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Purging LP tank

Post in 'It's a Gas!' started by FanMan, Mar 18, 2012.

  1. FanMan

    FanMan Feeling the Heat

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    OK, so I have my new gas fireplace installed and a brand new 420# propane tank ready to be connected. The tank is prepurged, vacuum inside (or should I say vacuum "not" inside?). Anyway, the tag on the tank says to initially fill through the service valve, adding methanol. Should I expect the gas company (I didn't buy the tank from them) to do this? Any reason I couldn't put some propane in from another tank (like a 20# portable tank), first adding the appropriate amount of methanol into the connection fitting?

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  2. greg13

    greg13 Feeling the Heat

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    I doubt there would be enough propane in a 20# tank to properly purge it. I would let the delivery company do it.
  3. FanMan

    FanMan Feeling the Heat

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    Sure there would... Tanks should be purged to 15 psig or 2atm; at that pressure at reasonable temperatures there will be no liquid.. 20# of liquid propane makes 174ft³ or 1299 gallons of gas at 1 atmosphere, enough to fill a 120 gallon tank to 10atm, over 130 psi. You could do it with four 1# Bernzomatic bottles.
  4. pen

    pen There are some who call me...mod. Staff Member

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    sounds like you don't need the f
    sounds like you really aren't looking for the forum's help.

    pen
  5. FanMan

    FanMan Feeling the Heat

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    ^^ If I didn't I wouldn't have posted. That there is sufficient gas in a 20# tank is clear. Whether it's advisable or inadvisable for other reasons is less clear, and that's why I asked.
  6. pen

    pen There are some who call me...mod. Staff Member

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    That makes more sense.
  7. coaly

    coaly Fisher Moderator Staff Member

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    Your tank is already purged. (if still in a vacuum) It only needs to be filled. You can connect a 20# cylinder with a Hogtail between valves. (copper line with POL fitting on each end) If you connect it to the new tank first, you can fill the line with methanol before connecting to the 20# valve. That isn't much, it should have a couple ounces.
    Purging means to remove the air. Some new tanks are pressurized with dry air. They need to be purged. The only way air gets into a used tank or cylinder is if the service valve is left open. (when you run out, it still has 100% propane vapor in it) As it cools down at night, the pressure drops inside and draws air (cool damp night air) in. During the day, the propane / air mix heats up and comes out. (leaving moistur behind) Each night more air enters until the vapor / air mixture is so lean, the cylinder needs purging to get the air out. Bringing up to pressure with propane and expelling it to the atmosphere removes 1/2 the volume of air each time. Doing this 3 times is considered removing enough air to fill the vessel with propane. If a valve is left open, methanol should be added upon filling since the night air going in is moisture laden. Propane companies will have a small pump with a reservoir that holds the methanol to pump it into a tank that is up to pressure. It can be added anytime.
    You're not transferring any propane fluid from the 20# cylinder to the larger tank. Only pressurizing it with vapor. Inverting the 20# cylinder will dump the fluid into the empty vessel. This is referred to as "Drifting". It is how a propane company gets fuel in an empty tank on an out of gas call at night without a bobtail.
    Purging is ONLY done with vapor.
  8. FanMan

    FanMan Feeling the Heat

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    Coaly, thank you. That's pretty much what I had in mind... and then drifting in the contents of one or two 20# tanks to test the system before the gas company comes out to completely fill the tank.

    I saw the hogtails for sale when I was buying the regulators and pigtail, and figured they could be used as you described... is that the primary purpose of the hogtail? Though it seems you should have a pressure gauge to verify the initial fill pressure...
  9. coaly

    coaly Fisher Moderator Staff Member

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    A hogtail is used to tie more than one cylinder together. There is a T with one male spud, and two female POL straight through ends. (open inside - no check valve) This T is installed in one cylinder, a hogtail goes to a second cylinder, and a pigtail goes from the T to regulator. This way you can manifold as many cylinders in line as you want. They all draw down evenly, so are kept filled the same. This is the normal connection when you see 2 420# cylinders or more.
    That is exactly how to install a 420# in a bad spot wihtout a motorized handcart. Normally the cylinders can be transported and placed about 20% full. If you have steps or a long yard to hand cart them, they can be installed empty, and drift enough in to do a leak down pressure test and get the customer in service until the bulk truck gets there. Instead of inverting the cylinder and holding it, a 100# cylinder with tube going to the bottom is made from a regular cylinder to get liquid to the empty vessel. The top of a drift tank around luquid valve is normally red, so you know it's not a vapor supply. They also have a seperate service valve (or multi fill valve to be able to fill with bobtail) in communication with the vapor space, since the service valve that normally incorporates the relief valve would have liquid and not vapor in it.
    You can use a first stage regulator (10 pound) from source to vessel being purged. (this regulator is used when the vessel is far from the appliance, a high pressure 10 lb. (red regulator) line is ran from tank to a low pressure (green) regulator. A very long supply line can be ran with small copper tubing at 10 psi and supply many second stage regulators as in hotel rooms with seperate heaters or a pool heater connected many feet away from the source when you don't want a tank near a pool or hot tub) When purging, you just turn on both valves and let it come up to 10 psi. no gauge required. I am in a very rural area and released to the atmosphere. They have stacks that burn the vapor off at the top in populated areas. It may even be required for all purging now.
    There are a few appliances like corn dryers and hot air ballons that use 10 pounds at the oriface. These are Million BTU burners and more. In extreme cases an Evaporator is used. A liquid reservoir over a burner (like a pressure cooker) to induce more heat into the liquid to vaporize it faster, when not enough atmospheric heat is available to keep the pressure up at high loads.

