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transcript
Calgary Pump Symposium 2013
Influence of Impeller Suction Specific
Speed on Vibration Performance (& LCC)
Simon Bradshaw
Calgary Pump Symposium 2015 1
Calgary Pump Symposium 2013Calgary Pump Symposium 2015
Simon Bradshaw
Director of API
Product Development
& Technology for ITT
Goulds Pumps, in
Seneca Falls NY
His responsibilities include the design and development
of new products and processes. Prior to joining ITT
Goulds, he worked for both Sulzer Pumps and Weir
Pumps.
He has accumulated 27 years in the pump industry. He
attributes this to having never exhausted the fun
inherent in moving fluid between two improbable
locations.
Mr. Bradshaw has a BEng (Hons) degree (Mechanical
Engineering) from Heriot Watt University. He is a
registered Chartered Engineer in the UK, a member of
the Institute of Engineering Designers and a member of
TEES Pump Symposium Advisory Committee.
Presenter
2
Calgary Pump Symposium 2013
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1982 1992 2002 2012
Pu
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Pu
mp
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Quiz• What are the red and blue lines ?
Vehicle fuel
efficiency
Pump
Nss limit
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Calgary Pump Symposium 2013
Suction-Specific Speed: What Is It?
• A measure of a pump’s suction performance
• Used since centrifugal pump theory was first developed
• Originally helped pump designers to predict & compare pump performances
• Now employed by contractors & end-users
• Commonly specified as a predictor of API pump reliability
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75.0NPSHR
QRPMN SS =
• Calculate ONLY at maximum diameter and Best Efficiency Point (BEP) flow
• For double suction pumps, divide Q by 2
Calgary Pump Symposium 2013
• Lower NPSHr is desirable to reduce 1st cost:
– Smaller pipework
– Lower tank elevations
– Less excavation
• But 1950-1980’s hydraulic design was limited
Historic Context #1
D1D1
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Calgary Pump Symposium 2013
• 1981 Fraser
– Demonstrated a method to predict suction recirculation
• 1982 Hallam
– Showed that pump reliability was correlated with Nss
Historic Context #2
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< 8000 8-9000 9-10000 10-11000 11-12000 12-13000 13-14000 >14000
Fail
ure
fre
qu
en
cy
Suction specific speed ranges (US units)
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Calgary Pump Symposium 2013
• 1985 Lobanoff & Ross
– Showed pump operating range (vibration) was strongly a
function of Nss
Historic Context #3
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NP
SHr
(m)
NP
SHr
(ft)
Pump Flow % of BEP
20000 (387)
Stable Operation
Window
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Calgary Pump Symposium 2013
Motivation
• Evaluate the effect of improved hydraulic design and
pump construction standards - (follow the red line)
• No modern large scale study on Nss vs. reliability
exists and none forthcoming…
• Validate the correctness company’s tradeoff (SGsT)
curves for impeller design
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Calgary Pump Symposium 2013
Advances in Impeller Design
Vane Leading Edge Profiles
Blunt
Circular
Parabola
Ellipse
• Vane development
• Low blade loadings near the impeller inlet
• Small incidence angles and approach fluid angles
• Tip geometry
• 2D and 3D computer
simulations
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Calgary Pump Symposium 2013
Impeller Leading Edge Profiles – 2011 Testing
Blunt
Circular
Parabola
Ellipse
Cast Impeller with blunt
Leading edge
Circular Leading edge profile Blunt Leading edge profile
Ellipse Leading edge profileParabola Leading edge profile
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Calgary Pump Symposium 2013
0.75
0.8
0.85
0.9
0.95
1
1.05
0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
H/H
BE
P
Cavitation Number σ
σ = 0.18 (3% head drop)σ = 0.22 (1% head drop)σ = 0.17 (head breakdown)
Cavitation Development at BEP flow in the impeller with Parabola
profile as suction pressure is reduced – 2011 Testing
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Calgary Pump Symposium 2013
Impeller Profile
NPSH 3%ft (m)
Nss (S)
Blunt 36.8 (11.2) 10386 (201)
Circular 33.4 (10.2) 11170 (216)
Ellipse 30 (9.1) 12104 (234)
Parabola 28.3 (8.6) 12644 (245)
Impeller Nss – 2011 Testing
• Parabola has the best cavitation performance
• Impeller life is doubled from Circular to Parabolic profile
• A 15% improvement in NPSHr is achievable with no change in
allowable operating range (or vibration performance)
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Calgary Pump Symposium 2013
• Pump standards have changed significantly
– Mandated smaller L3/d4 (API 610 11th edition)
Pump construction standards #1 - 2015
1.0E+00
1.0E+01
1.0E+02
1.0E+03
1.0E+04
1.0E+01 1.0E+02 1.0E+03 1.0E+04 1.0E+05
L3/d
4(i
n-1
)
QH/N (USGPM x ft / RPM)
API 610 App. K acceptance line
Test Pump
Older generation Pump
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Calgary Pump Symposium 2013
– Mandated smaller deflection under nozzle loads (API 610
7th edition)
– Mandated no rear foot (API 610 9th edition)
Pump construction standards #2 - 2015
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Calgary Pump Symposium 2013
Lobanoff & Ross – 2015 version
• Using the parabolic leading edge vane profile
• Utilizing modern impeller design techniques that achieve the required NPSHr while minimizing D1
• Considering the improvements in pump construction standards since 1982
• How will the operating envelope with acceptable vibration change ?
