Understanding SF6 Gas Pressure and Phase Changes in Industrial Cylinders

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In summary, the conversation discussed the use of industrial gas cylinders filled with SF6 gas and the process of transferring the gas from the cylinder to a containment unit. It was explained that the gas is initially in a liquid state, but as it is released from the cylinder and the pressure decreases, it becomes a gas again. The pressure curve and datasheet for SF6 gas were also referenced. It was noted that inside the cylinders, the gas is kept at a higher pressure to maintain a compressed liquid state.
  • #1
Ian_Brooks
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I'm trying to get my head around a concept.

We use Industrial Gas cylinders filled with SF6 gas - the gas cylinder is 70-80% liquid and the rest is in a gaseous state.

If this is kept at a pressure of approx 40Bar and is passed through a regulator that allows the gas to be filled into a containment unit at 1bar -

How does the liquid SF6 become a gas when kept at the same temperature? Obviously the pressure inside the cylinder will decrease as the gas is released.

I realize the liquid freezes if the pressure drop inside the cylinder is too fast.

The pressure curve is as follows.
http://img80.imageshack.us/img80/4661/sf6pressuretempliq.jpg

Datasheet available here:
http://www.solvaychemicals.com/docroot/fluor/static_files/attachments/sf6_e.pdf
 
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  • #2
40 bar is off the top of the graph...at room temperature the gas/liquid mixture must be at about 22 bar. And as you release some, the pressure drops and you go from the border of the liquid/gas region into the gas region, so some liquid boils until you get back to equilibrium.
 
  • #3
Thanks,

Inside the cylinders we keep them at a higher pressure when compared to that graph
 
  • #4
The extra gas is at same temperature as liquid but has gained energy from its suroundings. The liquid will have cooled very slightly but this is compensated by gaining some heat (energy) from the cylinder case and then from the environment. (energy is conserved)
The latent heat factor may be such that temporary cooling of liquid not measurable in the practical situation.
Darmog
 
  • #5
Ian_Brooks said:
Thanks,

Inside the cylinders we keep them at a higher pressure when compared to that graph
So then you keep it as a compressed liquid, not a mixture?
 

1. What is SF6 gas and why is it used in high-voltage equipment?

SF6 (sulfur hexafluoride) is a colorless, odorless, non-toxic, and non-flammable gas that is commonly used in high-voltage electrical equipment such as circuit breakers, switchgear, and transformers. It has excellent electrical insulation properties, making it ideal for use in these types of equipment.

2. How does SF6 gas maintain a higher pressure than other gases at the same temperature?

SF6 gas has a high density and low boiling point, which allows it to maintain a higher pressure than other gases at the same temperature. This is due to the strong intermolecular forces between the sulfur and fluorine atoms in SF6 molecules, which results in a tightly packed arrangement.

3. What is the basic pressure question in relation to SF6 gas?

The basic pressure question in relation to SF6 gas is how pressure affects its physical properties and behavior. This is important in understanding the performance and reliability of high-voltage equipment that uses SF6 gas.

4. How is SF6 gas measured and monitored in high-voltage equipment?

SF6 gas is typically measured using pressure gauges or density monitors, which provide information on the gas pressure and density inside the equipment. It can also be monitored using gas sensors that detect any leaks or abnormal levels of SF6 gas.

5. What are the safety concerns associated with SF6 gas?

While SF6 gas is non-toxic and non-flammable, it is a powerful greenhouse gas with a high global warming potential. Therefore, safety measures must be taken when handling and storing SF6 gas to prevent its release into the atmosphere. Additionally, due to its high density, SF6 gas can displace oxygen in enclosed spaces, presenting a potential asphyxiation hazard.

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