The Fundamentals of Pump Sizing Feb 2014

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Slurry handling solutionsThe fundamentals of pump sizing


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The fundamentals of pump sizing

Introduction

This presentation is aimed at providing abrief overview of pump sizing

Pumps may well be the oldest mechanicalinvention for the conversion of naturalenergy to useful work, but they are also the

most misunderstood of all machines

Mechanically very simple, pumps can alsobe very complicated hydraulically, andcertainly troublesome under the wrongconditions

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Centrifugal slurry pumps

There are literally hundreds of centrifugal pumpmanufacturers, but very few who manufacture a true

slurry pump Slurry pumps represent 5% of all centrifugal pumps

installed in the process industry

Of the 5% installed, slurry pumps represent 80% ofthe total operating costs

These operating costs increase significantly when aslurry pump is sized incorrectly, or misapplied

It is therefore essential to have a clear understandingof all aspects relating to pump sizing and indeedpump design and application

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Its never easy

All pump ranges have limited hydraulic coverage (capacity and head).Coverage across various ranges is generally good but not infinite

Slurry pump sizing is often very much an art form, there are guidelinesbut no fixed or firm rules especially when extremes are encountered

Common sense must prevail to succeed. Pump sizing software is oftenavailable, but this will not help if the basic selection is wrong

Good pump sizing relies on not only a basic understanding of hydraulics,but also an understanding of site needs and applications

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The right pump for the job

Every manufacturer has various pump ranges designed for a wide varietyof slurry applications, from dirty water to mill discharge

Numerous adjectives are used to describe a particular range including;- Solution- Mining

- Heavy Duty- Extra Heavy Duty- Severe Duty

Materials of construction may vary, there may be extra material at known

points of wear, but most manufacturers refer to the impeller aspect ratio the ratio of impeller diameter to impeller eye diameter (pump inlet)

Good practice requires a high aspect ratio for the most abrasive duties

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Increasingimpeller

diameter


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Materials of construction

Material selection plays a big part in pump sizing, get it wrong and thepump will fail much sooner then expected

Elastomers are superior for abrasion but are more likely to fail due topoor adjustment, vacuum, temperature, additives or oversized solids

Metal can withstand a lot more abuse and as such it is often a bestcompromise except where corrosion also exists

While manufacturers make selection at or close to BEP, as processeschange, so does the duty point relative to BEP

A pump that is too large can wear out just as quickly as a pump that istoo small

Efficiency reflects turbulence which in turn reflects rate of wear. Goodefficiency with smooth hydraulic flow and the minimum of recirculationwill always provide good service

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Pumps are simple machines, buttheir sizing for and application in a

system (their working environment)can be complicated, and indeedvery difficult with slurries

Often considered only one up fromvalves, the smallest of pumps canshut down a complete plant


Simple machines

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To every action there is always opposed an equal reaction

In our world, whatever we do when designing and or specifying plant,there will always be a consequence

Pump sizing is therefore often a compromise, to accommodate avariety of parameters set by others

Careful thought beforehand is a must, the more information madeavailable when sizing a pump, the greater the chances of success


Isaac Newton discovered that

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Details of the application willalways help with pump sizing

In short, what is the pumpexpected to do?

- Feed spray bars

- Dewater a pit- Feed a cyclone- Transfer slurry- Feed a press


What is the application?

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Where from, where to and everything in between?

