process control handbooks of knowledge

8/12/2019 Process Control Handbooks of Knowledge

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Process Controls Handbook of Knowledge

Content Sections

1. Pressures in flow systems2. Cavitation3. Fluid properties4. Pumps for sanitary processes5. Centrifugal pumps6. Sizing centrifugal pumps

7. Rotary-lobe pumps8. Sizing rotary-lobe pumps9. Pump motors10. Pump-shaft seals11. Troubleshooting pumps12. Flow control13. Materials for flow equipment14. Valves for sanitary processes15. Single-seat valves16. Mix-proof valves17. Regulating valves18. Troubleshooting valves19. Valve automation20. Standards and regulations


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1. Why is it important to know about pressures in flow systems? 2

2. Who should know about pressure? 23. What is pressure? 4

4. What is head? 6

5. What types of pressure are there? 8

6. What types of head are there? 10

7. What is a vacuum? 12

8. What is pressure drop? 14

9. What is NPSH? 16

10. What are pressure shocks? 18

Glossary 20

Other handbooks in the series 22

1

Pressures in flow systems

Contents


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Example: Effects of pressure

1. A pump that can deliver a head of 35m (115 ft) will give differentpressures for fluids with different densities.2. Certain processes work under vacuum. This influences selection of

suitable flow equipment and how it should be operated.3. Design of processes and design of flow equipment affect pressure

drops and consequently performance of processes and flowequipment.

4. Design of processes and handling of flow equipment affect risk ofpressure shocks. Pressure shocks affect performance of processesand flow equipment.

5. Pressure changes the behaviour and characteristics of fluids, e.g. thetemperature at which a fluid boils, the viscosity etc.

Closing valve

Flow

Pressure

increase

Pressure

drop

Fig.1.1. Pressure/head Fig.1.2. Vacuum

Fig.1.3. Pressure drop Fig.1.4. Pressure shocks

3


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4

Atmospheric pressure: Varies with location and climatic conditions.

Typical value is 0.95 - 1.05 bar a (13.96 - 15.43 psia).

Standard at sea level is 1.013 bar a, often rounded off to 1 bar a (14.7

psia).

Gauge pressure: Atmospheric pressure is zero reference.

Pressure measured by a gauge in a system, e.g. pressure of fluid.

Pressure that exceeds atmospheric pressure.

Units normally used are barg (bar gauge) or psig (psi gauge).

Absolute pressure: Total pressure, e.g. of a fluid. Absolute pressure = gauge pressure + atmospheric pressure.

Pressure measured relative to perfect vacuum.

Units normally used are bar a (bar absolute) or psia (psi absolute).

3. What is pressure?

Definition: force applied to a unit area of a surfaceSymbol: PNormal unit: bar, psi, Pascal (Pa)

Conversion between different units of pressure/head:

bar kg/cm2 lb/in

2

(psi)atm

(water)ft

(water)m mm Hg in Hg kPa

1.0 1.0197 14.504 0.9869 33.455 10.197 750.06 29.530 100


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Example: Various formulas for pressure

1. Pressure:AFP= (Pa), where F = force (N), A = area of surface (m2)

2. Static pressure: P = x g x h (Pa), where = fluid density (kg/m3)

g = gravity (m/s2)

h = fluid height (m)3. Static pressure: (bar), SG = specific gravity

4. Static pressure:31.2

SG)ft(hP

= (psi), SG = specific gravity

Area

Force

h = HeightP = Static

pressure

= Fluiddensity

g= GravityFig.3.1. Pressure/force Fig.3.2. Gauge pressure

Fig.3.3. Static pressure

5


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General: For a fluid at rest (static) the same vertical height will give the same

pressure regardless of pipe design.

Head is also defined as differential pressure.

Head is affected by different parameters that describe the installationconditions of the process.

Head is often a known value and can be calculated by different formulas ifthe installation conditions are specified.

Formula:

SG

gPH

= , where P = pressure, g = gravity, SG = specific gravity

H (m) = P (bar) x 10

H (ft) = P (psi) x 2.31

4. What is head?

Definition: The height of the surface of a liquid above aspecific point; the pressure of water, caused byheight or velocity, measured in terms of a verticalcolumn of water.

Symbol: H

Normal unit: m or ft

6


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Examples: Pressure and head

1. A pump that can deliver a head of 30m (98 ft) will give differentpressures for fluids with different densities.

2. A pump that can deliver a pressure of 3 bar (44 psi) will give differentheads for fluids with different densities.

3. Water: SG = 1, sugar solution: SG = 1.2, P = 1bar = 14.7psi

Water: m10

1

101)m(H =

=

ft341

31.27.14)ft(H =

=

Sugar: m82.1

101)m(H =

=

ft282.1

31.27.14)ft(H =

=

Conclusion:A pump with constant pressure of 1bar will give different heads fordifferent fluid densities.

