suction considerations for slurry and froth pumps

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Weir Minerals Division

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Presented to:

Prepared by:

Date: November 4, 2011

Suction Side Considerations

for Slurry and Froth Pumping

Calgary Pump Symposiu m November 2011

Peter Williams



2Weir Minerals Division

NPSH – is that all we need to consider?

For most mineral slurries, NPSHA is not much of a consideration unless

there isn’t enough, when it becomes a big deal.

In most mining operations, the slurries are relatively cool, the pump sizes &

flows are relatively low, and the suctions are flooded. Ignoring NPSH usually

doesn’t cause a problem.

However, when pumps are large, and flows are high, and temperatures are

elevated, it cannot be ignored. These situations can occur in large mill

circuits and in oil sands HT & Tails.

Air entrainment also causes suction problems that inter-relate with NPSH, butare not strictly an NPSH problem.

The NPSH portion is the work of Dr. Aleks Roudnev, Manager R&D- Applied Hydraulics, Weir Minerals North America,

and vice-chairman of HI Slurry Pump Standards Committee.

NPSH:




NPSH – It’s not the whole answer, we need to consider more.

Transport velocities in suction lines – at all anticipated flow rates, especially

minimum flow.

Sump design to:

Prevent settling

Prevent air entrainment

Allow air release

Provide surge capacity

Suction piping design to:

Prevent settling

Prevent disturbances that contribute to localized wear

Make maintenance possible

Not related to NPSH:




INTRODUCTION – Part 1

For slurries of settling type NPSH required by pump practically equals to that

on water

For viscous liquids, including Bingham Plastic mixtures, NPSH required bypump is not equal to that on water

Free air at pump inlet leads to increase in required NPSH

Current view:




HI Slurry Pump Standard

The ANSI/HI 12.1-12.6 Slurry PumpStandard deliberately offers little

specif ics relative to NPSH performance

- NPSH required by the pump when

handling slurry will increase, in mostcases.




NPSH TESTS ON SETTLING SLURRY

Mashin (1954)

Herbich and Cooper (1971)

Kirillov (1984)

Addie et al. (1999)

Governing equation

NPSH = (pa - pv)/ρg + Z0 - hf

NPSHRm = NPSHRw

NPSH required

on water and sand slurry,

5-inch pump,330 mm impeller

1450 RPM (from Mashin)

0

1

2

3

4

5

30 50 70

Flow Rate, l/s

N P S H R , m

w ater

sand slurry

density 1150

kg/m3,

d50=1.0mm Average value

BEP flow rate




NPSH TESTS ON SETTLING SLURRY

10

12

14

16

18

20

22

24

26

28

0 2 4 6 8 10 12

NPSH, m

H e a d , m

water

density 1070

kg/m3,

d50=0.96 mm

density 1180

kg/m3,

d50=0.96 mm

density 1080

kg/m3,

d50=1.80 mm

NPSH full

cavitation

NPSHRm = NPSHRw

0

10

20

30

40

50

60

70

80

90

100

0.01 0.1 1 10 100

Particl e size (d) mm

C u m u l a t i v e %

p a s s i n g

sand d50=0.20 mm gravel d50=0.96 mm

gravel d50=1.80 mm gravel d50=6.15 mm

NPSH curves for water-gravel

mixture, 5-inch pump, 1300 RPM Particle size distributions for

sand and gravel mixtures

(from Kirillov)




REQUIRED NPSH

AT 3% HEAD DROP

24

25

26

27

28

29

0 2 4 6 8

NPSH, m

H e

a d , m

water

density 1025

kg/m3

density 1060

kg/m3

density 1250

kg/m3NPSH 3%

NPSH const

Normalized to Hr

22

23

24

25

26

27

0 2 4 6 8

NPSH, m

H e a d , m

water

density 1040

kg/m3density 1085kg/m3density 1280

kg/m3NPSH 3%

NPSH const

Normalized to Hr

19

20

21

22

23

0 2 4 6 8

NPSH, m

H e a d , m

water

density 1035

kg/m3

density 1085kg/m3

density 1250kg/m3

NPSH 3%

NPSH const

Normalized to Hr

80% BEP flow rate

BEP

120% BEP flow rate

Sand Slurry




21

22

23

24

25

26

27

0 5

NPSH, m

H e a d , m

density 1070 kg/m3,

d50=0.96 mm

density 1180 kg/m3,

d50=0.96 mm

density 1080 kg/m3,d50=1.80 mm

NPSH 3%

NPSH const

Normalized to Hr

REQUIRED NPSH

AT 3% HEAD DROP

Gravel Slurry

80% BEP flow rate

BEP

120% BEP flow rate

23

24

25

26

27

28

29

0 5

NPSH, m

H e

a d , m

water

density 1088 kg/m3,

d50=0.96 mm

density 1160 kg/m3,

d50=0.96 mm


NPSH 3%

NPSH const

Normalized to Hr

19

20

21

22

23

0 2 4 6 8

NPSH, m

H e a d , m

density 1080 kg/m3,

d50=0.96 mm

density 1160 kg/m3,

d50=0.96 mm


density 1060 kg/m3,

d50=6.15 mm

NPSH 3%

NPSH const

Normalized to Hr




For slurries of settling type, most of the test work publ ished to date supports the

equivalent liquid approach, allowing considering NPSH required for slurry to be

equal to that on water.

A detailed compar ison of NPSH curves at a 3% head drop level showed some

trend towards increase in NPSHRm fol lowing the increase in head de-rating due

to solids coefficient Hr, but it also could be considered within the scatter of test

points.




EFFECT OF SLURRY SG ON NPSH

I assume that we all agree that you

must reduce the allowable lif t, by

using the SG?




EFFECT OF SLURRY SG ON NPSH

But what about this one? Do you get

addit ional credit for SG?

Our recommendation is that you donot.

Our assertion is that you do not.




NPSH TESTS ON BINGHAM

PLASTIC MIXTURES

Orlov (1955)

Mogilevsky (1972)

Herbich (1975)

Ladouani et al. (1995)

Reference equation

NPSH = (pa - pv)/ρg + Z0 - hf

NPSHRp ≠ NPSHRw

0

2

4

6

8

10

12

25 30 35 40 45 50 55 60 65 70

Flow rate Q, l/s

N P S

H 3 % , m

density 2340 kg/m3 density 2250 kg/m3


NPSHR on water BEP line

H-Q and NPSH performance on dense medium, 4” side inlet pump,4 vane closed impeller 335 mm diameter, 1450 RPM (from Mogi levsky)

0

5

10

15

20

25

30

35

40

0 10 20 30 40 50 60 70 80

Flow rate, l/s

H e a d

, m

0

10

20

30

40

50

60

70

80

E f f i c i e n

c y , %

water magnetite density 2030 kg/m3

magnetite density 2400 kg/m3 ferrosilicon density 3300 kg/m3




NPSH TESTS ON BINGHAM

PLASTIC MIXTURES

0

2

4

6

8

10

12

14

20 25 30 35 40 45 50 55 60

Flow rate Q, l/s

N P S H 3 % , m

densi ty 2450 kg/m3 density 2320 kg/m3

density 2170 kg/m3 NPSHR on water

BEP line

0

2

4

68

10

12

20 30 40 50 60

Flow rate Q, l/s

N P S H R 3 %

, m


NPSHR on water BEP line

NPSH performance

Dense medium of magnetite

4” side inlet pump

1450 RPM

6 vane closed impeller

330 mm diameter

2 vane channel type impeller

360 mm diameter




BINGHAM PLASTIC MIXTURE FLOW

AT PUMP INLET

pt. a - higher velocity,exceeding that of water

at the same flow rate,

low pressure

pt. b - velocity below vc,visco-plastic flow forms,

high pressure

Added blockage at the impeller inlet caused by visco-plastic flow

(schematic according to Jivotovsky and Smoilovskaya)

AP ∝ VCc1/w1




APPROXIMATION

OF TEST RESULTS

1.0

1.2

1.4

1.6

1.8

2.0

2.2

0.00 0.01 0.02 0.03 0.04 0.05

Added blockage coefficient

(vcc1/w12)

N P S H R p / N P S H R w peat slurry

magnetite densemedium

approximation,

Eqn (8)

NPSHP = NPSHW (1 + 24 vC c1/w12) Eqn 8




COMPARISON OF CALCULATED RESULTS WITH TEST

0.0

2.0

4.0

6.0

8.0

10.0

12.0

25 30 35 40 45 50 55 60 65 70

Flow rate Q, l/s

N P S H 3 % , m

Density 2340 kg/m3

test

Density 2340 kg/m3

Eqn 8

Density 2070 kg/m3

test

Density 2070 kg/m3

Eqn 8

NPSHR on water

NPSH required on dense medium of magnetite - test vs Eqn 8

4” side inlet pump @ 1450 RPM, 4 vane closed impeller 335 mm




If the slurry exhibits non-Newtonian properties, the required NPSH on slurry isexpected to increase compared to that on water, dependent on slurry rheology,and pump operating point on the curve.

