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SF6安全保障专家
专注于SF6气体检测的专业化
SF6 Knowledge of the relationship between gas pressure, volume, amount of matter and temperature
发布时间：2018-10-25
来源：ydlcb 浏览次数：54次
SF6 Knowledge of the relationship between gas pressure, volume, amount of matter and temperature
Sulfur hexafluoride, molecular formula SF6, relative molecular weight is 146.06, at normal temperature and pressure for colorless, tasteless, non-toxic, non-corrosive, non-flammable, non-explosive gas, density of about 5 times the air, the standard state density of 6.0886kg/ cubic meter. It is liquid at low temperature and under pressure and becomes a white solid when frozen. The sublimation temperature is -63.9℃, the melting point is -50.8℃, the critical temperature is 45.55℃, and the critical pressure is 3.759MPa. Sulfur hexafluoride has good chemical and thermal stability, excellent electrical insulation and arc extinguishing performance
Sulfur hexafluoride gas liquefaction temperature: it is under one atmospheric pressure (i.e. 0.1MPa), the liquefaction temperature is -62℃; Under the pressure of 1.2MPa, the liquefaction temperature is 0℃. Generally, the pressure of sulfur hexafluoride gas charged into the circuit breaker is 0.35 ~ 0.65MPa (specifically determined by the ambient temperature at the time of inflation), and the liquefaction temperature is -40℃.
The critical temperature is the highest temperature at which sulfur hexafluoride gas liquefaction occurs. The critical pressure indicates the gas pressure required for liquefaction to occur at this temperature. Sulfur hexafluoride can only remain gaseous when the temperature is above 45 degrees, under normal conditions of use, it has the possibility of liquefaction, so sulfur hexafluoride can not be used at low temperatures and too low pressure.
The electrical strength of sulfur hexafluoride is about 2.5 times that of air, and the arc extinguishing capacity is more than 100 times that of air, so it has completely replaced insulating oil and compressed air in the category of ultra-high pressure and ultra-high pressure, and has become the circuit breaker arc extinguishing medium.
Some problems in physical and chemical properties of sulfur hexafluoride gas should be used as an insulating medium in engineering practice, which should not only have high electrical strength, but also have good rationalization characteristics. Sulfur hexafluoride gas is widely used as a highly electronegative gas for this reason. C The physical and chemical properties of sulfur hexafluoride gas in practical applications are introduced below:
(1) Liquefaction problem
The pressure of modern sulfur hexafluoride high-voltage circuit breaker is about 0.7Mpa, and the inflation pressure of the rest of the GIS except the circuit breaker is generally not more than 0.45MPa. If the inflation pressure at 20 ° C is 0.75MPa (equivalent to the working pressure commonly used in circuit breakers), the corresponding liquefaction temperature is about -25 ° C, if the inflation pressure at 20 ° C is 0.45MPa, the corresponding liquefaction temperature is 40 ° C, it can be seen that there is generally no liquefaction problem. Only in high cold areas do circuit breakers need to be heated, or sulfur hexafluoride -N2 gas mixture to reduce the liquefaction temperature.
(2) toxic decomposing substances
Pure sulfur hexafluoride is a non-toxic, inert gas, and its compatibility with materials in electrical equipment is similar to nitrogen at temperatures below 180 degrees Celsius. However, the decomposition of sulfur hexafluoride is toxic and has a corrosive effect on materials, so measures must be taken to ensure the safety of people and equipment.
Ii. Sulfur hexafluoride gas mixture
1 Sulfur hexafluoride gas prices are higher
2 The liquefaction temperature is not low enough
3 Too sensitive to electrical unevenness
At present, sulfur hexafluoride mixture is being studied both at home and abroad, with a view to replacing sulfur hexafluoride gas with sulfur hexafluoride mixture in some occasions. At present, the industrial application is sulfur hexafluoride N2 mixture gas, mainly used as an insulating medium and arc extinguishing material for circuit breakers in cold areas, and the mixing ratio is usually 50% : 50% or 60% : 40%.
At atmospheric pressure -63℃, it becomes a colorless solid substance. It can be melted under pressure, and its three-phase point parameters are: t = -50.8℃,p = 0.23MPa.
The critical pressure and critical temperature of sulfur hexafluoride are high, the critical pressure is 3.9MPa, and the critical temperature is 45.6℃. The density of sulfur hexafluoride gas at critical pressure and temperature is 7.3g/L.
At a pressure above 3.9MPa, no matter how high, its liquefaction temperature is 45.6 ° C, which is a straight line. Therefore, the critical temperature is the highest temperature of liquefaction, while the critical pressure is the minimum pressure of liquefaction.
