influence of impeller suction specific speed on vibration performance … · ·...

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


0

2000

4000

6000

8000

10000

12000

14000

16000

0

5

10

15

20

25

30

35

40

45

50

1982 1992 2002 2012

Pu

mp

Nss

Ve

hic

le M

PG

/ t

on

Year

0

2000

4000

6000

8000

10000

12000

14000

16000

0

5

10

15

20

25

30

35

40

45

50

1982 1992 2002 2012

Pu

mp

Nss

lim

it

MP

G /

to

n

Year

Quiz• What are the red and blue lines ?

Vehicle fuel

efficiency

Pump

Nss limit

3Calgary Pump Symposium 2015 3


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

4 July 10, 2012Calgary Pump Symposium 2015 4

75.0NPSHR

QRPMN SS =

• Calculate ONLY at maximum diameter and Best Efficiency Point (BEP) flow

• For double suction pumps, divide Q by 2


• 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



• 1981 Fraser

– Demonstrated a method to predict suction recirculation

• 1982 Hallam

– Showed that pump reliability was correlated with Nss

Historic Context #2

0

0.5

1

< 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)



• 1985 Lobanoff & Ross

– Showed pump operating range (vibration) was strongly a

function of Nss

Historic Context #3

0

2

4

6

8

10

12

14

16

5

15

25

35

45

55

0 20 40 60 80 100 120 140

NP

SHr

(m)

NP

SHr

(ft)

Pump Flow % of BEP

20000 (387)

Stable Operation

Window



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



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



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



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



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)



• 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



– Mandated smaller deflection under nozzle loads (API 610

7th edition)

– Mandated no rear foot (API 610 9th edition)

Pump construction standards #2 - 2015



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 ?



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)



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



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



• 4 impellers manufactured by SLA for accuracy

• Parabolic leading edges

• Ns 1489 (nq 29)

Test Impellers

8000 (155) 11000 (213)

13000 (252) 15000 (290)



Test Setup• OH2 4x6-11 (100x150-280) – same size, speed and

power as used by Lobanoff & Ross



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

5

10

15

20

25

0.0 0.5 1.0 1.5N

PSH

3 (

ft)


Nss - 15000 CFD Nss - 13000 CFD

Nss - 15000 Test Nss - 13000 Test



Vibration results• 13000 (17066 actual) exceeds limit at 76% of BEP

• 15000 (17841 actual) exceeds limit at 86% of BEP

0

1

2

3

4

5

0

0.05

0.1

0.15

0.2

0 0.5 1 1.5

Ve

loci

ty (

in/s

RM

S)


Vane pass vibration

0

1

2

3

4

5

0

0.05

0.1

0.15

0.2

0 0.5 1 1.5

Ve

loci

ty (

in/s

RM

S)


Unfiltered vibration

Allowable vibration 15000 Nss vibration

13000 Nss vibration 11000 Nss vibration

8000 Nss vibration



5

15

25

35

45

55

0 20 40 60 80 100 120 140

NP

SH

r (f

t)

Pump Flow % of BEP

20000 (387)

Stable Operation

Window

5

15

25

35

45

55

0 20 40 60 80 100 120 140

NP

SH

r (f

t)

Pump Flow % of BEP

Stable Operation

Window

Conclusions• Stable operating window greatly increased

• Nss limits far above 11000 level



Conclusions – Now What ?• Testing provides some validation of SGsT curve

• Realizable Nss far above current industry norms

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

0 1000 2000 3000 4000

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


Nss=� �

��.�

Ns=� �

��.�


• 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

25Calgary Pump Symposium 2015 25


• 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.



• 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)



• 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


• ΔP = 45 KW



The cost of constraining Nss (RL example – OH2 )



• 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



The cost of constraining Nss (BB2 example)



• ΔP = 40 KW



The cost of constraining Nss (BB2 example)



Thank you for your attention

Questions ?

Special thanks to the following individuals for their assistance:

David Cowan Thomas Liebner

Susan Sullivan Dennis Fenner

Mark Ohlrich Martin Temple

Patricia BabowiczWebb John Salerno (Jr.)


influence of impeller suction specific speed on vibration performance … · ·...

Documents