Fig.4.1. Head/elevation/density effect on pressure

Fig.4.2. Pressure/density effect on head/elevation

7


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Inlet (suction) pressure: Pressure at which a fluid enters flow equipment.

Readings should be taken close to the equipment while the process andequipment are running.

Units normally used are absolute (bar a or psia) or gauge (barg or psig),depending on inlet conditions.

Outlet (discharge) pressure: Pressure at which a fluid leaves flow equipment.

Readings should be taken close to the equipment while the process andequipment are running.

Unit normally used is gauge (barg or psig).

Differential pressure: Difference between inlet and outlet pressures.

Total pressure reading for flow equipment.

Pdiff= Pout Pin

Pressure against which a pump has to operate.

Power requirements should be calculated based on differential pressure.

Vacuum:

See chapter 7.

General: P = 0 for open tank

P > 0 for pressure

P < 0 for vacuum

5. What types of pressure are there?

There are different types of pressure in a process. They must beidentified to enable correct selection of flow equipment for theprocess.

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Example: Types of pressures calculations

Pin= 1.5 barg = 22 psig (pressure)Pout= 4 barg = 59 psig

Differential pressure (barg) = 4 1.5 = 2.5 bargDifferential pressure (psig) = 59 22 = 37 psig

Pin= 0.5 barg = 7.4 psig (vacuum)Pout= 4 barg = 59 psig

Differential pressure (barg) = 4 0.5 = 3.5 bargDifferential pressure (psig) = 59 7.4 = 51.6 psig

Conclusion:The differential pressure is higher for vacuum (negative inlet pressure)

than for positive inlet pressure.

P>0 P


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Flooded inlet/suction:Positive inlet head where a fluid flows into the inlet of flow equipment.

Static suction head:

Difference in height between fluid level (e.g. in a tank) and centre of inlet offlow equipment.

Static outlet/discharge head:Difference in height between fluid level (e.g. in a tank) and centre of outlet offlow equipment.

Total static head:Difference in height between static inlet head and static outlet head.

Friction head:Pressure drops in inlet/outlet lines due to frictional losses in flow.

Total suction head: Static suction head less dynamic head.

Negative static head indicates fluid level below centre of flow equipment(suction lift).

Total discharge head:Sum of static discharge and dynamic heads.

Total head: Total pressure difference between total discharge and suction heads.

The head against which a pump has to operate.

6. What types of head are there?

There are different types of head in a process. They must beidentified to enable correct selection of flow equipment for theprocess.

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Examples: Types of head calculations

Suction line: hs= -2.5m = - 8.2 ft (lift)hfs= 0.5m = 1.6 ftPs= 0 (open tank)

Discharge line: ht= 20m = 65.6 fthft= 1.5m = 4.9 ftPt= 10m = 32.8 ft (pressure)

Discharge:Ht (m) = ht + hft+ Pt = 20 + 1.5 + 10 = 31.5 mHt(ft) = ht + hft+ Pt = 65.6 + 4.9 + 32.8 = 103.3 ft

Suction:Hs (m) = hs - hfs+ Ps = -2.5 - 0.5 + 0 = -2.5 mHs(ft) = hs - hfs+ Ps = -8.2 - 1.6 + 0 = -9.8 ft

Total:

H (m) = Ht - Hs= 31.5 - (-2.5) = 29 m

H (ft) = Ht - Hs=103.3 - (-9.8) = 113.1 ft

Fig.6.1. Flooded inlet,closed tanks

Fig.6.2. Suction lift,open tanks

H = Total headHs= Total suction

headHt= Total dischargehs= Static suction

headht= Static discharge

headhfs= Pressure drop in

suction linehft= Pressure drop in

discharge line

Ps= Vacuum/pressurein tank/suction sidePt = Pressure in tank

Total head H: H = Ht (Hs)Total discharge head Ht: Ht= ht+ hft+ PtTotal suction head Hs: Hs = hs- hfs+ (Ps)Friction head H: H = hft+ hfs

hs>0 for flooded inlet, hs0 for pressure, P


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Connection/relation:

0 psia = 760 mm Hg (29.9 in Hg)

14.7 psia = 0 mm Hg (0 in Hg)

7. What is a vacuum?

Definition: Pressure below atmospheric pressure in a flowsystem.Symbol: PNormal unit: Hg or psia

Vacuum is the difference between measured pressure and

atmospheric pressure, if this difference is negative.

