Propane Storage


Old 09-05-13, 09:32 PM
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Propane Storage

(We need a special forum for Propane; I've found propane related threads all over the place - hard to tell where to post.)
Anyway, I'm trying to become knowledgeable in propane stuff so I have a couple of basic questions:
1) If I had a brand-new, filled 20# propane tank and NEVER used it; how many years would it maintain its charge? (I realize this is a hypothetical.)
2) Disregarding travel distance, is it more economical to buy your own 20# tank (Costco has them for about $25 right now) and fill it where you can -or- buy the initial, filled 20# tank (about $50) and do the swap-for-refill routine thereafter (about $20 each swap)? I haven't checked (yet) with a station as to what a refill costs here in San Diego.
3) When I asked a driver for one of those 'swap' trucks at HD, he said the only thing that he was aware of was that the 'odorant' that was added would, over time, settle to the bottom of the tank; otherwise, they would last forever.
4) Are the 20# tanks particularly sensitive to heat. We're running low 80's right now; only rarely do we break 90, if ever. The tanks would be stored outside in the shade.

What I'm after here is to decide if I want to convert my gas generator to propane and stock up on about 4-6 propane (20#) tanks and wait for our next disaster power loss (granted rare around here).

I apologize if this has been answered somewhere else but I gave up trying to find all the propane threads; they're everywhere.

Thanks for your thoughts.
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Old 09-05-13, 10:00 PM
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There isn't a "charge" per say. Propane is pumped into the tank as a liquid. The liquid vaporizes into a gas at room temperature. Propane can sit there in tanks for many years and still be ok but.... the tanks are only steel and will rust.

Also.... tanks are certified for X amount of years. A new tank may be certified for 12 or so years. That means you can have it refilled again and again until it reaches its expiration date.

Those swap tanks you get are tanks that people have traded back in, have been re-certified for several years, painted and refilled. So it would not be to your advantage to buy a swap tank and use it for long term storage.

Dollar for dollar..... you will get more power out of gasoline then you will out of propane. I would definitely convert your generator to propane. It's not a permanent can still go back to gas.

I plan to convert my generator to trifuel. Since I have natural gas I want to use that with gasoline as backup.

I'm not sure of the exact ratio but you will lose like 10% of your generators KW rating by converting to LP.
Old 09-05-13, 11:29 PM
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1. The propane will not go stale like gasoline. Provided there are no leaks the tank will still contain 20 pounds of propane 100 years after filling and it will still burn as well. There IS a caveat however. New tanks contain air and if not properly purged this air will have a tendency to dilute the propane gas that is released. If you want to use this power a generator then you really need to either get your tank(s) properly purged or else use them a few times on a barbecue.

2. Buying the "swap" tanks is highly UNeconomical. You can often buy propane for around $3 to $4 a gallon. Consider that a 20 pound tank holds about five gallons and you can see that you pay a premium for the convenience of the "swap" tanks.

3. As is common, the big box mega-mart homecenter employee hasn't a clue. The odorant will NOT "settle out" in the bottom of the tank or anywhere else. The odorant is fully dissolved in the liquid propane and is released as vapor when the propane gas is used. PJ is correct that the tanks do not last forever but as long as they are kept clean and dry and not allowed to rust they can be recertified several times. Initial certification when manufactured is, if I recall correctly, 12 years and subsequent recertifications are usually good for 10 years. The down side is that recertification is not free and can eventually become non-cost effective when compared to just buying a new tank.

4. No tank is particularly sensitive to the heat but all tanks containing a gas or a combination of gas and liquid will change their internal pressure with differences of temperature. An ambient temperature of 80 to 90 degrees F. should not be any problem with standard propane tanks PROVIDED they are not overfilled. Federal law has required 20 pound tanks and smaller (maybe as large as 40 pound tanks and smaller) to be fitted with an OPD (Overfill Protective Device) valve to prevent overfilling. An overfilled tank, when heated, will cause the pressure safety device to release excess pressure and the release of the flammable gas is a very definite hazard.

