Boundary Conditions And The Hydraulic Behavior Of · PDF fileBoundary Conditions And The...

Boundary Conditions And The Hydraulic Behavior

Of The Pump

• Frank Hafner

1

Frank Hafner Frank has been working in hydraulic design engineering

at KSB AG, in Frankenthal, Germany, for twenty-three

years. During this time his responsibility has covered all

aspects of hydraulic engineering from CAD design to CFD

analysis and testing.

Frank is responsible for pumps in water transport,

energy, and industrial applications. He has a master’s

degree in mechanical engineering from University of

Karlsruhe, Germany.

Presenter

2

Or:

The performance curve – the unknown creature

Boundary Conditions And The Hydraulic Behavior

Of The Pump

Specific Speed

4

• Also called impeller type number.

• Characterizes the geometry of an impeller based on the performance data at BEP.

• The hydraulic behavior of the pump depends on the value of the specific speed.

Approximate reference values for ns:

up to 1300 / 25 Radial flow impeller (high head impeller)

up to 2000 / 40 Radial flow impeller (medium head impeller)

up to 3500 / 70 Radial flow impeller (low head impeller)

up to 8000 / 160 Mixed flow imp. (helical impeller, diagonal impeller)

up to 20000 / 400 Axial flow impeller (propeller)

High head Medium head Low head Mixed flow Axial flowimpeller

US / metricn - speed [1/min]Q - flow per impeller eye [USgpm] / [m3/s]H - head per stage [feet] / [m]

min]/1[3/4s

H

Qnn =

Specific Speed

5

1000/20

1500/30

2000/40

800/15

Lines of constant specific speed in product family curves.

=>Selecting a pump size implies selecting a specific speed.

4/3s HQ

nn = [ ]min1/

n = const.

Specific Speed and Performance Curve

6

The character of all aspects of the performance curve depends on the specific speed:

• Slope of head and power curves.

• Broadness of efficiency curve.

• Nature of NPSH curve

BEP BEP

BEP BEP

BEPBEP

BEPBEP

parameter: ns

Specific Speed and Performance Curve

7

Slope and parallel operation - example

2-pole, ns = 2200 rpm 4-pole, ns = 1100 rpm

No intersection with system curve

at single operation

Specific Speed and Efficiency

8

• The attainable efficiency of centrifugal pumps, both the specific speed and the Reynolds number are the determining parameters.

• The Reynolds number includes size and rotational speed.

• As a simplification for practical use most diagrams use the rate of flow instead of the Reynold number

Source:

Maximum practically attainable efficiency.Valid for: single stage, single suction, volute casing, overhung shaft, axial inflow

US units: 500 1000 2000 5000

Influence of Hub Diameter on Efficiency

9

The attainable efficiencyalso depends on the ratioof the impeller hub andsuction diameter.

Approximate values:• Overhung:ν≈0

• BB2, industrialBB4:ν≈0.45

• Boiler feed pump: ν≈0.65

→=ν1D

Dhub

Source: B. Neumann (1991) “ The Interaction between

Geometry and Performance of a Centrifugal Pump “;

Mechanical Engineering Publications Ltd, London; 268.

1DDh

ub

=ν2500/48

1800/35 1400/27

800/15

Parameter: ns

Influence Factors on Performance Curve

10

[ ]1u

12u

2th

cucug1H ⋅−⋅⋅=

Euler’s equation for turbo

machines

Conversion of velocities to geometry

values.

Only valid for the concept of “infinite

number of vanes”.

Good vehicle to explain influence of

geometry on performance curve.

( )22

22

th β t

an

bg60

nQg

DnH⋅⋅

⋅−⋅⋅Π=⇒ ∞

2

60

Outer diameter D2

• width at outlet b2 and

• vane angle at outlet ββββ2

2u

2th

cug1H ⋅⋅=

In many cases there is no circulation at the pump inlet.Therefore c1u ≡ 0


11

( )22

22

th β t

an

bg60

nQg

DnH⋅⋅

⋅−⋅⋅Π=⇒ ∞

2

60

Outer diameter D2



D2

b2


12

( )22

22

th β t

an

bg60

nQg

DnH⋅⋅

⋅−⋅⋅Π=⇒ ∞

2

60

Outer diameter D2



D2

b2

←←←←

Typically used

working range

←←←←


13

Finite Number of Vanes and Real

Performance

Deflection of streamlines close to impeller

outlet


14

• Infinite Number of Vanes

• Finite Number of Vanes

• Real Performance

Recirculation at impeller inlet &outlet

Comprising:

Friction losses HVR~ Q2

Shock losses HVST~ (Qdesign - Q)2

Gap losses QSP~ H


15

Example:

Double suction/horizontal split case pump

meeting customers requirement “maximum shutoff head”.