    I retired from the business in 2008. I recieved my training through Agway as an installer / serviceman at a local company, and later went on my own to install and service only. I was not a supplier when I was on my own, there was a division of supplier and service companies years ago in PA. Before that only suppliers did the install and maintenance. Years before that, gas appliances were only available from gas suppliers when the industry was in it's infancy.
  10. FanMan

    FanMan Feeling the Heat

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    Thanks again... I never thought of using the 1st stage regulator to purge but it makes sense. I am installing a two regulator system... 10psi gas to a tee on the house with legs running to each end, and second stage regulators at each end feeding the appliances. I'll probably make up an adapter with a gauge, though, so I can spike it with methanol and fill it to 15psi, make sure it holds pressure, then drift the rest of the tank in.

    As an engineer, I used to design gas stoves at one job I had so I understand the principles thoroughly (and am comfortable doing it all myself), but I'm less up on the codes and local legal requirements.
  11. coaly

    coaly Fisher Moderator Staff Member

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    When installation is complete, a legal leak down test is 10 minutes at 10" Water Column.
    With manometer or U-tube gauge (slack tube) on oriface of manual controlled burner (stove top burner) or at test port (usually 1/8" pipe at appliance service valve) you shut off gas at tank valve. Bleed it down to deplete pressure in regulator (tank pressure before needle and seat in reg.) When you see the pressure start to drop fast, stop bleeding and it must hold (usually around 8 to 10" pressure) for 10 minutes. You can't do a leak down test simply by shutting off the gas and watching for a pressure drop. The high pressure vapor in the valve body and line supplying regulator and regulator cavity up to the needle and seat will supply pressure to the system AFTER the regulator, not showing a pressure drop. Once you bleed the residual pressure from before regulator it's accurate. Any slight leak anywhere in a system will show it creep down. When you do have a system that doesn't hold pressure, ISOLATE by breaking the system in half and plugging it. This way you know which half holds pressure, and you can isolate it down in sections until you find the leak. There are test Tee's for tapping in, and connecting meter to 1/8" pipe nipple when testing partial systems. Flares with a professional tool, not a split die flare tool like most people have makes a huge difference at joints. The mandrel actually polishes the flare seat after flaring for a mirror finish.
    The reason for 10 minute test duration is due to temperature of fuel vapor increase or decrease going into cold lines (slow decrease) or cold vapor outside going into a warm building system that will build pressure when shut off at it warms. This increase in pressure can make up for the decrease in pressure from a leak. The gauge can look like it's holding pressure with a slow leak in the system, being masked by the expanding vapor increasing in pressure. It's considered after 10 minutes, the temperature differential from outside to inside is minimal and no longer affects the pressure reading. In large systems or exceptional temp differences, an inert gas is used for testing that does not expand and contract with temperature.
    You can tell if a system holds pressure on a gauge within a half minute. A slight tap on the gauge takes any mechanical play out of the needle and gauge. Each time you tap it, the needle will drop slightly showing a loss. It comes with experience.
  12. FanMan

    FanMan Feeling the Heat

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    Piece of cake, really. I used the tank's vacuum to draw in the methanol, then filled the tank to 15psi with a homemade hogtail with a tee for a pressure gauge. I then drifted the rest of the 20# bottle into the big tank. Pressure tested the line to the house (the second stage regulator is right at the appliance) at 25psi for about 4 hours while doing the above and then went out to dinner, still 25psi when I got home so I hooked it all up, wired the thermostat, and now I have heat!

    Probably still have to break the line at the tank and repeat the pressure test for the inspector, though...
  13. coaly

