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Starting gas

Old 10-20-2004, 08:14 AM
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Default Starting gas

My gas supplier has changed its brands and the propane-butane mix has altered.

I am a bit confused by the instruction manual. It says that we can use LPG, butane, or butane-propane mix with maximum 30% propane, but that pure propane should be avoided as it may freeze the gas solenoid. But after the instruction manual always refers to propane and not to butane, as if the gas normally used is propane.

My supplier has pure butane, or the butane-propane mix is now 65% butane 35% propane. Is pure butane okay to use, or is a butane-propane mix better due to the lower boiling point of propane? I typically fly in temperatures from freezing to 20C. I take it the mix of 35% propane is not recommended?

Harry
Old 10-20-2004, 09:57 AM
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Default RE: Starting gas

Harry: I don't remember the exact numbers but you want to keep the max pressure to 100 psi for inside air temp of 150 degrees F to keep the plastic lines from bursting from gas pressure. Butane is low pressure about 70 psi and propane is higher about 200 psi at 150 degrees.
The valves use gas pressure to help them close, and too high a pressure
may keep them from opening but not freeze as in cold.
Darryl Usher
Old 10-22-2004, 11:11 AM
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Hello PST? Helloooooooo, anybody there?!!
Old 10-22-2004, 08:49 PM
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Default RE: Starting gas

Hi Harry,

Sorry for the delay. Kraivuth is probably on a trip somewhere. He's a 777 Captain for Thailand airlines...

I'm no expert on the chemistry here and I didn't write the manual, so here's the best I can do...
From personal experience, I have used a variety of mixes and have come to use the Camping Gaz - CV470 canister, shown in this [link=http://www.caravanning-online.co.uk/acatalog/Camping_Gaz_Cartridges.html]link[/link]. (Note that this site is a UK supplier). I don't know what the mix percentages are, but it starts without a hitch here and has never frozen a valve. It doesn't smell anything like propane so the butane content is likely pretty high. Being in similar climate conditions I can say that it'll probably work just the same for you, although I don't use an on-board canister in my jets. I run a trigger valve on the large external tank, hooked up to the inlet of the SMC valve.

Regardless of the turbine manufacturer, I've found that in the lower end of the temperature spectrum sometimes even a large canister has trouble boiling off enough gas. Warming the canister by a few degrees will help. In cooler conditions, I usually leave it in the car till I need it and put it back in the car afterwards... If the tank is on-board, you may want to put it in a position where you can wrap your hand around it to sacrifice some body-heat.

As for the misconception in the manual, is probably just something that's been overlooked through the various edits. The term 'propane' should probably be 'start-gas' or something along those lines. It refers to the propane-butane mix used to start the motor. Hopefully Kraivuth will clarify it in the next manual revision. Thanks for pointing it out. From personal experience, I've tried pure propane a few times and various mixes, never freezing up a valve. Its probably possible, but unlikely. I'll leave that to Kraivuth to correct If I'm wrong...

Kelly
Old 10-25-2004, 08:20 PM
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Default RE: Starting gas

Harry and Kelly,
I live up in Colorado Springs, and fly at 6900 feet....so my lows temps can be down there too.
I find that below 60* F, straight propane works best ...between 60 and 65*, I'll get a missed start, first time, everytime...with propane no issues. It's run through the solenoid as Kelly described, by-passing my on board tank.
Hope this doesn't muddy things too much!

Greg
Old 10-26-2004, 02:02 AM
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Default RE: Starting gas

Harry,

The manual may be misleading when we use the word "propane" which should have been "startup gas". The startup gas' properties will vary depending on altitude and outside temperature. The best all around gas is the propane/butane mix for reasons mentioned above. Next manual revision will reflect this change.

Please understand that reply to RCU post might be a bit slow so for best reponse time, please use our direct email. Thank you.

B777
Old 10-26-2004, 11:03 AM
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Default RE: Starting gas

Thanks for all the info guys. I guess what I am hearing from you is that pure butane as recommended in the instructions is not so good at low temps, that a propane mix is better at low temps, and importantly is a mix of 35% propane okay to use despite the instruction manual saying max 30%?

I am wary of using an external gas bottle for starting, though I recognise the benefit of it being able to maintain gas pressure much better than the small on-board tank. The reason I am wary is that when I disconnect the tube for filling the tank, the gas/liquid pressure in the tube between the main bottle and the one-way valve bursts out. That is safe when nothing is running but is it a potential fire/explosion hazard when the engine is running?

H.
Old 10-27-2004, 02:26 PM
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Default RE: Starting gas

In my opinion, 30% vs. 35% is probably going to give you a negligible change in starting reliability. I don’t think there’s any firm ‘line in the sand’ to say what will or won’t work. In other words, I’d say go with the mix you have.

Regarding the possibility of an explosion, some have banned the use of cell phones at gas stations here in Canada and the US for the same reason. Hypothetically, there is a ‘chance’ that a gas station could blow up when a cell phone rings but the likelihood is miniscule. In order to have an ignition, you need to have roughly the right air fuel mixture to sustain the combustion. I can't recall any instance of this ever happening...

In the case of a turbine, I’m sure some of it gets sucked into the turbine’s inlet and burnt but I really don’t think there’s anything to be concerned about. The amount of gaseous fuel stored in the hose should dissipate very quickly, to a point where it can't go off all of a sudden. In one case, I’ve seen a KJ-66 homebuilder do this with the ‘start-gas’ (propane in his case) connection right on the intake of the engine. With the engine idling, he disconnected the propane and vented it without ignition. He’s done it a few hundred times now... By far the more significant danger in that situation is keeping hardware or fingers out of the intake. In short, you probably don’t need to worry about it...

Kelly
Old 10-27-2004, 08:18 PM
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Default RE: Starting gas

Harry,
I agree with Kelly about the very small probability of combustion from the start gas. I actually have less discharge using straight propane (I can close the valve prior to releasing the Festo connection to the solenoid), than I do from Powermax, and that is minimal.
Keep your length of tubing from the gas as short as you can (2 inches is great!) and you have minimal gas to worry about. Now add to it that you are 2-3 feet away from the engine, and if you aren't holding the lever down there is minimal gas to vent. How much gas do you smell? same goes for the engine. Be cautious always! but I have not had, nor seen a problem with disconnects...UNLESS the gas valve was left open.
My 2 cents.

Greg
Old 10-28-2004, 10:26 AM
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Default RE: Starting gas

Outside tank can keep better gas pressure but if you fill the on-board tank properly, you can get at least 3 starts. Removing the connecting outside tank after start is ok but take caution if your valve is near the engine exhaust....it is very hot back there and can be fire hazard. Safety first and have fun.

B777
Old 10-31-2004, 06:33 PM
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Default RE: Starting gas

Thanks for your help folks. My Sabre with PST J600 had its first flights today, in temp of 10C using the butane/propane with 35% propane the little on-board tank had no problem lighting up the engine.

H
Old 10-31-2004, 06:58 PM
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Default RE: Starting gas

Congrats Harry, glad we could be of help.

I just read about it in your "my first ever jet flies!" thread. If your experience was anything like the first time I flew my HotSpot, your probably still grinning...

Best of luck with the new jet.
Kelly

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