ept 07-t-06c centrifugal pumps

7/27/2019 EPT 07-T-06C Centrifugal Pumps

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Mobil Oil,1998 1 of 47

Centrifugal Pumps

EPT 07-T-06C

October 1992 Draft

Scope

This Engineering Practice Tutorial (EPT) is part of a series, starting with EPT 07-T-06A andcontinuing in EPT 07-T-06B. This EPT provides the project engineer with a basic understanding of

centrifugal pump selection. Refer to EPT 07-T-05A, EPT 07-T-05B and EPT 07-T-05C forinformation on Reciprocating Pumps.


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EPT 07-T-06C Centrifugal Pumps October 1992 Draft


Table of Contents

Scope...................................................................................................................................................1

Table of Figures ................................................................................................................................4

1. References..................................................................................................................................5

1.1. Mobil Tutorials .................................................................................................................5

1.2. APIAmerican Petroleum Institute ...............................................................................5

1.3. ASMEAmerican Society of Mechanical Engineers .................................................5

1.4. NACENational Association of Corrosion Engineers ...............................................5

2. General ........................................................................................................................................6

3. Materials of Construction .......................................................................................................6

3.1. Materials Selection Factors...........................................................................................6

3.2. Types of Corrosion .........................................................................................................7

3.3. Abrasive Wear ...............................................................................................................13

3.4. Material For Centrifugal Pump Parts .........................................................................14

3.5. Selection of Materials of Construction .......................................................................17

4. Standards..................................................................................................................................18

4.1. API STD 610 (Centrifugal Pumps for General Refinery Service) .........................18

4.2. Chemical Pump Standards..........................................................................................18

4.3. Comparison of API and ANSI Pump Standards ......................................................18

4.4. Data Sheets ...................................................................................................................20

5. Selecting a Centrifugal Pump..............................................................................................20

5.1. Number of Pumps Required........................................................................................22

5.2. Additional Considerations ............................................................................................22

Appendix A: Nomenclature ........................................................................................................23


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Appendix B: Example Problem (Metric)..................................................................................25

Appendix C: Example Problem (Customary).........................................................................29

Appendix D: Generic Types of Centrifugal Pumps ..............................................................33

1. ANSI Pump ...............................................................................................................................33

2. Single Stage API Pump.........................................................................................................35

3. Vertical In-Line Pump ............................................................................................................37

4. API Multistage Split Case Pumps.......................................................................................39

5. API Barrel Pumps ...................................................................................................................41

6. Sump Pump..............................................................................................................................42

7. API Vertical Turbine or Can Pump .....................................................................................44

8. Submersible Pump.................................................................................................................46


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Table of Figures

Figure 1: Pumping Head Vs. Flow Rate for Different Types of Pumps (Use as aGuide for Selecting the Most Economical Pump)..............................................21

Figure D-1: Schematic Diagram of an ANSI Pump (Courtesy of Gould Pumps).........33

Figure D-2: Example of an API single-stage pump (Courtesy of Gould Pumps).........35

Figure D-3: Schematic Diagram of an In-Line Pump (Courtesy of Gould Pumps)......37

Figure D-4: Example of an API Multistage Split Case Pump (Courtesy of GouldPumps) ..........................................................................................................................39

Figure D-5: Example of a Barrel Pump....................................................................................41

Figure D-6: Example of a Sump Pump (Courtesy of Gould Pumps) ...............................42

Figure D-7: Schematic Diagram of a Vertical Turbine or Can Pump...............................44

Figure D-8: Schematic Diagram of a Submersible Pump...................................................46


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1. ReferencesThe following Mobil EPTs and industry publications shall be considered a part of this EPT. Refer tothe latest editions unless otherwise specified herein.

1.1. Mobil Tutorials

EPT 07-T-05A Reciprocating Pumps

EPT 07-T-05B Reciprocating Pumps

EPT 07-T-05C Reciprocating Pumps

EPT 07-T-06A Centrifugal Pumps

EPT 07-T-06B Centrifugal Pumps

1.2. APIAmerican Petroleum Institute

API STD 610 Centrifugal Pumps for Petroleum, Heavy Duty Chemical, and Gas IndustryServices Eighth Edition

1.3. ASMEAmerican Society of Mechanical Engineers

ASME B73.1M Specification for Horizontal End Suction Centrifugal Pumps for Chemical

Process Errata - August 1992

ASME B73.2M Specification for Vertical In-Line Centrifugal Pumps for Chemical Process

ASME SEC VIII Boiler and Pressure Vessel Codes

1.4. NACENational Association of Corrosion Engineers

NACE RP0475 Selection of Metallic Materials to Be Used in All Phases of Water Handlingfor Injection into Oil-Bearing Formations


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2. GeneralCentrifugal pump selection shall be in accordance with requirements of this EPT, unless superceded

by more stringent local regulations.

Pumps serve many purposes in production facilities. The largest pumps are usually shipping or sales

pumps which increase the pressure of oil or condensate so that it can flow into a sales pipeline or beloaded into tankers, barges, railroad cars, or trucks. Other large pumps are used with water injection

systems for disposing of produced water or for water flooding. Smaller pumps are used to pumpliquids from low to higher pressure vessels, to pump liquids from tanks at a low elevation to tanks at a

higher elevation, or to transfer liquids for further processing.

A facility's utility system often has many pumps, which may be used for firewater wash down andutility water, heat medium, fuel oil or diesel, and hydraulic systems.

The project engineer shall be able to select the proper pump for each application, determinehorsepower requirements, design the piping system associated with the pump, and specify materials

and details of construction for bearings, seals, etc. On standard applications the project engineer mayallow the vendor to specify materials and construction details for the specified service conditions.

Even then, the project engineer shall be familiar with different alternatives so that he or she can betterevaluate proposals and alternate proposals of vendors.

3. Materials of Construction

3.1. Materials Selection Factors

Factors that affect selection of pump construction materials are:

1. Service condition

2. Safety

3. Service Life

4. Maintenance

Pump service conditions shall be reviewed to determine if pump liquid properties and

operating conditions can be handled with standard materials of construction offered by thepump manufacturer or if more expensive materials of construction will be required tohandle service conditions related to corrosion, erosion, or abrasion wear.