1. pipe, straight2. valve, butterfly3. pipe, straight4. bend, 905. pipe, straight6. bend, 90

7. pipe, straight8. bend, 90

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Total friction losses are converted into an equivalent length of straight pipe

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Static head

A. Static head is always liquid level to liquid level (high points shouldalways be specified)

B. Suction static (-/+) is liquid level on suction to pump centre line

C. Discharge static (-/+) is pump centre line to liquid level at point ofdischarge

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Slurry definitions

Slurry- Any mixture composed of solid

particles and a liquid

Concentration- The amount of solids present in a

slurry expressed as a percentage

Settling slurry- Solids settle rapidly and turbulent

conditions are required to prevent

settlement

Non settling slurry- Solids settle fairly slowly or not at all

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Always remember

To size a pump for a given application, it is essential to be provided with(as a minimum) the required volumetric flow rate and the total differential

head (TDH) against which the pump must produce this flow

Even with a given volumetric flow rate and TDH, numerous additionaldata is needed for a trouble free installation

Additional data should relate to both the liquid handled, solids presentand the system under which the pump is expected to operate

Various flows at the same total differential head are unlikely to berealistic. Static head may be the same for different flows but frictionvaries considerably

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Computer software

Computer design enables accuratetransition from theory to practice

While modern sizing proceduresfor slurry pumps are computerisedand easy to handle, it is importantthat we know the steps for sizing

slurry pumps and the relationshipbetween them

The following slides illustrate amanual procedure, which although

approximate gives reasonableaccuracy, except in extremeapplications

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Establish if the slurry / liquid is a:- Clear liquid

- Settling slurry- Non settling (viscous) slurry

(particle size < 50 micron)


Step 1 - fluid type

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Step 2 - duty details

Establish the duty details which will vary depending on the type of liquidaccording to step 1 (clear liquid, settling slurry or non settling slurry)

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Common duty details are:- Flow or tonnage

- Static lift (head)- Friction losses given or frompipe system known / selected)

- Chemical properties like pH

value, content of chlorides, oil,etc

- Temperature- Other liquid / slurry details


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Step 2 - liquid / slurry details

Clear liquid- For clear water no further fluid details are required

- For other clear liquids the liquid S.G. and the liquid dynamic viscosity (cp)are required

- If the kinematic viscosity (cSt) is given conversion factors can be applied

Slurries

- Numerous additional data is required for slurries, formulae is used to obtainmissing data as required for pump sizing

Non settling slurries- The plastic dynamic viscosity and maximum particle size is required

Settling slurries- The maximum particle size, 80% passing particle size (d80) and the

average particle size (d50) is required

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Step 2 - basic slurry formula

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Sm = Slurry S.G.

S = Solids S.G. Q = m3/h flow rate

TPH = Tonnes per hour

Cv = Concentration by volume (%)

Cw = Concentration by weight (%)


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Step 2 - solids tonnage or slurry flow?

Percent solids by weight is the normal way of explaining a slurry- e.g. Magnetite slurry, 40% solids by weight

This is due to the practice that production in general is measured intonnes (solids) / hour

- e.g. Magnetite feed to the circuit is 300 tonnes / hour as a slurry 40% by weight

These are meaningless figures for a slurry pump salesperson as pumps

are volumetric machines and are sized on the volumetric flow By using the solid S.G. you can calculate a volumetric flow rate

- e.g. Magnetite solids S.G. of 4.6, volumetric flow is 515 m3/h

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Check that the actual velocity in the pipe is greater than the criticalvelocity (refer to the adapted Wilson diagram)

If a pipe diameter has not been specified, select the first pipe sizegiving a velocity above 3 m/s, checking that the actual velocity isgreater than the critical velocity

If the actual velocity is less than the critical velocity the exercise should

be repeated for a pipe of smaller diameter (one size down) to checkthat settling does not take place

Likewise if the actual velocity is greater than the critical velocity theexercise should be repeated for a pipe of larger diameter (one size up)

to minimise friction losses NOTE - Always use the minimum anticipated flow rate to calculate the

pipe velocity


Step 3 - settling slurries only

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Step 3 - minimum settling velocity, Wilson diagram

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Chart for minimum settling velocity (adapted from Wilson, 1976)


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Step 3 - minimum settling velocity, Wilson diagram

Example data

- Pipe diameter = 0.25 m

- Particle size = 0.5mm (worst case)

- Particle S.G. = 3.6

- Minimum velocity = 4.25 m/sec

Minimum velocity to prevent settling is identified by;

1) Drawing a line from 250 mm pipe through 0.5 mm solids to the middle scale

(minimum velocity based on solids S.G. of 2.65)2) Drawing a second line from the middle scale through the actual solids S.G.

