Bilge Pump 1 1/2 pipe down to 1 1/8

[GordMay]

Originally Posted by Nord Sal

Head loss is inversely proportional to the square of the diameter of the conduit.

So, using the nominal dimensions and with all else equal (conduit length, material roughness, etc.) to the rated conditions, the capacity would be:

3,700 gpm x (1.125/1.5)^2 = 3,700 gpm x 56% = 2,080 gpm.

However, as correctly noted above Rule pump ratings aren't realistic to the real world. The actually capacity will be markedly lower.

Note that the calculation reveals a nearly 50% loss of capacity.

[Nord Sal]

Originally Posted by GordMay

Note that the calculation reveals a nearly 50% loss of capacity.

But the typo (gpm instead of gph) increases the capacity by 60X !

[rslifkin]

Centrifugal bilge pumps will never put out their full rated capacity in the real world with hose losses, lift to the discharge, etc. Typically, as-installed capacity will be 50 - 70% of nameplate rating. Real world output can be calculated, as Rule and some others publish a discharge head vs output flow graph. Add a little further de-rate to be conservative and you get an answer for how much water it'll really move. As installed, the Rule 3700 sitting in my main bilge as a backup should move about 2400 gal/hr.