For safety reasons, pressure-containing parts of pumps that handle flammable or toxicliquids need steel or alloy steel construction. Cast iron construction may be suitable forother services. Aluminum bronze or other corrosion-resistant alloys often are used in


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water handling/injection services for longer service life and ease of maintenance. These

factors, as well as economic factors of construction and design that are often based onexperience with an application, shall enter into the design selection process. In the final

analysis, there is no substitute for engineering judgement in making design decisions.

3.2. Types of Corrosion

Corrosion is the destructive attack of a metal by either a chemical or electrochemical

reaction. Types of corrosion are:

1. Uniform Corrosion

2. Erosion Corrosion

3. Stress Corrosion

4. Intergranular Corrosion

5. Cavitation Erosion

6. Graphitization

7. Galvanic Corrosion

8. Pitting Corrosion

9. Crevice or Concentration Cell Corrosion

3.2.1. Material SelectionAs a general rule, pump materials shall be selected primarily on the basis of

corrosion resistance to the liquid being pumped. If the liquid containsabrasive solids, the construction material shall be selected primarily for

abrasive-wear resistance, provided the corrosion resistance characteristics areacceptable.

3.2.2. Description and Impact of Corrosion Types

A brief description of each type of corrosion and its impact on pump designfollows:

1. Uniform Corrosion

a) Uniform corrosion is the overall attack on a metal by a corrodingliquid resulting in a relatively uniform metal loss over the exposedsurface. This is the most common type of corrosion; it can be

minimized through the selection of a material that offers resistance tothe corroding liquid.

b) The pH of a liquid is an indication of its corrosive qualities, eitheracidic or alkaline. It is a measure of the hydrogen or hydroxide ionconcentration in gram equivalents per liter. The pH value is


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expressed as the logarithm to the base 10 of the reciprocal of the

hydrogen ion concentration. The scale of pH values is from zero to14, with 7 as a neutral point. From 7 to zero denotes increasing

hydrogen ion concentration and increasing acidity; from 7 to 14denotes increasing hydroxide ion concentration and increasing

alkalinity.

c) Table 1 outlines materials of construction usually recommended forpumps handling liquids of known pH value. The pH value shall only

be used as a guide. For more corrosive solutions, temperature andchemical composition shall be carefully evaluated in the selection of

construction materials.

Table 1: Materials for Pumps Handling

Liquids of Known pH

pH Value Material of Construction

10 to 14 Corrosion Resistant Alloys

8 to 10 All Iron

6 to 8 Bronze Fitted or Standard Fitted

4 to 6 All Bronze

0 to 4 Corrosion Resistant Alloy Steels

3.2.3. Erosion-Corrosion

1. Erosion-corrosion results when a metal's protective film is destroyed byhigh velocity liquid. It is distinguished from abrasion, which is the

destruction of the base metal by liquids containing abrasives.

2. In centrifugal pumps the impeller is particularly susceptible to erosion-corrosion. Diffuser-type casings, with their many vanes, are more

susceptible to erosion-corrosion than are volute-type casings.

3. Wear rings also are susceptible to erosion-corrosion and shall receivespecial consideration in material selection. The high fluid velocities

through the small clearance annulus can result in high rates of wearunless the proper material is selected.

4. Most pump components of standard designs are limited to a hardness ofapproximately 350 Brinell. Above 350 Brinell the standard machiningoperations of turning, boring, drilling, and tapping become

uneconomical. Stuffing box sleeves and wear rings of greater hardnesscan be provided, and the finishing operation of cylindrical components

can be accomplished by grinding.


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3.2.4. Stress Corrosion

1. Stress corrosion (corrosion fatigue) is the failure of a material due to a

combination of stress and a corrosive environment.

2. When material for a pump component that is subjected to a cyclic stressis evaluated, the endurance limit of the material shall be considered. The

endurance limit is the maximum cyclic stress that the material can besubjected to and still not fail after an infinite number of cyclic stress

reversals. The endurance limit of steel, for example, is approximately 50percent of its tensile strength. A 690,000 kPa (100,000 psi) tensile steelcan be subjected to a static load in tension to produce 690,000 kPa(100,000 psi) stress, but the same steel subjected to a cyclic stress of

690,000 kPa (100,000 psi) would fail in a short period of time. If thestress were reduced to 345,000 kPa (50,000 psi), however, the endurance

limit would not be exceeded; the metal could be subjected to 345,000kPa (50,000 psi) stress reversals and not fail. If the same steel were

subjected to a cyclic stress of 345,000 kPa (50,000 psi) in a corrosiveenvironment, however, failure could occur quite rapidly.

3. In corrosion fatigue, minute cracks develop at the surface. In a corrosiveenvironment the surface of the metal exposed by the cracks corrodesrapidly. The crack then penetrates deeper, corrosion develops further

and the piece will ultimately fail.

4. Since the corrosion fatigue strength of any metal is dependent more onthe corrosion resistance of the metal than on its tensile strength, the life

of any pump component subjected to cyclic loading can only be

estimated. A pump shaft, for example, is subjected to a complete stressreversal for each revolution; it will have some definite life based on therotational speed of the pump and the corrosion fatigue strength of the

material in the particular application. The best way to guard againstshort shaft life is to protect the shaft against exposure to the liquid by

means of sleeves.

3.2.5. Intergranular Corrosion

1. Intergranular corrosion results in the complete destruction of themechanical properties of the steel for the depth of the attack. Solutionannealing or using extra-low-carbon stainless steels will minimize

intergranular corrosion. It may be necessary to use special metallurgyfor severe cases.

2. Intergranular corrosion of austenitic stainless steels occurs as a result ofcarbides precipitating out at the grain boundaries during the slow coolingof the casting. When exposed to a corrosive environment, the carbides

are preferentially attacked, and the strongly bonded matrix of the metalgrains is destroyed. The precipitation of carbides can be controlled by

heating the casting to 1100C (2000F) and then quenching. At 1100C(2000F) the carbides are held in solution, and the rapid quench preventstheir precipitation.


