Specifications
The SF6 produced by Solvay Fluor GmbH is manufactured in a plant that ensures consistent quality with a purity of min. 99.999%.
It corresponds to the following guarantee-analysis which in turn conforms to IEC 376, 1st Edition, Section 3 or to IEC 376, Chapter 3 and to VDE 0373, Part 1, Chapter 3 (according to this standard all values apply to the composition of the liquid phase).
In general, the impurities in Solvay sulphur hexafluoride are substantially less than the maximum values specified in the guaran-tee-analysis.
The table below shows Solvay's typical quality standards specification.
Prior to shipment, every batch of SF6 is tested for physiological safety (cf. Toxicity).
| |
Solvay Fluor
specifications |
IEC 376
specifications |
| SF6 |
% by weight |
>99.999
|
>99.90
|
| N2 |
ppm by weight |
<3
|
<500
|
| CO2 |
ppm by weight |
<0,5
|
|
| CO |
ppm by weight |
<0,1
|
|
|
CF4
|
ppm by weight |
<1
|
<500
|
| H2O |
ppm by weight |
<5
|
<15
|
| O2 |
ppm by weight |
<1
|
<10
|
| CH4 |
ppm by weight |
<1
|
<0.3
|
Hydrolyzable fluorides,
in terms of HF |
ppm by weight |
<0,5
|
<1
|
Electrical properties
Electron affinity
The excellent insulating properties of sulphur hexafluoride are attributable to the strong electron affinity (electronegativity) of the SF6 molecule. This is based mainly on two mechanisms, resonance capture and dissociative attachment of electrons, in accordance with the equations:
SF6 + e--> SF6 - (1)
SF6 + e--> SF5 -+ F (2)
The process represented by equation (1) applies to electron energies of 0.1 eV with an energy range of 0.05 eV, and that represented by equation (2) applies to an energy range of 0.1 eV[4].
Dielectric constants
The dielectric constant has a value of 1.0021 at 20oC, 1.0133 bar and 23.340 MHz; a rise in pressure to 20 bar leads to an increase of about 6 % in this value.
At -50oC, the dielectric constant of liquid sulphur hexafluoride throughout the range from 10 to 500 kHz remains unchanged at 1.81 ± 0.02[5].
 Fig. 23
50 Hz breakdown voltage of SF6 in a homogeneous field as a function of the distance between electrodes at various gas pressures (ETZ Supplement 3 [1966])
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Dielectric strength
The strong interaction of high-energy electrons with the polyatomic SF6 molecule causes their rapid deceleration to the lower energy of electron capture and dissociative attachment. SF6 -breakdown is therefore only possible at relatively high field strengths.
The breakdown voltages at 50 Hz and 1 bar in a homogenous field are thus 2.5 to 3 times higher than the corresponding values for air or nitrogen (Fig. 23).
Figure 24 shows the relationship of breakdown voltage to pressure in a non-homogeneous field in comparison with that of a N2 /CO2 mixture.
The breakdown strength of air is dramatically increased by the addition of small quantities of SF6. In contrast, air has only a limited influence on the breakdown strength of sulphur hexafluoride. The addition of 10 % of air by volume reduces the breakdown voltage of SF6 by about 3 %, the addition of 30 % air by about 10 %.
The breakdown voltage of SF6 reaches that of transformer oil at a pressure of only 3 bar (Fig. 25).
The behaviour of sulphur hexafluoride conforms over a wide range of pressures to Paschen's Law: at higher pressures, however, deviations have been observed under certain conditions[6, 7, 8].
The breakdown strength of SF6 is independent of frequency: it is therefore an ideal insulating gas for UHF equipment[9].
The Corona-onset voltage using SF6 in non-homogeneous fields is also considerably higher than that using air. Figures 26 and 27 show the respective dependence on pressure and radius of curvature of the electrodes in the case of SF6 and air in a point-to-plane electrode system.
Fig. 24
Relation of breakdown voltage to pressure (IEEE Trans. Pow. App. Syst. 66 [1963] 357) Comparison SF6 and N2 /CO2 -mixtures
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Fig. 25
Breakdown strength of transformer oil, air and SF6 as a function of gas pressure (Kali und Steinsalz, 3, issue 10 [1963] 319)
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Fig. 26
Dependence on pressure of the Corona-onset voltage in SF6 and air (ETZ, Supplement 3 [1966])
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Arc-quenching capacity
On account of its thermal properties and low ionisation temperature, sulphur hexafluoride exhibits outstanding charac-teristics for the extinguishing of electric arcs (Fig. 28).
All other conditions being equal, the arc-quenching time using SF6 is about 100 times less than that using air[10].
The superior arc-quenching performance of SF6 compared with other gases is impressively illustrated in figure 29.
Fig. 27
Corona-onset voltages for SF6 and air as a function of the radius of curvature rK at atmospheric pressure (ETZ, Supplement 3, [1966])
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Loss factor
The loss factor, tan  of sulphur hexa-fluoride is extremely low (less than 2.0.10-7). A value of tan <10-3was determined for liquid SF6 at -50oC[5].
Diagrams and data pertinent to the electrical properties of sulphur hexafluoride may be found in the Milek 'Sulphur hexafluoride data sheets'[11].