For Bingham plastic mixtures the required NPSH at low flow rates is equal tothat on water, but increases substantially wi th flow rate towards BEP.

For smaller size pumps a relationship based on slurry critical velocity, anddependent on slurry yield stress and plastic viscosity, allows for a good firstestimate of the NPSHRP .




INFLUENCE OF FREE AIR

ON SUCTION PERFORMANCE

Karelin (1963)

Petrov and Chebayevski (1973)

Budris and Mayleben (1998)

NPSHRG > NPSHRw

NPSHG full cavitation = NPSNW full cavitation/(1-1.5 )4/3 Eqn 6

where is volume fraction of air (gas) at pump inlet

Petrov and Chebayevski recommended the following

empirical expression for first estimate of NPSH increase:






4.65

5.4

5.8

6.2

6.6

77.4

7.8

8.2

0 0.01 0.02 0.03 0.04 0.05

Air volume fraction δ

N P S H , m

NPSH 3% power 3.2, Eqn (10)

NPSH full cavitation power 4/3, Eqn (6)

Influence of air injection

on allowable suction lift Pump NPSH change with

air volume fraction at inlet

6-inch end suction pump operating on cold water,

2950 RPM and constant flow rate of 30 l/s (from Karelin)

At 3% head drop NPSHRG = NPSHRW/(1- 1.5 )(3…3.5)

Eqn 10






0

5

10

15

20

25

0 200 400 600 800 1000 1200Flow rate, l/s

T o t a l h e a d ,

m

0

20

40

60

80

100

E f f i c i e n c y , %

no air with air 16

17

18

19

20

2 3 4 5 6 7 8

NPSH, m

T o

t a l h e a d , m

no air with air

Performance of a 24-inch pump

with and without air injection

Partial NPSH curves at BEP flow

rate with and without air injection

3.35/(1-0.01)3.5 = 3.47 (m)




Addi tion of free air to pumped slurry should affect NPSHR quite substantiall y.

The data for air injection when pumping water without solids suggest that the

NPSHR corresponding to 3% head drop wi ll increase.

When the approximate air content at pump inlet is known, this increase can be

roughly estimated.




The introduction of solids prevents the direct application of clear liquid knowledge in the

suction design of slurry systems. Here are some of the obvious pitfalls.

INTRODUCTION – Part 2 – non-NPSH issues

Desirable in clear liquid application Possible result in slur ry application

Large sumps with very low internal

velocities

Solids settle in the bottom of the sump

Minimum of 9 diameters of oversized

suction line

Sanded suction line or

Sliding bed in suction line – decreased lives

Eccentric reducer with flat on top to prevent

trapping air

Sanded suction line

Gauge taps Localized wear




Slurry pump hoppers or sumps should be designed to allow for the expected flow

rate variation without overflowing or running at low levels that allow air to enter

the pumps via vortices. In general the pump hopper should be designed tominimise settling of the solid particles in the hopper.

The minimum sump level should not be less that 2 intake pipe diameters above

the top of the intake pipe to the pump. The suction pipe from the hopper to the

pump should be inclined at an angle of 30 degrees (if practical) to prevent air

from accumulating in the suction pipe. The slurry should be introduced to the pump hopper in such a way as to

minimise the amount of air introduced to the slurry in the pump hopper. This can

be achieved by the use of baffles, extending the slurry inlet pipe below the

normal operating level in the sump or directing the slurry inlet away from the

entrance to the pump intake pipe.

When using an operating / standby arrangement for Mill Pumps it is advisable to

use a double hopper arrangement to allow the standby pump / hopper

combination to be completely drained on shutdown to prevent solids build up in

the pump, hopper or discharge pipe work

Suction Sump Hopper Designs – Warman Pumping Manual




Guidelines for Sump Design – Where Science Needs Art

Hopper must be both large enough to

provide adequate reserve, provide surge

capacity, allow air to be released at surface,

while simultaneously preventing sanding.

