ( HFA, HFC) water-based lubricating oils. In Fig. 2. and 3 show the dependence of viscosity on pressure for various hydraulic fluids.

Viscosity-temperature characteristics can also be described by an exponential expression:

η = η ο · e α P ,

Where η ο is the dynamic viscosity at atmospheric pressure, α is the coefficient of the “viscosity-pressure” relationship, R-pressure. For HFCα = 3.5 · 10 -4 atm -1 ;
For HFDα = 2.2·10 -3 atm -1 ; For HLPα = 1.7·10 -3 atm -1

3. Density
Losses of hydraulic fluids in the piping and in the elements of the hydraulic system are directly proportional to the density of the fluid. For example, pressure loss is directly proportional to density:

Δ P= (ρ/2) ξ With 2 ,

Where ρ is the fluid density, ξ is the drag coefficient, With is the fluid flow speed, and Δ P- loss of pressure.
Density ρ is the mass per unit volume of liquid.

ρ = m/V(kg/m3).

The density of hydraulic fluid is measured at a temperature of 15 °C. It depends on temperature and pressure, since the volume of a liquid increases with increasing temperature. Thus, the change in the volume of liquid as a result of heating occurs according to the equation

Δ V=V·β temp Δ T,

What leads to a change in density:

Δρ = ρ·β temp Δ T.

In hydrostatic conditions at temperatures from -5 to +150 °C, it is sufficient to use linear formula to the above equation. The coefficient of thermal volumetric expansion β temp can be applied to all types of hydraulic fluids.

Since the coefficient of thermal expansion of mineral oils is approximately 7 10 -4 K -1, the volume of hydraulic fluid increases by 0.7% if its temperature increases by 10 °C. In Fig. Figure 5 shows the dependence of the volume of hydraulic fluids on temperature.

The density-pressure relationship of hydraulic fluids should also be included in the hydrostatic assessment, since the compressibility of fluids negatively affects their dynamic characteristics. The dependence of density on pressure can be simply read from the corresponding curves (Fig. 6).

4. Compressibility
The compressibility of hydraulic fluids based on mineral oils depends on temperature and pressure. At pressures up to 400 atm and temperatures up to 70 °C, which are the limits for industrial systems, the compressibility is relevant to the system. The hydraulic fluids used in most hydraulic systems can be considered incompressible. However, at pressures from 1000 to 10,000 atm, changes in the compressibility of the medium can be observed. Compressibility is expressed by coefficient β or modulus M(Fig. 7, M = TO).

M= 1/β atm = 1/β · 10 5 N · m 2 = 1/β · 10 5 Pa.

The change in volume can be determined using the equation

Δ V=V · β( P max - R beginning)

Where Δ V— change in volume; R max—maximum pressure; R start - initial pressure.

5. Gas solubility, cavitation
Air and other gases can dissolve in liquids. The liquid can absorb gas to the point of saturation. This should not adversely affect the performance of the fluid. The solubility of a gas in a liquid depends on the basic components of gas type, pressure and temperature. At pressures up to ≈300 atm. The solubility of a gas is proportional to pressure and follows Henry's law.

V G= V F·α V · P/P o,

Where VG— volume of dissolved gas; V F is the volume of liquid, R o — Atmosphere pressure, P— fluid pressure; α V is the Bunsen distribution coefficient (1.013 mbar, 20 °C).
Bunsen ratio in high degree depends on the base liquid and shows how much (%) gas is dissolved per unit volume of liquid in normal conditions. Dissolved gas can be released from the hydraulic fluid at low static pressure ( high speed flow and high shear stress) until a new saturation point is reached. The rate at which a gas leaves a liquid is usually greater than the rate at which the gas is absorbed by the liquid. Gas leaving a liquid in the form of bubbles changes the compressibility of the liquid in a similar way to air bubbles. Even with low pressures a small amount of air can dramatically reduce the incompressibility of a liquid. In mobile systems with a high rate of liquid circulation, the content of undissolved air can reach values ​​of up to 5%. This undissolved air has a very negative effect on the performance, load-bearing capacity and dynamics of the system (see section 6 - deaeration and section 7 - foaming). Since the compressibility of fluids in systems usually occurs very quickly, air bubbles can suddenly become heated to high temperature(adiabatic compression). In extreme cases, the liquid's combustion temperature may be reached and microdiesel effects may occur.
Gas bubbles can also explode in pumps due to compression, which can lead to damage due to erosion (sometimes called cavitation or pseudo-cavitation). The situation can get worse if vapor bubbles form in the liquid. Thus, cavitation occurs when the pressure drops below the solubility of the gas or below the vapor pressure of the liquid.
Cavitation mainly occurs in open systems with a constant volume, that is, the danger of this phenomenon is relevant for inlet and outlet circuits and pumps. It can be caused by too low an absolute pressure resulting from losses in flow velocity in narrow cross sections, filters, manifolds and throttle valves, due to excess inlet pressure or pressure losses resulting from excessive fluid viscosity. Cavitation can lead to pump erosion, reduced efficiency, pressure peaks and excessive noise.
This phenomenon can adversely affect the stability of throttle regulators and cause foaming in containers if the liquid-water mixture is returned to the container at atmospheric pressure.

