problem with 30yr old well pump system

Its a common in well jet pump system. Most of the big brand residential and commercial water pump manufactures make a system like you have. Its cheap, simple and very effective. The guys recommending you get a 6 Hp pump have their greedy little heads up their asses.

If you chose to convert over to a submersible you would need a 1 HP multistage with an honest 120 meter head capacity or better. Thats a very common design also. However be aware that they are about 3.75 inches in diameter and need a well casing that got at least an honest 4 inches of clearance all the way down to where the pump sits.

Just do an online search for jet pumps and you will find loads of deep well jet systems to chose from. However you may need to change to a new pump head unit possibly in order to keep the jet and pump head set up properly matched.

I take the liberty to edit this this question thus:

I was given a call by a friend of mine when his well pump backed up. We talked over the phone, and I was told the well was 30 m deep and the house was 60 m above that. When I got there I found that for 30 yr this whole system without one problem had run on a single phase 0.75 hp electric pump made in Italy. It worked by having a twin outlet manifold; the twin outlets were 1-inch tube; one went to the house, the other went down the well. The pump inlet was 2-inch. We removed the pump and pulled the pipework up. Both pipes were joined at the base in a one-piece casting. The 1-inch pipe connected into the 2-inch pipe on the side; then 3 inches lower on the 2-inch pipe there was a nonreturn valve. So as the pump forced the water down the well, as it hit the larger pipe the pressure difference caused a siphon effect and forced more water up to the top, and excess water was fed up to the house. The lower casting was corroded and unusable. Has anybody else seen anything like this? The pump company recommends a 6-hp 3-phase pump to do the same job. I have found out the original design principle is 400-year-old Arabic. Is there an exact principle with the 2-to-1...?

Your research is quite good so far. This type of design is now known as a "jet pump" or an "eductor." By injecting higher-pressure liquid through a nozzle, a larger volume of lower-pressure liquid can be entrained from the side. This principle is useful in sucking wastewater from sumps, mixing liquids, and converting high-pressure/low-volume flow into low-pressure/high volume flow, etc. Ratios of up to about 4:1 can be obtained in certain conditions; the 2:1 you cite is not a magic or exact number, but it is probably reasonable here.

Ultimately you are using 0.75 hp to pump water 90 m higher (or less, if the water in the well rises on its own above the bottom of the well). This should allow for about 7 gpm, which should be adequate if there is a suitably sized storage tank. If the system has been fine in the past, there is no need for a larger pump. Instead, try to find a replacement for the corroded casting, even from another manufacturer. The original pipe sizes seem about right. (The casting constitutes the "jet" or "eductor.")

I see no reason for a 6-hp 3-phase pump, unless you now want to pump a great deal more water.