Selection criteria for OH3, OH4, and OH5


The OH2 pump is the designation given by API Standard 610 and ISO Standard
13709 to an overhung, centerline-mounted, single stage, horizontal centrifugal
pump. OH3, OH4, and OH5 are designations given to overhung, single stage,
vertical in-line pumps. The selection of OH3, OH4, or OH5 over OH2 is simply a
matter of selecting an in-line vertical pump over a horizontal pump. (The criteria for
selecting a horizontal over a vertical pump are discussed in another article on this
web site.)

The OH3, OH4, and OH5 are all overhung, single stage, in-line vertical pumps.
The question that might be asked is when to select an OH3, OH4, or OH5. To
answer this question, let us examine the three designations:

The OH3 is the designation given to a pump with a separate bearing bracket (or
housing) which is integral with the pump (as opposed to being integral with the
driver.) The pump and the driver are flexibly coupled. This design is preferred when
the pump is subjected to high suction pressure and high axial thrust load. This
design first gained wide acceptance in Europe where vertical motors have limited
thrust capacity unlike in the U.S. where vertical motors can be supplied with high
thrust capacity bearings. The separate bearing bracket (or housing) adds
significant cost to the pump, and increases the height of the pump. The OH4 is
typically suitable for heavy service.

Unlike the OH3, the OH4 pump does not have a separate bearing bracket (or
housing) and it is not flexibly coupled to the driver. The axial thrust is carried by the
driver which is rigidly coupled to the pump. This is by far the most popular and
widely used design among vertical in-line pumps. In applications where high
suction pressure would normally result in high axial thrust load, the thrust load
can be reduced to stay within the thrust capacity of be the driver by unbalancing
the impeller wear rings, omitting, the back wear rings, and/or reducing the size of
the mechanical seal (or packing rings) so that the need for a separate bearing
bracket (or housing) is avoided. The OH4 are typically suitable for medium.

The OH5 pump, on the other hand, is close-coupled to the driver. The term “close-
coupled” means that the driver (usually an electric motor) has an extended shaft
that serves as “the pump shaft.” The impeller is mounted directly on the driver
shaft and there is no coupling. The axial thrust is carried by the bearing/s of the
driver. Because of its compact, lightweight, and inexpensive design, the OH5 is
very popular in light service where, in some cases, the pump is mounted directly
on the piping similar to a valve installation and may be simply supported with
mounting brackets.


R-1109-SECR
File: pump API designation1

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Related topics

Introduction
Pump types
Horizontal vs. vertical
Axial  vs. radial split case
Parallel-series operation
Self-priming
OH1, OH2
OH2, OH3, OH4, OH5
VH2, VH3, VH7

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