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3. Susceptibility to intergranular corrosion in austenitic stainless steels alsocan be reduced by controlling the carbon content of the alloy. Standardaustenitic stainless steels of the 300 series have a carbon content in

excess of 0.08 percent. Without proper heat treatment, these steels aresusceptible to intergranular corrosion. Extra-low-carbon steels are

available in the 300 series and are identified by the suffix letter L. Thesesteels have a carbon content of less than 0.03 percent and are much less

susceptible to intergranular corrosion.

4. The possibility of intergranular corrosion shall be considered whencastings of austenitic stainless steels are indicated for impellers and

casings of centrifugal pumps or for the liquid ends of reciprocatingpumps.

3.2.6. Cavitation Erosion

1. Cavitation erosion is the removal of metal as a result of high localizedstresses produced in the metal surface from the collapse of cavitationvapor bubbles. In a corrosive environment the rate of damage is

accelerated as the corrosion products are continuously removed, and thecorrosion proceeds unabated.

2. While every effort shall be made in the design and application ofcentrifugal pumps to prevent cavitation, some is possible if the NPSHAis less than or equal to the NPSHR. In addition, it is not always possible

to assume that the NPSHA is always greater than the NPSHR atcapacities other than the rated maximum efficiency capacity of the pump.

Therefore, cavitation shall be considered in any evaluation of the

material for impellers.

3. Open-type mixed-flow impellers that produce heads in excess of 10.7 m(35 ft) are particularly susceptible to cavitation erosion in the clearancespace between the rotating vanes and the stationary housing. Any

evaluation of the impeller and housing material for a pump of this typeshall include the possibility of vanetip cavitation.

4. Laboratory tests of the resistance of a wide range of materials tocavitation erosion have produced the following tabulation of thecavitation resistance properties of pump materials, listed in order ofincreasing cavitation resistance:

a) Cast iron

b) Bronze

c) Cast steel

d) Manganese bronze

e) Monel

f) 400 series stainless steel

g) 300 series stainless steel


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h) Nickel-aluminum bronze

3.2.7. Graphitization

1. Gray iron consists of a matrix of iron and graphite. The graphite existsas flakes and produces the characteristic gray appearance of cast iron.The presence of the graphite also provides a lubricant during machining.

This property, plus the fragility of the chips, accounts for the excellentmachining qualities of cast iron.

2. These characteristics, in addition to low foundry costs, combine to makegray iron the most widely used metal in the pump industry. Aside fromthe low tensile strength and ductility of cast iron relative to the steels, the

corrosion- resistance properties of cast iron shall be carefully considered.The presence of graphite in the matrix of cast iron produces the unique

corrosion effect known as graphitic corrosion or, more simply,graphitization.

3. In the presence of an electrolyte, a galvanic cell exists between the ironand the graphite particles. In the combination of iron and graphite, iron

becomes the anode and the graphite becomes the cathode. A galvanic

current flows from the iron to the graphite, and the iron goes intosolution. The result is a gradual depletion of the iron in the matrix untilonly the graphite remains. The original casting in iron now has beenreduced to a porous graphite structure interspersed with the corrosion

products of iron. The physical properties of the graphite structure aregreatly inferior to those of cast iron, and the structure fails rapidly. In

fact, the effect is so dramatic that, while the casting appears sound from

outward appearances, pieces may be broken off with the fingers.

4. The effect of graphitization on a cast iron impeller pumping seawater hasbeen observed many times. The result is a rapid deterioration of theimpeller vanes as the soft graphite structure is scoured away,

progressively exposing new metal to further attack. The same impellerpumping a nonelectrolyte, such as fresh water, shows no effect ofgraphitization. Experience has shown that a cast iron impeller shall

never be used on brackish water or seawater; the result is inevitablydestruction by graphitization.

5. Experience also has shown that cast iron casings are much more resistantto destructive graphitization than are cast iron impellers. While it is true

that examination of the inside surface of the casing may reveal a layer ofgraphite, the velocities encountered in the casing very often are notsufficient to scour away the graphite, and the base material is protected

against further attack. This is true, however, only for pumps producing30 m (100 ft) of head or less. For heads in excess of 30 m (100 ft),

alternate casing materials shall be considered for brackish water orseawater services.


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3.2.8. Galvanic Corrosion

1. Galvanic corrosion is the electrochemical action produced when one

metal is in electrical contact with another more noble metal, with bothbeing immersed in the same corroding medium, called the electrolyte. A

galvanic cell is formed, and current flows between the two materials.The less noble material, called the anode, will corrode, while the more

noble cathode will be protected. It is important that the smaller wearingparts in a pump be of a more noble material than the larger, more

massive parts, as in an iron pump with bronze or stainless steel trim.

2. The following is a galvanic series listing the more common metals andalloys:

a) Corroded End (Anodic, or least noble)

b) Magnesium

c) Magnesium Alloys

d) Zinc

e) Aluminum 2S

f) Cadmium

g) Aluminum 17ST

h) Steel or Iron

i) Cast Ironj) Stainless Steel, 400 Series (Active)

k) Stainless Steel, Type 304 (Active)

l) Stainless Steel, Type 316 (Active)

m) Lead-tin Solders

n) Lead

o) Tin

p) Nickel

q) Nickel Base Alloy (active)

r) Brasses

s) Copper

t) Bronzes

u) Copper-Nickel Alloy

v) Monel


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w) Silver Solder

x) Nickel (Passive)

y) Nickel Base Alloy (Passive)

z) Stainless Steel, 400 Series (Passive)

aa)Stainless Steel, Type 304 (Passive)

bb)Stainless Steel, Type 316 (Passive)

cc)Silver

dd)Graphite

ee)Gold

ff) Platinum

gg)Protected End (Cathodic or most noble)

3.2.9. Pitting Corrosion

Pitting corrosion is a localized, rather than uniform, type of attack. It is

caused by a breakdown of a protective film, and it results in rapid pitformation at random locations on the surface.

3.2.10. Crevice or Concentration Cell Corrosion

Crevice or concentration cell corrosion occurs in joints or small surfaceimperfections. Portions of the liquid become trapped, and a difference in

potential is established due to the oxygen concentration difference in thesecells. The resulting corrosion may progress rapidly, leaving the surrounding

area unaffected.