I think that using 20 pound tanks for generator operation is foolish because they would need to be changed fairly often. I would use nothing less than a 40 pound tank and if you have a pick-up truck then I would strongly suggest using 100 pound tanks. The cost for a 100 pound tank is only a few dollars more than five 20 pound tanks and you only have one tank to recertify vs. five.

Now to correct a couple of misconceptions. Based upon current costs for propane vs. gasoline the cost of operation is about a wash with a slight edge towards propane. It is true that gasoline has a higher heat content per gallon than propane but that is offset by the (generally) lower price per gallon of propane.

Conversion of a generator engine to use propane can be either permanent or a multi-fuel approach. The simplest method destroys the original carburetor so if you wanted to go back to gasoline you would have to purchase a new carburetor. For some generator engines this could be a couple of hundred dollars or more. The second method uses an add-on venturi between the carburetor and the air cleaner. This method allows for easy changeover between gaseous fuel (propane or natural gas) and gasoline. The downside is that for some generators there is not enough room for this add-on venturi which is about 1-1/4 inch thick and so requires modification of the generator frame. This can sometimes be overcome by a relatively simple device called a "snorkel" kit. The third method is drilling the carburetor for a gaseous fuel inlet connection.

It is a myth that conversion of a small engine to gaseous fuel results in a loss of power. That myth comes about because of the lower BTU content of gaseous fuels compared to gasoline. The truth is that the power developed depends upon how much fuel and air passes through the carburetor and not the BTU content of the fuel. Most smaller engines have carburetors that are capable of passing vastly more air than the engine can use and the gaseous fuel governor can pass enough fuel to match the air flow so no loss in power.
Old 09-06-13, 03:30 AM
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Most propane dealers charge a higher rate for propane when filling small 20lb tanks than for filling larger tanks when they come to your home with the truck. Many dealers will rent or sell tanks. Then the propane is sold at two rates depending on whether or not you own or rent your tank. The best rate is for a tank you own. But, if you are just getting propane to power your generator for emergency use the cost of the propane should be a trivial concern since you will be using so little.
Old 09-08-13, 04:31 AM
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Since I, finally, tracked down my post (I hadn't subscribed properly - most confusing), here I am.
Furd - Q#2 - I'm a little confused. If Propane costs, say, $4/gallon, and a 20# tank holds 5 gallons, that's a cost of $20 to re-fill. The Home Depot/Lowes 'swap' cost is $19.97 ($20) - after you buy the first tank/filled for $30 (Costco has them for $25). Since you have to buy your first tank somewhere, that part seems to be a wash.
So I, at this point, don't see it as UNecomical - it seems to be, also, a wash. And I don't have to find a station that fills them.
It seems that only if I find propane cheaper than $3/gal would it pay to drive to a distant station (my HD is 5 minutes away) - or am I missing something?

Additionally, my Q#1 was aimed at the 'leak potential' aspect - if there is no interference, do these tank valves 'leak'? It's kind of like the aerosol cans - they teensy tiny leak so after a couple of years, you have a can full of paint but no propellant - it's so tiny you don't notice until you pick up a very old can and it doesn't spray. Are the propane valves 'non-leaking' (even if ever so slightly)?

Last edited by silvercbx; 09-08-13 at 04:53 AM.
Old 09-08-13, 05:59 AM
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Hi... If you look at the homestore tanks they sell and swap you will realize thos tanks are smaller. You get less propane for your money... I believe they are 15 lb tanks opposed to the 20 lb tanks... Which infact I do not think are even 20 lb anymore but 17.5 lb or so...

Basically since the advent of the overfill prevention device, 20lb tanks are filled to a max of 17.5lbs. When fuel prices sky rocketed a lot of the swap propane companies went to 15lbs basically keep their price lower and roll the wool over the consumers.

