1. Which process
ANSI
pump is right
for your
application?
vs
I
API? By Ross C. Mackay,
Contributing Editor
n the world of pumps, there are two types of
horizontal end suction centrifugal pumps that
are more commonly used than all the others put
In view of these figures, it is apparent that the
API pumps should be considered for higher pres-
sure and temperature services than the lighter duty
together. They are the ANSI pump that is designed ANSI pump.
and built to the standards of the American National
Standards Institute, and the API pump that meets The Liquids
the requirements of the American Petroleum In considering the various types of liquids han-
Institute Standard 610 for General Refinery Service. dled by these pumps, the fluids in the petrochemi-
Over the years, ANSI designs have become the cal industry are frequently classified as
preferred style of end suction pumps, not only for Hydrocarbons, Aggressive Chemicals and Mild
chemical process applications, but also for water Chemicals.
and other less aggressive services. The ANSI Hydrocarbons are petroleum-based products
Standard provides for dimensional interchangeabil- that are frequently classified as light, intermediate
ity of pumps from one manufacturer to another. or heavy. At atmospheric pressure and temperature,
The API pump is almost the exclusive choice light hydrocarbons tend to vaporize, intermediate
for applications in the oil refin- hydrocarbons are liquid, and
ery industry, where it handles heavy hydrocarbons are highly
higher temperatures and pressure Taking these fac- viscous or even solid.
applications of a more aggressive Aggressive Chemicals include
nature. While this specification tors into account, strong acids, alkalines or oxidiz-
also deals with some vertical ing agents that are destructive to
shaft models, this article will you can finish up both equipment and the environ-
focus on the more widely used ment. They are also dangerous to
horizontal designs. These single
with a profitable plant personnel if allowed to leak.
stage pumps are designed with a Mild Chemicals are generally
radially split casing to accommo-
and reliable easy to handle, and are not detri-
mental to either equipment or
date a back pullout arrangement
for ease of maintenance. The
operating system. the environment.
major difference between the Many of these liquids can
two styles is reflected in the casing pressure design produce toxic fluid exposure and vapors if they are
ratings, which are as follows: allowed to leak out of a pump. Vapor release is a
common danger with hydrocarbons that vaporize at
ANSI Pump Rating = 300 PSIG at 300° F atmospheric conditions or other chemicals that may
API Pump Rating = 750 PSIG at 500° F be exposed to very high operating temperatures.
36 MARCH 2004 www.pump-zone.com PUMPS & SYSTEMS

2. If a vapor release is exposed to a spark, the vapor discussed on this problem in recent years. This load
cloud may even explode or catch fires. is at its maximum when the pump is run at the
Consequently, in handling these liquids, we shutoff condition, and gradually decreases as the
must be extremely aware of much more than envi- flow rate approaches the Best Efficiency Point
ronmental damage and pumping efficiency. We (BEP). If the pump operates beyond the BEP, the
must also be very conscious about personal safety. load increases again, but in the opposite direction
Therefore, the choice between the ANSI pump and on the same plane. Examination of the resultant
the API pump must take into account the specific shaft deflection problems has indicated that the
fluid properties, as well as the operating conditions. radial plane on which the out-of-balance load acts
The main difference between these choices is is approximately 60° counterclockwise from the
predominantly a result of the differences in casing cut-water of the volute.
design.
Volute Cases
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 con-
stant velocity at the periphery of the impeller. This
velocity energy is then changed into a pressure ener-
gy by the time the fluid enters the discharge nozzle.
Figure 2. Double volute case
Most of the larger API pumps are produced
with a double volute design to reduce these loads on
high-flow and high-head units. (See Figure 2.) This
is accomplished by balancing the opposing out-of-
balance loads from each volute. While the cost of
this is a slight reduction in efficiency, it is consid-
ered a small price to pay for the increased reliabili-
ty that ensues.
Another casing feature found in many API
pumps is the top suction/top discharge arrange-
ment, which has also been used in a slightly differ-
Figure 1. Single volute case ent configuration in a vertical inline pump design.
In this arrangement with a horizontal pump, the
The peculiar shape of the volute also produces suction nozzle is located at the top of the casing
an uneven pressure distribution around the adjacent to the discharge nozzle, rather than on the
impeller, which in turn results in an imbalance of end. On the vertical inline design, the suction noz-
the thrust loads around the impeller and at right zle is once again on the side, but now it is opposite
angles to the shaft. This load must be accommodat- to the discharge nozzle, thus creating the “inline”
ed by the shaft and bearings, and much has been appearance.
PUMPS & SYSTEMS www.pump-zone.com MARCH 2004 37