The melting point of sulfur hexafluoride, its parameters are TM=-50.8℃, PM=0.23 MPa, which is the coexistence of gas, liquid and solid three phases. Point B is the boiling point of sulfur hexafluoride, TB=-63.8℃, and the saturated vapor pressure is equal to 0.1MPa.
In general, many gases can be regarded as ideal gases, and there is a simple relationship between their state parameters, that is, the ideal gas equation of state:
pV=mRT/M=nRT
Where: m -- gas mass, g
P -- gas pressure, MPa
T -- temperature, K
V -- volume of gas, L
M -- molar mass of gas, g/mol
R -- Molar gas constant (=0.0082MPa·L/ (K·mol))
Mathematically, when an equation contains only 1 unknown quantity, it is possible to calculate this unknown quantity. Therefore, of the four quantities of pressure, volume, temperature, and amount of matter contained, only three of them are known to calculate the fourth quantity. Depending on the target to be calculated, this equation can be converted into the following four equivalent formulas:
Pressure: p=nRT/v
Find the volume: v=nRT/p
Find the amount of substance contained: n=pv/RT
Temperature: T=pv/nR
According to the gas state equation, the relationship between the parameters can be deduced when the gas state changes. For example, when a gas is isothermal compressed (or isothermal expanded), the pressure is proportional to the density.
When the pressure is higher than 0.3~ 0.5MPa, due to the relationship between molecular pressure and density change of sulfur hexafluoride (t=20℃)
The attraction becomes more and more significant as the density increases, that is, the distance between molecules decreases. 1 - varies by ideal gas
The actual gas pressure change characteristics, and according to the ideal gas change law 2 - sulfur hexafluoride gas pressure change
The deduced equations used to calculate the parameters of sulfur hexafluoride will produce large errors.
In practice, the empirical formula is often used to calculate the state parameters of sulfur hexafluoride more accurately, and the following formula is more practical.
P = 56.2 * 10 ^ gamma - 6 * * T * (1 + B) - gamma ^ 2 * A
A = 74.9 * 10 ^ - 6 * (1-0.727 * 10 ^ 3 * gamma)
B = 2.51 * 10 ^ 3 * gamma * (1-0.846 * 10 ^ 3 * gamma)
Where P is the pressure MPa of sulfur hexafluoride gas
Gamma is the density of the gas kg/m^3
T is the temperature K of the gas
T = T + 273.15
Application of sulfur hexafluoride gas state parameter curve
The state parameters of sulfur hexafluoride and the temperature of liquefaction or solidification can be calculated easily by using the state parameter curve.
1. Calculate the filling volume of sulfur hexafluoride gas in the circuit breaker
For example, a sulfur hexafluoride circuit breaker, the working pressure at 20 ° C is 0.45MPa, (gauge pressure), the sulfur hexafluoride gas filling amount is 31kg, and the internal inflation volume of the circuit breaker is required.
When the working pressure is 0.45MPa at 20℃, the absolute pressure is 0.55MPa. The working point S in the break linear cluster can be obtained from the pressure of 0.55MPa at 20℃. It is estimated that the density of the oblique line parallel to the oblique linear cluster passing through the point S is 35kg/ m3
The gas filling volume of the sulfur hexafluoride circuit breaker is: 31/35=0.886 m3.
2. The allowable range of the internal inflation pressure of the sulfur hexafluoride circuit breaker changing with the external temperature
For example, at 20 ° C, the above inflatable working pressure is 0.45 MPa, and the absolute pressure is 0.55 MPa. When the ambient temperature rises to 30 ° C and the density =35 kg/ m3 is kept unchanged, the absolute pressure is 0.58 MPa and the working pressure is 0.48MPa when 30 ° C is obtained along the diagonal line to the right of point S in Figure 1-4. When the temperature drops to -10℃, along the density =35 kg/ m3 oblique line can be found on the left side of point S at -10℃, the absolute pressure is 0.49 Mpa, and the working pressure is 0.39MPa. The results show that when the external temperature changes between -10℃ and 30℃, the working pressure of sulfur hexafluoride circuit breaker can change between 0.39 MPa and 0.48 MPa. (Inflation pressure 0.45MPa at 20℃).
3. Understand the temperature of sulfur hexafluoride gas liquefaction under different working pressures
In the above example, the sulfur hexafluoride circuit breaker has a working pressure of 0.45MPa at 20 ° C, a density of 35 kg/ m3, a working point S and a diagonal line crossing the point S intersect the AMB curve at point T, and the temperature at this point t=-33 ° C, and the corresponding working pressure is 0.35MPa. That is, the sulfur hexafluoride gas in this circuit breaker begins to liquefy at -33 ° C. The T point indicates the liquefaction point where condensation occurs as the temperature drops.
