Freezing Propane Tanks. A Different Solution

[maddog]

It's been a long time since I took my graduate level heat transport course, but I think that I can still figure out an estimate if I can locate my book.

Now you're putting the pressure on.

No pressure . I found your analysis of the heat draw as the propane evaporates very interesting. It would be great to have even a back of the envelope estimate for how hot and perhaps how fast the air must flow around the tank. If the tank can be kept at operating temp without any active heating that would be great. At least it would calm everybody down I wish I had taken a heat transport class at any level! As for using a plain fan, I could just do the experiment. I like to keep a fan blowing anyway to disperse any leaks that might pool and also to move the CO out.

....It is a brewbelt, it keeps your fermenting brew around 70 degrees. I ordered one and will try it out on my 20lb tank. Since it is below freezing now in MD and I work in an unheated garage it might work. I will let you know how it works out. Cost around 25 bucks. What says you? ...

It sounds like it definitely could be a solution. Of course fermentation tanks go the opposite way from propane tanks in that they generate heat internaly. But the propane tank just has to stay above 0F to deliver a useful pressure. I had thought of using a heating pad - one of those things you sit on or wrapping it with pipe warming cable. Also one of those mini heater that you can sit on your desk might be enough. Thats why it would be nice to have an estimate of the required heat input if Jack can manage it. I am sure none of these ideas are sanctioned by OSHA, DOT, DOE, NFPA, FBI or XYZ either. So don't try these ideas at home unless you are a mad dog like me!! :D

[Jack Evers]

No pressure . I found your analysis of the heat draw as the propane evaporates very interesting. It would be great to have even a back of the envelope estimate for how hot and perhaps how fast the air must flow around the tank. If the tank can be kept at operating temp without any active heating that would be great. At least it would calm everybody down I wish I had taken a heat transport class at any level! As for using a plain fan, I could just do the experiment. I like to keep a fan blowing anyway to disperse any leaks that might pool and also to move the CO out.

A quick preliminary estimate (hopefully a SWAG -scientific wild xxx guess - rather than a WAG)suggests a heat transfer under a strong fan of about 10 BTU/ft 2/degree F. i.e. if you have a sq ft of surface in contact with the liquid (20 pound bottle about half full) are using about a pound of propane per hour and need zero degrees on the liquid, the air temp is 20 degrees, you can transfer about 200 BTU per hour which would be just about right. I'll do some experimentation.

[Woody]

There is a considerable amount of mis-information on this thread.

DOT placards are required for more than 1000 lbs of Hazardous Material not 100 lbs. Except for Explosives, Poison Gases and Radlioactive 3 materials which require placards for any amount.

What we call Propane is not pure Propane, it is LPG, Liquified Petroleum Gas, which is a mixture of Butane, Propane and other light ends. The mixture varies from location to location with more Butane in the mixture in the Southern areas of the country because Butane boils at a much higher temperature than Propane. Therefore, "Propane" tanks in a Southern location would tend to freeze up at a higher temperature than those in a location farther North. Butane boils at 31 degrees F. Propane boils at -44 degrees F.

A BLEVE is possible with any propane container with or without a relilef valve. All it takes is for a flame to impinge on the contaner above the liquid level causing the metal to become red hot and unable to contain the pressure in the container. The contaner then ruptures and the liquid which is well above it's boiling point immediately vaporizes and ignites.

Propane is held in liquid state by pressure. The warmer the ambient temperature, the more pressure is required to hold it in the liquid state. When things are compressed, they heat up, when pressure is released, things cool off. The reason that Propane tanks freeze up is because as the gas is drawn off, the pressure is released causing the temperature of the liquid to fall. When the temperature of the liquid falls below the boiling point of the propane, no more liquid boils to gas.

[Jack Evers]

OK back to the freezing propane problem. It has more unknowns than a 25 cent hamburger so you need to fill in your own situation. First to address Woody's concerns about propane/butane ratio.

A few facts:

air to cleanly burn Methane, the primary component of natural gas, equals 10 times the volume of methane
air to cleanly burn propane equals 25 times the volume of vaporized propane
air to cleanly burn butane equals 32.5 times the volume of vaporized butane

We all know that we can't connect a natural gas appliance to LPG (propane), but to a lessor extent the same holds true for for a propane rich LPG vs a Butane rich LPG. Although LPG is a broad term, it's application in the US and Canada is somewhat narrow. As appliances become more refined the fuel mix is narrowed and the LPG you buy is more defined.

Here are some comments from a former LPG certified technician in West Texas and an LPG dispatcher in the Northern Mid
west.


