I found Kevin Delaney's article on the dependability difference of 3600 RPM and 1800 RPM pumps very interesting. I have always thought it common knowledge that to double the RPM would increase the wear rate by a factor of four. I think a lesson could be learned from Dr. Lev Nelik with his response to a letter to the editor when he says that, knowledge should be based on field input on what works and why. As Dr. Nelik explains, after spending lots of time in the field, he realized how little is actually known by engineers for major pump manufacturers who know little outside the comfort of their own desks. Dr. Nelik goes on to say that "factual input would be refreshing from those who know it from personal experience", not hearsay from those behind a desk. I have personally installed and repaired thousands of pumps both 3600 and 1800 RPM in the last thirty years. It has been my experience that 1800 RPM pumps do usually last longer and can handle abrasives better than 3600 RPM pumps. However, it has also been my experience that RPM has less to do with premature pump and motor failures than does cycling on and off or problems associated with variable speed drives. Greg Towsley of Grundfos Pumps Corporation makes an excellent case for 3600 RPM pumps in his letter "Place Your Bets". I have installed many of the CR Series pumps that Greg mentioned and have found them to be very dependable even when operated at close to shut off conditions for extended periods of time. I did find it interesting that Greg Towsley said that "MOST of their annual warranty cost are integrated variable speed motor issues, not the pump". This follows exactly what I have seen with thousands of installations in the field. 1800 and 3600 RPM pumps are both very dependable when not allowed to cycle on and off, and when not having to deal with all the negative side effects created by variable speed drives.
Cycle Stop Valves, Inc.