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Calgary Pump Symposium 2013
Impeller Design 1– Pump Size: 4x6-11 (100x150-280) in a single stage
overhung configuration with centerline mount (OH2)
– Design Criteria
• Modern design of 4 impellers with identical flow and head
requirements
• Vary the suction performance by adjusting:
– inlet angles
– inlet diameter
– meridonal profile
– and vane tip
Parameter Value
Running Speed 3560 RPM
BEP Head 450 ft (137 m)
BEP Flow 1670 USGPM (380m3/h)
BEP power @ 1.0 SG 232 HP (173 kW)
Specific Speed Ns (nq) 1489 (28.8)
Design Pressure 750 psig (51.7 barg)
Materials of Construction API 610 code S6
Shaft dia. @ mechanical seal 2.362” (60mm)
L3/d4 ratio 42 in-1 (1.65 mm-1)
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Calgary Pump Symposium 2013
Impeller Design 2
– Design Criteria
Design 1 Design 2 Design 3 Design 4
Nominal Nss (S)8000
(155)
11,000
(213)
13,000
(252)
15,000
(290)
D2 Impeller outlet
diameter (in)11 11 11 11
B2 Impeller outlet width
(in)1 0.9 0.85 0.95
β2 Impeller vane angle @
outlet (deg)24 26.3 29 27.5
β1t Impeller vane angle
@ inlet (deg)29 13.2 14.7 11.7
D1 Impeller inlet eye
diameter (in)4.9 5.3 5.5 5.8
D1 / D2
Impeller inlet / impeller
outlet dia.
0.44 0.48 0.5 0.53
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Calgary Pump Symposium 2013
Nominal Suction
Specific Speed
Fraser
Suction Recirc.
(% of BEP)
CFD
Suction Recirc.
(% of BEP)
8000 (155) 48% ≈48%
11,000 (213) 60% ≈63%
13,000 (252) 66% ≈63%
15,000 (290) 75% ≈74%
Suction recirculation• Fraser vs. CFD
8000 (155)
@50% of BEP
13000 (252)
@65% of BEP
11000 (213)
@55% of BEP
15000 (290)
@75% of BEP
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Calgary Pump Symposium 2013
• 4 impellers manufactured by SLA for accuracy
• Parabolic leading edges
• Ns 1489 (nq 29)
Test Impellers
8000 (155) 11000 (213)
13000 (252) 15000 (290)
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Calgary Pump Symposium 2013
Test Setup• OH2 4x6-11 (100x150-280) – same size, speed and
power as used by Lobanoff & Ross
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Calgary Pump Symposium 2013
5101520253035404550
0.0 0.5 1.0 1.5
NP
SH3
(ft
)
Flow rate relative to BEP
Nss - 11000 CFD Nss - 8000 CFD
Nss - 11000 Test Nss - 8000 Test
Nominal Suction
Specific Speed
Target NPSHr
@BEP ft (m)
Tested NPSHr
@ BEP ft (m)
Tested Suction
Specific Speed
% decrease in NPSHr
(tested vs. nominal)
8000 (155) 47.8 (14.6) 37.4 (11.4) 9568 (185) 22%
11,000 (213) 31.3 (9.5) 21.1 (6.4) 14,776 (286) 33%
13,000 (252) 25.0 (7.6) 17.6 (5.4) 17,066 (331) 30%
15,000 (290) 20.7 (6.3) 16.4 (5.0) 17,841 (346) 21%
NPSHr results
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PSH
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Flow rate relative to BEP
Nss - 15000 CFD Nss - 13000 CFD
Nss - 15000 Test Nss - 13000 Test
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Calgary Pump Symposium 2013
Vibration results• 13000 (17066 actual) exceeds limit at 76% of BEP
• 15000 (17841 actual) exceeds limit at 86% of BEP
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Flow rate relative to BEP
Vane pass vibration
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Unfiltered vibration
Allowable vibration 15000 Nss vibration
13000 Nss vibration 11000 Nss vibration
8000 Nss vibration
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NP
SH
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Pump Flow % of BEP
20000 (387)
Stable Operation
Window
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NP
SH
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Pump Flow % of BEP
Stable Operation
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Conclusions• Stable operating window greatly increased
• Nss limits far above 11000 level
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Calgary Pump Symposium 2013
Conclusions – Now What ?• Testing provides some validation of SGsT curve
• Realizable Nss far above current industry norms
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Att
ain
ab
le P
um
p N
ss (
US
un
its)
Pump Ns (US units)
ITT Goulds Nss vs. Ns tradeoff chart (SGsT curve)
Existing SGsT line
Nss = 17841 (345)
Nss = 17066 (330)
Nss = 14766 (286)
Nss = 9568 (185)Attainable &
acceptable
performance
Not attainable
with acceptable
performance
Calgary Pump Symposium 2015 24
Nss=� �
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Ns=� �
��.�
Calgary Pump Symposium 2013
• Use Warren Fraser’s calculation to find the onset of suction side recirculation
• Older designs will have an onset at >80% of BEP• Good modern designs will have an onset at ≈ 60% of BEP
• If recirculation is in between 80% and 60%, consider the criticality and power of the pump and if in doubt treat it as if it were an older design
How To Tell If A Pump Design is Modern
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Calgary Pump Symposium 2013
• Goulds uses the graph shown below as an average for an industrial pump with a 10 year life
• Energy cost amounts to around 1/3rd of the LCC - based on US industrial electricity prices. If you live in a region of the world where electricity costs substantially more, this will be a larger portion.