"The majority of propane appliances (including vehicles) these days require a high percentage propane fuel to optimize efficiency. I have been out of the business for a few years, but I would allow that there few suppliers of propane selling a fuel less than 90%."



"yep, you're right for our neck of the woods. we use the HD 5. i just talked to CHS propane division. that's cenex harvest states.

there is a commercial grade of propane used in southern Minnesota that is to only go for corn dryers (which is burned in a liquid form-unlike a vapor for houses and cylinders) and if you were to accidentally get it in your tank for your house, it would not be nearly as efficient and make your appliances all burn yellow.

so our HD-5 up here cannot be less than 90% propane, it's 5% propylene and the max it can have in it for butane is 2.5% and the mixture does not change during the year. in fact, the guy i talked to,*********, says the mixture hasn't changed in many years. the butane is added to propane because it adds to the BTU content, making it more efficient. "


The HD 5 spec mentioned is a fuel spec for the US and Canada which requires the propane to be > 90%. If your supplier may be supplying vehicle fuel, the mix is mostly propane. If you're buying from some off brand such as the corn dryer guys or you're in some other country you need to do your own research, Europe along the Mediterranean allows some extremely high Butane mixes. Seems that driving North to South over there with an LPG fueled vehicle could be a problem.

That said, if you can keep the liquid that I'll call propane to suggest it is mostly propane, at zero F or above, you should be all right for delivery pressure. As we draw off vapor the propane comes first and some computer simulations by a Chemical engineering friend suggest that as we use up a bottle that started at 90% propane we may be down to 70% or less propane 30% or more butane when the bottle is a quarter full. The boiling point is closer to -15 degrees than the -44 for pure propane.

Heat transport theory suggests that most of the heat transfer into the bottle will be into the liquid so a bottle a quarter full will only receive about a quarter of the outside heat that a full bottle will. A nearly empty bottle costs in three ways, limited heat transfer, a limited latent heat reservoir in the liquid, and an enriched propane fraction meaning lower pressure.or higher required temps. Since transfer is related to wetted wall and our 20, 30, 40 lb bottles are all the same diameter 1/2 full refers to 10 pounds of liquid. 1/4 full in a 40 lb bottle would be the same.

Some quick and dirty experiments that I've run suggest that the overall heat transfer coefficient U for a full bottle is around 5 BTU/hr-degree for free convection (sitting in still air) and perhaps 20 with a fan (I used a 20 incher about 4 feet away)

Now there are a number of possibilities for the tank:

1) For safety reasons it stays outside and vapor is piped into the shop. You really can't work much on sub zero nights.

2) It's in the back of a pickup, same problems as case 1

3) It's inside a heated shop. Generally no problem until liquid levels are very low and demand is high.

4) It's in an unheated shop that will warm up as you work

In cases 1 and 2 with air temps in the 20's or above, the tank at least half full and for case 4, a good fan blowing on the tank will probably keep you in business. One pound of propane vaporized and burned per hr will require a BTU input of 185 BTU/hr, at a heat transfer coefficient of 10 (half full tank and a fan) the required temperature differential will be 18.5 degrees. 20 degree air and zero degree liquid works. At a quarter full, you'd need air at 35 - 40 degrees. For folks looking at electric heating pads and such. 185 BTU/hr is about 60 watts. or a 5 amp heater. A brew belt is only 25 watts. It would help though.That assumes all of the heat goes into the tank.

Guess after all the number crunching and asking questions, the only new idea I have is use of a fan (forced convection) It was a fun project though. So keep your tanks fairly full (or use larger tanks) and remember that while a 20 degree wind may be unpleasant to you your propane will love it.

[maddog]

Jack, thanks for doing all that work. It's useful info.

My 20# tanks will freeze in the summer time. The frost line is about one third of the tank height. Taking the curved bottom into account this is probably close to 1/4 full.

As for the possibility of a bleve. The overpressure valve is designed to release at 375psi to prevent a bleve. It doesnt matter how the tank is heated. 10 years ago my town was burnt in a forest fire and propane tanks were significant hazard for the fire fighters but I dont recall any accounts of a bleve. From the videos I saw, the tanks simply turned into a ball of fire, much as you would expect a bucket of gasoline to behave under the same conditions. The process was rapid and catastrophic but there was no explosion. I assume that the overpressure valve vented at a high rate and the burning propane heated the tank accelerating the process.

I have now replaced the hand dryer with a small electric space heater which is more convenient and compact. The tank feels neither warm nor cool to touch. Since it sits on the floor, it blows mostly on the lower part of the tank where the liquid is pooled. In light of Jack's analysis an even smaller heater should be sufficient. Perhaps a mini desktop heater. I will see how far I can turn this one down. I am confident that with reasonable care, this setup is perfectly safe but that is just my OPINION.

