Can compressed air safely pressurize gasoline without causing auto-ignition?

[charliep]

Guys, I need some help. I've designed a container that I plan to use for a starting system that uses gasoline. I'm storing the gas in a sealed container and plan to pressurize the container with compressed air from a compressor. The container is rated for 600 psi and I plan to use a shop air compressor that has a max pressure of 120 psi. I want to pressurize the tank of gas to 120 psi. ai've tried the system using water at the 120 psi and it works great. The container is 8 ounces in volume and I fill it 2/3 with gas leaving room for compressed air.

I want to understand more before trying gasoline. I understand that when gas is compressed it generates heat. Will my quick pressurization of compressed air cause any temperature rise problems causing the gas to auto ignite? Gas auto-ignite temperature is around 500 deg F. What do you guy's think??

Thanks,

CharlieP

 

[capnahab]

120PSI seems pretty high to me with gasoline when 5PSI is called high pressure natural gas. Are you using fuel injection? But I still would not pressurize the gasoline tank with 120PSI, a rupture would be catastrophic. But if you were only pressurizing the fuel line to the injectors then the volume of gasoline woud be small and you would not need a fuel pump.

After reading your thread more carfully, 8 ounces is not much but even it the pressure vessle is rated at 600PSI you have to consider all of the components of the system. You don't want your toy race car to go out in a blaze of glory, been there and done that with my advanced homemade alteration of a model rocket when I was 13.

 

[pixel01]

I have worked quite a lot with compressors and those things. Per my experience, with normal compressure (up to 120psi) you can not heat the air inside to about 100-150oC. Of course the temperature depends on the rate at which you pump the air, but for a commercial compressor, it's quite safe.
What I am concerned is the gasoline can destroy some rubber seals if they are not designed for special purposes.

 

[charliep]

Thanks. The o-rings and valve that I'm using are made of Viton so gasoline will be fine. The air supply will be pre-presurized air from a compressor air hose. Would it be helpful to limit the air fill fitting by using a small hole to slow the filing time of the cylinder?

 

[pixel01]

Thanks. The o-rings and valve that I'm using are made of Viton so gasoline will be fine. The air supply will be pre-presurized air from a compressor air hose. Would it be helpful to limit the air fill fitting by using a small hole to slow the filing time of the cylinder?

Sure, with a needle valve you can pressurize slowly enough that the tank is kept almost at the same temperature.

 

[charliep]

Actually this is far from a toy car! Our cars operate on Methanol and during the start and end of the season when outside tenmperatures are 50 F and below the engines have difficulty starting. The standard practice is to squirt some gasoline down the carb (Not fuel injected). Instead of pouring gas down the carb I'm going to squirt gas from an orifice under the carb. The pressurized vessel supplies or pushes fuel out of the orifice. The increased pressure allows for many squirts per charge.

 

[Q_Goest]

Hi Charlie,
I can't help but think that your situation is a serious step-out. Normally in industry, we'd perform a http://en.wikipedia.org/wiki/Hazop" (hazardous operability analysis) on anything new like this and cover it under an MOC (management of change) which is signed off by the safety and engineering community where you work - even for something as small as an 8 oz container.

I'd agree with pixel that the temperature isn't going to approach the auto ignition temperature you're quoting, but that's not the only concern. The flow of any fluid, including air, through a pipe can generate static electricity. Other considerations would include electrical equipment in the vicinity which should be per NEC code (NFPA actually). Have you done any kind of analysis to ensure you've identified every potential source of ignition, overpressure, failure mechanisms, etc, and what would happen if a leak developed at one of the seals?

Have you considered using nitrogen instead of air? At least then you'd eliminate the mixing of an oxidizer with your fuel.

One other thing - Viton is notorious for getting hard and loosing it's ability to seal at fairly modest temperature. If you use this where temperature goes below freezing, you may find Viton seals open up. Check material compatibility in the http://www.parker.com/o-ring/Literature/00-5700.pdf".