Fig. 28
Radial temperature profile in SF6 and N2 electric arcs (schematic representation: from Z. Angew. Physik 12, [1960] 5, pp 231 to 237)
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 Fig. 29
Quenching capacity of SF6, air and a mixture of both gases (Insulating Materials for Design and Engineering Practice, N.Y. [1962], p. 116)
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Other Physical Properties
Sulphur hexafluoride is a colourless, odourless, non-toxic and non-flammable gas. With a molecular weight of 146.05, SF6 is about 5 times heavier than air and one of the heaviest known gases.
Mechanical and caloric data
|
Sublimation point (1.0133 bar)
|
- 63.9 °C
|
| Melting point (2.26 bar) |
- 50.8 °C
|
| Vapour pressure |
(see Fig. 31)
|
| Heat of sublimation |
153.2 kJ/kg
|
| Heat of fusion |
34.37 kJ/kg
|
|
Heat of vaporization[12]:
Temperature (°C)
Heat of vaporization (kJ/kg)
|
-20 0 +20 +40
91.71 78.96 62.54 34.08 |
Critical Data[12]:
Critical temperature
Critical pressure
Critical density |
45.58 °C
37.59 bar
0.74 kg/l
|
Density:
Gas density (20°C, 1 bar)
Liquid density (0°C, 12.65 bar)
Solid density (-100°C)[13] |
(see Figs. 30 and 32)
6.07 g/l
1.56 kg/l
2.77 kg/l
|
| Viscosity |
(see Fig. 33)
|
| Thermal conductivity |
(see Fig. 34)
|
| Heat transfer capacity |
(see Fig. 35)
|
Acoustic velocity in SF6
(0°C, 1.0 bar) |
129.06 m/sec.
|
Isentropic exponent ( )[12]:
The dynamic compressibility of
SF6 is particularly high on account
of the low value of the isentropic
exponent |
= 1.08 (30°C, 1.0 bar)
|
Heat of formation (  HB, 25°C)*
Entropy of reaction ( SB, 25°C)*
* for formation from rhombic sulphur and
gaseous fluorine[13]. |
-1221.58 ± 1.0 kJ/mol
- 349.01 J/mol k
|
Chemical behaviour
Under normal conditions, sulphur hexafluoride is chemically inert and stable; its reactivity is among the lowest of all substances.
Behaviour at elevated temperatures
SF6 can be heated to 500 °C in quartz containers without any decomposition occurring. At temperatures of up to approximately 150 °C, generally used materials such as metals, ceramics, glass, rubber and cast resins are completely stable in the presence of sulphur hexafluoride. Not until the temperature exceeds 200 °C do some metals begin to have a decomposing effect on SF6; however, the usual working metals and alloys do not have a significant decomposing effect until the temperature reaches 400 to 600 °C.
Since SF6 reacts with metals at high temperatures, it is used as a protective gas for melts. In particular, it is used in magnesium foundries because it forms a thin and impervious layer on the surface of the molten magnesium. This layer acts very effectively in preventing further reaction with air[19]. In spite of the high temperature of the molten magnesium alloys, there is only a minimal level of decomposition of the SF6.
Behaviour under theinfluence of electricaldischarges[20]
Electrical discharges cause a decomposition of the gas to an extent proportional to the converted energy. Under the influence of an electric arc, part of the sulphur hexafluoride is dissociated into its atomic constituents, as shown in the following equation:
This reaction is reversible. After the discharge, the dissociation products recombine, provided that no secondary reactions with vaporized electrode metal, the container wall or other constructional components occur.
Both solid and gaseous products can result from these secondary reactions:
- metal fluorides, metal sulphides and metal oxides
- sulphur fluorides such as SF4
- sulphur oxyfluorides such as SOF2, SO2F2, SOF4
Such decomposition products resulting from high-energy discharges are also good dielectrics, so that dust-like deposits on the surface of insulators do not impair the operational efficiency of affected equipment.
However, this applies only if the humidity in the gas chamber is very low. If exposed to moisture, the above-mentioned decomposition products hydrolyse and form secondary products, for example as illustrated in the following equations:
The hydrogen fluoride (HF) formed in these reactions vigorously attacks any materials containing silicon dioxide (SiO2) (e.g. glass and porcelain). The use of these materials in equipment in which SF6 is to be used for arc-quenching is therefore only suitable under certain conditions.
Corrosion characteristics of SF6 and its decomposition products
As already indicated, pure SF6 is chemically inert: it cannot, therefore, cause corrosion.
In the presence of moisture, however, the primary and secondary decomposition products of sulphur hexafluoride form corrosive electrolytes which may cause damage and operational failure, particularly in electrical equipment. If the formation of decomposition products cannot be avoided by the use of appropriate construction methods, corrosion can be largely eliminated by the careful exclusion of moisture and the employment of suitable materials.
Commonly used metals such as aluminium, steel, copper and brass remain virtually free of attack. In contrast, materials such as glass, porcelain, insulating paper and similar materials may be severely damaged, depending on the concentration of the corrosive substances. Insulating materials such as epoxy-resin, PTFE, polyethylene, polyvinyl chloride and polymethylene oxide are either only slightly or undetectably affected[21].
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