I'm approaching the GRM community because I've received a fair bit of conflicting advice from the mechanical engineers I work with and the mechanical contractors I deal with at work.
I have a 140' drilled well with a submersible pump of unknown age. I do know that it's at least 25 years old but it may be closer to 40. My question is, what kind of life span do these things have? I'm thinking of buying a new one and replacing the existing one before it fails then having the old one rebuilt and kept as a spare. This sounds great but I have a bunch of other things to spend money on before the snow flys so I'd prefer to put this off until spring. The pump isn't causing me any issues(knocking on wood). It's drawing 12.5 Amps at 230V but that doesn't tell me much since I don't know what the rated horsepower is.
I actually trust the 25 plus year old pump more than a new offshore built unit but I may be wrong in my thinking.
Thanks
My father's original (1977) failed after about 25 years.
Pull it, check the impeller for wear, and replace if necessary. A good submersible should run forever.
They are pretty easy to replace, and not terribly expensive. As such, I'd ignore it until I actually needed to pull it and replace it. I would not do it premptively, nor keep a spare in case it fails. They are far to readily available for that need.
They usually don't last that long. 10 to 15 years is probably about average for the ones I have replaced. Either the motor seizes up or the top of the pump rots out spraying pumped water inside the well and not up the pipe. Just about everything residential is disposable nowadays. Yea you can rebuild the nozzle end of a shallow well pump, but the parts and labor you have to charge exceed the cost of a new pump. Pumps goes bad. Throw away pump and install a new one. At least with a 140 foot well, you can pull the pump by hand.
The one at our cabin near Jamestown, NY (erected in 1954) was a high end stainless Sears model (and the original well pump). It died two years ago and was replaced for $1500. That's 55 years 