[Jack Evers]

Different boards, someone just suggested a hood for the gas forge with a squirrel cage blower and dryer vent hose to carry the warm air to your tanks. Sounds like a pretty good solution.

[pkrankow]

A fan sounds 1) simple 2) cheap 3) risk free

I'll have to try it at some point.

Phil

[ThomasPowers]

Frankly I like my exhaust gasses to EXHAUST! Any re-running of ehaust through the system ups the CO production MASSIVELY! Not exactly risk free...

But could you put a piece of stainless pipe through the outflow and send air through that towards the tank? ie a heat exchanger?

[maddog]

Different boards, someone just suggested a hood for the gas forge with a squirrel cage blower and dryer vent hose to carry the warm air to your tanks. Sounds like a pretty good solution.

I considered that idea before I set up the heater. There is something frustrating about generating all that heat with the forge and then having to use a separate heat source to warm the tank. But after thinking about it, it seemed more complex and a lot more cumbersome. A small space heater is neat and easy and if you are running a blower the electricity is right there.

[Ratel10mm]

You could try a bottle blanket which would be sold by your local refrigeration parts wholesaler. It's essentially an electric blanket that goes around your bottle. I have seen these with thermal cut-outs to avoid excessive heating - something that I think would be a very good idea for a LPG bottle.

You'll need to do some research because they're not intended for bottles of flammable gas, & I take no responsibility should you decide to try one. However I'd think it'd be more efficient than a fan.

[Corin]

There are a lot of good points on this thread, some well made. There is some misinformation also.

Yes it is best to use more cylinders, or larger cylinders to get the vaporization you need.

Yes a fan works well to increase vaporization. like wind running through your radiator.

Yes lying cylinders down is bad. (liquid relieving to atmosphere expands 274 times to vapor, 2.4% - 9.6 % vapor to air makes an explosive mix, hence much much greater risk. For forktruck cylinders you will note the relief valve is at the top when lying down in the vapor space always present (see below).

Cylinders are never filled completely. WC on the cylinder collar denotes water capacity. Cylinders are filled to 80% to allow for thermal expansion of liquid.

No,a relief valve going off is not like a BLEVE, It is scary as all hell if it finds a source of ignition, and the ensuing fireball has and will kill. I have tested relief valves, seen first hand a big one going off, been involved in forensic cases where people have been killed, and I have videos that will make you think twice! LP Gas relief valves are generally pop acting, they can go from closed to fully open in a split second.

Yes Heating a propane tank is perfectly safe, in much the same way as walking up the middle of a busy highway is safe..... as long as nothing hits you. The issue here is purely if something goes wrong. sure at 113 F you have only 200 PSI in the tank, but at 149 F you have 375 PSI and that relief valve will open (hopefully). If the contents reach about 204 F No amount of pressure will keep propane as a liquid.
My link


If you do heat, and I am not at all in any way, suggesting you should, make sure. Really sure, like absolutely, you only have one shot at this, positively sure, that you cannot over heat. That heating elements can't short out. That you can't possibly continue to heat an already hot cylinder. That thermostats are backed up by over-temp cutouts. That the over-temp cutouts will absolutely positively work no matter were the fault occurs. Bad things can happen if even a small section of wetted cylinder gets really hot quickly enough.

I feel I have a good understanding of propane, I am very comfortable with heating in very controlled conditions. I work on Direct fired propane vaporizers that literally boil propane in a tank with a flame underneath. As comfortable as I am with that, I know it is not worth my time or the equipment to make a system to safely heat cylinders at home, I just use two 20# and keep them over half full. easy. It does not get below freezing here so it is not as hard for me as many of you.

If you have cash and a good gasfitter you could always consider the right tool for the job. my link

I have set up a thread for propane questions, if you have any, feel free to ask, Propane is my day job...
I can calculate your forges BTU's and vaporization capacity for your cylinders or tanks. I am happy to help wherever I can.

Ask Propane questions here!

[freemab]

Well, I am a little late coming into this discussion, but I will contribute my 2 cents anyway.

I am one of the ones who advocates the use of "cold" water to "warm" a propane cylinder. Obviously, if the water is so cold that the vapor pressure of the propane isn't sufficient for the burner, you could have a problem (but see below). On the other hand, if it's too warm (i.e., hot) you WILL have a problem. So what goes? And what is safe?

First of all, a disclaimer: I'm a chemist. I'm not an expert in propane, propane safety, hydrocarbons, fuels, burners, or anything of the sort. However, I have worked with this stuff myself, more than some of you, less than others, and I am able to parlay what I know into advice that can be safe and useful. Enough on me.