 

[charliep]

Thanks for your input. The bottle is aluminum mounted to the steel frame of a dragster. The top of the bottle is nickel plated brass and all of the hoses are high pressure (3000 psi) steel braided teflon lined hose. 3/16 ID. The solenoid is brass and rated for gasoline. It operates using 12 vdc from the race car. The bottle (8 oz total capacity) is 3/4 filled with gasoline and pressurized to 120 psi using a standard air hose thru a schrader valve (Same way you put air in a tire). The gasoline is directed under the carb using a small atomizer nozzle that's fed by a small jet (.018" diameter). Each one second activation of the solenoid is like pumping the gas pedal once. Our cars easily start on gas and as soon as vacuum is available Methanol begins fueling the motor through the carb. Keep in mind that most of our cars also use Co2 bottles pressurized to 1200-1500 psi, fuel pumps rated at 50 psi, Nitrous oxide bottles at 2000 psi with heaters wraped around them to keep them warm. Most of out motors are generating 900-1200 HP normally asperated.

 

[Q_Goest]

Hi Charlie,
Thanks for the clarification. Sounds like you've got things under control. Just a thought though. Why not use nitrogen or the CO2? You obviously aren't concerned with the additional nickle, and I have to believe nitrogen is readily available with what you're doing.

 

[Q_Goest]

Scratch the CO2, it might liquify or solidify.

 

[pixel01]

Instead, I see the good aspect of liquidified CO2: you can store more gas at the same tank capicity.
One possible problem is that you need enhanced pressure for starting only. Then at nornal operation, you have to depressurize the gasoline tank. Have you thought of this task?

 

[charliep]

The pressurized container is used to store some gasoline just used before starting the engine. I'm pressurizing the bottle so I do not have to use a fuel pump. The tank remains under pressure for the next time I need to start the engine in cold weather. Usually once the motor is started and get's some heat it starts the rest of the day just on Methanol. I have thought about using the CO2 that we already have on the vehicle. It's already regulated @ 100 psi and I could simply keep it connected to the bottle with the gasoline in it. I just wasn't sure of the chemical reaction between the CO2 and gasoline? We generally do not have Nitrogen available. It is starting to become popular to fill our tires with nitrogen but the people who use it are few and far. We all have air compressors at our trailers and it seemed easy enough to fill the small tank up once in a while and pressurize it with air. Keep in mind that one fill of the 8 ounce container will last us several weeks of starting. Keep up with the great ideas.

 

[Dale]

Hi CharlieP,

You have, in general, gotten some good advice here. However, I would strongly caution you against risking life or limb by taking the word of a bunch of unknown individuals on an internet chat room, none of whom have actually seen your design. You should not place anyone (including yourself) at risk until you believe that you fully understand all the physical principles involved and that your design has an acceptable margin of safety.

 

[charliep]

Well understood DaleSpam. I have already received data from the manufacture of the tank that they are regulary tested to 600 PSI burst pressure. At 150 psi that a safety factor of 4. In addition I have filled the tank remotely many times using 120 PSI compressed air with water in the bottle and also with the bottle empty. I have not been able to measure any increase in bottle temperatur after pressurizing. I will however keep looking to back up my observations with real principles

 

[capnahab]

As far as I know, the only advantage with using nitrogen as compared to atmospheric air as the pressurizing agent (omitting food grade) is that there is no moisture if it is quality nitrogen. There is a lot of moisture in pressurized air unless there are stages of air drying yet still the water moisture creeps through. Argon may be a better substitute since it is inert and a bottle of Argon is commercially available at a welding supply shop, been over ten years since I bought a bottle of argon but back then it was less than $100 to fill one (you only rent the bottle and not buy it), a mere pittance when considering the value of a dragster. Helium would probably be the best but it is more expensive and it is also inert but gives you the added advantage of being capable of singing "Tip Toe through the Tulips", like Tiny Tim, at the pit stop party after you win the race! A Happy and prosperous New Year to All!

