OH1/OH2 single stage, horizontal overhung pumps
Single stage, horizontal overhung pumps (or cantilever pumps) are widely used in
broad array of service in various industries because they are compact, lightweight,
have fewer parts, have small footprint, and inexpensive. They are easy to install,
operate, and maintain.
OH1 or OH2
API Standard 610 and ISO Standard 13709 classify these pumps as OH1
(foot-mounted), or OH2 (centerline-mounted.) Per section 18.104.22.168 of the standard,
OH1 pumps are required in service temperature below 300 degrees Fahrenheit;
and OH2 at 300 degrees, or above.
Basis for temperature criteria
A pump casing expands radially in direct proportion to its temperature. A
foot-mounted casing expands upward (it cannot expand downward.) If the upward
expansion is excessive, as in the case of high temperature service, the casing
can be subjected to harmful thermal stress and excessive nozzle load. The shaft
can also get misaligned with the driver.
To avoid these problems, centerline-mounted pumps should be used in high
temperature service. Centerline-mounted pump have their mounting feet located
at the casing horizontal centerline (in line with the shaft centerline) to allow the
casing to expand equally in both upward and downward directions.
Foot-mounted pumps non-API 610 and non-ISO 13709 standard
Section 22.214.171.124 can be used as selection criteria for pumps not built to API 610 or
ISO 13709 standard with regard to service temperature limit. But this section is
unrealistically restrictive because it does not consider the maximum impeller
diameter size of the pumps.
OH1 and OH2 pumps are typically available in sizes from [*]" to [*] maximum
impeller diameter. (Above [*] between-bearing pumps are almost always used.) A
[*] pump will have minimal thermal expansion compared to that of a [*] pump.
Therefore, it is reasonable to expect that smaller foot-mounted pumps can be
operated safely at higher temperature, above 300 degrees, compared to bigger
A good ballpark figure for foot-mounted pumps not built to API 610, or ISO 13709,
is that [*] pumps can be operated safely up to [*]} degrees and [*] pumps should
be limited to 300 degrees Fahrenheit maximum. These figures can be
interpolated for sizes, or temperature, in-between.
Advantages of foot-mounted over centerline-mounted
In spite of its limited service temperature, OH1 pumps have many advantages
over OH2 which make them popular, among which are: they have smaller foot-
print and lower center of gravity; they do not require mounting pedestals and can
be mounted directly on baseplate, or skid; they are easier to handle and transport
having some feet to stand on.
[*] Some information are excluded in this article. Read more.
F: Pump API designation3
Top photo: Example of OH2 centrifugal pump
Bottom: Generic images of OH1 and OH2 pumps
OH1 - foot-mounted
OH2 - centerline-mounted