(3.6) to the right hand scale and read of the minimum velocity (4.25 m/sec)for the given solids S.G.

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Similar to voltage drop in a power cable, there arefriction losses in a pipe system

The friction losses in a straight pipe vary with- Diameter- Length- Material (roughness)

- Flow rate (velocity) Friction losses can either be looked up in a table,

extracted from a diagram or calculated from formulae

Friction losses are multiplied by the total equivalentpipe length to give a friction head in metres which isadded to the static head


Step 4 - total discharge head

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Additional process equipment needing pressure must also beconsidered, e.g. filter press or hydrocyclones

Pressure needs to be converted to a head in metres (dividing thepressure by the specific gravity of the fluid) and added to the static headand friction head


Step 4 - total discharge head

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Example data

- Volumetric flow = 2000 l/min

- Pipe diameter = 150 mm

Friction losses identified by;

1) Drawing a line up from thevolumetric flow to the pipediameter

2) Draw a second line horizontallyacross to determine the friction

losses (2.2 m/100 m)

3) Draw a third line diagonally todetermine the flow velocity (1.9m/s)


Step 4 - friction losses for clear water, calculation chart

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Step 4 - friction losses for clear water, formula

For water and settling slurries at higher velocities, the following formulamay be used for assessing friction loss in pipe where;

- HF = Metres loss/100 metres- Q = m3/h flow rate- D = Pipe diameter in metres- C = Friction factor

Typical values for C are;- Polyurethane lined and plastic = 150- Seamless steel and spun cast iron = 140

- Spiral welded steel and cast iron = 130- Rippled bore rubber hose = 110

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Step 4 - friction losses for clear water, formula

Example- Q = 120 m3/h (2000 l/min)

- D = 0.15- C = 140

If pipe length is 200m, the friction head is 4.26m which is added to thestatic head

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Correction factor identified by;

1) Drawing a line across from thegiven concentration by volume

(20%)

2) At point of intersection draw asecond line down to determine thefriction loss correction factor (1.15)


Step 4 - friction losses for settling slurries

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When calculating the pipe friction losses for a slurry it is advisable toallow for a certain increase when compared with the losses for clean

water Concentrations of up to 15% by volume behave like water

For higher concentrations friction losses should be corrected by a factor

NOTE - Calculated values must be used for rough estimates only


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Step 4 - friction losses for non settling slurries

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Friction loss assessments for non settling slurriesare best accomplished with the aid of computer

software There are numerous methods of making

assessments manually although these can provedifficult with all the variables

Whatever method is used full rheology of theviscous solution is necessary for any accurateassesment

Assumptions can be made but these can prove tobe very inaccurate


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Step 4 - total equivalent length

When a system includes valvesand fittings, an allowance for

additional friction is needed The most common method is called

the equivalent pipe length

The valve or fitting is treated as a

length of straight pipe giving anequivalent resistance to flow

TEL - Total Equivalent Length- TEL = Straight pipe length + equivalent

length of all pipe fittings

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Select the wet end wear partsmaterial subject to the maximum

particle size

For clear liquids metal pumps arethe first choice

Check available chemicalresistance tables for the selectedwear material


Step 5 - wear material

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The application will dictate if this isa horizontal pump, sump pump,

tank pump or froth pump

It can also be a pump for extreme,heavy or normal wear conditions

Some designs and ranges arealso specific and tailored for agiven industry


Step 6 - selecting the right pump

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Pump performance curves are basedon clear water, corrections arerequired if other liquids or a slurry ispumped

For clear water mark the flow andtotal discharge head on the uppersection of the curve