16

• Increase of BEP flow

• Modification of the volute

• Utilization of a volute modification requires knowledge about the impeller working range (MCSF, flow of shockfree entry)

• In this case a new impeller is employed

• Graphic shows comparison with calibrated CFD results

Cutback of tongue


17

Different stator

elements:

• Vaned diffuser

• Volute

• Ring (annular

casing) without

vanes

Viscosity of Pumped Medium

18

• For kinamatic viscosities of ν > 20 mm²/s a correction of the performance curve is required.

• Head decreases

• Power increases

• Efficiency decreases

• Methods for recalculation of the curves are provided by HI and KSB.

Flow rate Q

Flow rate Q

Flow rate Q

Sha

ftpo

wer

Effi

cien

cyH

ead

Speed n [rpm]

0

200

400

600

800

1000

1200

1400

1600

Impe

ller

diam

eter

[mm

]

0

2

4

6

8

10

12

14

16

NP

SH

avai

labl

e [m

]

80

81

82

83

84

85

86

87

88

89

90

Effi

cien

cy η

[%]

715 890 990 1190 1490

Example:Example:

Double Suction Water PumpDouble Suction Water Pump

Flow Q = 5000 m3/h = 22000 gpmFlow Q = 5000 m3/h = 22000 gpm

Head H = 150 m = 492 ftHead H = 150 m = 492 ft

Correlation of NPSH System,

Rotational Speed and Size

19

720/14 900/17 1000/19 1200/23 1500/29ns [US/metric]

10

20

3

0

40

5

0[ft

]

Efficiency

Impeller diameter NPSHavailable

Example:double suctionwater pump

22000 gpm500 ft

5000 m³/h152 m

States of Cavitation

20

0 .0 5 .0 1 0 .0 1 5 .0 2 0 .0

N P S H A [m ]

3 5 .0

4 0 .0

H [m ]

3 % F ö rd e rh ö h e n a b fa l l3 % F ö rd e rh ö h e n a b fa l l3% head drop

NPSH3%

m4NPSHA = m10NPSHA =m6NPSHA =

Head break-down

curve at:

Q = 405 m³/h

n = 1490 1/min

Influence of Air on NPSH

Influence of dissolved air on

the suction performance of

a radial centrifugal pump.

n=1450 rpm, QBEP=210 m³/h

Flow rate Q [m³/h]

NP

SH

3 [m

]

Volume fraction ofair [%]

Influence of Surface Roughness

22

Source: Technical University of Darmstadt, Germany, department for Fluid

system technology.

poliert Körnung F500k = 50 µm

Körnung F230k = 110 µm

10 mm

n = 2000 min-1

ns (metric)= 26 min-1

ns (US) = 1340 min-1

q = Q/Qopt = 0.8h = H/Hmax = 0.995

Vanes polished medium roughness high roughness

NPSH and nss

23

• Suction specific speed:

The basic idea: pumps with high suction specific speed nss must have impellers with large

inlet diameters. This again results in an early start of part load recirculation at impeller

eye.

min]

/[13/4ss

NPSH

Qnn =

Suction specific speed as a selection criteria:

the good, the bad and the ugly

NPSH and nss

24

• Pfleiderer‘s formula:

• The first part of the formula covers the losses at the suction side of the pump. c1 stands for the meridional flow velocity at impeller eye, i.e. for impeller eye diameter.

i.e. c1 decreases with increasing impeller eye diameter.

• w1 stands for relative flow velocity at the suction edge of the vanes, i.e. for the shape of the vane.

w1 ~ u1 i.e. the circumferential velocity at inlet.

• The factor λw covers the pressure drop at the suction edge of the pump, i.e. the shape of the vane.

→ NPSH can be decreased by an optimized vane design (the good) or by increasing the impeller eye diameter (the bad) or both (a so called “suction impeller”).