When I advocated the use of cold water it was specifically to avoid some yahoo from misinterpreting "warm" to mean "boiling hot", dumping his propane cylinder into a tub of boiling water, blowing out the safety plug, resulting in propane filling his shop and igniting on the first flame or spark it encountered.

What I know that some don't appreciate is (first) that if the water is of sufficient volume, it's heat capacity will compensate to a degree for a "low" temperature, and (second) that the temperature of the water should not be an issue.

Take #2 first: When your water freezes around the cylinder, the pressure inside the propane cylinder is still 50 psi. You need at most 20 psi at your burner and at most 20 psi pressure drop across your regulator. Hence you will not have a problem with water temperature short of freezing the water.

Now #1: As you evaporate propane in the cylinder, it absorbs heat and cools the propane. When the propane cools below the temperature of the water, heat will begin to be transferred through the cylinder wall into the propane. Hence, further tapping of propane will further cool the water. But the heat capacity of water is high, so this is not particularly a problem. If your water freezes, you should continue to get propane at 50psi until the layer of ice insulates the tank from the water.

Okay, but in practice we do see a reduction in propane flow. What of this? Well, first make sure you're addressing the right problem. If the freezing is occurring in your regulator or in a valve, warming the propane is not the answer. Fortunately, it is not particularly dangerous to warm a valve, and even a regulator can be warmed gently. Warm air is appropriate for this. Do not get these devices hot to the touch -- the plastic components inside could fail catastrophically.

Now as to the suggestion of directly warming the tank with warm air. Even that is not necessarily a problem. Suppose we decide to live carefully. You can measure the temperature change of the water around your cylinder over a measured time period, and, knowing the weight of water, calculate approximately how much energy was used per unit time, which is power. Now compare this to the electrical power used by the warm-air blower. If expressed in the same units, then as long as the electrical power approximates the power consumed evaporating the propane, you're okay. In fact, there will be considerable inefficiencies of heat transfer, so these numbers do not have to match anywhere near exactly. But if you're applying many times more electrical power (heat) than the propane is absorbing to evaporate, then you're likely to be heating the propane, which can be dangerous.

Those with a background in physics can vet my statements above. No offense taken for corrections to errors.

[DClaville]

here in Denmark you can buy a heat belt for gas tanks that goes in a 220v power socket they are made and sold by BP-Gas and made for use on propane tanks i think they are thermo controlled so they go on and off automatically at certain temps
i dont have one but might get one if needed
but will be looking into using two tanks first as it will be more convenient for me

[Drq]

Just to throw a little more gas on the fire....lol

I spent a bit of time burning brush for the Oilpatch in Northern Alberta. Its gets plenty cold up here to freeze tanks, not just frost them up but freeze them solid. (Three years ago we were working in -57c). Nobody was running a forge out there but we used a lot of tiger torches. The common thing to keep them from freezing was to make a circle of three or four tanks and then aim one of the tiger torches into them. Never saw any of them go off, but sure kept a wide berth from them and the guys who were working with them.

It wasn't what you would call a very intellectual work place, but the storys I could tell...

Drq

[demac777]

Here is my attempt to make sense of the subject in layman terms. Freemab had an excellent explanation and I'm essentially re-stating his description.

The conversion from liquid in the tank to vapor in the available head space in the tank is simply by evaporation, the same process where by our skin is cooled through sweating. The tank by nature cannot produce more heat, as our bodies can, so the energy contained in the "system" (composed of the tank, gas, and vapor) is fixed based on the ambient temperature. As the liquid vaporises, the energy (heat) contained in the system is reduced. By immersing the tank in water, we are simply adding more energy by increasing the thermal mass of the "system" as the water becomes part of that system by transfering it's energy through conduction. The manifolding of multiple tanks decreases the rate of evaporation, and thereby slows the reduction of the energy contained in the system (which in turn is increased due to more thermal mass of the larger volume),in the same way in which we do not cool as efficiently by sweating when the relative humidity is high, preventing rapid evaporation of our sweat.

As far as safety, I use those little tank buddy heaters which attach directly to the tank valve and they put out around 16,000 btu. I dont sweat those that much. When I'm fully up and running, I plan on my 100 tanks sitting on the outside, hardlined into the shop. That will separate any possible tank leaks/failures/blow-offs from ignition sources in the shop.

