when refueling a propane tank, how do you keep from forcing some air into the tank? I know there must be a way to aviod pushing that is in the fuel inlet into the tank. there would only be a little air in the inlet, but after several time filling the tank, the little bit of air put in each time would need to be bled off. How is this done? I know that only 80% of the volumn of the tank is filled with liquid, the other 20% is vaporized propane. You can not just open a valve, or the liquid would vaproize with the loss of pressure. Does anyone know?
Not sure I see your problem. While I guess a small amount of air could enter, I think it would mix in with the propane and, under pressure, you wouldn't notice it. Maybe you're thinking there would be a bubble of air?
When I was running LPG ag tractors, we just vented the tank to atmosphere to fill. The storage tank was pressurized by the LP and supplied from the bottom (under the liquid level. Venting the tractor tank released pressure and allowed the pressure in the storage tank to push the liquid over. We stopped when we got liquid coming out the vent (the vapor turned to a white cloud).
I really don't think it's an issue in the situation you describe. I'm not sure if the automotive LPG systems tap off the vapor area of the tank or run off the liquid area. Our tractors could do both. We ran off vapor till warmed up and then switched to liquid. If autos tap the vapor off the top of the tank, then any air pumped into the tank will be used by the car.
when you fill a propane tank, there is a small bleed off valve which is supposed to be opened while fueling. it prevents the tank from becoming over-pressurized and thus opening the pressure relief valve. opening the bleed off valve also allows you to properly fill the tank to the correct level by bleeding off liquid once the level is reached.
The little bleeder valve your are talking about is only used to verify volume in the tank. As said early LP tanks are only filled to 80%. If you over fill one especially in the summer heat you will diffinently have the relief valve pop off. The amount of air introduced when filling is negligable and not and issue.
However if you ever have to blow a tank down and remove a valve the tank will need to be re-purged to remove any air that was allowed to enter. Most automotive fuel systems use a vaporiser that heats and regulates the liquid LP to supply vapor to the engine