SF6, critical temp 2

[Audun]

What happens to SF6 above 45.6 deg.C, since this is stated as a critical temp for use in a SF6 CB?

Audun

 

[tgott]

General definition: The critical temperature is the temperature above which it is no longer possible to liquify the substance in question by increasing the pressure.

 

[cuky2000]

Sulfur Hexafluoride (SF6) is chemically inert and is completely stable in the presence of most materials to temperatures of about +204°C at 28 bar, (+400°F at 392 psig ). It is shipped as a liquefied compressed gas at its vapor pressure of 298 psig at +70°F (21.5 bar at +21.3°C).

The SF₆ critical parameters are as follow:

1- Critical temperature: +114.0ºF, (+45.55ºC) . .​

2- Critical pressure: 544.3 psia, (37.53 bar), (36.557 atm)​

3- Critical density: 45.8 lb/ft3, (734 kg/m3 ), (0.755 g/mL)​

The operational pressure on a typical SF6 dead-tank circuit breaker vessel is as follow:

Operational Pressure Temp=20oC Temp=45.50C​

Rated……………………... 87 psig 95 psig​

Alarm…………. ………… 75 psig 83 psig​

Minimum/cut-out…………72 psig 78 psig​

CASE I: Breaker without operation at ambient temperature.
At rated filled pressure equivalent to 95 psig at 45.5oC there will not be any condensation since this is produced at critical pressure of 544.3 psig.

CASE II: Circuit breaker interruption operation at rated SC.
Temperature above 5000K converts the gas in stage of conductive plasma. After rapid cooling to 1500K, the material starts recovering the gas estate.

CASE III: During freezing condition.
For temperatures bellow –25C there is risk of condensation. Thermal insulated blanket with heater and mix with other dielectric gases could be used for critical cases with temperatures down to –50C.

 

[electricpete]

Another great detailed post, cuky...
You're a star in my book.