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Old 11-05-2013, 11:38 AM   #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, 11:41 AM   #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, 11:47 AM   #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, 03:54 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, 04:18 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, 10:05 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.

Quote:
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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Originally Posted by Parker View Post
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 11-06-2013, 11:08 PM   #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, 08:33 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, 05:08 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, 06:27 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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Old 11-07-2013, 06:50 PM   #11
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Originally Posted by fudge_brownie View Post
Life is too short to worry about the piddly annoyances.
I really am concerned by this issue, but I want to pursue it only when that pursuit is likely to be constructive. Venting liquid propane from an overfilled cylinder is a pretty significant annoyance.

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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.
Excellent - your cylinders must have a proper 80% level bleed valve; that's a valid alternative method to weighing, and I have heard it is standard practice in some areas even for these little cylinders. Depending on just the OPD - as my guy was doing before I questioned him - usually works too... it just isn't reliable.
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Old 11-07-2013, 07:12 PM   #12
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http://allpropanemowers.indigofiles....ane_Safely.pdf
This site completely explains propane dispensing. Most folks are unaware of the properties of LP gas, it's incredible expansion rates due to temperature changes (reason why only fill tank 80%)and it ability to give the dispenser 3rd Degree burns if they are exposed to the liquid propane(-44 F). Having managed a hardware store (33 years)with a propane dispensing station got to see some interesting tanks both free standing and permanently mounted and hear a lot of misinformation. Too many businesses dispensing propane with poorly trained employees out there for sure!
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Old 11-07-2013, 07:38 PM   #13
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Howard - very informative - thanks!
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Old 11-07-2013, 07:58 PM   #14
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Originally Posted by ADKzookeeper View Post
http://allpropanemowers.indigofiles....ane_Safely.pdf
This site completely explains propane dispensing.
Excellent! Most available material is aimed at consumers, and avoids details of proper dispensing... after all, we are supposed to be able to depend on the "trained professionals".

The source is the Propane Education & Research Council (PERC). I was unable to find this document in their website, which suggests that it is supposed to be available only to registered commercial members; the company which posted it may be violating the terms of their access to this material. I believe it is a component of a training package which must be purchased from PERC.

This is U.S. material - so it refers to the U.S. National Fire Protection Association code 58 and various U.S. government agencies, but propane is the same everywhere and I believe that the content is generally applicable to Canadian practices.

For anyone wanting to quickly get the portion of interest, our tanks are DOT cylinders (see section 4) and are "small cylinders" for filling purposes (see section 5).

A couple of quotes of note:
from page 19:
OPDs are not the primary means of preventing overfilling. The dispenser operator's responsibility is to close the hose end valve when the proper filling level has been reached.
from page 26:
Cylinders less than 200 pounds water capacity and subject to DOT jurisdiction must be filled by weight. Check with your supervisor for any exceptions.
This document may be somewhat obsolete (or perhaps this is a Canada/U.S. difference), because it does not mention the use of approved stop-fill devices for permanently mounted tanks (such as motorhomes). This isn't relevant to your Escape's portable tanks, but is a caution about the current validity of this document.
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Old 11-11-2013, 04:01 PM   #15
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Information from manufacturers

As promised...

I asked this question of Manchester Tank and Worthington Cylinders, the two major manufacturers of small (30 pound and under) DOT-standard cylinders used for propane on travel trailers such as an Escape:
Quote:
I have several small portable DOT-style Manchester propane cylinders (20-pound and 30-pound capacity) with similar valve assemblies. In addition to the vapour service port and valve handle, and the relief valve, there is an additional port closed by a slot-head screw.

When I recently had one of these cylinders filled, the attendant used this screw as a bleed valve to monitoring filling. Is this valve connected to a dip tube to the 80% fill level for use as a fixed liquid level gauge, or is simply connected to the top of the tank volume (perhaps as a purge valve)?
In the question to Worthington, which was by e-mail and so allowed me to attach a photo, I illustrated the subject port with the photo attached to this post. The question to Manchester was a text-only web form.

The answer is unfortunately inconsistent:
From Manchester Tank:
Quote:
The bleeder screw should be secondary measure, the tanks should be filled by weight.
No, the bleeder screw is in the valve, if liquid is coming out then the there is a chance the tank may have been overfilled.
From Worthington Cylinders:
Quote:
The slot head screw is often referred to as a “spitter” valve and it is a part of the dip tube. All 20 lb. and 30 lb. cylinders should be filled by weight, however, come propane fillers will just fill the cylinder until the propane comes out of the spitter valve which will give you rougly 80% fill + or -. In saying this, we do not recommend this method of filling a propane cylinder.
The conclusion:
The guy at the station in BC overfilled my tank because he depended on the OPD which failed to shut off flow at the proper 80% mark. There may be no dip tube leading to that bleed screw (so it is just a top-of-cylinder bleed), if so, if you fill until it spits liquid the cylinder is probably over-filled.

