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Old 12-29-2004, 09:53 AM   #29
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Just one more thing to point out. The way the system is susposed to operate allows the gas to boil off the liquid propane in the tank and supply propane gas to your appliances. With the bottle on its side the gas will still boil off but it will push liquid propane through the lines to your appliances. This was the primary reason for the OPD valves installation to prevent liquid propane from entering the lines going to the coach. I have horizontal tanks on my unit but they are designed to work in a horizontal position.
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Old 12-29-2004, 10:09 AM   #30
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Quote:
Originally Posted by TomW
Does the relief valve work by gently releasing pressure, or does it pop suddenly at a particular pressure, and reset when the pressure drops below some value?
Tom,

The relief valve will open and close as needed to allow the excess pressure to bleed off.

This is something I have experienced when I had a trailer. The tanks were filled in the great white north, I then towed the trailer to more temperate climates and the tank not yet used would vent when the sun hit it. The propane was only filled to the 80% line, but as the tank was warmed the gas expanded to the point the pressure relief did it's thing. This was in the days before OPD as as I remember the guys filling the tanks would fill until the pressure relief popped or the tanks weighed the right amount. Whichever happened first.
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Old 12-29-2004, 10:19 AM   #31
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Quote:
Originally Posted by jcanavera
Lynn I find your comment interesting since I've always read that filling by volume is the only safe way to fill. In every case where I have had a situation where the pressure relief valve opened, I could point back to the dealer who filled the tank used the weight method rather than using the 80% screw. When that method is used the point that liquid shows up is when the tank is 80% full. Problem is the clowns who fill these tanks don't have accurate scales or don't check the tank closely to be sure the weight they are using (as full) is accurate. This is a big reason why the OPD valves have come about. If dealers filled by volume, we wouldn't have had to go to these valves.
Sorry to leave the long quote intact, but there are pluses and minuses for both means of "filling" a NGL tank - I say NGL (Natural Gas Liquids) because in almost all instances the "Propane" (Liquid) is actually a mixture of Propane and Butane with a smattering of both higher and lower weight gasses (liquids) thrown into the mix. This mix is customized for the areas that the NGL is sold in, as in the example above, where the tanks were filled with a "cold weather" NGL mix, and then expanded to an equilibrium point (vented the easier to vaporize gases via the "pop off") when the tanks were moved to a warmer climate.

If useing the "weight" method, almost any balanced arm scale can be used...the tare weight (TW) is added to the weight of a full fill (X lbs of NGL mix), plus the known weight of the fill device. This weight is set on the "balance arm" of the scale (a scale similar to one of the mechanical devices found in most Doctor's offices). When the proper weight is reached, the "arm" tips from the "up" position to the "down" position. The problem is, that the tank is usually filling at too fast a rate to be shut down quickly enough so that the "pop off" does not go off....again, lack of training (or laziness) to do the job properly (slowing the fill rate down so that the pump will be shut down prior to the "pop off" spewing gas (or liquid) into the atmosphere.....THIS lack of common sense is what causes "preventable accidents" that we all pay for (either directly or indirectly) through litigation and insurance costs. The "weight" method of filling (and paying) is accurate, but requires a certain level of expertise that will not normally be available in entry level (interpretation - minimum wage) field operators and office staff.

On the downside of the "volume" fill...unless a tank is TOTALLY empty, you cannot use the "volume" method - simply because (unless the filler acurately weighs the tank) there is no accurate method to determine what the "start" volume of residual liquids left in the tank....to just let the residuals "vent" to the atmosphere to totally empty the tank prior to filling (a lot of places do this) wastes this natural resource and damages the environment. The "venting" (emptying the tank to atmosphere) also drops out some higher end hydrocarbon chains (Pentanes and Hexanes) due to the temperature of the interior of the tank dropping due to the rapid evacuation of the tank. What this does, over a period of time, is let an appreciable amount of higher end hydrocarbons accumulate in the tank, and, as the tank is drawn off in extremely cold weather, these higher end liquids will not evaporate, and cause the various burners to shut off due to lack of available "gas" coming through the regulator - in other words, you may have a quarter to a half tank of liquids that will not turn to gas due to the decreased temperature caused by a heavy demand situation (ie stove, water heater, and two furnaces trying to suck from the same straw at the same time).

Hope this suffices to confuse everyone and muddle the murky waters even more.
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Old 12-29-2004, 10:33 AM   #32
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Quote:
Originally Posted by 87MH
... also drops out some higher end hydrocarbon chains (Pentanes and Hexanes) due to the temperature of the interior of the tank dropping due to the rapid evacuation of the tank...
Well, I was going to say that, but I thought everyone already knew that!

Seriously, the part about "quarter to a half tank of liquids that will not turn to gas due to the decreased temperature". I assume it is the liquid in the tank getting cold? Or is the valve freezing up? Does this only happen when it is really cold outside?

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Old 12-29-2004, 10:49 AM   #33
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Quote:
Originally Posted by TomW
...the part about "quarter to a half tank of liquids that will not turn to gas due to the decreased temperature". I assume it is the liquid in the tank getting cold? Or is the valve freezing up? Does this only happen when it is really cold outside?
It COULD happen when it is warm, but most likely when it is REALLY cold out (well below freezing)...and it is because the liquids will not turn to gas - a combination of the cold ambient temps and the temperature drop inside the tank brought on by rapid drawdown (like, when you really need to run the furnace).

The valve is not as likely to freeze up, due to the very low content of free water (or water vapor) in the NGL's - the "freezing" point of NGL's is VERY low - in reality hydrate freeze problems are very rare in processed gasses (or liquids).

I actually experienced a "no gas situation" several years ago - borrowed my dad's pop up (tanks last filled in Missouri with a "summer mix" of NGL's), had a half tank of liquids in each tank - when we awoke in Cade's Cove in the Smokies on Thanksgiving day the dog's water bowl (6 inches deep) on the floor of the trailer was frozen solid....the interior of the pop up tent fabric was covered with ice crystals that had condensed from our breathing overnight. Outside temp was around zero farenheit.

Couldn't even get enough gas out of the bottles to run ONE burner to make coffee....still, it was a good camping trip - we had so many blankets on the bed we could not even turn over easily due to the weight of the covers.
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