If infact you are considering propane to run a generator only the the advice to get a 100 lb tank is your best option... Generators use 10k btu per hp roughly. So a 10 hp gen will need 100k btu. Like running your homes boiler or furnace... If you do the calculation you can figure how long you can run on propane, you will see 20 lbs tanks will be a pain...

But heck try it out... It may be yrs till the next major power outage... Hurricane season here on the east has been a dude this yr... Sandy gave us enough for a lifetime...
Old 09-08-13, 12:07 PM
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Good information from Mike (Lawrosa). A properly filled propane tank is only 80% of its ultimate capacity to allow for expansion due to temperature increase. If you look at the stamping on the handle of the tank you will see the letters WC and a figure such as 26.2 (this is on my 11 pound tank) The WC stands for Water Capacity and the number is pounds of water. Since water weighs approximately 8.35 pounds per gallon it is fairly easy to convert to the ultimate capacity of the tank. In my example of my tank that would be 3.14 gallons ultimate capacity. Take 80% of that and you see that the maximum allowable capacity is 2.5 gallons.

Propane weighs approximately 4.2 pounds per gallon so a tank that will safely hold forty pounds of propane would contain about 4.75 gallons. To find the ultimate WC of that tank you multiply the 4.75 times 125% (1.25 because 125% is the reciprocal of 80%) and find 5.95 gallons and then multiply that by 8.35 to get the ultimate WC in weight. Doing this simple arithmetic will let you know the TRUE volume of the tank in question.

Propane is sold by either weight or volume. Years ago I used to buy propane at my local welding supplies store. They would weigh the tank prior to filling and then set the scale to the tare weight of the tank PLUS the total weight of propane the tank could safely contain. They would then fill until the scale tripped and that was full up. Subtracting the initial weight of the partially filled tank from the total weight gave the weight of the added propane and that was multiplied by the cost per pound to arrive at the cost. Now almost everyone that sells propane has a totalizing flowmeter calibrated in gallons. Tanks now are filled to either the 80% level as determined by the fill bleed valve or else until the OPD valve shuts off. Pure and simple, you WON'T get five gallons into a "standard" 20 pound tank.

My price range of $3 to $4 a gallon is pretty much what I see locally. The higher figure is calculated from the cost of the "swap" tanks based upon them actually holding five gallons, which they do not, so the upper cost figure is actually higher. Buying at a bulk plant is going to be much closer to the lower figure and buying at the gasoline station (many gasoline stations sell propane for RVs and trailers) will be somewhere in between. I have seen advertised prices for propane much lower, some as low as $2.25 a gallon in some areas but not in my local area.

As for the valves, either the main valve or the 80% bleeder valve is fairly rare but always possible. I have no data but I suspect that the swap tanks are worse in this respect simply because people do not take as much care with items that they do not own, but just borrow. They make solid brass plugs that screw into the standard POL tank valve to seal the valve against leakage during transit, in fact, it is unlawful to transport a propane tank without the main valve sealed with at least a plastic screw-in plug or a rubber dust cap over the ACME threaded valve.

And that brings up another item. All the 20 pound tanks sold today have the ACME external thread valve and use the flow-limiting ACME nut in most applications. This flow-limiting nut may not pass enough propane to run a larger generator. There is also the physics of boiling temperature of the propane liquid to consider. For any given size tank there is a maximum gas flow rate from the tank versus the ambient temperature. As the ambient temperature drops the flow rate decreases because the propane liquid cannot absorb enough heat to boil into vapor. The outlet pressure of the tank will drop significantly and if the generator engine is "pulling" too much gas then it will simply run out of gas despite the fact that the tank is far from empty. The larger the tank the more heat it can absorb from the surrounding air and the greater the gas flow even at lower temperatures.

Again, I strongly suggest that IF you want to store propane for an emergency that you not use the swap tanks but go with a minimum of 40 pound tanks or far better 100 pound tanks.

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