My PC replaced almost all my plumbing during a pool refinish but kept my existing pump, filter and heater. At first startup, I was getting a continuous bubble stream at my returns and my sand filter would fill with air in a few hours. Hadn’t done that before the refinish.
PC told me it was the pump strainer o-ring or the pump itself. Changed the o-ring with no effect. I wasn’t willing to throw away a pump that had previously worked well so I started doing some troubleshooting (Note how quickly the PC walks away from the problem!). I carefully wrapped each welded pipe joint with electrical tape to see if there was any change. Nada. I was very worried I had an underground leak, but I wasn’t able to wrap the valves very well, nor the threaded connection into the pump’s strainer basket so it wasn’t totally conclusive yet.
I decided to replace my entire suction side piping from the pump’s strainer to the water surface with a temporary pipe that ran across the pool deck and down into the pool. This I thought would establish once and for all whether it was the pump or the piping system. I was quite bummed when that setup resulted in no bubbles, now I was thinking I had an underground leak.
Took the test piping off and put the PC’s piping back on and noticed the bubble stream took a lot longer to appear, two days instead of 3 hours. Hmmm. something changed from the disconnection/connection process.
Then I started looking a little closer. The PC had connected to my pump's suction side using a Praher 150-906 union. I do like the Praher unions because they’re very compact, not the massive plastic assemblies that are standard unions. They incorporate an o-ring boss seal on the threaded end that screws into the pump and would theoretically seal against a boss coming out of the pump. On closer inspection I saw the o-ring wasn’t seated against the pump even though I remember I tightened it with a lot of force.
I took the union back out of the pump and looked at it carefully. It had a straight pipe thread, which would make sense for an o-ring face type seal, however my pump had a female tapered pipe thread at it’s suction port, so I had two, somewhat incompatible threaded parts.
Standard tapered pipe threads are tapered on both the male and female sides so they eventually cinch up and seal across a great number of threads. However, when a straight pipe thread is screwed into a tapered pipe thread, as ended up happening here, any sealing only happens across a very short thread distance, so sealing is far less certain. The only sure seal for my situation would be the o-ring, but because the threads would bind up before the o-ring seated, I had a leak path.
I filed the boss area on the pump flat (it had part numbers cast into the housing exactly where the o-ring seated) and tightened it much farther than I normally would think wise, but I was able to get the o-ring to seat and the leak was eliminated. I was very happy not to have an underground leak!
Praher’s catalog calls the thread on this fitting an MIP thread, which is an old designation that stands for “Male Iron Pipe” and is fairly non-descript. It’s generally accepted to mean a standard tapered pipe thread but in this case I don’t believe there’s any taper at all. I called Praher and spoke to someone who wasn’t really technical, but he agreed their thread was not a standard tapered pipe thread.
An Internet search revealed there are some pumps that have straight pipe thread ports with a boss, but they seem to be in the minority.
So the moral of the story is it pays to look closely at your threaded components and don’t assume the catalog has all the right information.