Note!It is very important to know how vacuum will affect the fluids andflow equipment to be used, so that flow equipment can be selected

and handled correctly.

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Examples: Effect of vacuum

1. Vacuum may be used in processes to minimise energy consumption,as fluids boil at lower temperatures under lower system pressures.

2. The combination of vacuum and high temperature will give a lowavailable inlet/suction pressure. This will increase the risk ofcavitation before and inside flow equipment. This may damage theflow equipment.

3. Vacuum, e.g. in a pump, may increase the risk of contamination of

fluids from the atmosphere. A water-flushed shaft seal as a barrier isa suitable solution.

Vacuum and high temperatureRisk of cavitation!Pin < Pvp

Fluid (liquid form)

Pvp

Fig.7.1. Vacuum in suction line Fig.7.2. Vapour pressure

Fig.7.3. Flushed shaft seal/barrier (principle)

AtmospherePumped

media

Seal face

area

Flush

barrier

Shaft with

impeller

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General: Suppliers of flow equipment usually have pressure data available as

diagrams or calculations.

In practise, friction losses and total pressure drop may be determined byconverting losses into equivalent tube length.

Pressure-drop for water-like fluids may be calculated according to apressure-drop curve.

Friction loss: Depends on the flow type in the flow system.

Can be calculated for laminar flow.

As turbulent flow is typical in flow systems, friction losses and pressure

dropsare normally calculated by computer programs.

Correction factors for higher viscosity fluids:

Viscosity (cP) 1 - 100 101 - 2000 2001 - 20000 20001 - 100000

Correctionfactor

1.0 0.75 0.5 0.25

Installation conditions:Installations should be done correctly to ensure:

Pressure drops are minimised.

Running conditions are correct (no air pockets etc). Cavitation is avoided.

Gentle treatment of fluids.

8. What is pressure drop?

Definition: Loss of pressure due to friction in piping and flowequipment.

Symbol: H or P ( = delta = difference)Normal unit: m, ft, bar or psi

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Examples: Pressure drop/friction loss

Tubing and equipment: 51mm (2in)Pumping water from tank A to Tank EPump capacity: 10 m

3/h (44 US gal/min)

Equivalent tube lengths for equipment are estimated (data from supplier)

Total pressure drop in system:

100

434)m(H

= = 1.4 m (10m

3/h From curve: H = 4m/100m tube)

328

13112)ft(H

= = 4.4 ft (44 gpm From curve: H = 13 ft/328 ft tube)

For sugar solution with viscosity of 5000 cP:Correction factor: 0.5 (see table)

H = 1.4m x 0.5 = 4.4 ft x 0.5 = 0.7m = 2.2 ft

H = 0.7m = 2.2 ft should be added to the straight piping given (which is10m = 34 ft).

Equipment m ftA outlet 1 3

A-E tube 10 33A-D bends 3 10C check valve 10 33D seat valve 10 33Total 34 112

10m /h44gpm

H=4mH=13ft

51mm

Fig.8.1. Pressure drop in51mm 2 s stem

Fig.8.2. Estimated equivalent tubelength (from supplier)

Fig.8.3. Pressure drop curve 100m (328ft) ISO/DIN tube

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NPSHr: Required minimum pressure that must be present in the suction line to

avoid cavitation.

Result of equipment design (e.g. pumps). Should be as low as possible.

Normally specified in pump flow charts.

Increases with increasing capacity.

NPSHa: Available suction pressure at actual process conditions.

Should be as high as possible.

Result of suction conditions can be calculated.

Formula:

Where:Pa= Pressure absolute above fluid level (bar a or psia)

hs= Static suction head (m or ft)hfs= Pressure drop in suction line (m or ft)Pvp= Vapour pressure (bar a or psia)

9. What is NPSH?

Definition: NPSH (Net Positive Suction Head) is the suctionpressure required (NPSHr) or available (NPSHa) in thesuction line of a process.

Normal unit: m or ft

Pressureon fluid (Pa)

Static head(hs)

-Pressuredrop (hfs)

+ -

-Vapourpressure (Pvp)

NPSHa=

NPSHa= Pa hs hfs Pvp

Fig.9.1. Calculation of NPSHa

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Examples: NPSH calculations/cavitation check

Process:

Water at 45 C (113 F)Pa= 1 bar a = 10m = 33 ft (open tank)hs= - 3m = - 10 ft (lift)hfs= 1.5m = 5 ftPvp= 0.096 bar a = 1m = 3.3 ft

Pump selected: NPSHr= 3.5m = 11.5 ft

NPSHa(m) = 10-3-1.5-1 = 4.5m > 3.5m No cavitation!

