Hello Artisi,
You are a curious fellow. A lifetime of that stuff and you probably know a lot.
As you know, double volutes are incredibly balanced, practically no radial thrust at any flow rate. Costs a few points in efficiency, cost a little more to make. Sooo, they have dissappeared from the market place. I and others have cried about this decline in the pump industry.
Anyway, talking to engineers in a very large pump company, they claimed that radial thrust is only a problem in large pumps, but that is not true.
Our experience with end suction volute casing pumps is that systems with variable flow rates, say 30% of BEP up to BEP, the bearings do not last more than about 5 years maximum, and a few fail in 1 year, probably because they operated for very long periods at 30% of BEP.
We have decided this is due to radial thrust loads, and strangly, the front bearing intended to take the radial thrust load is not the bearing that fails, it is the smaller back bearing, probably the vibration and torque is not entirely negated by the huge front bearing and makes it back to the tiny little back bearing and kills it.
For five years we have been refusing to sell the larger volutes and instead have gone to turbines and multistage pumps with diffusers to eliminate radial thrust. But we have walked away from a lot of work because our systems cost much more. In some cases, I would like to be more competitive with the cheap and dirty single volute guys with their extremely high efficiencies and yet for a few bucks more we can compete and not have short lived motors like the other guys.
We recently walked away from a $100,000 job because we did not want anyone to be angry with us over pump motors and bearings that lasted only 1-3 years. When we asked them if that was acceptable they said it was, but still we just could not get ourselves to do it. Imagine your name on something that fails like clockwork every three years where a quality product would last ten-twenty years. And I must remind myself of this one thing, those people that said 2 years of life was Ok, why are they coming to me when they already are in that mode now? Because they are tired of the short lived pumps and are constantly replacing the entire pump sets, and they admitted that to me. Our price drove them away. But, with a double volute I can compete and know that the pump will last 10-20 years.
Is that why some cry about the loss of double volutes in the market place? If I cannot find small double volutes, I may design and make my own, I can sell it.
God Bless.
PUMPDESIGNER