Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

PVC Cap Design

  1. Sep 4, 2005 #1


    User Avatar
    Science Advisor

    I am building this project which involved to PVC pipe chambers to be semi-pressurized (I know PVC isn’t intended for pressure, but I am not talking 200 psi, 40 to 60 psi theoretical MAX, usually around 5 to 10 psi).

    I have a pretty good depiction of the set up made in ASCII art, but I previewed it and it didn’t show up very well since all the extra spaces were removed.

    The problem is that the PVC pipe cap is too loose of a fit at even low pressures to stay on, it will fly off into the air. To make matters worse, to ensure there is a seal between the PVC Pipe and the cap, I smear a bunch of petroleum jelly around the cap, so it becomes even easier to slide the cap off.
    So I need a way to hold the cap on tight, or strap it down somehow to it wont go shooting of into the air.

    My Solution: I taped it. I got some heavy duty tape and strapped it down. It has worked so far in normal operation (< 5 psi, probably, I didn’t measure), but is not a very good long term solution, and it is a hassle to remove when I actually want to remove the cap. Also the tape wears out after a while.

    What makes securing the cap so hard is that I can’t drill any holes through the side of the PVC pipe since that will cause a leak for the pressure to vent from.

    Another idea I had was to get some type of rope and strap it down by going over the top of the cap and tying it down somehow below.
    Or perhaps something like a latch that is attached to the pipe and will grab on something on the cap. Have you seen those coolers that the handle is pointing up, it locks the lid down in place and when you turn the handle to the side it releases the lid, like that. But that would be hard to build.

    So, as is my style, when I can’t think of a solution after blankly starting at the pipe for a while, I will pass it on to your guys, the finest engineers on the internet, for help.

    If you need me too, I can go into more detail on what the apparatus looks like, or try to draw some pictures for you.
  2. jcsd
  3. Sep 4, 2005 #2
    Does this cap need to be removed? PVC used in plumbing is usually glued together with PVC cement.
    Depending on the size of the pipe, you can get a slip to thread coupler for pipe from 1-6 inches and screw a plug into the threaded end.
    More info will get you a better answer.
  4. Sep 5, 2005 #3


    User Avatar
    Science Advisor

    Yes, the top cap needs to be removable.
    The cap on the other end, which is technically a "Flange" I glued in with PVC cement and that works really well, but I will need to take off the cap semi-often.

    The screw plug you speak of is ideal I think, that is what I originally planned on using, but Home Depot didnt have any, or rather thay had it but it didnt fit. I am using 4" PVC pipe and the 4" threaded coupling and screw cap was too big for the pipe, I dont understand.

    I attached a text file with the ascii art image to give you a better idea of what it looks like.

    Attached Files:

  5. Sep 5, 2005 #4
    My potato cannon (the first one I built) used 4 inch threaded caps. All pvc componants were purchased at Home Depot. The 4 inch male and female PVC threaded caps are very common plumbing items, I am sure all HD's stock them.
  6. Sep 5, 2005 #5


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    If you have a hard time finding pipe plugs that large at your local hardware store, you might try McMaster Carr or some other internet provider.
  7. Sep 5, 2005 #6


    User Avatar
    Science Advisor

    You would think so, but like I said, they had the correct parts but when you go to fit it on the pipe, it didnt work.

    I am going to check out another store today and see if they have the parts I need.
    Hopefully I didnt put my gas outlet too far up the tube so that when the threaded cap adapter is put on it doenst fit.
  8. Sep 5, 2005 #7


    User Avatar
    Science Advisor

    I have returned.

    Lowes, a had the parts I needed that fit correctly, so it looks like I am set for now.

    By the way, GENIERE, about hoe much pressure do you thinnk your potato cannon got up to? Did you do anything special to the screw on lid to make it air tight?
  9. Sep 6, 2005 #8
    Somewhere I read that a 2-liter Coke bottle can withstand 80psi.

    Just a wild guess – 150-200psi. It shoots 2 to 3inch by 1 and 1/12inch potato cylinders about 200 yards and maybe 200 feet straight up; I made no attempt ever to measure the distances. The ignition chamber is a 4inch PVC “Tee” with threaded caps on two ends. The vertical part of the “Tee” is necked down to a 1 and 1/12 inch 4 foot PVC barrel. I mounted a bar-b-q ignition in one cap and used the other for clean out. I made no attempt to seal them, they are just hand tightened with minimal effort. I tried several fuels, ending up with automobile starter fluid that I think is ether based but the label did not list the contents. I tried propane and would caution you not to unless you want to shoot mashed potatoes. It was a violent explosion and I think the cannon would not have survived long with dangerous consequences (shrapnel). The pvc glue eventually failed because I think the fuel dissolved it; I just re-glued. I fire it while my grandkids are around and use my body as a shield, making sure they are always behind me. I inspect it after each shot.

  10. Sep 6, 2005 #9


    User Avatar
    Science Advisor

    80 psi...so that would be 551.6 kPa (I think in kPa, despite what you might think from my other posts).
    I know from personal experience that a 2 liter soda bottle can take quite a bit. Using a highly scientific approch a testing its strength under pressure, I once pressurized one using a hand-bicycle pump and then shot a BB gun at the bottle. The bottle survived unharmed.

    I do have a pressure guage on my pipe, so I will be able to find the pressure that I get up to, so I'll be sure to make a note of it.
  11. Sep 7, 2005 #10


    User Avatar
    Science Advisor

    The one thing you have to watch with a 2 litre under pressure like that is that any impact to the bottle while charged will be enough for it to catastrophically fail.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook