Lobanoff / Ross Centrifugal Pumps

Design and Application
2. Auflage 2013
ISBN: 978-0-08-050085-0
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 640 Seiten

Centrifugal Pumps: Design and Application, Second Edition focuses on the design of chemical pumps, composite materials, manufacturing techniques employed in nonmetallic pump applications, mechanical seals, and hydraulic design. The publication first offers information on the elements of pump design, specific speed and modeling laws, and impeller design. Discussions focus on shape of head capacity curve, pump speed, viscosity, specific gravity, correction for impeller trim, model law, and design suggestions. The book then takes a look at general pump design, volute design, and design of multi-stage casing. The manuscript examines double-suction pumps and side-suction design, net positive suction head, and vertical pumps. Topics include configurations, design features, pump vibration, effect of viscosity, suction piping, high speed pumps, and side suction and suction nozzle layout. The publication also ponders on high speed pumps, double-case pumps, hydraulic power recovery turbines, and shaft design and axial thrust. The book is a valuable source of data for pump designers, students, and rotating equipment engineers.

Autoren/Hrsg.

Lobanoff, Val S.

Ross, Robert R.

Inhaltsverzeichnis

1;Front Cover;1
2;Centrifugal Pumps: Design & Application;4
3;Copyright Page
;5
4;Table of Contents;6
5;Preface;12
6;Part 1: Elements of Pump Design;16
6.1;Chapter 1. Introduction;18
6.1.1;System Analysis for Pump Selection;18
6.1.2;Differential Head Required;18
6.1.3;NPSHA;19
6.1.4;Shape of Head Capacity Curve;19
6.1.5;Pump Speed;21
6.1.6;Liquid Characteristics;22
6.1.7;Viscosity;22
6.1.8;Specific Gravity;22
6.1.9;Construction;23
6.1.10;Pump Selection;25
6.2;Chapter 2. Specific Speed and Modeling Laws;26
6.2.1;Definition of Pump Specific Speed and Suction Specific Speed;26
6.2.2;The Affinity Law;27
6.2.3;Specific Speed Charts;29
6.2.4;Correction for Impeller Trim;33
6.2.5;Model Law;34
6.2.6;Factoring Law;36
6.2.7;Conclusion;40
6.3;Chapter 3. Impeller Design;43
6.3.1;Impeller Layout;52
6.3.2;Design Suggestions;54
6.3.3;Notation;59
6.4;Chapter 4. General Pump Design;60
6.4.1;Performance Chart;61
6.5;Chapter 5. Volute Design;65
6.5.1;Types of Volute Designs;67
6.5.2;General Design Considerations;71
6.5.3;The Use of Universal Volute Sections for Standard Volute Designs;73
6.5.4;General Considerations in Casing Design;76
6.5.5;Notation;79
6.5.6;Reference;79
6.6;Chapter 6. Design of Multi-Stage Casing;80
6.6.1;General Considerations in Crossover Design;82
6.6.2;Specific Crossover Designs;85
6.6.3;Crossovers with Radial Diffusion Sections;86
6.6.4;Crossovers with Diagonal Diffusion Sections;87
6.6.5;Mechanical Suggestions;87
6.6.6;Notation;91
6.7;Chapter 7. Double-Suction Pumps and Side-Suction Design;92
6.7.1;Double-Suction Pump Design;92
6.7.2;Side Suction and Suction Nozzle Layout;94
6.8;Chapter 8. NPSH;100
6.8.1;Establishing NPSHA;100
6.8.2;Moderate Speed Pumps;103
6.8.3;Influence of Suction Specific Speed (Nss)
;105
6.8.4;High Speed Pumps;105
6.8.5;Suction Piping;118
6.8.6;Effect of Viscosity;118
6.8.7;Notation;124
6.8.8;References;124
7;Part 2: Application;126
7.1;Chapter 9. Vertical Pumps;128
7.1.1;Configurations;129
7.1.2;Applications;137
7.1.3;Design Features;146
7.1.4;Pump Vibration;151
7.1.5;References;153
7.2;Chapter 10. Popeline, Waterflood, and CO2 Pumps;154
7.2.1;Pipeline Pumps;154
7.2.2;Waterflood Pumps;179
7.2.3;CO2 Pumps
;181
7.2.4;Notations;187
7.2.5;References;187
7.3;Chapter 11. High Speed Pumps;188
7.3.1;References;219
7.4;Chapter 12. Double-Case Pumps;221
7.4.1;Configurations;221
7.4.2;Applications;223
7.4.3;Design Features;225
7.4.4;Double-Case Pump Rotordynamic Analysis;234
7.4.5;Comparison of Diffuser Casings with Volute Casings;237
7.4.6;References;239
7.5;Chapter 13. Slurry Pumps;241
7.5.1;Slurry Abrasivity;241
7.5.2;Pump Materials to Resist Abrasive Wear;243
7.5.3;Slurry Pump Types;247
7.5.4;Specific Speed and Wear;247
7.5.5;Areas of Wear;249
7.6;Chapter 14. Hydraulic Power Recovery Turbines;261
7.6.1;Selection Process;263
7.6.2;Turbine Performance Prediction;266
7.6.3;Optimizing and Adjusting Performance Characteristics;269
7.6.4;Design Features (Hydraulic and Mechanical);271
7.6.5;Operating Considerations;286
7.6.6;Performance Testing;286
7.6.7;Applications;287
7.6.8;Operation and Control Equipment;294
7.6.9;Conclusion;296
7.6.10;References;297
7.7;Chapter 15. Chemical Pumps Metallic and Nonmetallic;298
7.7.1;ANSI Pumps;298
7.7.2;General Construction;305
7.7.3;Casing Covers;312
7.7.4;Frame;312
7.7.5;Bearing Housing;313
7.7.6;Bedplates;319
7.7.7;Other Types of Chemical Pumps;320
7.7.8;Nonmetallic Pumps;324
7.7.9;General Construction of Nonmetallic Pumps;332
7.7.10;Nonmetallic Immersion Sump Pumps;336
7.7.11;Processes;338
7.7.12;References;344
8;Part 3: Mechanical Design;346
8.1;Chapter 16. Shaft Design and Axial Thrust;348
8.1.1;Shaft Design;348
8.1.2;Axial Thrust;358
8.1.3;Notation;366
8.1.4;References;368
8.2;Chapter 17. Mechanical Seals;369
8.2.1;References;436
8.3;Chapter 18. Vibration and Noise in Pumps;437
8.3.1;Introduction;437
8.3.2;Sources of Pump Noise;438
8.3.3;Causes of Vibrations;439
8.3.4;Rotordynamic Analysis;456
8.3.5;Diagnosis of Pump Vibration Problems;484
8.3.6;Appendix Acoustic Velocity of Liquids;501
8.3.7;References;506
9;Part 4: Extending Pump Life;510
9.1;Chapter 19. Alignment;512
9.1.1;Definitions;512
9.1.2;Why Bother With Precise Alignment?;512
9.1.3;Causes of Misalignment;513
9.1.4;Pre-Alignment Steps;522
9.1.5;Methods of Primary Alignment Measurement;526
9.1.6;Methods of Calculating Alignment Movements;531
9.1.7;Jig Posts;531
9.1.8;Numerical Examples;534
9.1.9;Thermal Growth;534
9.1.10;References;537
9.2;Chapter 20. Rolling Element Bearings and Lubrication;539
9.2.1;Friction Torque;540
9.2.2;Function of the Lubricant;540
9.2.3;Oil Versus Grease;540
9.2.4;Oil Characteristics;541
9.2.5;General Lubricant Considerations;544
9.2.6;Application Limits for Greases;548
9.2.7;Life-Time Lubricated, "Sealed" Bearings;549
9.2.8;Oil Viscosity Selection;551
9.2.9;Applications of Liquid Lubricants in Pumps;551
9.2.10;Oil Bath Lubrication;552
9.2.11;Drip Feed Lubrication;554
9.2.12;Forced Feed Circulation;554
9.2.13;Oil Mist Lubrication;554
9.2.14;Selecting Rolling Element Bearings for Reduced Failure Risk;560
9.2.15;Magnetic Shaft Seals in the Lubrication Environment;564
9.2.16;References;570
9.3;Chapter 21. Mechanical Seal Relibility;571
9.3.1;Failure Analysis;572
9.3.2;Seal Hardware Failures;574
9.3.3;Seal Failures from Installation Problems;576
9.3.4;Seal Failures Related to Pump Hardware;578
9.3.5;Seal Failures Caused by Pump Repair and Installation;579
9.3.6;Seal Failures Caused by Pump Operation;580
9.3.7;Reliability;582
10;Index;584