Once the sulfur hexafluoride gas begins to liquefy, as the temperature continues to drop, the sulfur hexafluoride gas continues to condense into a liquid, the density of the gas no longer remains constant but constantly decreases, and the pressure of the gas drops faster. When the temperature drops to the liquefaction point, it does not mean that all the gas is immediately condensed into a liquid, but only the beginning of condensation. However, when the temperature continues to decrease and the pressure and density of the gas decline faster, the insulation and arc extinguishing performance of the sulfur hexafluoride gas decline rapidly, so the sulfur hexafluoride circuit breaker does not allow the operating temperature to be lower than the liquefaction point.
It can be seen from the curve AMB that the higher the working pressure (gauge pressure) of the sulfur hexafluoride circuit breaker, the higher the liquefaction temperature. The liquefaction temperature is related to the working pressure of the circuit breaker. If the liquefaction temperature is not high and -20℃ is calculated, the corresponding absolute pressure at 20℃ should not be higher than 0.82 MPa, and the working pressure (gauge pressure) should not be higher than 0.72 MPa.
When the operating pressure of the circuit breaker is very low, the temperature may not be liquefied and directly condensed into a solid.
It can be seen that the operating environmental conditions of sulfur 6 fluoride circuit breaker are -30℃ ~ +40℃, rated pressure 0.45MPa, and locking pressure 0.4MPa. The analysis results can also be calculated and verified by Boil-Mallott gas equation of state PV/T=P1V1/T1. Where: P is pressure; V is the volume; T is the temperature (absolute temperature); P1 is the pressure after the change; V1 is the changed volume; T1 is the temperature after the change. When the volume is unchanged and the sulfur hexafluoride gas pressure changes with the change of temperature, the sulfur hexafluoride gas pressure change value of LW8-35 circuit breaker can be calculated, and the parameters are substituted into the formula to obtain: P1=P×T1/T0.45MPa×(273℃-39℃)/(273℃+20℃)=0.36MPa When the temperature of sulfur hexafluoride gas changes from 20℃ to -39℃, the pressure of sulfur hexafluoride gas changes from 0.45MPa to 0.36MPa, which is lower than the locking pressure of 0.4MPa.
Conclusion:
1, the insulation and arc extinguishing performance of sulfur hexafluoride mainly depends on its purity and density. It has nothing to do with stress.
2, most of the current sulfur hexafluoride switch gas status monitoring device for the "gas density meter", in fact, it reflects the "converted to 20℃, the pressure of the gas in the switch Mpa", rather than the real pressure in the current barrel. It is composed of a sensing device for the current pressure inside the switch and a temperature compensation device composed of a bimetal strip.
3, "switch inflation pressure", "rated pressure on the switch nameplate" and other usually called pressure refers to the "gas density indicator pressure". In fact, it is the difference between the absolute pressure of the gas inside the switch and the external atmospheric pressure. Therefore, the indicating pressure of the gas density meter (referred to as "gauge pressure" or "working pressure") plus the external atmospheric pressure is the absolute pressure in the switch.
4, the absolute pressure of the gas refers to "the orthogonal pressure of the gas on the container wall".
5, 1 standard atmospheric pressure (atm) = 101325Pa
1 Engineering Atmospheric Pressure (ata) = 98000Pa
1Bar=100000Pa
6, the switch due to liquefaction caused by the reduction of gas density and gas leakage on the insulation and arc extinguishing performance of the gas is exactly the same. Therefore, when an exact decrease in gas density due to liquefaction occurs and the switch emits a "lock signal", the lock should not be manually removed.
7, because the measuring temperature range of the gas density table does not meet the actual requirements of the site, the appropriate density table should be replaced. For example, the nominal temperature range of the density table is -20 to 40 degrees, and the actual temperature range of the switch installation site may be -35 to 45 degrees, so that in extreme cases, the density table will not truly reflect the density of the gas. In short, it is necessary to make the temperature range of the density table cover the actual possible ambient temperature.
8, In order to enable the safe operation of sulfur hexafluoride switches at extremely low temperatures, there are two practical ways:
A Heat the switch bucket at the bottom of the switch; Some areas have adopted this method, and the effect is good. However, it is necessary to install heating devices and insulation measures.
B The use of sulfur hexafluoride - nitrogen mixed gas switch can reduce the liquefaction temperature of gas under working pressure to about -42 degrees. Greatly adapted to low temperature environment.