3.3. Abrasive Wear

Abrasive wear is the mechanical removal of metal from the cutting or abrading action ofsolids carried in suspension in the pumped liquid. An undulating matte-finish wear

pattern can usually be identified as abrasive wear. The rate of wear for any material is

dependent upon the following characteristics of the suspended solids:

1. Solids concentration

2. Solids size and mass

3. Solids shape: spherical, angular, or sharp fractured surfaces

4. Solids hardness

5. Relative velocity between solids and metal surface

6. Angle of impingement


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3.3.1. Rate of Wear

The rate of wear depends on the materials selected for the rotating and

stationary components of a centrifugal pump. Although metal hardness isnot the sole criterion of resistance to abrasive wear, hardness does provide a

convenient index in the selection of ductile materials normally used incentrifugal pumps.

3.3.2. Material Selection Guide

While it is not normally possible to establish any direct relation between thelife of pump components and the quantity and characteristics of abrasive

particles pumped, the following tabulation can be used as a guide in materialselection, listed in order of increasing abrasive-wear resistance:

1. Cast iron

2. Bronze

3. Manganese bronze

4. Nickel-aluminum bronze

5. Cast steel

6. 300 series stainless steel

7. 400 series stainless steel

3.4. Material For Centrifugal Pump Parts

3.4.1. Impellers

1. The following criteria shall be considered in the selection of the materialfor the impeller:

a) Corrosion resistance

b) Abrasive-wear resistance

c) Cavitation resistance

d) Casting and machining properties

e) Cost

2. For most water and other noncorrosive services, bronze satisfies thesecriteria and, as a result, is the most widely used impeller material forthese services. Bronze impellers, however, shall not be used for

pumping temperatures above 120C (250F). This limitation existsprimarily because of the different expansion rates of the bronze impeller

and the steel shaft. Above 120C (250F), this difference produces an


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unacceptable clearance between impeller and shaft. The result is a loose

impeller on the shaft.

3. Cast iron impellers are used to a limited extent in small, low-cost pumps.Since cast iron is inferior to bronze in corrosion, erosion and cavitationresistance, low initial cost would be the only justification for a cast ironimpeller.

4. Stainless steel impellers of the 400 series are widely used when bronzedoes not satisfy the requirements for corrosion, erosion or cavitationresistance. The 400 series of stainless steels is not used for seawater,

since pitting will limit their performance life. Nevertheless, the 400

series shall be used when pumping temperatures exceed 120C (250F),since the differential expansion problem no longer exists with a steelimpeller on a steel shaft.

5. The austenitic stainless steels of the 300 series are the next step up on thecorrosion and cavitation-resistance scale. Initial cost is a factor here thatshall be evaluated against the increased life.

3.4.2. Casings

1. The following criteria shall be considered in the selection of material forcentrifugal pump casings:

a) Strength

b) Corrosion resistance

c) Abrasive-wear resistance


e) Cost

2. For most pumping applications, cast iron is the preferred material forpump casings when evaluated against initial cost. For single-stagepumps, cast iron is usually of sufficient strength for the pressuresdeveloped. For corrosive liquids and hydrocarbons it may be necessaryto specify ductile iron, cast steel, or cast stainless steels of the 400 or 300

series.

3. Cast iron casings for multistage pumps are limited to approximately 6900kPa (1000 psi) discharge pressure and 177C (350F). For temperatures

above 177C (350F) and pressures above 13,800 kPa (2000 psi)discharge pressure, cast or forged steel is usually specified.

4. In any evaluation of cast iron versus steel casings, consideration shall begiven to the probability of casing erosion during operation. Erosion canoccur either from abrasive particles in the fluid or from wire drawing

across the flange of a split-case pump. (Wire drawing is the condition inwhich, at high pressures, the fluid may begin to leak at the flanges. The

fluid causes erosion of small channels in the flange surfaces. Cast iron


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casings are very susceptible to wire drawing because it is very difficult to

obtain a perfect seal on a split-case pump.)

5. While the initial cost of a steel casing is higher than that of a cast ironcasing, a steel casing often can be salvaged by welding the erodedportions and then re-machining. Salvaging a cast iron casing by weldingis not practical, and the casing usually shall be replaced.

6. The ductile irons are useful casing materials for pressure and temperatureratings between cast iron and the steels. While the modulus of ela sticityfor the ductile irons is essentially the same as that for cast iron, the

tensile strength of the former is approximately double that of the latter.In any evaluation of the ductile irons as a substitute for the steels in the

intermediate pressure and temperature range, it shall be remembered thatductile iron casings cannot be effectively repair-welded in the field.

Some companies specify ductile iron for hydrocarbon service even in

temperature and pressure ranges at which other companies accept castiron.

3.4.3. Shafts

1. The following criteria shall be considered in the selection of the materialfor a centrifugal pump shaft.

a) Endurance limit

b) Corrosion resistance

c) Notch sensitivity

2. The endurance limit is the stress below which the shaft will withstand aninfinite number of stress reversals without failure. Since one stressreversal occurs for each revolution of the shaft, the shaft ideally will

never fail if the maximum bending stress in the shaft is less than theendurance limit of the shaft material.

3. In practice, however, the endurance limit is substantially reducedbecause of corrosion and stress raisers such as threads, keyways andshoulders on the shaft. In the selection of shaft material, consideration

shall be given to the corrosion resistance of the material in the fluidbeing pumped as well as the notch sensitivity.

4. Often the shaft (and wear rings) is made of a less corrosion resistantmaterial, and corrosion control material is added to the critical areas ofthe shaft. This process is called overlaying or facing. It is usually done

by weld metal deposition and machining to tolerance; but it can be doneusing metal sprays, electrostatic deposition, diffusion and so on. Proper

use of this technique can result in longer service life or more economicinvestment decisions.


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3.4.4. Wear Rings

1. The following criteria shall be considered in the selection of the material

for the wear rings:

a) Corrosion resistance

b) Abrasive-water resistance

c) Galling characteristics


2. Since the purpose of wear rings is to provide a close running clearance tominimize leakage from the discharge to the suction of the impeller, anincrease in leakage as a result of wear in the rings has a direct effect on

the head, capacity, and efficiency of the pump.