→ There are many parameters influencing NPSH and nss.

]m[g

wg

cwc 22

21

21 λ+λ=NPSH

1

1

D~1c

Suction specific speed as a selection criteria:

the good, the bad and the ugly

Influence of Pump size and Speed on NPSH

25

The attainable

suction specific

speed increases

with the size of

the pump.

Source: NPSH for Rotodynamic Pumps, EUROPUMP booklet

Flow rate, size

n ss

Part Load Recirculation and nss

26

• Incipient cavitationfor three different impeller entrydesigns .

• Eperimentallydetermined byvisual inspection.

• Same suctiondiameter for all variations!

⇒ The onset of partload recirculationnot only dependson suctiondiameter.

Flow rate Q →

NP

SHin

cip

ien

t→

Original Impeller

Vane modified var. 1

Vane modified var. 2

Onset of part load recirculation

Influence of Hub Diameter on NPSH

27

Q/Qdesign →

1DDN=ν

The NPSH value

depends on the ratio of

impeller hub diameter

(DN)

and suction diameter

(D1).

Influence of Suction Casing Design on NPSH

28

With

suction

casing

Axial inlet

nss(suction casing) / nss (axial) ≈ 0.75

The good or the bad?

nss

29

The ugly:

Poor casting quality

= good nss?

• Before rework:

nss = 10800

• After rework:

nss = 12000

Suction edge of

impeller as delivered

Influence of the Installation Upstream

on the Performance

30

• GE: axial inlet w/o ellbow and butterfly• 0DK: with ellbow & butterfly valve horizontal (L=0)• 0DS: with ellbow & butterfly valve vertical (L=0)

M. Roth et al.: „Study of the influence of the installation

on the performance of centrifugal pumps in water supply

facilities“ (paper in german language)

Pump Users International Forum 2004, Karlsruhe,

Germany

Average of vibration amplitude

Q/QBEP →

Butterfly valve

Ellbow

Pump

The influence of the installation on the performance should not be overseen.

NPSH Criteria

31

API 610 11th Edition(EN ISO 13709 / 2009)

EN ISO 9906 / 2012

ANSI/HI 14.6 / 2011

ANSI/HI 9.6.1 / 2012

EN ISO 9905 / 2011

EN ISO 5199 / 2002

EN ISO 9908 / 1993

API 610 10th Edition (withdrawn) (EN ISO 13709 / 2003)

ANSI/HI 1.6 / 2000 (withdrawn)


ANSI/HI 9.6.1 / 1998 (withdrawn)

NPSHR: …is the NPSH that will cause the total head

to be reduced by 3%, due to flow blockage from

cavitation vapour in the impeller vanes…

NPSHR: …is the minimum NPSH given by the

manufacturer/supplier for a pump achieving a

specified performance at a specified flow rate,

speed end pumped liquid…

API 610 11th Edition

EN ISO 9906 / 2012

ANSI/HI 14.6 / 2011

ANSI/HI 9.6.1 / 2012

NPSH Criteria

32

NPSHR toEN ISO 9905 / 2011

EN ISO 5199 / 2002

EN ISO 9908 / 1993

API 610 10th Edition (withdrawn)



ANSI/HI 9.6.1 / 1998 (withdrawn)

NPSHR toEN ISO 9906 / 2012

ANSI/HI 14.6 / 2011

ANSI/HI 9.6.1 / 2012

- Reduced head

- Increased vibrations

- Increased noise

- Reduced lifetime

NPSH required

33

The circumferential

velocity at impeller

inlet u1 is an

important parameter

determining NPSH

required for a specific

material loss.

u1 = D1 P n/60u1

Cavitation resistance

34

The resistance of the material against cavitationerosion is another important parameter.

Highest to lowest resistance:

• cast duplex stainless steelcast stainless steel

• aluminum bronze

• tin bronze

• grey cast iron100

101

102

cavi

tatio

n w

ear

rate

[mm

/a]

30 35 40 45 50 55 60

bubble trail length [mm]

grey cast iron

tin bronze

aluminiumbronze

stainless steel(comp. CF8M)

duplex stainless steel(comp. CD4MCuN)

cavitation tests

water 40 °C

cast materials

Values derived from test probes in a „cavitation mill“

Thank You !

35

Lesson learned?

Guess the specific speed

of this pump!

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