6. Deaeration
When hydraulic fluids return to reservoirs, the flow of fluid can carry air with it. This can occur due to leaks in the piping during constriction and partial vacuum. Turbulence in the tank or local cavitation indicates the formation of air bubbles in the liquid.
The trapped air must be released to the surface of the liquid, otherwise, if it enters the pump, it can cause damage to other components of the system. The speed at which air bubbles rise to the surface depends on the diameter of the bubbles, the viscosity of the liquid, and the density and quality of the base oil. The higher the quality and purity of the base oil, the faster the deaeration occurs. Low viscosity oils generally de-aerate faster than high viscosity base oils. This is due to the rate at which the bubbles rise.

C = (ρ FL -ρ L )Χ/η,

Where ρ FL— fluid density; ρ L— air density; η—dynamic viscosity; X is a constant depending on the density and viscosity of the liquid.
Systems must be designed so that air does not enter the liquid and, if it does, entrained air bubbles can easily escape. Critical areas are tanks, which must be equipped with baffles and air deflectors, and the configuration of piping and circuits. Additives cannot have a positive effect on the deaeration properties of hydraulic fluids. Surfactants (particularly silicone-based antifoam additives) and contaminants (such as greases and corrosion inhibitors) adversely affect the release characteristics of hydraulic oils. Mineral oils generally have better air release properties than fire retardant fluids. Deaeration properties HPLD hydraulic fluid can be comparable to the properties of hydraulic fluids HLP.
A test to determine deaeration properties is described in the standard DIN 51 381. This method involves injecting air into the oil. The deaeration number is the time it takes for air (minus 0.2%) to leave a liquid at a temperature of 50 °C under specified conditions.
The proportion of dispersed air is determined by measuring the density of the oil-air mixture.

7. Foaming
Surface foaming occurs when the rate of deaeration is higher than the rate at which air bubbles burst on the surface of the liquid, that is, when there are more bubbles formed than destroyed. In a worst-case scenario, this foam can be forced out of the tank through the holes or carried into the pump. Silicone-based or silicone-free antifoam additives can accelerate bubble breakdown by reducing the surface tension of the foam. They also negatively affect the deaeration properties of the fluid, which can cause compressibility problems and cavitation. Therefore, antifoam additives are used in very low concentrations (≈ 0.001%). The concentration of antifoam additive can progressively decrease as a result of aging and deposition on metal surfaces, and foaming problems often arise when using old, already used fluids. Subsequent introduction of an anti-foam additive should only be carried out after consultation with the hydraulic fluid manufacturer.
The volume of foam formed on the surface of the liquid is measured over time (immediately, after 10 minutes) and at different temperatures (25 and 95 °C). Surfactants, detergents or dispersants, contaminants such as grease, corrosion inhibitors, cleaning agents, cutting fluids, oxidation by-products, etc. can negatively affect the effectiveness of antifoam additives.

8. Demulsification
Demulsification is the ability of a hydraulic fluid to repel intruded water. Water can enter hydraulic fluid through heat exchanger leaks, condensed water in reservoirs due to significant changes in oil levels, poor filtration, water contamination due to faulty seals, and extreme environmental conditions. Water in hydraulic fluid can cause corrosion, cavitation in pumps, increase friction and wear, and accelerate the breakdown of elastomers and plastics. Free water should be removed from hydraulic fluid containers as quickly as possible through drain valves. Contamination with water-soluble coolants, especially on machine tools, can cause sticky residues to form after the water evaporates. This can cause problems in pumps, valves and cylinders. The hydraulic fluid must quickly and completely repel water that has penetrated it. Demulsification is determined by DIN 51,599, but this method is not applicable to hydraulic fluids containing detergent-dispersant ( DD) additives. Demulsification is the time it takes to separate mixtures of oil and water. Demulsification parameters are:
. viscosity up to 95 mm 2 /s at 40 °C; test temperature 54 °C;
. viscosity > 95 mm 2 /s; temperature 82 °C.
In hydraulic oils containing DD additives, water, liquid and solid contaminants are held in suspension. They can be removed using suitable filter systems without using the hydraulic function of the machine, excluding negative impact to hydraulic fluid. That's why DD Hydraulic fluids are often used in hydrostatic machine tools and mobile hydraulic systems.
For machines with high circulation rates, which require constant availability and are constantly exposed to the risk of water and other contaminants, the use of cleaning hydraulic fluids is a primary area. Hydraulic fluids with demulsifying properties are recommended for use in steelmaking and rolling shops, where large volumes of water are present and a low circulation rate allows the separation of emulsions in the tank. Demulsifying properties in modified form are used to determine equipment compatibility with hydraulic oils. Aging of hydraulic fluid negatively affects demulsifying properties.

