About the question of "what is the problem and what is the question?"
rhatcher - 21 Mar 11 said:
With regard to the application, the pumps (two identical units) do successfully remove water from the drydock. The problem is that the motors draw overcurrent and run hot. The application and the associated problem have obviously existed for quite some time..... This is the reason that I want to know what the triangle points represent. If they represent the application design points, then this is, in my opinion, further evidence of a total application/pump/motor mismatch.
rhatcher - 30 Mar 11 said:
My interest in the triangles was because if they represented the design points then I could calculatute the minimum impeller diameter that would meet the application head requirement to be sure that we did not cut too much from the impeller.
Also, with respect to another comment; the outlet of the pumps is below the surface of the river into which they are pumping. Presumably this is because this amount of water dumping into the river from an elevated pipe at the edge of the docks would cause some surface turbulence and/or otherwise hinder adjacent boat operations.
My understanding is that the difference in head pressure is based on the difference in surface level betwen the 'well' being pumped and the reservoir into which it is pumped. The location of the piping at the bottom or the top of the reservior does not matter, it is the difference in the surface levels. Specifically, if the level of water at the bottom of the dock is 40 feet below the level of the river into which it is pumped, then the static head (not including friction loss) would be 40 feet regardless of whether the pump outlet was at the bottom of the river (40 feet deep) or at the surface.
I would add that if you piped the outlet above the river surface the then the static head would be the difference in well depth and river depth plus the elevation of the outlet above the river surface. Keep in mind that there are back-flow preventers in the underwater pump outlet so it is not necessary for the outlet pipe to be above the level of the reservoir (river) to which it is pumping. Am I wrong about this?
Finally, the question has been asked bout how this thread evolved from a simple question about triangles on a curve to a technical discussion. My answer is that you guys took the ball and ran with it. This has been to my benefit since I am not a 'pump guy' and the benefit of your experiences, comments, and insights has helped reinforce my developing theoretical understanding of the situation at hand.
Anyway...I have been away for some time on other projects while one pump impeller has been removed to be cut and balanced. The motor has been inspected during the same time with no problems found. I am thinking that the installation of the motor and pump will be in the next week or two (it's a big job).
The plan is to run each pump individually and compare the motor loading and heating to see if the change was effective. We will also look at the differential pressure to presumably (?) get an idea of the change in flow rate and/or the new flow rate. I'll have to figure that part out when the time comes.
Thanks again for the help, comments, and suggestions. I will be sure to post again once the pump is installed and the results are in.