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.