Leseproben

2

Specific Speed and Modeling Laws

Specific speed and suction specific speed are very useful parameters for engineers involved in centrifugal pump design and/or application. For the pump designer an intimate knowledge of the function of specific speed is the only road to successful pump design. For the application or product engineer specific speed provides a useful means of evaluating various pump lines. For the user specific speed is a tool for use in comparing various pumps and selecting the most efficient and economical pumping equipment for his plant applications.

A theoretical knowledge of pump design and extensive experience in the application of pumps both indicate that the numerical values of specific speed are very critical. In fact, a detailed study of specific speed will lead to the necessary design parameters for all types of pumps.

Definition of Pump Specific Speed and Suction Specific Speed

Pump specific speed (Ns) as it is applied to centrifugal pumps is defined in U.S. units as:

Specificspeed is always calculated at the best efficiency point (BEP) with maximum impeller diameter and single stage only. As specific speed can be calculated in any consistent units, it is useful to convert the calculated number to some other system of units. See Table 2-1. The suction specific speed (Nss) is calculated by the same formula as pump specific speed (Ns) but uses NPSHR values in feet in place of head (H) in feet. To calculate pump specific speed (Ns) use full capacity (GPM) for either single- or double-suction pumps. To calculate suction specific speed (Nss) use one half of capacity (GPM) for double-suction pumps.

Table 2-1

Specific Speed Conversion.

It is well known that specific speed is a reference number that describes the hydraulic features of a pump, whether radial, semi-axial (Francis type), or propeller type. The term, although widely used, is usually considered (except by designers) only as a characteristic number without any associated concrete reference or picture. This is partly due to its definition as the speed (RPM) of a geometrically similar pump which will deliver one gallon per minute against one foot of head.

To connect the term specific speed with a definite picture, and give it more concrete meaning such as GPM for rate of flow or RPM for rate of speed, two well known and important laws of centrifugal pump design must be borne in mind—the affinity law and the model law (the model law will be discussed later).

The Affinity Law

This is used to refigure the performance of a pump from one speed to another. This law states that for similar conditions of flow (i.e. substantially same efficiency) the capacity will vary directly with the ratio of speed and/or impeller diameter and the head with the square of this ratio at the point of best efficiency. Other points to the left or right of the best efficiency point will correspond similarly. The flow cut-off point is usually determined by the pump suction conditions. From this definition, the rules in Table 2-2 can be used to refigure pump performance with impeller diameter or speed change.

Table 2-2

Formulas for Refiguring Pump Performance with Impeller Diameter or Speed Change.

=.

Q2, H2, bhp2, D2, and N2 =

Example

A pump operating at 3,550 RPM has a performance as shown in solid lines in Figure 2-1. Calculate the new performance of the pump if the operating speed is increased to 4,000 RPM.

Figure 2-1 New pump factored from model pump—different speed.

Step 1

From the performance curve, tabulate performance at 3,550 RPM (Table 2-3).