9. Pour point
The pour point is the lowest temperature at which a liquid is still fluid. A sample of the liquid is systematically cooled and tested for fluidity at a temperature decrease of every 3 °C. Parameters such as pour point and limiting viscosity determine the most low temperature, at which normal use of oil is possible.

10. Copper corrosion (copper plate test)
Copper and copper-containing materials are often used in hydraulic systems. Materials such as brass, cast bronze or sintered bronze are found in bearing elements, guides or control units, slides, hydraulic pumps and motors. Copper pipes are used in cooling systems. Copper corrosion can lead to failure of the entire hydraulic system, so the copper strip corrosion test is performed to provide information about the corrosiveness of base fluids and additives to copper-containing materials. The test method for the corrosiveness of mineral-based hydraulic fluids, i.e., biodegradable fluids, in relation to non-ferrous metals is known as the Linde method (a screening method for testing biodegradable oils for corrosiveness in relation to copper alloys) ( SAE Technical Bulletin 981516, April 1998), also known as VDMA 24570 (VDMA 24570 - biodegradable hydraulic fluids - effect on non-ferrous alloys 03-1999 in German).
According to standard DIN 51 759, corrosion on the copper plate may be in the form of discoloration or flake formation. The copper grinding plate is immersed in the test liquid for specified time at a given temperature. Hydraulic and lubricating oils are usually tested at a temperature of 100 °C. The degree of corrosion is assessed in points:
1 - slight color change;
2 - moderate color change;
3 - strong color change;
4 - corrosion (darkening).

11. Water content (Karl Fischer method)
If water enters a hydraulic system partially finely dispersed to the point that it penetrates into the oil phase, then depending on the density of the hydraulic fluid, water may also be released from the oil phase. This possibility must be taken into account when taking samples to determine water content.
Determination of water content in mg/kg (mass) by the Karl Fischer method involves the introduction of a Karl Fischer solution through direct or indirect titration.

12. Resistance to aging (Baader method)
This is an attempt to replicate the study of the effects of air, temperature and oxygen on hydraulic fluids in a laboratory setting. An attempt has been made to artificially accelerate the aging of hydraulic oils by raising temperatures above levels practical application, as well as oxygen levels in the presence of metal catalysts. The increase in viscosity and increase in acid number (free acid) are recorded and evaluated. Laboratory test results are translated into practical conditions. The Baader Method is a practical way to test hydraulic and lubricating oils for aging.
For a given period of time, the samples are aged at a given temperature and air flow pressure while periodically immersing a copper coil in oil, which acts as an oxidation accelerator. In accordance with DIN 51 554-3 C, CL And CLP liquids and H.L., HLP, NM Hydraulic oils are tested for oxidative stability at a temperature of 95 °C. The saponification number is expressed in mg KOH/g.

13. Resistance to aging (method TOST)
The oxidative stability of steam turbine oils and hydraulic oils containing additives is determined in accordance with DIN 51 587. Method TOST has been used for many years to test turbine oils and hydraulic fluids based on mineral oils. In modified form (without water) dry TOST The method is used to determine the oxidative resistance of ester-based hydraulic oils.
Aging of lubricating oils is characterized by an increase in acid number when the oil is exposed to oxygen, water, steel and copper for a maximum of 1000 hours at 95°C (aging neutralization curve). The maximum permissible increase in acid number is 2 mg KOH/g after 1000 hours.

14. Acid number (neutralization number)
The acid number of hydraulic oil increases as a result of aging, overheating or oxidation. The resulting aging products can have an aggressive effect on the pumps and bearings of the hydraulic system. Therefore, the acid number is an important criterion for assessing the condition of a hydraulic fluid.
The acid number indicates the amount of acidic or alkaline substances in the lubricating oil. Acids in mineral oils can attack hydraulic system materials. High acid content is undesirable as it may result from oxidation.

