Below are some pump differences more explanation.Pump Rating
ANSI Pump Rating = 300 PSIG at 300������ F
API Pump Rating = 750 PSIG at 500������ F
Volute Cases & Suction/Discharge Flanges
Both pump styles have a radial split casing, and most ANSI pumps and some API pumps employ a single volute design of the interior passages. This is particularly evident in the smaller sizes that involve low-flow rates and lower specific speeds of the impeller. As shown in Figure 1, the area of the volute increases at a rate that is proportional to the rate of discharge from the impeller, thus producing a constant velocity at the periphery of the impeller. This velocity energy is then changed into a pressure energy by the time the fluid enters the discharge nozzle. Most of the larger API pumps are produced with a double volute design to reduce these loads on high-flow and high-head units.
The top suction/top discharge arrangement, which has also been used in a slightly different configuration in a vertical inline pump design. In this arrangement with a horizontal pump, the suction nozzle is located at the top of the casing adjacent to the discharge nozzle, rather than on the end. On the vertical inline design, the suction nozzle is once again on the side, but now it is opposite to the discharge nozzle, thus creating the ���������inline��������� appearance. The drawback of this design is, for most of these pumps, that the NPSH required is often considerably greater than it would be in the end suction arrangement. More NPSH is needed in order to accommodate the friction losses in the tortuous path from the suction flange to the eye of the impeller.
ANSI Pump Rating = 300 PSIG at 300������ F
API Pump Rating = 750 PSIG at 500������ F
Volute Cases & Suction/Discharge Flanges
Both pump styles have a radial split casing, and most ANSI pumps and some API pumps employ a single volute design of the interior passages. This is particularly evident in the smaller sizes that involve low-flow rates and lower specific speeds of the impeller. As shown in Figure 1, the area of the volute increases at a rate that is proportional to the rate of discharge from the impeller, thus producing a constant velocity at the periphery of the impeller. This velocity energy is then changed into a pressure energy by the time the fluid enters the discharge nozzle. Most of the larger API pumps are produced with a double volute design to reduce these loads on high-flow and high-head units.
The top suction/top discharge arrangement, which has also been used in a slightly different configuration in a vertical inline pump design. In this arrangement with a horizontal pump, the suction nozzle is located at the top of the casing adjacent to the discharge nozzle, rather than on the end. On the vertical inline design, the suction nozzle is once again on the side, but now it is opposite to the discharge nozzle, thus creating the ���������inline��������� appearance. The drawback of this design is, for most of these pumps, that the NPSH required is often considerably greater than it would be in the end suction arrangement. More NPSH is needed in order to accommodate the friction losses in the tortuous path from the suction flange to the eye of the impeller.
Back Cover Arrangements
One of the major differences between the ANSI and API pump casings is in the manner in which the back cover is secured to the casing. In the ANSI design shown in Figure 3, the back cover and gasket are held against the pump casing by the bearing frame adaptor, which is most frequently supplied in cast iron. This usually results in a gap between the mating faces of the frame adaptor and the pump casing that has the potential to permit uneven torquing of the bolts. In the event of a higher-than-normal pressurization of the casing by the process system, this may cause a fracture of the adaptor.
One of the major differences between the ANSI and API pump casings is in the manner in which the back cover is secured to the casing. In the ANSI design shown in Figure 3, the back cover and gasket are held against the pump casing by the bearing frame adaptor, which is most frequently supplied in cast iron. This usually results in a gap between the mating faces of the frame adaptor and the pump casing that has the potential to permit uneven torquing of the bolts. In the event of a higher-than-normal pressurization of the casing by the process system, this may cause a fracture of the adaptor.
The API design in Figure 4 bolts the back cover directly to the casing and uses a confined controlled compression gasket with metal to metal fits. The adaptor is bolted independently to the back cover and does not play a part in the pressure boundary of the pump casing.
Mounting Feet & Bearing Housing
Another difference 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 that 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 that 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. The ability to handle higher temperature services is also evident in the bearing housings of the API pumps, which tend to be much more robust in design and also accommodate cooling jackets with a greater capacity of cooling water.
Materials of Construction
Pump manufacturers can provide ANSI and API pumps in a wide assortment of materials, the selection of which depends on the operating stress and effects, as well as the type of wear from the product being pumped. The most common materials used in these centrifugal pumps are:
��������� Cast iron
��������� Ductile iron
��������� Bronze
��������� Carbon and low alloy steels such as 4140
��������� Chrome steels such as 11%, 12% or 13%
��������� Martenistic stainless steels: the 400 series
��������� Precipitation hardening stainless steels: 17-4 PH
��������� Austenitic stainless steels: the 300 series or alloy 20
��������� Duplex stainless steels: CD4MCu
��������� exotic alloys: Hastelloy, Titanium, dll.
Repair Considerations
It is important to remember, before any repair procedures are performed on any pump component, that the material of construction must be accurately identified by means of the appropriate tests. Prior to any repairs being conducted on a pump casing, it is also
advisable to consider the economic advantage of the repair under consideration. Smaller and medium-sized ANSI pumps are designed with a high degree of interchangeability
and produced in volume. Consequently, it can frequently be more cost effective to replace the entire pump rather than a combination of the impeller, casing and back cover. In addition, both the individual parts and complete pumps are available fairly quickly. This can make it more cost effective to replace rather than repair the parts, unless the wet
ends are made of the more exotic alloys. It is clear, in the case of non-metallic pumps (which may also conform to ANSI standards), that the components must be replaced, as they generally cannot be repaired. API pumps, however, are generally more economical to repair than to replace. These units are usually installed in more rugged duties and hazardous applications in refineries or other petrochemical industries, and are consequently more durable and more expensive. Delivery periods are also frequently longer, and the parts more costly than their ANSI equivalents���������particularly the cases and impellers.
This makes it very tempting to source these parts from an after-market supplier rather than the Original Equipment Manufacturer (OEM). It should be noted, though, that the major parts of a centrifugal pump (i.e. the casing, the impeller and the back cover) are all cast from patterns involving intricate hydraulic designs, which are of a proprietary nature. These parts are also the ones that provide the hydraulic performance of the pump. While the parts might be available from after-market suppliers at slightly lower prices than they are from the OEM, that cost saving will fade into insignificance if the pump does not meet its hydraulic performance. Your OEM can accept the responsibility for thesubsequent hydraulic performance of these replacement parts.
Price
API pumps is more expensive than ANSI pump.
CONCLUSION
So, Let���������s Stay Focused and Practical. Pump selection is not a beauty contest���������ANSI and API are not brands to be ���������preferred.��������� Instead, it���������s up to the system designer and equipment supplier to cooperate as much as possible to ensure that the best possible and most reliable pump selection ensues.
Trims
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