Mositure testing in GIS 3

[proelect8717]

I have a question from all respected memebers. The pressure in GIS is always above atmospheric pressure (6.5 bar in Circuit Breakers and Isolators and 5.5 in VT/PT and Busbars), while atmospheric is always less (say 1 atm). In case of leakage, it is confirm that always SF6 will be released into atmosphere and air cannot go inside GIS. My question is:

When we check the dew point of SF6 gas and some times results are not good. So from where does the moisture enters the GIS?

Kindly answer dear engineers.

 

[DTR2011]

Are you correcting for temperature? What kind of instrument are you using to check dew point. Was the SF6 dew point checked prior to installation?

 

[zeusfaber]

Perhaps a confluence of two effects:

The tendency of gaseous contaminants to diffuse from one side of a barrier to the other is a function of the partial pressures of that contaminant on either side of the barrier - not of the total pressures. The partial pressure of water vapour in atmospheric air can be quite high and the desired partial pressure on the dry gas side of the system remarkably low.

Water is a very small molecule compared to SF6, which means that, for a given temperature, the average speed of water vapour molecules is much higher than that of those of the rest of the gas.

Where you have leaks, the first factor creates a preference for water vapour to diffuse through them into a dry system. The second makes it possible for that tendency to overcome the flushing effect of the general outflow of gas.

That all ignores the ability of water vapour to adsorb onto surfaces (like the inside of hoses) exposed to atmosphere and later outgas into a dry gas stream (for instance when you use the hose to connect your hygrometer to your GIS).

A.

 

[crshears]

My utility has often encountered moisture migration from atmosphere into the 1000# dry air pressure system we use for our air blast breakers; system is tight, with no discernible leaks. When no breaker operations take place for an extended period of time, moisture gradually builds up in the lines, and system blowdowns [or sometimes performing otherwise unmerited air blast breaker operations, just to change the air] are required; a living, field example of partial pressure diffusion / osmosis.

Anecdote: The dryouts were once deferred for too long; the fog and mist that appeared when a breaker subsequently operated during a contingency was so pronounced that the operators began to jocularly ask up the chain of command when we had started installing water-blast breakers...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[proelect8717]

thanks all dear engineers for your replies,

are there any chances that moisture is contained by solid components inside the bay like copper contacts, insulators etc all which are packed inside the GIS bays.