3. To reduce the wear rate of the wear rings and thereby increase the life ofthe pump, special consideration shall be given to the corrosion and

abrasive-wear characteristics of the ring material in any evaluation ofwear ring selection.

4. Bronze is the most widely used material for wear rings because itexhibits good corrosion resistance for a wide range of water services andgood wear characteristics in clear liquids. The bronzes also are low on

the galling scale if metal-to-metal contact occurs. This is especially truefor the 8 to 12 percent leaded bronzes, and they shall be used whenever

possible. The casting and machining properties of most grades of bronzeare excellent. However, bronze wears rapidly when abrasive particles

are present.

5. In applications where bronze is not suitable because of either corrosionor abrasive-wear limitations, or where pumping temperatures exceed

250F (120C), stainless steel rings are required. Unlike bronze, thestainless steels of the 300 and 400 series exhibit a great tendency to gall.

The risk of wear ring seizure can be minimized by increasing theclearance between the rings, specifying a difference in hardness of at

least 125 to 150 Brinell between the two rings, or providing separationsin one of the wear ring surfaces. It is apparent that increasing the

clearance between the wear rings is the least costly procedure to reducethe risk of galling or seizure, but increasing the wear ring clearance

reduces the output and efficiency of the pump.

3.5. Selection of Materials of Construction

The selection of the materials for pumps is a compromise between the cost of manufactureand the anticipated maintenance costs. Many pump installations start out with a lowservice factor and, through operating experience, are gradually upgraded in materials untilan acceptable and scheduled replacement program is achieved. It shall be anticipated that,

for the more corrosive services, modification and replacement of the wetted parts isnecessary during the life of the pumps.


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API STD 610 includes tables that can help in the selection of materials for centrifugal

pump applications. Another useful source of material selection guidelines is NACERP0475.

4. Standards

4.1. API STD 610 (Centrifugal Pumps for GeneralRefinery Service)

API STD 610 for centrifugalpumps was first issued in 1954 and since then has been

updated several times. Its intent is to outline minimum design and mechanicalrequirements, standards, and quality control to limit maintenance and insure reliability incritical services.

The standard states that equipment furnished to this standard is designed and constructed

for a minimum service life of 20 years and at least three years of uninterrupted operation.

4.2. Chemical Pump Standards

In 1962, a committee of the Manufacturing Chemists Association (MCA) reachedagreement with a special committee of the Hydraulic Institute on a proposed American

Standards Association (ASA) standard for chemical process pumps. This document wasreferred to as the American Voluntary Standard or the Manufacturing Chemists

Association Standard. In 1971 it was accepted by the American National StandardsInstitute (ANSI) and issued as ANSI Standard B123.1. In 1974, it was renumbered as

ANSI B73.1. (Currently, information can be found in ASME B73.1M.) Many pumpmanufacturers in the United States and a few in foreign countries are building pumps thatmeet these dimensional and design criteria.

This standard assumes that pumps of similar size from all sources of supply aredimensionally interchangeable with respect to mounting dimensions, size, and

location of suction and discharge nozzles, input shafts, base plates, and foundationbolts. It also outlines certain design features.

The standard states that the minimum bearing life, under the most adverse operatingconditions, shall be not less than two years.

A similar document, ASME B73.2M, covers vertical in- line centrifugal pumps forchemical process.

4.3. Comparison of API and ANSI Pump Standards

Table 7 compares some of the major requirements of these two standards.


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The API standard is more stringent in design requirements and quality control. Thepump is normally used for critical services where reliability is important.

Information found in ANSI B73.1 is currently found in ASME B73.1M.

ANSI pumps for pressure ratings of ANSI Class 150 are less expensive and muchmore readily available. Some manufacturers make an ANSI-type pump for higher

pressure ratings. Whenever service conditions allow, considerable time and costsavings are possible when an ANSI pump is specified.

Table 2: Comparison of API and ANSI Pump Standards

API 610 ANSI B73.1

Pump Casings Rating All pressures and temperatures

normally encountered

ANSI class 150 and temperatures up

to 500F

ASME SEC VIII, Div. 1 Ductile iron, carbons steel, or alloysteel required for flammable or toxicservice

Pump casings

Carbon or alloy steel required inflammable or toxic service

Single piece castings May be keyed or threaded to shaftImpellers

Secured to shaft with a key

Wear Rings Minimum hardness of 400 BHN orhardness difference of 50

No requirements

Mechanical Seals Required No requirements

Shaft Critical Speed Lateral critical speed greater than120 percent of maximum pump

speed

No requirements

Three-year life for ball bearings Ball bearings with two-year lifeBearings

Hydrodynamic and thrust bearingswhere bearing diameter (mm) x

pump rpm > 300,000, or pump ratedhp x pump rpm > 2.7x10 6

Drip lip or drain pan required No requirementsBaseplates

Adequate to limit shaft displacementat coupling to 0.005 in

Hydrostatic test to 1.5 items

allowable casing pressure for 30minutes

Hydrostatic test to 1.5 times

allowable casing pressure for 10minutes

Testing

Performance test required No performance test required


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4.4. Data Sheets

Many companies provide detailed data sheets for specifying centrifugal pumps. If no datasheets are available, the following items shall be considered.

1. Information Required by Pump Manufacturer for Setting Pump Size; Flow, head,NPSH Available, liquid pumped and its corrosivity, pumping temperature.

2. Construction grade: API STD 610, ASME B73.1M, other

3. Pump type (if there is a preference); See Appendix D of this EPT

4. Preferred major construction features if appropriate

a) Materials of construction: Case, shaft, impeller, packing gland/mechanical seal,gland

b) Sealing system: Mechanical seal or packing and, for a mechanical seal, API sealcode seal piping plan.

c) Bearing type: Rolling contact or hydrodynamic

d) Pump baseplate and skid: Drain pan, skid material, skid leveling screws

e) Coupling type

5. Required testing: Performance test, hydrostatic test, NPSH test. Which tests will bewitnessed?

6. Driver type

5. Selecting a Centrifugal Pump

To select the pump required for a specific installation it is necessary first to determine the desired

flow rate or head. The NPSH available shall be determined, and if a centrifugal selection is possible,a system head/flow rate curve shall be developed.

