Quote:
Originally Posted by categ50
My question; What is the correct pressure of the tank for lampwork, and what is over filling the tank. I would like to be armed with this info for when I take the tank back to AGL Tank Filling Guys and not feel like I am hitting a .
Cate
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Edit: The following assumes you are using a bulk propylene (or similar mapp-like gas) tank. If not, skip it. Sorry.
Well, it's a short question, but it will be along answer. And there will hopefully be others to answer too.
First, the correct pressure is "tank pressure". That's about all that is ever specified when the question comes up. Not trying to be evasive, but the tank pressure is whatever it needs to be to force a gas to be a liquid. And as the temperature changes, that pressure changes. (Which is why a frosty 1# mapp canister has to be warmed up to get the pressure back up, to get the flow through a HH).
At 60 degrees, the tank pressure could/would be about 97 psig. But at 110 degress (in the sun) the tank pressure could/would be over 220 psig. When they filled the tank, they probably did it by weight, at the temperature of the day/time. "Usually", on a "normal day", the bulk tank pressure is "about" 170 psig. That's why you will get odd looks in you try to pin down the pressure... it's in constant motion, between pretty wide swings, keeping the gas in a liquid state. I have even measured pressure at retail studios that was over 270 psig (the pressure relief valve on the tank will pop open at about 320-380 pisg - I forget the limit for propylene tanks). IMHO, some of the "my HH flame doesn't look like your HH flame" situations is because the pressure of each person's system, depending on the current temperature, could be different by up to 180 psig - and that will make a big difference in lighting and running the flame. For reference, two-gas torch nominally operates at only 5 to 10 psig and they are regulated to stay at a that set pressure +/- 1 to 2 psig, with a pressure regulator. Hotheads operate at tank pressure, without a regulator, so the pressure swing could be quite dramatic.
Liquid in your line. Yes. There is a theoretical reason and practical reason this could/would occur.
Theoretically, as the liquid becomes a high pressure gas and leaves the tank and travels through the hose, it will encounter a temperature change along the way, Usually, during the summer, it goes from a hot outside to a cooler inside. We are now back to the temperature thing... some of the gas could convert back into a liquid, sort of like condensation. Some, not all, but enough to be really annoying. Theoretically.
Practically, when the gas shop fills the tank, they are dealing with a large bulk tank normally used for welding. Since it is exempt from the federal overfill prevention device (OPD) regulation, and since no welder ever wants to be shorted on his gas fill, there is a
tendency to overfill... from a lampworkers standpoint, but not from the filler or welders standpoint. All welders will put a regulator on it (which also has a internal filter) and will reduce the pressure/flow from the tank, so the "slight" over fill is no problem... for welders. Filling anything other than a BBQ grill tank (which you cannot use for propylene), is a little hit or miss anyway when it's done by weight versus volume, unless you very carefully calcuate the temperature effect. Couple the "overfill", with transport and set-up, without keeping the tank perfectly upright (I've seen them laying on thier side in a cars backseat), and the liquid then migrates into the valve - or so I been told, but I don't know that for certain. If it's so, then the first thing usually out of the tank could be liquid, if the tank valve is not burped before the hose is attached.
And there is a hypothetical answer too. Propane is not all propane. Propane (like two-gas torch users use, and you use on a BBQ grill) is a blend of liquid fuels, propane and butane, and has three grades (but no one ever reall specifies grade unless they are willing to pay a premium price), and "todays blend" could be 90%/10% - Propane/Butane. Propylene is a mix of
several different gases (not just two, like "propane"). So the blend could be quite different, tanker truck to tanker truck. Hypothetically, something is "separating out" in the line, and maybe it's the "special additive of the day".
Hope this helps a little. This is not meant in anyway to be a knock against the Hothead. It's just a part of the art.
note: edited for assumption and typos (but probably not all).