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Many details need to be considered during the selection process, but it would be helpful to first understand the differences between ANSI and API pumps. Each of these pumps is designed according to different standards. ANSI pumps meet standards set forth by the American National Standards Institute, while API pumps are engineered to standards defined by the American Petroleum Institute.

As you may have guessed, the API 610 Standard focuses more specifically on refinery and oil & gas applications, whereas the ANSI B73.1 standard applies to a wider range of industries. The ANSI B73.1 Standard allows for interchangeability across 27 centrifugal pump sizes, making this kind of pump ideal for many applications, including food processing, textiles, chemical, pulp & paper, and other types of process plants.

While ANSI pumps can handle a range of different fluids, they do not move thick and highly viscous materials well – unlike API pumps. API pumps are heavier duty and tend to operate more reliably under higher pressures, higher temperatures, and with the capacity to control emissions per EPA requirements. Typically, more expensive than ANSI Pumps, API pumps have some major differences when it comes to the casing design, bearing housings, impellers, mounting, and back cover arrangements. Much of this is covered in Heinz P. Bloch’s book “Pump User’s Handbook Life Extension”, co-authored with Allan R. Budris, but below is a brief recap of some of these major differences.

Casing Design Ratings for Pressure and Temperature

An ANSI pump is rated for 300 psig at 300℉, while an API pump is rated for 600 psig at 600℉.


Manufactured without wear rings, ANSI pump impellers are open or semi-open; whereas API pumps feature closed impellers with replaceable casing wear rings.

Bearing Housings

The bearing housings of API pumps tend to be more robust steel design and often accommodate cooling jackets with greater capacity to handle higher temperatures than ANSI pumps.


ANSI pumps are generally foot-mounted, which makes it hard to keep aligned at elevated and varying temperatures as thermal stresses can significantly decrease operational life. API pumps are centerline mounted to counter the effects of thermal expansion and minimize the amount of stress to the casing.

Note: there are some ANSI pumps that are manufactured with centerline mounting.

Back Cover Arrangements

As pump expert Ross Mackay explains in slide 3 of his slideshare presentation, the back covers in each type of pump are secured differently. In ANSI pumps, the back cover and gasket are held against the pump casing by a cast iron bearing frame adaptor; while in API pumps, the back cover is bolted directly to the casing and uses a compression gasket with metal-to-metal fits.

When choosing a pump, the most important factor to consider is safety. In many cases, an API pump is chosen even when an ANSI style can ‘do the job’. Decisions on which type of pump to use must take into account not only the specific operating conditions and properties of the fluid being handled, but the safety and well-being of plant personnel and the surrounding environment.