3. 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 cas-
ings 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
Figure 3. Typical ANSI pump potential to permit uneven torquing
of the bolts. In the event of a higher-
The drawback of this design is, for most of than-normal pressurization of the casing by the
these pumps, that the NPSH required is often con- process system, this may cause a fracture of the
siderably greater than it would be in the end suction adaptor
arrangement. More NPSH is needed in order to The API design in Figure 4 bolts the back cover
accommodate the friction losses in the tortuous directly to the casing and uses a confined controlled
Circle 392 on Reader Service Card
38 MARCH 2004 www.pump-zone.com PUMPS & SYSTEMS

4. March04PUMPS&SYSp26-41 2/17/04 10:45 AM Page 39
compression gasket with
metal to metal fits. The
adaptor is bolted indepen-
dently to the back cover and
does not play a part in the
pressure boundary of the
pump casing.
Mounting Feet
Another difference be-
tween the two pump styles is
the configuration of the
mounting feet.
All ANSI pump casings
are mounted on feet project- Figure 4. Typical API pump
ing from the underside of
the casing and bolted to the
baseplate. If these pumps are
used on high-temperature applications, the casing On the other hand, API pumps are mounted at
will expand upwards from the mounting feet and the horizontal centerline of the casing on feet pro-
cause severe thermal stresses in the casing that will jecting from each side of the casing and bolted to
detrimentally affect the reliability of the pump. pedestals that form part of the baseplate. This
Operation at lower temperatures will not be affect- arrangement provides the API pump with the
ed by this feature. advantage of being able to operate with pumpage at
Circle 519 on Reader Service Card
PUMPS & SYSTEMS www.pump-zone.com MARCH 2004 39

5. elevated temperatures. As the pump comes up to vices is also evident in the bearing housings of the
temperature in such cases, any expansion of the API pumps, which tend to be much more robust in
metal will be above and below the casing centerline, design and also accommodate cooling jackets with
and will exert minimal amounts of stress to the cas- a greater capacity of cooling water.
ing, thus contributing to optimum reliability of the
pump. Materials of Construction
The ability to handle higher temperature ser- Pump manufacturers can provide ANSI and API
pumps in a wide assortment of mate-
rials, 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
In order to common materials used in these cen-
low NPSHR achieve
suction de - values, an
end
trifugal pumps are:
sign
enclosed im incorporating an
pe
eye and in ller with a large
jection bo
standard res is
• Cast iron
• Ductile iron
• Bronze
• Carbon and low alloy steels
such as 4140
• Chrome steels such as 11%,
12% or 13%
• Martenistic stainless steels in
the 400 series
• Precipitation hardening stain-
less steels like 17-4 PH
• Austenitic stainless steels like
the 300 series or alloy 20
• Duplex stainless steels such as
CD4MCu
• Other more exotic alloys such
as Hastelloy, Titanium, etc.
Repair Considerations
It is important to remember,
before any repair procedures are
performed on any pump compo-
nent, that the material of construc-
tion must be accurately identified
by means of the appropriate tests.
Prior to any repairs being conduct-
ed on a pump casing, it is also
Circle 210 on Reader Service Card
advisable to consider the economic
advantage of the repair under con-
sideration.
Smaller and medium-sized
ANSI pumps are designed with a
high degree of interchangeability
and produced in volume. Conse-
quently, it can frequently be more
cost effective to replace the entire
40 MARCH 2004 www.pump-zone.com PUMPS & SYSTEMS

6. pump rather than a combination of the impeller, Contributing Editor Ross Mackay has over 40
casing and back cover. In addition, both the indi- years in the pump industry—many of them working
vidual parts and complete pumps are available fairly with both ANSI and API pump applications. Today,
quickly. This can make it more cost effective to he specializes in helping companies increase their
replace rather than repair the parts, unless the wet pump reliability and reduce operating and mainte-
ends are made of the more exotic alloys. It is clear, nance costs through consulting and education. He can
in the case of non-metallic pumps (which may also be reached at 1-800-465-6260 or through his web site
conform to ANSI standards), that the components at www.rossmackay.com
must be replaced, as they general-
ly cannot be repaired.
API pumps, however, are gen-
erally more economical to repair
than to replace. These units are
usually installed in more rugged
duties and hazardous applications
in refineries or other petrochemi-
cal industries, and are conse-
quently 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 Manu-
facturer (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 pat-
terns 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
Circle 537 on Reader Service Card
accept the responsibility for the
subsequent hydraulic perfor-
mance of these replacement parts.
By taking these factors into
account, you can finish up with
reliable pumps that have been
properly selected to provide a prof-
itable and reliable operating system.
P&S
PUMPS & SYSTEMS www.pump-zone.com MARCH 2004 41