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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
Influence Factors on Performance Curve
11
( )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
D2
b2
Influence Factors on Performance Curve
12
( )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
D2
b2
←←←←
Typically used
working range
←←←←
Influence Factors on Performance Curve
13
Finite Number of Vanes and Real
Performance
Deflection of streamlines close to impeller
outlet
Influence Factors on Performance Curve
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
Influence Factors on Performance Curve
15
Example:
Double suction/horizontal split case pump
meeting customers requirement “maximum shutoff head”.
Influence Factors on Performance Curve
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
Influence Factors on Performance Curve
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 2.6 / 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 1.6 / 2000 (withdrawn)
ANSI/HI 2.6 / 2000 (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!