2020-05-25
190
Определение индекса вязкости VI
Viscosity
Viscosity is the most important physical constant that characterizes the physical properties of fuels, petroleum oils, and a number of other petroleum products. In oil dispersed systems, the viscosity is a variable and depends on certain conditions. This property is manifested when the fluid moves and is a measure of resistance to flow in crude oil due to internal friction.
The viscosity value is used to judge the possibility of spraying and pumping oil and petroleum products. Oils are naturally oily in texture and have widely varying viscosities. Oil on the surface is usually more viscous than oil in warm underground reservoirs. Most oils are lighter than water. The surface viscosity values range from 1.4 to 19400 centistocks and depend not only on the temperature, but also on the age and depth of the oil.
There are dynamic, kinematic, conditional and effective (structural) viscosity.
Dynamic (absolute) viscosity [n], or internal friction, is a property of real liquids to resist shear forces. The dynamic viscosity in the SI system is measured in [N⸱s/m2]. The viscous friction force F acting on the liquid is proportional to the speed of relative movement of the liquid layers V and the area S of the layers and inversely proportional to the distance between the layers x.
It takes the value equal to 1 if two layers with an area of 1 m2 located at a distance of 1 m under the influence of an external force of 1 N move relative to each other at a speed of 1 m/s.
F = η ⸱S⸱ dV/dх, (8.7.1)
where,
F – friction force between two layers, H
S – surface square, m2
dV/dх – shear rate gradient, s-1
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ηef – effective viscosity, H⸱s/m2
Given that 1N/m2 =1 Pа, the dynamic viscosity is often expressed in [Pа·s] or [MPa·s]. In the CGS system, the dimension of the dynamic viscosity is [DIN·s/m2]. This unit is called poise (1P = 0.1Pa·s). The conversion coefficients for calculating the dynamic viscosity [η] are given in Tab. 14.
Table 14
Conversion factors for the calculation of dynamic [η] viscosity
| Units | Микропуаз (мкП) | Centipoise (sP) | Poise (g/cm·s) | Pa⸱s (kg/m·s) | kg/(m⸱h) | kg⸱s /m2 |
| Micropoise (mcP) | 1 | 10-4 | 10-6 | 107 | 3,6·10-4 | 1,02·10-8 |
| Centipoise (sP) | 104 | 1 | 10-2 | 10-3 | 3,6 | 1,02·10-4 |
| Poise (g/cm·s) | 106 | 102 | 1 | 103 | 3,6·102 | 1,02·10-2 |
| Pa⸱s (kg/m·s) | 107 | 103 | 10 | 13 | 3,6·103 | 1,02·10-1 |
| kg/(m⸱h) | 2,78·103 | 2,78·10-1 | 2,78·10-3 | 2,78·10-4 | 1 | 2,84·10-3 |
| kg⸱s /m2 | 9,81·107 | 9,81·103 | 9,81·102 | 9,81·101 | 3,53·104 | 1 |
Kinematic viscosity [ ν ] is the value equal to the ratio of the dynamic viscosity of a liquid [η] to its density [ρ] at the same temperature:
ν = η/ρ, (8.7.2)
The unit of kinematic viscosity is m2/s. The kinematic viscosity of a liquid is 1 m2/s if the dynamic viscosity of the liquid is 1 N⸱s /m2 and the density is 1 kg/m3 (N = kg⸱m/s2). In the CGS system, the kinematic viscosity is expressed in [cm2/s]. This unit is called Stokes (1St = 10-4 m2/s; 1sSt = 1 mm2/s). The conversion coefficients for calculating the kinematic viscosity [ν] are given in Table 15.
Table 15
Conversion factors for calculation of kinematic viscosity
| Units | mm2/s (sSt) | sm2/s (St) | m2/s | m2/h |
| mm2/s (sSt) | 1 | 10-2 | 10-6 | 3,6·10-3 |
| sm2/s (St) | 102 | 1 | 10-4 | 0,36 |
| m2/s | 106 | 104 | 1 | 3,6·103 |
| m2/h | 2,78·102 | 2,78 | 2,78·104 | 1 |
The viscosity of a crude oil is influenced by the amount of dissolved gases at the prevailing temperature. Crudes characterized by high amount of dissolved gases have high A.P.I gravities and low viscosity or moderately high fluidity.
At high temperatures molecular agitation (or velocity) of the crude increases, making for a volumetric expansion and reduction in internal molecular friction, thus, reducing the oil viscosity. The greater the quantity of a high-molecular weight hydrocarbons group in a crude, the denser and more viscous it is.
Oil and oil products are often characterized by conditional viscosity. The conditional viscosity is measured in degrees CV (° CV) if the test is carried out in a standard viscometer of Engler, Seibolt seconds and Redwood seconds if the test is carried out on Seibolt and Redwood viscometers.
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To determine the viscosity use special standard devices-viscometers, differing in principle.
Kinematic viscosity is determined for relatively low viscosity light oil products and oils using capillary viscometers. Its action is based on measuring the time of fluid flow through the capillary (Fig. 8.7.1).