BTW, I can't swear all the info above is 100% correct, but my chemistry and physics background usually serves me pretty well, so take it FWIW. I'll have to assume your not paying for it, I hope! ;)

[wd&mlteach]

Is it normal for the regulator, gas line, and shut off valve to have a layer of frost when the gas forge is in operation? I have an NC Tools Whisper Momma and a 40# LP tank(Just Filled) running a 7lbs pressure for about five minutes. I have only used the forge three times since I relined it and I had a borrowed 20# tank. I never ran the forge for a long (more than 10 minute at a time) and I never saw any frost on the lines with the 20# tank. I am completely new with a gas forge and when I saw the frost, it was a new developement and I shut it down. I am now asking you folks is this normal?

Forge image attached the way I was using it, only the garage door was opened at the time of operation.

[Nakedanvil - Grant Sarver]

It's normal. Varies with temperature and humidity.

[Ric Furrer]

If you notice a drop in pressure you may need to link two tanks together or get one larger tank....depends how full the tank is and how much gas you are trying to pull off it.
It is not dangerous, but it may cut your forging short.
Keep notes and decide after a while what you wish to do.

Ric

[brian robertson]

You are drawing too much propane at too high a pressure. The frosting is expensive in 2 ways: it damages the diaphragm in the regulator and it's a sign of too much unburned fuel. Dial back the psi until the forge get up to forging temp then only if you're going to be welding crank it up. Call Nc for recommended psi settings.

[wd&mlteach]

Thanks for the advice, but I think I might have figured out the problem and found a new one. As I said I just had the tank filled and I checked the receipt. They filled the tank with 11.3 gallons of propane, from what I found on this site and other manufactures a 40# tank should hold closer to 9.7 gallons of propane. What is the safe way to handle this problem?

http://www.missiongas.com/lpgascylinders.htm

[Ten Hammers]

If what you say is true, you were charged for roughly 50 lbs of fuel. Check the data on your bottle (likely on the handle area). There should be a stamping entitled WC. This is the water capacity of the bottle. This information is used with a chart by the filler to determine how much fuel to fill with (in weight). The other option is that the filler vent filled the bottle (open the vent, fill till the vent spits) not caring about weight. Some valves have no vent. OK now that I read my own typing I have not seen an OPD valve with a vent. Some areas may get by with filling a 40lb RV bottle without an OPD valve. I can't really see the valve on your bottle.

After you have determined the WC of your bottle, go back to the gas filler and ask them how they got 50lbs of fuel in a 40lb bottle (if this is indeed the case. Show them your receipt). Take the bottle and have them weigh it after your few uses. Explain the situation to the supplier regarding frost.

As stated above, you can only draw so much gas off a given bottle size before frosting. Linking tanks can help (or larger bottle). These are all gas draw bottles I am speaking of. An overfilled bottle (gas draw) is possible to draw liquid through the dip tube.

Is (was ) this a new bottle and did you buy it from this gas supplier ?

[wd&mlteach]

I had the tank filled at a local hardware store. The guy who filled the tank did not fill it by weight he just hooked it up to the storage tank and ran an electric motor/pump device until it stopped at 11.3 gallons. The WC rating for the tank is 95.2. I know I will catch it for this but the bottle was not new, has a manufacture date of 2007. OPD Valve installed, may have failed?

[kraythe]

You are drawing too much propane at too high a pressure. The frosting is expensive in 2 ways: it damages the diaphragm in the regulator and it's a sign of too much unburned fuel. Dial back the psi until the forge get up to forging temp then only if you're going to be welding crank it up. Call Nc for recommended psi settings.

Uh no. I practically freeze my tanks every time I forge weld. Its a result of Bernouli's Principle. Its normal and I wouldnt worry about it.

[r smith]

You are drawing too much propane at too high a pressure. The frosting is expensive in 2 ways: it damages the diaphragm in the regulator and it's a sign of too much unburned fuel. Dial back the psi until the forge get up to forging temp then only if you're going to be welding crank it up. Call Nc for recommended psi settings.

What does frost on the line have to do with amount of fuel burned/unburned?

[Drewed]

Converting liquid propane to a gas is a endothermic process ( it requires heat.) The process draws heat from anything it can ( air, metal from the tank and valves, etc. ) Once the material is cooled to below the dew point, water vapor condenses on the part. Get it cold enough ( from expanding enough gas ) and frost will form. Really shouldn't hurt anything, but the chill on the tanks will lower the pressure on them.

Back when I was paintballing, we would used compressed co2 to power the markers. With rapid firing, I could chill a 20oz co2 tank to the point of causing frost bite in the middle of summer. In winter, we used to use electric gloves on our tanks to heat them while playing. High humidity days will increase this problem.

Liquidfied gasses should always be sold ( and there for filled ) in portable tanks by weight, not volume.