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-   -   sniffle valve required? (https://vansairforce.net/community/showthread.php?t=178538)

DanH 01-12-2020 09:46 AM

Explosion risk is a function of fuel-air ratio by weight. Fuel in liquid phase has no practical explosion risk. The fuel of interest is that which is in vapor phase, above the liquid, and combined with air in a proportion within the combustible range.

Put another way, the risk is about the same for two tablespoons of fuel in the air plenum, or a quart.

Correction: See post #57, this thread. The quart is less risky.

scsmith 01-12-2020 01:26 PM

Quote:

Originally Posted by DanH (Post 1399360)
Explosion risk is a function of fuel-air ratio by weight. Fuel in liquid phase has no practical explosion risk. The fuel of interest is that which is in vapor phase, above the liquid, and combined with air in a proportion within the combustible range.

Put another way, the risk is about the same for two tablespoons of fuel in the air plenum, or a quart.

Except that if there is an ignition event in the vapor-filled plenum, the pooled liquid will pretty quickly vaporize and contribute.

I have a sniffle valve for the main concern being that even a fraction of a tablespoon of liquid drain-back, then vaporized by the warm sump and filling the plenum, could get ignited on a subsequent start. Think about a fuel stop or quick turn dropping off a passenger.

I don't know how likely it is. I have had one backfire on a hot start, and I kept cranking, sucking the fire back into the engine.

It is less than ideal for taildraggers that the sniffle valve boss is not along the back edge of the plenum. But at least that limits the volume of liquid that can pool, then evaporate.

DSmith 01-12-2020 02:11 PM

Water in the sump is a real exposure if you tie down on the ramp and there comes a heavy blowing rain storm. A friend of mine had that happen to him while on a trip. Fortunately the hydraulic lock was not catastrophic due to a weak battery and the fact he recognized the problem quickly.

Sniffle valves are cheap and good insurance.

jcarne 01-12-2020 05:12 PM

I have heard a few people mention now that these sniffle valves are cheap. Is there an alternative that I am not aware of? Spruce is selling the valve right now for $115. Cheap in the grand scheme of things but not cheap for what it is.

RV8Squaz 01-12-2020 06:15 PM

Quote:

Originally Posted by hgerhardt (Post 1399222)
I haven't had one in 13 years and 810 hours. Last CI I removed the intake tubes to replace the gaskets as I do about every other year (someday I'll install Ross' deal) and as usual there was no more than about a teaspoon of brown motor oil in the back of the plenum. No other "crud".

Also, this being a taildragger, all of that small amount of oil was in the back of the plenum, or several inches behind the sniffle valve boss, so even if there was a valve, nothing would have come out of it.

The only time I could think having a sniffle valve might be useful is IF the airplane were parked in a torrential rainstorm and somehow overwhelmed the drain hole in the fiberglass snorkel. Or if a guy forgot to drill that drain hole... Vic, in your myriad inspections, how many times have you found where there was no drain hole in that snorkel?

Torque,

My experience in 1700 hours and 13 years has been exactly as described above, except I think I had a 1/4 teaspoon of oil along the back of the sump. I don't see how a sniffle valve would help in the ground attitude of a taildragger unless there was severe flooding with either water or fuel. I guess it can't hurt, but I haven't experienced a need for it.

DanH 01-13-2020 06:37 AM

Quote:

Originally Posted by scsmith (Post 1399410)
Except that if there is an ignition event in the vapor-filled plenum, the pooled liquid will pretty quickly vaporize and contribute.

Enclosed space. Where is the required oxygen?

Quote:

I have a sniffle valve for the main concern being that even a fraction of a tablespoon of liquid drain-back, then vaporized by the warm sump and filling the plenum, could get ignited on a subsequent start.
No matter, as our sumps are designed are to be unharmed. For sure a sniffle doesn't prevent the occasional "thump from the sump", or air filter blowouts. Most of the cranking fires I've seen (two maybe) were flooded engines with fuel dripping on the ground prior to the prop turning. The average sniffle valve installation dumps fuel inside the cowl or just aft of it. Plenty of air there.

Quote:

Originally Posted by DSmith (Post 1399418)
Water in the sump is a real exposure if you tie down on the ramp and there comes a heavy blowing rain storm. A friend of mine had that happen to him while on a trip. Fortunately the hydraulic lock was not catastrophic due to a weak battery and the fact he recognized the problem quickly.

Sniffle valves are cheap and good insurance.

Agree.

I note another thread going right now, in which the OP (apparently not the builder) doesn't seem to know what he does or doesn't have, or to have been aware of water risk. Plus, an unskilled operator could just as easily hydraulic lock one with excess fuel. Although many of us successfully operate without a sniffle, henceforth I'll probably recommend their use, just because so many RV's are now in the hands of non-builders.

rv8ch 01-13-2020 07:15 AM

sniffle
 
Quote:

Originally Posted by DanH (Post 1399626)
... Although many of us successfully operate without a sniffle, henceforth I'll probably recommend their use, just because so many RV's are now in the hands of non-builders.

I agree - it would be interesting to know that number - it's almost certain that the percentage will get closer and closer to 100%.

scottmillhouse 01-13-2020 10:09 AM

Interesting that most folks do not use one. It appears to be specified by both Vans and Lycoming. I only have a little more than 40 hours since new but I’m still getting some fuel and oil mixed draining out after most flights. Generally about a teaspoon. For those with sniffle valves, is this normal?

I guess I exaggerated the amount as a teaspoon, as others noted a 1” drip spot most days is probably about what I see too.

flyinga 01-13-2020 10:50 AM

Scott,
I often get a mall amount of fuel/oil out of my sniffle valve. Usually only a drop or two. Makes about a 1" dia. spot on hangar floor.

DanH 01-13-2020 10:54 AM

Both you guys are using a constant flow injection without a purge valve?


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