In the world of pumps, there are two types of horizontal end suction centrifugal pumps that are more commonly used than all the others put together; the ANSI pump that is designed and built to the standards of the American National Standards Institute, and the API pump that meets the requirments of the American Petroleum Institute Standard 610 for General Refinery Service.
Over the years, the ANSI pump has become the preferred style of end suction pumps, not only for chemical process applications, but also for water and other less aggressive services. The Standard provides for dimensional interchangeability of pumps from one manufacturer to another.
The API pump is almost the exclusive choice for applications in the oil refinery industry, where it handles higher temperature and pressure applications of a more aggressive nature.
While both of these horizontal, single stage, centrifugal pumps are designed with a radially split casing to accommodate a back pullout arrangement for ease of maintenance, the major difference between them is the casing pressure design ratings.
ANSI Pump Rating = 300 psig at 300 degs. F.
API Pump Rating = 750 psig at 500 degs. F.
In view of these figures, it is apparent that the API pumps should be considered for higher pressure and temperature services than the lighter duty ANSI pump. Unfortunately, that does not always happen.
When working with many clients in the process industries, I am frequently called on to troubleshoot the occasional “bad actor”. This is a term given to a pump that seems to always be in trouble with frequent and repetitive failures. In many cases I find an ANSI pump trying valiantly to handle a liquid well beyond the range of its design rating.of 300 degs. F.
One of the principal differences between the two pump styles is the configuration of the mounting feet. All ANSI pump casings are mounted on feet projecting from the underside of the casing and bolted to the baseplate. If these pumps are used on high temperature applications, the casing will expand upwards from the mounting feet and cause severe thermal stresses in the casing which will detrimentally affect the reliability of the pump. Operation at lower temperatures will not be affected by this feature.
On the other hand, API pumps are mounted at the horizontal centerline of the casing on feet projecting from each side of the casing and bolted to pedestals which form part of the baseplate. This arrangement provides the API pump with the advantage of being able to operate with pumpage at elevated temperatures. As the pump comes up to temperature in such cases, any expansion of the metal will be above and below the casing centerline, and will exert minimal amounts of stress to the casing, thus contributing to optimum reliability of the pump.
Consequently, when you are choosing the pump you need, remember the effects that high temperatures may have on the reliability of the two designs and be guided accordingly.
Author of “The Practical Pumping Handbook” and a specialist in Pumping Reliability, Ross Mackay can be reached at www.practicalpumping.com or at 1-800-465-6260