The cost of constraining Nss (energy costs matter)
November 9, 2015Optimizing Pump Hydraulics 26
• The US industrial electricity cost has been around $0.07 per KWh in 2014
• In Europe it is around twice this at $0.16 per KWh.
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Calgary Pump Symposium 2013
• How does limiting the maximum Nss affect the pump energy cost ?
• Nss is a function of the pump speed (as well as BEP flow and NPSHr). So the normal way to achieve a target Nss limit such as 11,000 (when flow and NPSHr are fixed), is to slow the pump down
The cost of constraining Nss (energy costs matter)
November 9, 2015Optimizing Pump Hydraulics 27
• When the pump is slowed down the pump Specific Speed (Ns) also reduces. This reduction affects the efficiency that the pump can attain as shown in the chart
• For any given pump BEP flowrate, lowering the pump Specific Speed will lower the attainable efficiency, sometimes significantly
(1000) (2000) (3000)
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Calgary Pump Symposium 2013
• Head 651 ft (198.4 m), Flow 1598 USGPM (363 m3/hr), Maximum NPSHr 26 ft (8 m), SG 0.754, 60 Hz
• The pump sold was a 3x8-27A running at 1785 RPM with an efficiency of 64% and an absorbed power of 310 HP (231 KW). The specific speed of this selection is 482 US units (9 metric)
• Because the Nss was limited to 11,000 (US units) a 2 pole selection was not possible. However if that limit was raised to the SGsT limit, a valid selection would the 4x6-13H at 3560 RPM, efficiency = 79.5%, power = 249 HP (186 KW)
• ΔP = 45 KW
• For a pump running 8000 hrs/year 45 x 8000 x $0.07 = $25,200 per year
• For the 20 year life this will cost an additional $0.5 million in energy usage
The cost of constraining Nss (RL example – OH2 )
November 9, 2015Optimizing Pump Hydraulics 28Calgary Pump Symposium 2015 28
Calgary Pump Symposium 2013
• ΔP = 45 KW
• For a pump running 8000 hrs/year 45 x 8000 x $0.07 = $25,200 per year
• For the 20 year life this will cost an additional $0.5 million in energy usage
The cost of constraining Nss (RL example – OH2 )
November 9, 2015Optimizing Pump Hydraulics 29Calgary Pump Symposium 2015 29
Calgary Pump Symposium 2013
• Head 1191 ft (363 m), Flow 1940 USGPM (441 m3/hr), Maximum NPSHr 18 ft (5.5 m),
SG 0.65, 60 Hz
• The pump sold was a 8x10-27CD running at 1785 RPM with an efficiency of 64.5% and
an absorbed power of 574 HP (438 KW). The specific speed of this selection is 598 US
units (12 metric)
• Raising the Nss limit above 11,000 would allow a 2 pole selection with an efficiency of
71.5% and an absorbed power of 533 HP (398 KW)
• ΔP = 40 KW
• For a pump running 8000 hrs/year 40 x 8000 x $0.07 = $22,400 per year
• For the 20 year life this will cost an additional $0.45 million in energy usage
The cost of constraining Nss (BB2 example)
November 9, 2015Optimizing Pump Hydraulics 30Calgary Pump Symposium 2015 30
Calgary Pump Symposium 2013
• ΔP = 40 KW
• For a pump running 8000 hrs/year 40 x 8000 x $0.07 = $22,400 per year
• For the 20 year life this will cost an additional $0.45 million in energy usage
The cost of constraining Nss (BB2 example)
November 9, 2015Optimizing Pump Hydraulics 31Calgary Pump Symposium 2015 31