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Old 11-05-2013, 12:38 PM   #1
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Curious About Propane Refilling

I recently had one of our propane tanks refilled, and I asked the person doing it about why some refill folks open the vent screw on the side of the tank valve (he didn't happen to be one). He replied, and I've heard it before, that some people do this to "let the air out". I find this really curious. A new tank, or I guess one that has been allowed to be totally depleted and left with the valve open, would need to be purged, probably with nitrogen. I understand that. But what's up with just routinely venting gaseous propane out of the top of the tank while filling it? I can think of a couple of things. It's an old wive's tale, and just because there is a screw there they think it's a good idea to use it? They get to sell a tiny bit more propane? Maybe their pump is too weak to get propane to flow into the tank against the vapor pressure of gas above the liquid, so venting off some pressure might allow them to pump more into the tank? I don't buy the "air in the tank" idea at all. That would mean we're all going around with potentially combustible mixtures in our tanks, and exactly how would the air get in there to begin with? Does anyone really know where this thing started?
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Old 11-05-2013, 12:41 PM   #2
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Interesting question. I get mine filled at the closed U-Haul - and only one time has the person opened that screw to vent. That guy claimed it let him fill the tank fuller. And yes - they charge to purge new tanks, so it should not be venting out air.
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Old 11-05-2013, 12:47 PM   #3
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The bleed valve is opened when filling to indicate when the tank is full, as liquid will spray out at that point. If you fill by weight, then you just fill to the specified weight, and the bleed valve need not be used. Either method should result in approximately the same fill level.
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Old 11-05-2013, 04:54 PM   #4
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Originally Posted by Jim Bennett View Post
The bleed valve is opened when filling to indicate when the tank is full, as liquid will spray out at that point. If you fill by weight, then you just fill to the specified weight, and the bleed valve need not be used. Either method should result in approximately the same fill level.
Aha, just found it on a balloonists site. You're right. The bleed screw is connected to a dip tube that goes down to what is the 85% full level. I figured the overflow protection device (float valve, in other words) would cut off the propane, but I understand that's not it's purpose. The propane should be cut off before reaching the point where the OPD closes. So now I'm wondering what happens when they don't open the screw and don't go by weight (which I witnessed yesterday). The flow of propane eventually just stops. I wonder if they're relying on the OPD in that case, which is supposed to be hard on the pump. Another mystery, but thanks for the tip on the use of the bleed screw. I certainly wasn't buying the story about air in the tank.
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Old 11-05-2013, 05:18 PM   #5
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I think my U-Haul place charges by the gallon now.
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Old 11-06-2013, 11:05 PM   #6
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Quote:
Originally Posted by Parker View Post
He replied, and I've heard it before, that some people do this to "let the air out".
As you know, there is never any air in a propane tank once it has been purged for use the first time.

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Originally Posted by Parker View Post
Does anyone really know where this thing started?
Poorly trained people filling tanks, and the tendency for misinformation to expand to fill all available space. Also, releasing propane vapour from a cylinder being filled should increase the rate of filling, by reducing the amount of vapour to be compressed into a liquid, and in turn reducing heating of the receiving cylinder... but this should be irrelevant when filling a cylinder with a pump, and all filling stations use pumps.

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Originally Posted by Jim Bennett View Post
The bleed valve is opened when filling to indicate when the tank is full, as liquid will spray out at that point. If you fill by weight, then you just fill to the specified weight, and the bleed valve need not be used. Either method should result in approximately the same fill level.
Yes, exactly, but only if that is an 80% level bleed valve, also known as a fixed level gauge. Here, small portable cylinders don't typically have those (although all large cylinders and all permanently mounted cylinders do); I would be concerned that what looks like a bleed valve might be a purge valve, which is not the same (the purge valve would not be connected to that dip tube).

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Aha, just found it on a balloonists site. You're right. The bleed screw is connected to a dip tube that goes down to what is the 85% full level.
Well, it should be 80%, but yes that's right... again, if it is a bleed valve for level checking. An 80% fill bleed valve typically has a tiny knob (often triangular) which is turned with the fingers; a purge valve used to exhaust air from the tank before its first use may just have a slot head which is turned with a screwdriver.

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I figured the overflow protection device (float valve, in other words) would cut off the propane, but I understand that's not it's purpose. The propane should be cut off before reaching the point where the OPD closes.
The Overfill Prevention Device (OPD) on a portable cylinder is a backup device for safety, not to be used as the primary method of determining fill level. Many untrained or just lazy filling station operators depend on it, but that's potentially dangerous, as these devices are not reliable enough for this purpose.

Tanks which are permanently mounted (such as to a motorhome, or to any vehicle as motor fuel) now commonly have a "stop fill" device, which is approved as the method for stopping the fill. I haven't seen either the designs or the prices, but I would bet that an approved stop fill device is better and more expensive than an OPD - I wouldn't be surprised if the stop fill device alone costs more than an entire 20-pound cylinder with OPD-equipped service valve.

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Originally Posted by Parker View Post
So now I'm wondering what happens when they don't open the screw and don't go by weight (which I witnessed yesterday). The flow of propane eventually just stops. I wonder if they're relying on the OPD in that case, which is supposed to be hard on the pump.
I don't understand why it would be any harder on the pump than releasing the handle on the end of the hose, but as mentioned above that's what they're doing and they shouldn't be doing it.


I had a 30-pound cylinder filled in British Columbia a few days ago. There was a scale there for the purpose of determining fill level but the kid just put the cylinder on the ground, connected the hose, and started filling; I asked him if he was depending on the OPD to stop the fill and he said yes, that's what he had been taught. I'm absolutely sure he was not taught this in any approved course, but the other kid who showed him how to fill tanks probably said that. I explained that this was dangerous and illegal, and in response he opened the bleed valve. Sure enough it eventually spit liquid and he stopped, but when I got it back to the RV I weighed it and it was 2 pounds overweight. I was concerned enough by this that I intend to contact the tank manufacturer to determine whether the valve he opened is a fixed level gauge bleed or a purge valve. The OPD apparently didn't work.
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Old 11-07-2013, 12:08 AM   #7
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I hope you also contacted the gas station company and appropriate government agency.
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Old 11-07-2013, 09:33 AM   #8
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I hope you also contacted the gas station company and appropriate government agency.
I did not, but I should have. It did occur to me, but I decided not to for two reasons:
  1. I have not yet confirmed the nature of the bleed valve on my cylinder, so any report would be confusing... even though there is still the fundamental error of using neither weight nor a bleed valve before I questioned his method.
  2. Even before starting to fill, he failed to check either the condition of the cylinder or the manufacture/certification date. These steps are almost never done - in my experience - so if I am to report this event I should report almost every station that I visit... and frankly, I want someone to continue in business selling me propane. That's irresponsible of me, but the alternatives seem likely to a frustrating bureaucratic run-around in which nothing is done, or a response by station operators that training and regulating staff is not worth the hassle so they will get out of the bulk business and just sell those massively overpriced pre-filled exchange tanks. I have been known to be somewhat cynical...

I suppose that I am - at least for now complicit - in the station's unsafe practice.
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Old 11-07-2013, 06:08 PM   #9
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I like your logic. Life is too short to worry about the piddly annoyances.
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Old 11-07-2013, 07:27 PM   #10
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I had 3 cylinders filled this afternoon. The two newer ones took exactly 20 pounds. The older one took 19.2
He did not weigh them, but used the valve on the side and shut the gas off when they were full.
I weighed them when I got home.

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