Generally, positive displacement pumps are better suited for high head and low flow rate applications.

Figure 59 is a guide for the type of pump that will probably be most economical. Naturally, theeconomics and thus the choice of pump type vary from installation to installation. Figure 1 shall be

used merely to provide guidance and not to justify any one specific decision.


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Figure 1: Pumping Head vs. Flow Rate for Different Types of Pumps (Use as a

Guide for Selecting the Most Economical Pump)

Generally, select centrifugal pumps unless:


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1. Capacity at pumping head files within area labeled "positive displacement pumps."

2. Viscosity is greater than 1000 cp at pumping temperature.

3. Percentage of undissolved gas by volume is greater than 5 percent, or gas is not well dispersed.

4. The percentage of solids by volume is greater than 50 percent, or solids are not well dispersed.

5. The pump is expected to run dry without automatic shutdown provisions, e.g., pumpout pumps.

In choosing a pump type it is necessary to consider the system's physical constraints. Space availableor weight limitations may dictate a centrifugal or rotary pump; the need to pump solids may dictate a

centrifugal or diaphragm pump, and the need to run dry may dictate a diaphragm pump. Whendischarge pressures vary over a large range but flow shall remain constant, a positive displacement

pump is probably a good choice.

5.1. Number of Pumps Required

The number of pumps required for a given installation depends on a balance of capital

cost and operating flexibility. For most small installations a choice shall be made whetherthe cost of a standby pump is justified by the potential lost income if the pump shall bemaintained. On larger installations several other alternatives shall be investigated. These

can include:

1. One pump rated at 100 percent throughput.

2. Two pumps rated at 100 percent throughput each (one pump is standby).

3. Two pumps rated at 50 percent throughput each (if one pump is down, throughput isdecreased to 50 percent of design).

4. Three pumps rated at 50 percent throughput each (two operate, one is standby).

5. Three pumps rated at 33 percent throughput each.

5.2. Additional Considerations

In making the selection it is important to consider the actual throughput requirements over

the life of the installation and not merely the peak design throughputs. Finally, the abilityto obtain spare parts and service at the location and the desires of operating personnel

shall be considered.


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Appendix A: Nomenclature

BHP Brake horsepower, kW (HP)

BHPvis Viscous brake horsepower - the horsepower required by the pumpfor the viscous conditions, kW (HP)

C Bearing constant

CE Efficiency correction factor

CH Head correction factor

CQ Capacity correction factor

D Diameter of impeller, mm (in)

EM Pump mechanical efficiency

Evis Viscous efficiency in percent

EW Water efficiency - the efficiency when pumping water, percent

g Acceleration due to gravity, 9.81 m/sec2 (32.2 ft/sec 2)

H Total head, m (ft)

HHP Hydraulic horsepower, kW (HP)

HA The head on the surface of the liquid supply level, m (ft)

HEV Potential head, m (ft)

Hf Pipe friction loss, m (ft)

Hp Total head required for pump, m (ft)

HSH Static pressure head, m (ft)

HVH Velocity head, m (ft)

HVIS Viscous head - the head when pumping a viscous fluid, m (ft)

Hvpa The vapor pressure head, m (ft)

HW Water head - the head when pumping water, m (ft)

Ln Life of bearing, millions of revolutions

N Pump speed, rps (rpm)

NPSH Net positive suction head, m (ft)

NPSHA Net positive suction head available, m (ft)

NPSHR Net positive suction head required, m (ft)

Ns Pump specific speed


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P Static pressure, kPa (psi)

Q Flow rate, m3/hr (ft3/sec)

Q Flow rate, m3/hr (BPD)

q Flow rate, m3/hr (gpm)

qvis Viscous capacity - the capacity when pumping a viscous liquid,

m3/hr (gpm)

qw Water capacity of pump, m3/hr (gpm)

Rp Pump speed, rps (rpm)

SG Specific gravity of liquid relative to water

V Average liquid velocity, m/sec (ft/sec)

W Bearing load, N (lb)Z Elevation above or below pump centerline datum, m (ft)

Density of liquid, kg/m3

(lb/ft3)


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Appendix B: Example Problem (Metric)

Given:

An offshore production facility processes 400 m3/hr of a 0.94 specific gravity crude oil. A charge

pump pumps produced oil from a storage tank and delivers it through a metering skid to the mainshipping pumps. The inlet pressure to the shipping pump is 1415 kPa (g). Crude oil pumping

temperature is 27C.

The shipping pump for the above facility shall be centrifugal. This pump shall meet the following

criteria:

1. The maximum size motor that can be installed on the pump is 150 kW. (The platform generatoris not capable of starting a larger motor.)

2. The pump's discharge pressure is to be 4035 kPa.

3. Platform space is at a premium.

4. The pump is to be direct-driven by an electric motor turning at 3600 rpm.

5. Pump components and auxiliaries are to be in accordance with safe practices for off shoreplatforms.

Steps to Solve:1. Approximate number of pumps.

2. Pump configuration.

3. Head and flow for each pump.

4. Pump type.

5. Mechanical features of the pump:

a) Seal system with API code

b) Bearingsc) Material by ASTM number for this application, as recommended by API STD 610, for the

following parts:

i. Pressure Casing

ii. Impeller

iii. Shaft


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Step 1: Determine Approximate Number of Pumps

Total Hydraulic Power Required

3600

PqHP

=

q = 400 m3/hr

4035 - 1415 = 2620 kPa

( )( )kW2913600

2620400

=

Number of pumps required at 150 kW each is

Pumps3Use2.8,pump/kW150

kW416=

Assuming pumps are selected with an average efficiency of 0.7.

kW4167.0

291=

Step 2: Determine Pump Configuration

Since three pumps are required, the basic pumping configuration shall be established.

Option I: Three Pumps Operating in Parallel

This configuration requires three identical pumps. Thus, if one pump is down, pump capacitywill be reduced by approximately 33 percent. Additionally, spare parts for each pump are

identical.

