Jet Fuel and Prist

MirageCM

MirageCM

Well-Known Member
What exactly does prist do in Jet A? I have always heard the term but what exactly does it do?
 
It doesn't lower the freezing level of jet fuel really. It lowers the freezing level of water (to around -45C.)

After takeoff dissolved water will seperate from the jet fuel and if prist was applied evenly throughout the fuel it will seperate and dissolve in the water, thus reducing the freezing point.

Jet A freezes at -40C, water with prist in it a little under that, like -45C. It's to prevent water from freezing in the fuel lines.
 
I had been told once that long ago when I was a lineguy it use to kill bacteria that would grow in tanks as well as lower the freezing point, but it doesn't do that anymore I guess. Maybe some old school mechanic can say if that was a heap or not.

When I was fueling I remember having to add it when I pumped corporate jets. Some companies had it mandatory they got prist everytime they fueld up, jets and choppers, but most did not. But I remember airliners (727s) didn't get it.

I also think they have cans of prist that you can dump into the fuel tank yourself to add it for even 100LL guys.
 
Unless the solution is spread evenly throughout the fuel -- IE mixed -- it is ineffective. Dumping some on the top of a tank of jet fuel or into your tank after you fill it won't be much use.
 
Molecularly Prist is kinda like soap. Part of it is attracted to water and part is attracted to oil (fuel). Its keeps the water in suspension in much higher quantities than kerosene does. The net result is two things, the fuel 'gels' at lower temperatures, (handy for unheated tanks common in small bizjets) and water does not settle in the tanks. This is where the killing bacteria myth comes from, it doesn't actually kill it, but it prevents it from forming in pools of water in the tanks.
 
deek said:
I had been told once that long ago when I was a lineguy it use to kill bacteria that would grow in tanks as well as lower the freezing point, but it doesn't do that anymore I guess. Maybe some old school mechanic can say if that was a heap or not.
Click to expand...

not prist, but there is an additive in jet-a to kill algae.
 
deek said:
I had been told once that long ago when I was a lineguy it use to kill bacteria that would grow in tanks as well as lower the freezing point, but it doesn't do that anymore I guess. Maybe some old school mechanic can say if that was a heap or not.
Click to expand...

They used to market it as anti-microbial but at some point they changed the formula because they don't do that anymore.
 
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