15. Protective antioxidant properties against steel/ferrous metals
The antioxidant properties of turbine and hydraulic oils containing additives in relation to steel/ferrous metals are determined in accordance with the standard DIN 51 585.
Hydraulic fluids often contain dispersed, dissolved or free water, so the hydraulic fluid must provide corrosion protection to all wetted parts under all operating conditions, including water contamination. This test method determines the performance of anti-corrosion additives under a number of different operating conditions.
The test oil is mixed with distilled water (method A) or artificial sea ​​water(method B), stirring continuously (for 24 hours at 60 °C) with a steel rod immersed in the mixture. Afterwards, the steel rod is examined for corrosion. The results allow us to evaluate the anti-corrosion protective properties of the oil in relation to steel components in contact with water or water vapor:
Corrosion degree 0 means no corrosion,
grade 1 - minor corrosion;
grade 2 - moderate corrosion;
degree 3 - severe corrosion.

16. Anti-wear properties (four ball machine Shell; VKA, DIN 51350)
Company's four-ball machine Shell serves to measure the anti-wear and extreme pressure properties of hydraulic fluids. The load-bearing capacity of hydraulic fluids is tested under conditions of boundary friction. The method is used to determine values ​​for lubricating oils with additives that can withstand high pressure under conditions of boundary friction between sliding surfaces. Lubricating oil is tested in a four-ball apparatus, which consists of one (central) rotating ball and three stationary balls arranged in a ring. Under constant test conditions and with a given duration, the diameter of the contact patch on three stationary balls or the load on a rotating ball, which can be increased until welding with the remaining three balls, is measured.

17. Shear stability of lubricating oils containing polymers
To improve the viscosity-temperature characteristics, polymers are introduced into lubricating oils and used as additives that improve the viscosity index. As you increase molecular weight these substances become increasingly sensitive to mechanical stress, for example to those that exist between the piston and its cylinder. To assess the shear stability of oils under various conditions, there are several test methods:
DIN 5350-6, four-ball method, DIN 5354-3,FZG method and DIN 51 382, ​​diesel fuel injection method.
Reduction in relative viscosity due to shear after a 20-hour test DIN 5350-6 (Determination of shear stability of lubricating oils containing polymers used for tapered roller bearings) applies in accordance with DIN 51 524-3 (2006); A viscosity reduction due to shear of less than 15% is recommended.

18. Mechanical tests of hydraulic fluids in rotary vane pumps ( DIN 51 389-2)
Testing on a Vickers pump and pumps from other manufacturers allows a realistic assessment of the performance of hydraulic fluids. However, alternative testing methods are currently under development (in particular, the DGMK 514 - mechanical tests of hydraulic fluids).
The Vickers method is used to determine the anti-wear properties of hydraulic fluids in a rotary vane pump at given temperatures and pressures (140 atm, 250 hours, working fluid viscosity of 13 mm 2 /s at varying temperatures). At the end of the test, inspect the rings and wings for wear ( Vickers V-104WITH 10 or Vickers V-105WITH 10). Maximum permissible wear values:< 120 мг для кольца и < 30 мг для крыльев.

19. Anti-wear properties (test on gear FZG stand; DIN 534-1i-2)
Hydraulic fluids, especially high-viscosity grades, are used as hydraulic and lubricating oils in combined systems. Dynamic viscosity is the main factor in anti-wear performance in hydrodynamic lubrication mode. At low sliding speeds or high pressures under conditions of boundary friction, the anti-wear properties of the fluid depend on the additives used (formation of a reactive layer). These boundary conditions are reproduced when tested for FZG stand.
This method is mainly used to determine the boundary characteristics of lubricants. Certain gears, rotating at a certain speed, are lubricated by splashing or atomizing oil, the initial temperature of which is recorded. The load on the teeth legs is increased stepwise and the characteristics are recorded appearance teeth legs. This procedure is repeated until the final 12th load stage: the Hertzian pressure at the 10th load stage in the meshing band is 1,539 N/mm2; at stage 11 - 1,691 N/mm 2; at the 12th stage - 1,841 N/mm 2. The initial temperature at stage 4 is 90 °C, the peripheral speed is 8.3 m/s, the limiting temperature is not determined; gear geometry A is used.
The load failure stage is determined by DIN 51 524-2. For positive result it must be at least level 10. Hydraulic fluids that meet the requirements ISO VG 46, which do not contain anti-wear additives, typically reach load stage 6 (≈ 929 N/mm2). Hydraulic fluids containing zinc usually reach at least the 10-11th load stage before failure. So-called zinc-free ZAF hydraulic fluids can withstand load stage 12 or higher.

Roman Maslov.
Based on materials from foreign publications.