What should be done in filling one of our tanks?
  1. The primary method to determine the filling level is the weight of the cylinder, according to all sources, and regardless of the bleed valve configuration.
  2. The bleed valve is a suitable monitoring tool only if it is connected to an 80% level dip tube; even then, it is not recommended to use this as the primary or only method to determine when to stop filling.
  3. The OPD is a safety backup which might save someone filling a tank from their own incompetence or errors, but cannot safely be depended upon as the primary (let alone only) method to determine when to stop filling. It must not be be only method used.
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Old 11-11-2013, 07:05 PM   #16
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Brian, I really appreciate your research and posting. In my experience, RV camping for the last four years, and lots of grilling tanks refilled before that, I don't recall anyone ever looking at the empty weight stamped on the cylinder to determine how many pounds to add. They do usually set the tank on scales, so maybe they use a nominal cylinder weight and fill based on that. More often I see the bleed screw employed, or simply pumping propane in until the flow stops as indicated by the gallons meter. Even the weight idea is a little suspect considering the weight of the fill hose and associated valves attached to the tank. I guess we're fortunate that more accidents don't happen in this process.
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Old 11-11-2013, 07:18 PM   #17
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Quote:
Originally Posted by Parker View Post
In my experience, RV camping for the last four years, and lots of grilling tanks refilled before that, I don't recall anyone ever looking at the empty weight stamped on the cylinder to determine how many pounds to add. They do usually set the tank on scales, so maybe they use a nominal cylinder weight and fill based on that...
Even the weight idea is a little suspect considering the weight of the fill hose and associated valves attached to the tank.
Yes, everyone I see assumes a standard weight. The nozzle and hose are supposed to be included in the target filled weight.

This assumption that all tanks are the same is not good if you have a lightweight (aluminum or fiberglass) tank, although of course the vast majority of people have a steel tank such as those which come on Escapes. For non-standard tanks, correct use of a proper 80% bleed valve would be a better method, if the attendant will not bother to do the job properly.

Quote:
Originally Posted by Parker View Post
More often I see the bleed screw employed, or simply pumping propane in until the flow stops as indicated by the gallons meter.
That appears to be a regional thing. Here in Alberta, I've never seen the bleed screw used - it is always weighed... and it normally is in B.C. (Escape's home), despite my improperly trained guy last week. Propane for tanks filled through the vapour service fitting (RV, barbecue, and other small portable cylinders) is not even run through the meter here; they charge a flat rate by cylinder size regardless of how little it actually takes.

Stopping based on the meter reading is scary unless they are very sure the tank starts empty.

Quote:
Originally Posted by Parker View Post
I guess we're fortunate that more accidents don't happen in this process.
Yes, and the accidents tend to happen later, when an overfilled tank gets hot and vents liquid propane. The overfilling may never be known, so the true rate of incidents is not likely known, but there are not many propane incidents compared to the number of RV and barbecue tanks out there, so the luck is running good so far!
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Old 11-11-2013, 07:26 PM   #18
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Make informed decisions

Propane Bottle and LP Gas Cylinder Filling

Keep reading and make informed decisions

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Old 11-12-2013, 03:12 PM   #19
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Very interesting, and enlightening, discussion. I can safely say I have NEVER had a propane tank weighed at a filling station here in California. I can't recall even seeing a scale. Most of the time the tank is either placed on the ground, or on some sort of elevated platform. One favorite location uses a folding chair! They all utilize the bleed valve. I'm going to start shopping about and paying a little more attention.
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Old 09-26-2018, 12:20 PM   #20
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Propane tank overfill?

I'm bumping this thread due to what happened to me yesterday. I took a propane tank to be filled and was not watching while the filling was taking place. When the guy was done the meter read 5.6 gallons. I thought this was high, as I recall a normal fill for me is from 4.5-4.7 gallons. The man said the pump would not let it overfill. I paid for the refill and left.

I was worried about this, though. Today was predicted to be a warm day and at one point in the morning I thought I smelled propane in my trailer, but the detector didn't go off. I took the tank back to where I had it filled and asked the man to bleed off some of the propane, which he did. I didn't recall reading this thread and its associated documents beforehand; I wish I had. Comments?
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