Centrifugal Pumps Design and Application 2nd ed - Val S. Lobanoff, Robert R. Ross (Butterworth-Heinemann, 1992)
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 (N s ) as it is applied to centrifugal pumps is defined in U.S. units as: Specific speed is always calculated at the best efficiency point (BEP) with maximum impeller diameter and single stage only. As specific speea 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 (N ss ) is calculated by the same formula as pump specific speed (N s ) 11
12 Centrifugal Pumps: Design and Application Table 2-1 Specific Speed Conversion Capacity UNITS Head/ Stage U.S. to Metric Multiply By .0472 .0194 ,15 1.1615 Metric to U.S. Multiply By 21.19 51.65 6.67 .8609 Ft 3 /Sec M 3 /Sec M 3 /Min M 3 /Hr Feet Meters Meters Meters but uses NPSHR values in feet in place of head (H) in feet. To calculate pump specific speed (N s ) use full capacity (GPM) for either singleor double-suction pumps. To calculate suction specific speed (N ss ) use one half of capacity (GPM) for double-suction pumps. 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.
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- Page 6 and 7: Contents Preface —..... —......
- Page 8 and 9: ern Pumps, Mine Dewatering Pumps. W
- Page 10 and 11: Stage Pumps. Single-Suction Single-
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- Page 16 and 17: Part 1 Elements of Pump Design
- Page 18 and 19: 1 Introduction System Analysis for
- Page 20 and 21: Introduction 5 Figure 1-2. The syst
- Page 22 and 23: Introduction 7 mate responsibility
- Page 24 and 25: Introduction 9 Figure 1-6. Maximum
- Page 28 and 29: Specific Speed and Modeling Laws 13
- Page 30 and 31: Specific Speed and Modeling Laws 15
- Page 32 and 33: Specific Speed and Modeling Laws 17
- Page 34 and 35: Specific Speed and Modeling Laws 19
- Page 36 and 37: Specific Speed and Modeling Laws 21
- Page 38 and 39: Figyre 2-7, New pump from model pum
- Page 40 and 41: Specific Speed and Modeling Laws 25
- Page 42 and 43: Specific Speed and Modeling Laws 27
- Page 44 and 45: Impeller Design 29 Figure 3-1. Requ
- Page 46 and 47: Impeller Design 31 Figure 3-4. Capa
- Page 48 and 49: Impeller Design 33 Step 8: Estimate
- Page 50 and 51: Impeller Design 35 Figure 3-7. Volu
- Page 52 and 53: impeller Design 37 (2) 5 , as final
- Page 54 and 55: Impeller Design 39 The vane develop
- Page 56 and 57: Impeller Design 41 Figure 3-12. Are
- Page 58 and 59: impeller Design 43 Figure 3-16. Inf
- Page 60 and 61: 4 General Pump Design It is not a d
- Page 62 and 63: General Pump Design 4? Figure 4-1.
- Page 64 and 65: General Pump Design 49 designed and
- Page 66 and 67: Volute Design 51 Figure 5-1. Volute
- Page 68 and 69: Volute Design 53 Figure 5-2. Radial
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- Page 72 and 73: Volute Design 57 Figure 5-4. Effici
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12 <strong>Centrifugal</strong> <strong>Pumps</strong>: <strong>Design</strong> <strong>and</strong> <strong>Application</strong><br />
Table 2-1<br />
Specific Spe<strong>ed</strong> Conversion<br />
Capacity<br />
UNITS<br />
Head/<br />
Stage<br />
U.S. to Metric<br />
Multiply By<br />
.0472<br />
.0194<br />
,15<br />
1.1615<br />
Metric to U.S.<br />
Multiply By<br />
21.19<br />
51.65<br />
6.67<br />
.8609<br />
Ft 3 /Sec<br />
M 3 /Sec<br />
M 3 /Min<br />
M 3 /Hr<br />
Feet<br />
Meters<br />
Meters<br />
Meters<br />
but uses NPSHR values in feet in place of head (H) in feet. To calculate<br />
pump specific spe<strong>ed</strong> (N s ) use full capacity (GPM) for either singleor<br />
double-suction pumps. To calculate suction specific spe<strong>ed</strong> (N ss ) use<br />
one half of capacity (GPM) for double-suction pumps.<br />
It is well known that specific spe<strong>ed</strong> is a reference number that describes<br />
the hydraulic features of a pump, whether radial, semi-axial<br />
(Francis type), or propeller type. The term, although widely us<strong>ed</strong>, is usually<br />
consider<strong>ed</strong> (except by designers) only as a characteristic number<br />
without any associat<strong>ed</strong> concrete reference or picture. This is partly due to<br />
its definition as the spe<strong>ed</strong> (RPM) of a geometrically similar pump which<br />
will deliver one gallon per minute against one foot of head.<br />
To connect the term specific spe<strong>ed</strong> with a definite picture, <strong>and</strong> give it<br />
more concrete meaning such as GPM for rate of flow or RPM for rate of<br />
spe<strong>ed</strong>, two well known <strong>and</strong> important laws of centrifugal pump design<br />
must be borne in mind—the affinity law <strong>and</strong> the model law (the model<br />
law will be discuss<strong>ed</strong> later).<br />
The Affinity Law<br />
This is us<strong>ed</strong> to refigure the performance of a pump from one spe<strong>ed</strong> to<br />
another. This law states that for similar conditions of flow (i.e. substantially<br />
same efficiency) the capacity will vary directly with the ratio of<br />
spe<strong>ed</strong> <strong>and</strong>/or impeller diameter <strong>and</strong> the head with the square of this ratio<br />
at the point of best efficiency. Other points to the left or right of the best<br />
efficiency point will correspond similarly. The flow cut-off point is usually<br />
determin<strong>ed</strong> by the pump suction conditions. From this definition, the<br />
rules in Table 2-2 can be us<strong>ed</strong> to refigure pump performance with impeller<br />
diameter or spe<strong>ed</strong> change.
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