Beware Of The L3/D4 Formula

Recent trade publication articles have suggested the use of the simplified L3/D4 formula when evaluating competitive pumps. Caveat emptor -- let the buyer beware.

Shaft deflection is critical to performance but this abbreviated version of the full formula is only valid if you are comparing two identical centrifugal pumps -- identical in design and, most important, identical in materials of construction. A recent technical article, TL-126 from Vanton's Chief Engineer, makes it very clear that using L3/D4 to compare thermoplastic pumps with metal ones will cause you to make the wrong decision every time.