 

[BriK]

How about a high-pressure/low-flow gas-duty pump instead of air pressure?

Pump the gas in a circle out of the tank and back to the tank continually with the return being thru a needle valve or small orifice or capillary (very small) tube to give the pump something to push against, then T-off the loop after the pump to your engine using a gas-duty solenoid valve. Open the solenoid valve to get the high pressure gas flow to the engine...

 

[charliep]

My concept is to develope a pumpless simple solution by just storing air.

 

[BriK]

Whenever I pressurize a pressure vessel, I always always always have a safety relief valve in place...even if the compressor cannot develope the pressure needed to burst the vessel...its just a good general safety practice...most safety codes likley require it...

Picture if, god forbide, it did ignite internally...your small tank is now a grenade...

I have worked as an HVAC engineer for years and have actually seen rooms destroyed by compressor systems that failed to releif the pressure when flow is impeded, or cooling fails and pressure in the system builds...its not pretty...even your window air conditioner has a means to releif pressure whether it be a soft solder joint or an actual safety valve...

Make sure the safety valve has high flow so that if it does fail you do your best to avoid the vessel failure pressure in time...

The direction of safety valve flow is also a consideration...like a safety blowoff on a boiler would be a concern...you don't want to burn someone walking or working nearby...

I bet there are a lot of studies on this for car gas tanks...I would guess that gas tanks are made to fail (releif mechanism of some kind) pretty easily to avoid making a worse explosion...

 

[eddyz71]

Be safe.

I would highly recommend using nitrogen if you insist on pressurizing a bottle of gasoline. Argon is $$. Helium is $$ and leaks quite easily due to small molecule size. In the case of gasoline, N2 is as inert as argon. It is used for inerting purposes at every refinery in the world. I would go to a welding supply store and get: a 125 cu. ft. or 251 cu. ft N2 cylinder, inert gas regulator (delivery pressure 0-200psi), and safety literature on compressed gases.
You can also get safety literature from CGA - the Compressed Gas Association.
Excellent point about the relief valve - ALL pressurized vessels require this feature. The outlet should dump the contents away from any/all potential ignition source ie: exhaust header, electronics, etc.
Since you've already considered the heat of compression during pressurization, also please consider any radiant heat from the motor or exhaust on the gasoline bottle not because it may be enough to ignite the gas tank, but because it will raise the pressure in the tank to the point of venting through the pressure relief valve. (heat=expansion=higher pressure)

 

[FredGarvin]

As far as I know, the only advantage with using nitrogen as compared to atmospheric air as the pressurizing agent (omitting food grade) is that there is no moisture if it is quality nitrogen.

There is also the inert property in that it will not support combustion.

We use N2 blankets on some of our fuel systems with some frequency. N2 is definitely the way to go.

Excellent point about the relief valve. I can't stress enough the need for a properly sized relief.

Also, please remember that, when using gasoline, that even though the canister is empty, there can still be a considerable amount of vapor inside the tank that can still ignite. Ideally you should establish a safe way to completely purge the tank, i.e. to a flare.

 

[RonL]

Not having time to do any research right now, i'll pass on an idea that you might look into. I have in my shop an old blow torch that i picked up in a garage sale, the same kind of torch i had seen my dad use to thaw frozen pipes, back when i was a kid.
It is a brass container, with a hand pump with a leather seal on the piston, it holds about a quart of white gas and when pumped up to pressure, a needle valve is slowly opened, and as the gas flows thru a venturi it draws a proper amount of air, and when you lite it, it burns in a bright blue flame about 6" long. Never put a gauge on one but the pressure likely goes to a 100psi or more.

Do a little research on a "hand pump blow torch" or something along that line, and you might find what you are looking for.
Hope this helps a little.

RonL