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Old 08-19-2010, 11:52 AM   #11
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Iggy, I don't know if I agree with your description 100%.

I do agree that most moisture should be evacuated from a propane tank. The "purge" is to do that, and also get most of the air out.

Propane boils @ -44F. Without pressure, it will be a gas unless you are really having a bad winter. When propane is pumped to your tank, it is a liquid because it is under pressure, even though the temperature outside is bearable. While the propane is in a pressurized tank, the bottom is liquid, but the top portion is a gas....that is why you never want to fill a tank to need the room at the top for the gas part.

By the way I understand it, the gas in the tank is not any colder than the tank temperature. Why any moisture in the tank can "freeze" a regulator is because of the decompression of the propane. As any gas goes from a high pressure to a lower pressure, it gets cold.....that is the way our absorption refrigerators operate.

I think I have posted these before. These were taken at a fire school I attended years ago. The 1st picture is a crew going in fighting a propane fire, and the 2nd photo is that same tank after the fire. Even with all of that heat, look at the ice around the top of the tank....that is what decompressing a gas can do.

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Old 01-17-2015, 06:58 PM   #12
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existing propane, air & moisture, purge???

Today, I had the uhaul guy purge my used tank as he was filling. I can see maybe at the beginning to rid the air from the coupler, but he did this for about .4 of a gallon - about $1.70 at there price (My tank typically holds about 4.3 gallons and he filled it to 4.7 gallons). A pretty significant amount. 20-30 seconds or more. It seemed like at least half the time it took to fill the tank. It was more than just air, as it was ejecting LP that had expanded, and thus freezing moisture in the air against the outside of the tank. (If I am not mistaken, 1 gallon of propane typically produces about 270 gallons of gaseous propane) -- Either way, I feel gypped.

I am wondering if this .4 gallon purge is normal. It seems like it may be .... at least a common practice, but does it actually do any good, and any purge after the first second is a waste let along 30 seconds, or a second & third purge mid fill.

Ok, I guess I have several questions that may help the thread make sense for others that find the thread.

A: Existing propane in the tank? If there is existing propane in the tank, great, you need less.

B: Why did you think it was empty? Did you try to use it? If so, then I could understand more than propane being in the tank.

C: Where is the purge and use tube? It seems that the grill type propane just gets the gas out. (The Vehicular {3rd photo} type pulls directly from the liquid) -- If its at the top, it will automatically purge any air in the tank. -- Gas Propane is 1.5x heavier than air.

D: Once the tank is empty, its not very likely that much if any outside air gets in. It would have to travel down the tube, and into a tank without an exit.

E: Water weighs about 4 lb's more per gallon than LP, and thus purging the upright tank does not get rid of that. It appears that none of the diagrams show a purge line extending to the bottom.


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Old 01-17-2015, 07:18 PM   #13
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I am under the impression that once a tank has been used there is no more need to "purge" it.

If he let gas escape at the very end of the fill then he just ran up the cost to you for nothing and may have overfilled the tank.
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Old 01-17-2015, 09:05 PM   #14
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My wife worked at a propane company and has filled thousands of LP tanks. If a tank is new or had been opened to outside air it needs to be purged. The way she purged it was to put a squirt of propane in the tank, shake it up to mix it, and then open the bleeder to release the moisture and air out. She would do three times to insure the moisture and air was all gone. Then she would fill the tank to 20 lbs of propane (4.7 gallons). This was calculated by adding the tare weight of the tank plus 20 lbs to equal the total of everything. In order to get the tank full she would use a screw driver to open the bleeder and release the propane GAS so the LIQUID propane could fill that space.
If the U-Haul guy only charged .4 gallon ($1.70) to purge the tank you got a steal. The wife had a flat rate of $10 to purge a tank.
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Old 01-17-2015, 10:09 PM   #15
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I took 4 bottles in to fill a couple months ago at our local propane dealer (pretty good sized outfit from what ive been told, several of the bigger than train car, storage tanks on site).
A couple bottle weren't empty, but i might as well fill everything since i was going. My wifes friend works there and filled them, and topped off the two that still had gas in them. They just use a scale and weigh before the fill (if not empty). The bottle weight is stamped on them. They just fill till 20 or 30lbs is in there (depening on size), and yes she bled off the tank on all of them. And being a weight fill, you dont get the charge for the excaped gas- im sure its already figured into thier pricing.

Im not real sure of all the moisture/purge requirements on propane. I am currently Oklahoma state certified for CNG and so i know whats required for that, but those are 3000 or 3600 psi systems so everything is a little more critical.
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Old 01-17-2015, 10:19 PM   #16
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Another reason to not watch. Ace hardware in my hometown charges a flat rate. No, I don't want to be to close to an explosive gas, especially when a teenager is doing the filling .....
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Old 01-17-2015, 10:27 PM   #17
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Originally Posted by donanddona View Post
Another reason to not watch. Ace hardware in my hometown charges a flat rate. No, I don't want to be to close to an explosive gas, especially when a teenager is doing the filling .....
Ahhh, as long as your not smoking or have an open flame close, your fine. The gas disperses and dilutes pretty quickly, unless its really cold.

Im only about 3 inches away from gas when I light my propane grille (and with a butane lighter in my hand )
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Old 01-17-2015, 10:45 PM   #18
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If I have a bottle being filled and the person filling it does not open the bleed and is relying on the float I tell him to open the bleed, when the escaping gas turns from clear to cloudy this is when the liquid is at 80% full in the bottle. I do not trust the float and I want to see the liquid discharge to make sure the float is not allowing an overfill of the tank. I just purchased a used MH and when I picked it up the LP tank was over full the needle on the gauge was pointing just above empty then I asked the tech (I use this word loosely) why it hadn't been filled as promised he said it was so I had him open the bleeder and it flowed liquid, it had been overfilled and pushed the needle past full then past empty. An overfilled tank can bring with it a whole new set of hazards that vapor space at the top is for more than room for vapors for your burner, it also allows for expansion and contraction of the liquid as outdoor temperatures change. With this in mind I don't mind paying for that little bit of lost LP.
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Old 01-17-2015, 11:41 PM   #19
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This was the first 30 seconds or so. Not the end, and it was not filled a bit, shook, bled, and then filled.

These are gallon meters ... I don't remember if the u-haul had a scale, but the other place does, it just cuts off before that. at 4.3 gallons (4.7 at the u-haul, but that includes the vented portion).
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Old 01-18-2015, 04:00 AM   #20
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Lyon's Butane Company always uses the bleed while fill my tanks. I have never felt like I didn't get a full tank. I go to them because I fell I get a properly filled tank and full every time. I use Lyons Butane year round and go thru a lot of propane heating my barn house and RV and BBQ, smoker. I have 20#, 30#, 100# tanks constantly rotating to be filled. Propane can be volatile if no handled properly, you usually don't get a second chance to do it the right way. Be safe don't mess up a nice week end of camping. A rainy day camping is better then a sunny day spent at work.

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