Table 2-3

Tabulated Performance at 3,550 RPM

Step 2

Establish the correction factors for operation at 4,000 RPM.

Step 3

Calculate new conditions at 4,000 RPM from:

Results are tabulated in Table 2-4 and shown as a dotted line, in Figure 2-1. Note that the pump efficiency remains the same with the increase in speed.

Table 2-4

Tabulated Performance at 4,000 RPM

Specific Speed Charts

We have prepared a nomograph (Figure 2-2) relating pump specific speed and suction specific speed to capacity, speed, and head. The nomograph is very simple to use: Locate capacity at the bottom of the graph, go vertically to the rotating speed, horizontally to TDH, and vertically to the pump specific speed. To obtain suction specific speed continue from the rotating speed to NPSHR and vertically to the suction specific speed. Pump specific speed is the same for either single-suction or double-suction designs.

Figure 2-2 Specific speed and suction specific speed nomograph.

For estimating the expected pump efficiencies at the best efficiency points, many textbooks have plotted charts showing efficiency as a function of specific speed (Ns) and capacity (GPM). We have prepared similar charts, but ours are based on test results of many different types of pumps and many years of experience.

Figure 2-3 shows efficiencies vs. specific speed as applied to end-suction process pumps (API-types). Figure 2-4 shows them as applied to single-stage double-suction pumps, and Figure 2-5 shows them as applied to double-volute-type horizontally split multi-stage pumps.

Figure 2-3 Efficiency for overhung process pumps.

Figure 2-4 Efficiency for single-stage double-suction pumps.

Figure 2-5 Efficiency for double-volute-type horizontally split multi-stage pumps.

Figure 2-5 is based on competitive data. It is interesting to note that although the specific speed of multi-stage pumps stays within a rather narrow range, the pump efficiencies are very high, equal almost to those of the double-suction pumps. The data shown are based on pumps having six stages or less and operating at 3,560 RPM. For additional stages or higher speed, horsepower requirements may dictate an increase in shaft size. This has a negative effect on pump performance and the efficiency shown will be reduced.

As can be seen, efficiency increases very rapidly up to Ns 2,000, stays reasonably constant up to Ns 3,500, and after that begins to fall off slowly. The drop at high specific speeds is explained by the fact that hydraulic friction and shock losses for high specific speed (low head) pumps contribute a greater percentage of total head than for low specific speed (high head) pumps. The drop at low specific speeds is explained by the fact that pump mechanical losses do not vary much over the range of specific speeds and are therefore a greater percentage of total power consumption at the lower specific speeds.

Correction for Impeller Trim

The affinity laws described earlier require correction when performance is being figured on an impeller diameter change. Test results have shown that there is a discrepancy between the calculated impeller diameter and the achieved performance. The larger the impeller cut, the larger the discrepancy as shown in Figure 2-6.

Figure 2-6 Impeller trim correction.

Example

What impeller trim is required on a 7-in. impeller to reduce head from 135 ft to 90 ft?

Step 1.

From affinity laws:

Calculated percent of original diameter = 5.72/7 = .82

Step 2.

Establish correction factor:

From Figure 2-6 calculated diameter .82 = Actual required diameter .84.

Impeller trims less than 80% of original diameter should be avoided since they result in a considerable drop in efficiency and might create unstable pump performance. Also, for pumps of high specific speed (2,500–4,000), impeller trim should be limited to 90% of original diameter. This is due to possible hydraulic problems associated with inadequate vane overlap.

Model Law

Another index related to specific speed is the pump modeling law. The “model law” is not very well known and usually applies to very large pumps used in hydroelectric applications. It states that two geometrically similar pumps working against the same head will have similar flow conditions (same velocities in every pump section) if they run at speeds inversely proportional to their size, and in that case their capacity will vary with the square of their size. This is easily understood if we realize that the peripheral velocities, which are the product of impeller diameter and RPM, will be the same for the two pumps if the diameter increase is inversely proportional to the RPM increase. The head, being...