    coaly Fisher Moderator Staff Member

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    If anyone does a pressure test , it should be on the low pressure side. You don't break a connection anywhere. You use a test port on a shut off valve at the appliance, (1/8" pipe plug on valve or appliance valve, or T in with a test port. (You then soap the fitting you used to break into the system when done) This tests from the tank valve connection to appliance. IF there is a kitchen range on the system, simply remove one of the burners, slip the manometer hose on the oriface (they are the correct size for over an oriface) and open the burner valve to show system pressure. When you bleed off the pressure, (after the green low pressure regulator) the high side supplies the pressure until the entire system is lower then what is supplied by the low pressure regulator. You can "see" the pressure drop on a gauge when it's almost down to zero better than you can when the pressure is high. I'm referring to a Manometer showing inches of water, very minute pressures, not a gauge reading PSI. 1 PSI will peg a manometer, and it takes a lot of bleeding to get it down to 1/2 PSI, (like an open pilot for a few seconds) then you will see it drop quicker as the same volume of vapor is left out.
    It's like watching water drain out of a bath tub. When it's full with a lot of water, you can't see it go down. The last little bit looks like it's draining fast. The water is draining the same speed, you just can't see it due to the volume of water. Same as pressure. You can't see the gauge drop until the very last of the pressure in the line depletes. Then it drops like a rock. That's the pressure that shows a leak on a gauge. If you play with bleeding off pressure while watching the gauge, a little at a time, you will notice it drops faster as it runs out of pressure. 10" W.C is about 1/2 PSI and below 8 " it drops fast. That's where you do your pressure test. It may seem strange to test a fitting at such a low pressure when it operates at 300#, but the "volume" of vapor at high pressure is like a cushion and won't show a decrease as it leaks, just like the water in a full tub. The higher the pressure, the more has to leak out before it's noticable on a gauge.
    Temperature determines drifting speed. If the empty tank is in the sun, and your full tank was in the back of a truck getting cold air over it, the pressure differential isn't as great. After the first cylinder, the pressure is the same in both tanks. A gallon of propane liquid is going to be the same pressure as 50,000 gallons at the same temperature. So you need a temperature differential to move it from one tank to another. If the drifting slows, and you can hear it only dripping into the tank, a black plastic garbage bag over the supply tank will absorb heat causing it to boil and increase pressure to move over to the other tank. Of course no heat calls always happen at night when you're on call, and it's 15 degrees or so, so sometimes you can only drift 1/2 of a 100# cylinder. Once you equalize the pressure, it's not going to move. It's enough to get them going until morning when the bulk truck gets there. Degree day information is used by fuel companies to determine how much a customer "should" have. But the addition of a dryer, log set, or outdoor BBQ not reported to the supplier may leave a customer running out before they should. An outdoor 1000 gallon ASME tank on a zero degree night running a 100,000 BTU boiler is going to need the bulk truck if it can't keep the pressure up. They try not to let that kind of customer get below 10%. Not all servicemen on call have a CDL to drive a bobtail. I left the company just as CDL licensing was starting, so when on call, I could go to the shop and get a bobtail if I needed to fill someone in the night. Laws make it more difficult now.
  14. coaly

    coaly Fisher Moderator Staff Member

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    Here's the type ball valve that should be installed at the appliance. Simply remove the test plug with allen wrench, install a short 1/8" pipe nipple and connect rubber hose on manometer to system. Ball Valve with  test tap.jpg This is used to set pressure regulator, flow check pressure with appliance main burner on, or leak down testing. Soap bubble test the port plug when done.
  15. FanMan

    FanMan Feeling the Heat

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    Good point on testing at low pressure. I don't have a manometer, though I could improvise one with some clear tubing and water like we used to do in the aero engineering lab. The ball valve I have (rated for 1/2 psi gas) doesn't have a test port, but I did soap test everything. I'm satisfied there are no leaks but it's the inspector that has to be satisfied... he said something about testing at 30 psi so I'll see what he says tomorrow.

    I drifted it in with both tanks at the same temperature (and at night so no sunlight to help) but with the small tank on a stepladder so gravity did the trick. Took about 20 minutes to fully drain into the big tank, I thought of pouring warm water on it but I was in no hurry.
  16. FanMan

    FanMan Feeling the Heat

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    I disconnected the line on the downstream side of the 1st stage regulator, put in 20psi and bled it back down to 3/4 psi on the gauge (a 30psi gauge with 1/2 psi graduations). The inspector arrived an hour later, the needle hadn't moved, he took one look at the gauge, glanced at the piping, and signed it off.
  17. coaly

    coaly Fisher Moderator Staff Member

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    So he in effect certified the entire system leak free, knowing you had to make a final connection, that wouldn't be tested. Amazing.
    Another thing that always amazed me was when they would approve a system without a pressure tap on a service valve. The code in PA adopts NFPA 54 which calls for all appliances to be installed as per installation instructions supplied by manufacturer. Most appliance instructions call for this type shut off valve at appliance. So if it has a regular ball valve with no pressure tap, it's not installed as per manufacturers instructions. I've never had an inspector ask to see installation instructions to see what equipment was required by the manufacturer. Perhaps they feel asking to see the book makes them look stupid ??
    And............ there was an installation at a airport of overhead radiant heaters. After hanging them as the manufacturer suggested, the local code enforcement officer didn't like their low height in the hanger. I went over his head to Labor and Industry. They didn't like "the way they were supported". I went over their head to the FAA. Long story short, they were fine. But Labor and Industry failed the system for "regulator not supported correctly". This was a high pressure regulator under the dome of a large ASME tank. You simply make a coil with a pigtail and the regulator connects to it. There is just enough room to close the lid and no such thing as supporting the regulator ! By the time I was done showing him NFPA 58 and 54, he asked for my number and if he could call me to use as a reference if he had questions on an inspection some day. He never saw the entire book ! He only had poor photocopied pages of a few things his instructors dwelled on.

    The old saying goes; Those who can't do, teach. Those who can't teach, inspect.

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