"Multiplicity ought not to be posited without necessity."
— Occam's Razor
API Standard 610 and Overhung Pumps
It is common knowledge that two-stage overhung pumps have a tarnished reputation for reliability. This was the basis for the API Standard 610, Sixth and Seventh Editions, proscribing their use since January, 1981.
The API Standard 610, Eighth Edition, allows for the use of purchaser-specified, two-stage overhung designs, providing rotor stiffness criteria are addressed. (See more on API pecifications.)
(Figure 1) Horizontal single-stage overhung centrifugal process pump, featuring centerline mounted, back pull-out radial split case (Click on image to enlarge.)
Figure 1 illustrates an API Standard single-stage overhung pump and the same pump incorporating an auxiliary (booster) impeller.
Figure 2 illustrates a conventional two-stage overhung design. It is obvious that the two-impeller (Split Flow) design is not a two-stage overhung type, where each of the two stages, in series, pump 100 percent of the design flow.
The API 610 Taskforce has noted that the two-impeller design for dual service "...has a niche where it is desirable and does not want to discourage its application by inclusion in Table 1-1." (reference Eighth Edition). Pumps modified to incorporate the Split Flow feature satisfy the rotor stiffness criteria for two-stage overhung types, in accordance with API specifications.