KSB - Centrifugal Pump Design.pdf - banduras1

Centrifugal Pump Design

pump&)

valves

Contents

1 Symbols, Units and Designations

Page

General

National and International Standards for

Centrifugal Pumps

Shaft Deflection

Improving the NPSH Requirement

impeller Types

Pump Types

Pump Installation Arrangements

Pump Sump Configuration

Suction Pipe Layout

Shaft Couplings

Page

2 Design

2.1 Pump Capacity

2.2 Pump Head

2.3 Svstem Head

2.4 ~beed

2.5 Selectina the PumD Size

2.6 calculating the power Consumption

2.6.1 Pump Power lnput

2.6.2 Calculating the Drive Rating

2.7 Pump Characteristic Curve

9 Technical Data

9.1 Vapour pressure p, and Density p of Water

9.2 Vapour pressure p, of Various Liquids

9.3 Density p of Various Liquids at Atmospheric

Pressure

9.4 Extract of Main Legal Units for Centrifugal

Pumps

9.5 Conversion of British and U.S. Units

9.6 Graph for Calculating Flow Velocity v

9.7 Graph for Calcuiating Velocity Head v212g

9.8 Graph for Calculating Velocity Head

Differential A v212 g

9.9 Graph for Calculating Head Losses H,

9.10 Graph for Calcuiating Conversion Factors

fern, ~H,W and f,,,~ for Viscous Liquids

9.1 1 Graph for Calculating Conversion Factors for

and f~~for Viscous Liquids

9.12 Graph for Calculating Specific Speed n,

- Schedule for Calculatingthe Operating Point

or Pump Size for Viscous Liquids

2.8 System Characteristic (Piping Characteristic)

2.9 Operating Point

2.1 0 Parallel Operation of Centrifugal Pumps

3 Suction Characteristics

3.1 NPSH Required

3.2 NPSH Available

4.1 Head Losses H, in Straight Pipes

4.2 head Losses H, In p~ast-c P pes

4 3 Head Losses H, lor VISCOUS Llqulds

in Straight Pipes

4.4 Head Losses H, in Valves and Fittings

Pressure Losses

5 Changing the Pump Performance

5.1 Changing the Speed

5.2 Trimming the Impellers

Handling Viscous Liquids

7 Typical Selection Examples

7.1 Selecting the Pump Size

7.2 Calculating the Power Consumption

7.2.1 Pump Power lnput

7.2.2 Calculating the Drive Rating

7.3 Calculating the NPSH,

7.3.1 Suction Lift from OpenfClosed Tank

7.3.2 Positive Suction Operation from OpenlCiosed

Tank

7.3.3 Positive Suction Operation from Closed Tank

at Vapour Pressure

7.4 Changing the Speed

7.5 Trimming the Impeller

7.6 Handling Viscous Liquids

7.6.1 Calcuiating the Operating Point

7.6.2 Establishingthe Pump Size

1 Symbols, Units and Designations

Area

2 Design

2.1 Pump Capacity

The capacity Q is the external volume flow per unit of time in

ms/s (I/s and m3/h are also commonly used). Balance water,

leakage water etc. do not count as part of the capacity.

Width

Impeller outlet width

mm (m)

impeller diameter,

pipe diameter

(mm)

Nominal bore of pipe

Smallest inner diametel

Conversion factor for head

2.2 Pump Head

Conversion factor for flow rate

The head H of a pump is the useful mechanical energy trans-

mitted by the pump to the medium handled, related to the

Conversion factor for efficiency

weight of the medium, expressed in m. It is independent of

mIs2

Gravitational constant = 9.81 mlsz

Head

the density p of the medium handled, i.e. a centrifugal pump

System head

will generate the same head H for all fluids irrespectiveof the

density p. The density p determines the pressure within the

pump

p=p.g.H

and influencesthe pump power input P.

H....

Static head

~o'"

Hs ,,,

Hz ,en

H",,

NPSHreq

NPSH,

Q,,"

G.d

llmin

bar (N/m2)

bar (NIm2)

bar (Nlm2)

I/s (mVh)

Ils (m31h)

Ils (m3/h)

mls

l/h

- - -

m21s

kg/m3

(kg/dm3)

Shut-off head

Static suction lift

Static positive suction head

Head loss

Head loss - suction side

Differential head

Coefficient

Absolute roughness

Length of pipe

Speed

NPSH required

NPSH available

Specific speed

Pump power input

Pressure

Barometric pressure

Vapour pressure of liquid

Pressure loss

Differential capacity

Capacity/Flow rate

Minimum flow rate

Radius

Reynolds number

Circumference

Flow velocity

Stroke

Switching frequency

Height differential between pump

suction and discharge nozzles

Loss coefficient

Pump efficiency

Pipe friction coefficient

Correction coefficient

Kinematic viscosity

Density

2.3 System Head

The total head of the system H, is made up of the following

(see Figs. 1 and 2):

Static head = height difference between the suction

and discharge fluid levels. If the discharge pipe emerges

above the liquid level, then H

is referredto the centreline

of the outflow section.