Option II: Two Pumps Operating in Parallel with 1 Pump in Series

This configuration requires two different pumps. The final pump will have larger impellers andcasing than the first stage pumps. This will require additional spare parts. Additionally, if one

pump is down, pump capacity is reduced by 50 to 100 percent.

Option III: Three Pumps Operating in Series

This configuration requires three identical pumps. Each will be selected with the capability topump the design flow rate at 33 percent of the rated differential pressure. If one pump is down,


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two pumps operating in series will be able to reach the required discharge pressure, but the flow

rate will be reduced considerably. Spare parts for each pump will be identical.

Option IV: Four Pumps Operating in Series or ParallelThis is similar to Options I and III as far as pump selections is concerned. The only difference is

that one standby pump is provided, so capacity can be maintained if one pump is out of service.

Selection

Only Options I and III will be considered. The selection will depend on pump system interactioncurves.

Option I:

Step 3 (Option I): Determine Head and Flow for Each Pump

SG = .94

pump/m28494.

)14154035(102.0

S

P102.0Head =

=

=

Flow = 400 m3/hr x

1/3 pumps = 133 m

3/hr

Step 4 (Option I): Determine Pump Type

A review of the centrifugal pump types shows that the following are available:

1. ANSI Pump

2. API Single Stage

3. In-line Pump

4. API Multistage Split Case

5. API Barrel Pump

6. Vertical Can Pump

7. Sump Pump

8. Submersible Pump

Types 1-3 can be eliminated as they are single stage pumps and a multistage most likely will berequired.

Types 7 and 8 are not designed for applications such as this.

Types 4-6 are all suitable for application. Types 4 and 5 are horizontal pumps with horizontal electric

motors, while Type 6 is a vertical pump with a vertical motor.

Since the vertical pump uses considerably less space than the horizontal pumps, it probably would bepreferred for this offshore application.


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Step 5 (Option I): Determine Mechanical Features of the Pump

1. Seal system: Per seal usage guide, a balanced tandem seal shall be provided. Since the fluid

pumped is not extremely hot and not sour, the o-ring material can be Teflon or Kelrez, fourthletter G or I.

Since crude is liable to contain some sand or other abrasives (e.g., products of corrosion, the

rubbing surfaces of the seal), the fifth letter shall be at least an L. The total seal system is thusBTTGL with API seal piping plan 12 or 13.

2. Bearings: Manufacturer's standard. Bearings shall have both a radial and thrust capacity.

3. The material class from API STD 610 is S-1. Therefore, the following material shall bespecified:

a) Pressure Casing = Carbon Steel

b) Impeller = Cast Iron

c) Shaft = Carbon Steel

From API STD 610 the following conversions can be made:

1. Pressure Casing = ASTM A-216 Grade WCA or WCB

2. Impeller (Forging) = ASTM A-105

3. Shaft (bar stock) = ASTM A-576 Grade 1015

Option III:

Step 3 (Option III): Determine Head and Flow

)94(.x3

)14154035)(102.0(Head

=

Flow = 400 m3/hr

Step 4 (Option III): Determine Pump Type

This type of pump shall be identical to that of Option I.

Step 5 (Option III): Determine Mechanical Features

Mechanical features of this pump shall be id entical to those of Option I.


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Appendix C: Example Problem (Customary)

Given:

An offshore production facility processes 60,000 BOPD of a 0.94 specific gravity crude oil. A charge

pump pumps produced oil from a storage tank and delivers it through a metering skid to the mainshipping pumps. The inlet pressure to the shipping pump is 205 psig. Crude oil pumping

temperature is 80F.

The shipping pump for the above facility shall be centrifugal. This pump shall meet the following

criteria:

1. The maximum size motor that can be installed on the pump is 200 HP. (The platform generator isnot capable of starting a larger motor.)

2. The pump's discharge pressure is to be 585 psig.

3. Platform space is at a premium.

4. The pump is to be direct-given by an electric motor turning at 3600 rpm.

5. Pump components and auxiliaries are to be in accordance with safe practices for offshoreplatforms.

Steps to Solve:1. Approximate number of pumps.

2. Pump configuration.

3. Head and flow for each pump.

4. Pump type.

5. Mechanical features of the pump:

a) Seal system with API code

b) Bearingsc) Material by ASTM number for this application, as recommended by API STD 610, for the

following parts:

i. Pressure Casing,

ii. Impeller

iii. Shaft


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Step 1: Determine Approximate Number of Pumps

Total Hydraulic Horsepower Required

58,756

PQHP

=

Q' = 60,000 BPD

P = 585 - 205 = 380 psi

HHP388756,58

)380)(000,60(HP ==

Assuming pumps are selected with an average efficiency of .7

BHP/E = 388/.7 = 554 HP

Number of pumps required at 200 HP each is:

Pumps3Use2.7,pump/HP200

HP554=

Step 2: Determine Pump Configuration

Option I: Three Pumps Operating in Parallel

This configuration requires three identical pumps. Thus, if one pump is down, pump capacitywill be reduced by approximately 33 percent. Additionally, spare parts for each pump are

identical.

Option II: Two Pumps Operating in Parallel with 1 Pump in Series

This configuration requires two different pumps. The final pump will have larger impellers andcasing than the first stage pumps. This will require additional spare parts. Additionally, if one

pump is down, pump capacity is reduced by 50 to 100 percent. Option III: Three Pumps Operating in Series

This configuration requires three identical pumps. Each will be selected with the capability topump the design flow rate at 33 percent of the rated differential pressure. If one pump is down,

two pumps operating in series will be able to reach the required discharge pressure, but the flowrate will be reduced considerably. Spare parts for each pump will be identical.

Option IV: Four Pumps Operating in Series or Parallel

This is similar to Options I and III as far as pump selection is concerned. The only difference isthat one standby pump is provided, so capacity can be maintained if one pump is out of service.


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Selection

Only Options I and III will be considered. The selection will depend on pump system interaction

curves.

Option I:

Step 3 (Option I): Determine Head and Flow for Each Pump

SG = .94

( )

gpm582BPD

gpm0.0292pumps

3

1BPD60,000Flow

pump/ft933.94

205-5852.31

SG

P2.31Head

==

==

=

Step 4 (Option I): Determine Pump Type

A review of the centrifugal pump types shows that the following are available:

1. ANSI Pump

2. API Single Stage3. In-line Pump

4. API Multistage Split Case

5. API Barrel Pump

6. Vertical Can Pump

7. Sump Pump

8. Submersible Pump

Types 1-3 can be eliminated as they are single stage pumps and a multistage most likely will berequired.

