My parents' house has a well that supplies their water. About a year ago, they had a less-than-competent contractor install a storage cistern and a booster pump.
The contractor didn't anchor down the pump base, depending entirely on the weak PVC pipe and gravity to hold it in place against constant vibration. The electrical work he did had to be completely redone by someone else. The concrete cistern has leaked since day one, even after several repair attempts by the original contractor
The pump started leaking much more recently, and much worse. What looks like a blurry water drop at the rightmost corner is a constant stream. This is what the leak looked like with the booster pump turned off and the piping downstream depressured.
The first step I took was to disconnect the piping from the downstream union and I began to unscrew it from the pump. The pipe was loose! It wasn't even screwed in tightly. The piping configuration makes it impossible for the pipe to unscrew itself from the pump without disconnecting it downstream. Here is an overview of the pump and piping configuration. The piece that I removed comes out of the top of the pump and connects to the half of the union seen above and to the right of the opening.
Once I had the piping removed, I took a closer look at the opening on the pump. This is what I saw.
There was metal sticking out into the bottom portion of the threaded opening. It looked like it might be flash left over from when the pump housing was cast. It deserved a closer look.
That's strange... It looks like the threads abruptly stop.
Yup. The threads definitely stop. My only thought is that this might have been done to prevent threading the pipe in too far. If the pipe went past the inside surface of the pump case, it could hit the impeller (the part that spins.)
Let's take a close look at the pipe I removed.
The first thread at the end of the pipe is rather screwed up. It's as if it were tightened past that point where the threads in the pump stopped. It's almost exactly like that.
At this point it is important to understand how tapered pipe threads are supposed to seal. The seal occurs at the threads as they fit tightly together. This site provides excellent diagrams of what should be happening. I have modified them below to illustrate what was happening in this specific case. When the pipe bottomed out, the threads were still loosely engaged. This allowed the water to leak out past the threads. I used a saw to shorten the pipe, removing the two smallest threads at the end. It was then able to seal tightly. A small amount of Teflon tape or joint compound seals any small gaps left in the imperfect threads.
Below you can see the before and after pictures of the pipe.
Based on what I showed above, this is what I think happened. The contractor screwed the pipe in place, tightened it to the point it became difficult, and then discovered that it leaked. When it leaked, he tightened the crap out of it until it stopped leaking, damaging the plastic pipe thread as it bottomed out in the hole. However, the threads were still not properly tight. The only reason it stopped leaking was due to the rather excessive amount of joint compound that was used.
This is pipe joint compound. More is not better...
The contractor saw the opening of the pump when he assembled the system. He failed to notice the rather obvious metal bits extending into where the pipe should be threaded. This is one more example of the shoddy job that was done all-around.
Here is the final result after my repairs. If you compare this to the original picture of the leak, you can see that the pipe threaded in further. This created a good tight seal that should last.
The leak started shortly after we had replaced the filter downstream of the pump. What we did must have been just enough to disturb the loose joint and start the leak. Both the inlet and outlet connections on the pump had the same problem, and were fixed the same way.