.-,

the pressure head difference between the suction

P'S

and discharge fluid levels in closed tanks.

.ZH,, the sum of all pressure head losses (pipe friction,

friction in valves, fittings etc. in suction and discharge

pipes).

va2 - ve2

.--

,the difference in velocity heads in the tanks.

The system head HAis thus:

IJ. "

In practice the difference between the velocity heads can be

ignored, leaving

for closed tanks

HA = H

, +

P'S

+ ZH,,

HA - Hseo + ZHV

Temperature fact01

Opening angle

for open tanks

Indices

gee

opt

sch

z 1,2,3

at outlet cross section of the systemlbranching off

at operating point

at discharge nozzle of pump/flowing through

at inlet cross section of planVbranching off

for cast iron

geodetic

for plastic

suction side, at suction nozzle of pump

at best efficiency point

radial

for sulphuric acid

for water

for viscous liquids

consecutive numbers, items

2.4 Speed

With three-phase motor drives (asynchronous squirrel cage

motor) the approximate pump speeds are as follows:

No. of poles

Frequenw

~eterencespeeds

in curve

960

1160

documentation

725

875

llmin

480

580

at 50 Hz

at 60 Hz

2900

3500

1450

1750

580

700

415

500

In practice, however, motors usually run at slightly higher

speeds which - upon consent of the customer - are taken

into account by the pump manufacturer at the design stage

(see section 7.4).

Different speeds are possible using a speed adjustment

device, gearbox or belt drive.

Force

Fig 1 Pumping system with suction lin

Flg. 2 Pumping system with p~itive

suction

2.5 Selecting the Pump Size (see 7.1)

The data needed for selecting the pump size - capacity Q and

head H at the required duty point - is known, as is the mains

frequency. The pump size and speed can be determined from

the performance chart (also called selection chart) (see 8.0

Fig. 26); then the other parameters of the pump selected, such

as efficiency q, input power P and NPSH, can be established

from the appropriate individual performance curve (see 8.0,

Fig. 3).

Unless there is a particular reason to the contrary, arrange

the operating point near Qopt(b.e.p.).

For pumps handling viscous liquids see sections 6 and 7.6.2

2.7 Pump Characteristic Curve

In contrast to positive-displacementpumps (e.g. reciprocating

pumps) at constant speed (n = const.) centrifugal pumps

have a capacity Q which will increase if the head decreases.

They are thus capable of self-regulation. The pump power

input P, and therefore the efficiency q, plus the NPSHreq

depend on the capacity.

The behaviourand relationshipof all thesevariables are shown

by the curves (see Fig. 3) which thus illustrate the operating

characteristics of a centrifugal pump.

The characteristic curves apply to the density p and kinematic

viscosity v of water, unless stated otherwise.

2.6 Calculating the Power Consumption

2.6.1 Pump Power Input

(see example in 7.2.1)

The pump power input P of a centrifugal pump is the mechan-

ical energy at the pump coupling or pump shaft absorbed

from the drive. It is determined using the following equation:

with p in kgIdm3

g in m/s2

Q in 11s

H inm

q between 0 and 1

or another equation which is still used:

with p in kgIdm3

Q in m3/h

H inm

367 conversion factor (constant)

The pump power input P in kW can also be directly read with

sufficient accuracy off the characteristiccurves (see2.7) where

the de-nsity p = 1000 kgIm3. The pump power input P must

be cbnverted (see 7.2.1) for other densities p.

2.6.2 Calculating the Drive Rating

(see example under 7.2.2)

Since it is possible that the system volume flow, and thus the

operating point, will fluctuate, which could mean an increase

in the pump power input P, it is standard practice to use the

following safety margins when determining the motor size,

unless the customer specifies otherwise:

up to 7.5 kW approx. 20%

from 7.5 to 40 kW approx. 15%

from 40 kW approx. 10%.

If extreme volume flow fluctuations are expected, the motor

size must be selected with reference to the maximum possible

pump capacity on the characteristic curves, taking the follow-

ing into consideration:

impeller diameter required,

condition NPSHav L NPSH,,, (see 3.2),

0 permissible P/n values for the bearings.

Handling 'liquids with a high proportion of solids, as well as

handling pulp, means using special pumps and/or special

impellers.

129001 llmin Laufrad Impeller Roue

Largeur Anchura mm 9

Rodete 0 mm 130-169

Fig. 3 Centrifugal pump characteristic curves

The duty conditions determine which is the more favourable

- a flat or a steep curve. With a steep curve the capacity

changes less than with a flat curve under the same differen-

tial head conditions AH (see Fig. 4). The steep curve thus

possesses better control characteristics.

II KSB

Breite Width

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