Types 7 and 8 are not designed for applications such as this.

Types 4-6 are all suitable for application. Types 4 and 5 are horizontal pumps with horizontal electricmotors, while type 6 is a vertical pump with a vertical motor.

Since the vertical pump uses considerably less space than the horizontal pumps, it probably would bepreferred for this offshore application.


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Step 5 (Option I): Determine Mechanical Features of the Pump

1. Seal System: Per seal usage guide, a balanced tandem seal shall be provided. Since the fluid

pumped is not extremely hot or not sour, the o-ring material can be Teflon or Kelrez, fourth letterG or I.

Since crude is liable to contain some sand or other abrasives (e.g., products of corrosion, the

rubbing surfaces of the seal), the fifth letter shall be at least an L. The total seal system is thusBTTGL with API seal piping plan 12 or 13.

2. Bearings: Manufacturer's standard. Bearings shall have both a radial and thrust capacity.

3. The material class from API STD 610 is S-1. Therefore, the following material shall bespecified:

a) Pressure Casing = Carbon Steel

b) Impeller = Cast Iron

c) Shaft = Carbon Steel

From API STD 610 the following conversions can be made:

1. Pressure Casing = ASTM A-216, Grade WCA or WCB

2. Impeller (Forging) = ASTM A-105

3. Shaft (bar stock) = ASTM A-576 Grade 1015

Option III

Step 3 (Option III): Determine Head and Flow

( )

( )

gpm1746BPD

gpm0.0292BPD60,000Flow

pump/ft311.943

205-5852.31Head

==

=

=

Step 4 (Option III): Determine Pump Type

This type of pump shall be identical to that of Option I.

Step 5 (Option III): Determine Mechanical Features

Mechanical features of this pump shall be identical to those of Option I.


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Appendix D: Generic Types of Centrifugal Pumps

1. ANSI Pump

Figure D-1: Schematic Diagram of an ANSI Pump (Courtesy of Gould Pumps)


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Mobil Oil, 1998 34 of 47

DESCRIPTION:

1. Mounting = Horizontal

2. Casing Split = Radial

3. Impeller Type = Radial

4. Mounting Feet = Bottom of pump casing

5. Number of Stages = One

APPLICATIONS:

1. Flow Conditions: 30 to 700 ft of head; 35 to 400 gpm

2. Service Rating: Non-Critical

3. Typical Uses:

a) Service Water

b) LACT Charge Pump

c) Oil Transfer

d) Cooling Tower Circulation Pumps

NOTE: General processing and transfer service at temperatures below 350F


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2. Single Stage API Pump

Figure D-2: Example of an API single-stage pump (Courtesy of Gould Pumps)

DESCRIPTION:




4. Mounting Feet = Centerline of pump casing

5. Number of Stages = Single

APPLICATIONS:

1. Flow Conditions: 100 to 20,000 ft of head; 40 to 900 gpm, high temperature

2. Service Rating: Critical

3. Typical Uses:

a) Hot Oil Pumps


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b) Rich Oil Pumps

c) Raw Products Pumps

d) Cooling Tower Circulation Pumps

NOTE: Hydrocarbon in the low to moderate flow and moderate head ranges


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3. Vertical In-Line Pump

Figure D-3: Schematic Diagram of an In-Line Pump (Courtesy of Gould Pumps)


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DESCRIPTION:

1. Mounting = Vertically, in-line with piping



4. Mounting Feet = None

5. Number of Stages = Single

APPLICATIONS:

1. Flow Conditions: 15 to 600 ft head; 20 to 1200 gpm


3. Typical Uses:

a) Service Water

b) LACT Charge Pump

c) Oil Transfer

NOTE: General processing and transfer service at temperature below 350F. Minimum spaceapplications.


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4. API Multistage Split Case Pumps

Figure D-4: Example of an API Multistage Split Case Pump (Courtesy of GouldPumps)

DESCRIPTION:



40/47



2. Casing Split = Axial



5. Number of Stages = Multiple

APPLICATIONS:

1. Flow Conditions: 200 to 1500 ft of head; 200 to 4500 gpm, high specific gravity


3. Typical Uses:

a) Pipeline Booster (Crudes and Refined Hydrocarbons)

b) Water Flood

c) Hot Oil Pumps

d) High Pressure Boiler Feedwater


41/47



5. API Barrel Pumps

Figure D-5: Example of a Barrel Pump

DESCRIPTION:






APPLICATIONS:

1. Flow Conditions: 200 to 1700 ft of head; up to 900 gpm, all specific gravities


3. Typical Uses:


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a) Pipeline Booster

b) Water Flood

c) Lean Oil Pumps

6. Sump Pump

Figure D-6: Example of a Sump Pump (Courtesy of Gould Pumps)


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DESCRIPTION:

1. Mounting = Vertical



4. Mounting Feet = Mounting Plate

5. Number of Stages = One

APPLICATIONS:

1. Flow Conditions: Low head and moderate flow rate


3. Typical Uses: Primarily used to pump water or hydrocarbons from shallow pits or compartments.


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7. API Vertical Turbine or Can Pump

Figure D-7: Schematic Diagram of a Vertical Turbine or Can Pump


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DESCRIPTION:



3. Impeller Type = Mixed or Radial



APPLICATIONS:

1. Flow Conditions: 100 to 30,000 ft of head; 10 to 1500 gpm, low NPSH available


3. Typical Uses:

a) Firewater Pump (Offshore)

b) Pipeline Pump


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8. Submersible Pump

Figure D-8: Schematic Diagram of a Submersible Pump


47/47


DESCRIPTION:






APPLICATIONS:

1. Flow Conditions: 100 to 30,000 ft of head; 10 to 2000 gpm, low NPSH available


3. Typical Uses:

a) Firewater Pump (Offshore)

b) Downhole in Well (Crude lifting)

c)

ept 07-t-06c centrifugal pumps

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