From this you can estimate therequired pump speed (1880 rpm)

On the lower section of the curve

mark the flow at the running speed toestimate the power absorbed (28 kW)


Step 7 - clear water

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Example data

- Average particle size (d50) = 0.9 mm

- Density of solids = 2.0

- Concentration by weight = 35%

HR/ER identified by;1) Drawing a line up from the average

particle size to the solids density

2) Draw a second line across to theconcentration by weight

3) Draw a third line up to determinethe HR/ER


Step 7 - settling slurries HR/ER

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After establishing the HR/ER, divide thetotal discharge head by the HR factor

Mark the flow and corrected totaldischarge head point on theperformance curve, from this you canestimate the pump speed (1880rpm)

From the flow and running speedestimate the power absorbed requiredfor clear water (28kW)

Multiply the required clear water power

absorbed by the relative density to givethe required slurry power

- Relative density = Slurry density______________ Clear water density


Step 7 - settling slurries HR/ER

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Pump performance is derated in accordance with the AmericanHydraulics Institute (HI) guidelines

Deration for the head, efficiency and flow are calculated from the ratedB.E.P. and not the duty point

For slurry pumps, these deration factors can be taken as being veryconservative as all development work by the HI was undertaken on

process pumps with narrow impellers Divide your duty flow and head with the correction factors and mark

them on the clear water curve to estimate the pump speed

With the corrected flow and the estimated pump speed estimate therequired clear water power

Multiply the required clear water power absorbed by the relative densityto give the required slurry power

- Relative density = Slurry density / clear water density


Step 7 - non settling (viscous) slurries

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It is recommended to add a 15%safety margin to the required powerand select the next available motorsize

- Required power = 18kW- Including safety margin = 20.7kW

- Motor size = 22kW For clear water duties the safety

margin is added to the clear waterrequired power

For slurry duties the safety margin isadded to the slurry required power


Step 8 - motor size

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There are two basic drive designs for slurrypumps

- Indirect drives (v-belt)- Direct drives (coupling)

Select a suitable drive to achieve therequired pump running speed

Typical maximum speed ratios for v-beltdrives is 5:1 with 1500rpm motors and 4:1with 1800rpm motors

For certain applications (varying flowconditions, long pipe lines) variable speeddrives should be used


Step 9 - drive

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An international standard for slurrypumps now effectively exists

Introduced back in 2005 by the HydraulicInstitute / American National StandardsInstitute (HI/ANSI) as guidelines

Today, most manufacturers include some

if not all of the recommendations made Pump sizing recommendations in

particular have been almost universallyadopted - all aimed at providing optimum

rates of wear and longevity in service


HI guidelines

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HI guidelines for duty classification

Class 4- Extremely abrasive

Class 3- Highly abrasive

Class 2- Abrasive

Class 1- Mildly abrasive

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Th f d l f i i


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Operating limits Casing TypeService Class

1 2 3 4

Recommendedpercent range of

BEP flow rate

Annular 20 120% 30 110% 40 100% 50 90%

Semi volute 30 130% 40 120% 50 110% 60 100%

Near volute 50 140% 60 130% 70 120% 80 110%


HI guidelines for optimum wear relative to BEP

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Th f d t l f i i


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HI guidelines for maximum operating values

Adjustments are also made to inlet velocity limit for volume and size of solids

Maximum impeller speed in natural rubber = 28 m/s

Maximum impeller speed in synthetic rubbers and PU = 30 m/s Maximum speed metal impeller in rubber lined pump = 31 m/s

Service Class

1 2 3 4

Maximum head per stage:

meter 123 66 52 40

feet 400 225 160 130

Maximum impeller peripheral speed:

All metal pump

m/s 43 38 33 28

ft/min 8500 7500 6500 5500

Rubber lined pump

m/s 31 28 26 23

ft/min 6000 5500 5000 4500

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The Fundamentals of Pump Sizing Feb 2014

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