Make as tall as practical

NPSHA

Surge capacity

Free power

Slope bottom 30°to prevent collection of

solids

Ideally have pipe come off at 30°to allow

air in suction pipe to come back into sump

Large enough surface area to promoterelease of air

Locate sump supply pipe to eliminate air

entrainment as slurry enters sump




Prior Envirotech Pumpsystems experience

Suction Sump Design for Mill Circuits




Mill Pump Sumps

To protect pumps from damage by large tramp material or impactfrom mill balls a suction strainer can be incorporated into the tank. Aperture sizes in the strainer must be adequate to preventblocking and to minimise intake head loss. (Refer photos below).This method is particularly recommended if there is no trommelscreen or vibrating screen on the discharge of a mill.




Guidelines for Suction Pipe Design – Where Science Needs Art

Must review minimum transport velocities at

all anticipated flow rates. Suction pipe is

usually the largest diameter so presents a

likely location for sanding. Most often the

pump is considered the determining factor

for minimum capacity. But most often,

sanding of lines is the real problem.

As velocity decreases, the flow regime

moved from heterogeneous suspension (no

deposition), through sliding bed, through

stationary bed, ending up with blocked pipe.

Partially self-limiting as effective areadecreases forcing velocity upwards.

Sliding beds usually cause decreased pump

component life.




Suction Pipe Reducers

Which is correct? Pump is on the left.

Flat on top

Flat on bot tom

Can’t decide




Suction Pipe Reducers

Which is correct? Pump is on the left. The answer is “ it depends”

Prevents air collecting at top of pipe, but

collects solids on bottom of pipe.

Minimizes trapping of solid, but can collect air.

Some of each? None of each? Can’t decide?

Flat on top

Flat on bot tom

Can’t decide




Mill Pump Sumps – added comments

• The minimum slurry level in thesump should not be less that 2intake pipe diameters above thetop of the intake pipe to thepump, more may be required inhighly agitated sumps. The

normal slurry level should be atleast 6 to 7 times the diameter ofthe pump intake pipe.

• Sump make-up/flush watershould be added below theslurry level in the sump and used

to control the overall sump level.• It is recommended to install aninclined baffle or grizzly screenwith a cleanout access in thesump.




Mill Pump Sumps – Comments from Ricardo Abarca• The solution, we found best, is to take both suction pipes inside the tank,close enough one to the other, so the flow movement of the operating pumpkeeps clean the entrance of the pump that is on a standby.




Air Entrainment – when the art becomes really “out there”




Air Entrainment

This discussion is aimed at sump design, not pump design. That is another Arts

and Science project. Papers have been presented on froth pump design at this

symposium in recent years.

One of the improvements to this situation that will benefit all pump types is having

a very deep sump. This can only be obtained if the liquid supply is high in the

plant. The added suction pressure reduces the bubble size. For example,

consider suction head equal to zero gauge (at pump centreline) and compare tothe same aerated stream at 1 atmosphere gauge suction pressure, the bubble is

half the size.

Another major benefit of this higher suction head is the free power that I mentioned

earlier. If my total head is 100 m, I require a certain amount of power for a given flow

rate and SG. If I can add 10 m to my suction tank, my total head goes to 90 m, and I

get an immediate 10% reduction in power consumption, regardless of pump efficiency.




Another Unusual Suction Condition

DIFFERENTIAL COLUMN HEAD LOSS

In a dredging application, we are picking up a slurry that has an SG higher than

water, but the surrounding liquid column is essentially water at 1.0 SG. This will

not support as high column of slurry as it would water, so NPSHA and total head

need to be adjusted.




Conclusions

The first part of the presentation is science. Much has been published on NPSH

for clear liquids, but very little specifically for slurry. If you want to investigate this

topic in more depth, see Dr. Roudnev’s paper “Slurry pump suction performance

considerations”, ©BHR Group, 2004 Hydrotransport 14. The bibliography in that

paper will lead you to 15 others.

The second part of the presentation is a mixture of arts and science. There are

too many variables in the nature of slurry to allow it to be reduced (or elevated) toscience.

From the pump manufacturers’ points of view, the answer is simple. Give us

properly designed pump boxes that are very tall, with high suction levels. The

reality is that it can be very expensive to do this in large installations.

But it is much more expensive to fix an installation than didn’t consider theserequirements during the design stages.

Weir is working towards collecting field data and publishing a paper with more

practical recommendations on sump design. Hopefully, it will be published next

year.




Caption – if desired

Questions and Comments

suction considerations for slurry and froth pumps

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