NPSHa(ft) = 33-10-5-3.3 = 14.7 ft > 11.5 ft No cavitation!

How to avoid cavitation:Reduce pressure drops in suction line.Keep static suction head as high as possible.Keep fluid temperature as low as possible.Reduce capacity in process.

Fig.9.2. NPSHa

NPSHa > NPSHr No cavitation!

General:

Pa= 1 for open tank

Pa> 1 for pressure

Pa< 1 for vacuum hs> 0 for flooded inlet

hs< 0 for suction lift

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Changes in fluid velocity are caused by:

Opening and closing of valves.

Start and stop of pumps. Resistance in flow equipment.

Changes in tube dimensions.

Changes in flow direction.

Effects of pressure shocks: Noise in tubing.

Damaged tubing.

Damaged flow equipment.

Cavitation.

How to avoid pressure shocks: Correct flow direction through seat valves.

Damping movement of valve plug.

Speed control of pumps.

10. What are pressure shocks?

Definition: The results of change in fluid velocity, especially inlong tubing systems.

Symbol: H or PNormal unit: bar, psi, m or ft

Most pressure shocks in flow systems are caused by rapidly closedor opened valves. Rapid starting and stopping of pumps also cause

some problems.

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Examples: Pressure shocks

1. Rapid closing/opening of valves can cause pressure shocks up to 50-60 bar (735-880 psi), depending on process conditions.

2. Water hammer from seat valves can be avoided or reduced by: Correct flow direction through the valve. Damping movement of valve plug.

3. Pressure shocks from pumps can be avoided by slowly stopping orstarting the pumps.

4. The pump/motor control can be done by a soft starter or by a

frequency inverter.

Correct

flow!

Closing valve

Flow

Pressureincrease

Pressuredrop

v

v

Flow

Damper

Valve plug

Fig.10.1. Closing valve Fig.10.2. Change in tube dimensionand direction

Fig.10.3. Correct flow direction Fig.10.4. Damping of valvemovement

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Absolute pressure Total pressure, e.g. of a fluid.

Atmospheric Surrounding (ambient) pressure, normally specifiedpressure as 1bar (14.7psi) at sea level.

Cavitation The formation and collapse of regions of lowpressure in a flowing liquid. Occurs when fluidpressure in flow equipment locally drops belowthe vapour pressure of the fluid at the current

temperature.

Density Mass per unit of volume of a fluid.

Flow equipment Equipment used in flow systems. Examples are heatexchangers, pumps, valves, tubing, fittings and tankparts.

Fluid Liquids/media (non-solid and non-gas) processed

in flow systems.

Flooded inlet Positive inlet pressure/head.

Flushed shaft seal Pump shaft seal which is cleaned and/or cooled, bye.g. water.

Gauge pressure Pressure measured by a gauge in a process.

Head Total vertical height which a fluid is lifted. Alsospecified as elevation.

Dynamic Energy required to keep a fluid in motion.Friction Pressure drop in inlet/outlet lines.Static outlet Difference in height between fluid/tank level

(discharge side) and centre, e.g. of a pump.

Glossary

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Static suction Difference in height between fluid/tank level (suction

side) and centre, e.g. of a pump.Total head Total pressure difference between total discharge

and suction heads.Total discharge Sum of static discharge and dynamic heads.Total static Difference in height between static inlet and outlet

head.Total suction Static suction head less dynamic head.

Laminar flow Flow behaviour which appears at relatively lowvelocity and/or relatively high viscosity.

NPSH Net Positive Suction Head.

NPSHr Required minimum pressure in suction line to avoidcavitation.

NPSHa Available suction pressure under the processconditions in question.

Pressure Force per unit area.Differential Difference between inlet and outlet pressures.Inlet Pressure at which a fluid enters flow equipment.Losses Result of frictional losses in piping and flow

equipment.Outlet Pressure at which a fluid leaves flow equipment.Shock Results of change in fluid velocity.

Specific gravity Ratio of a fluids density to the density of water.

Suction lift Negative inlet head/pressure.

Turbulent flow Most common flow type in practise due to relative

high process velocities.

Vacuum Pressure below atmospheric pressure.

Vapour pressure Minimum required external pressure to preventvaporisation of a fluid.

Water hammer Banging noise resulting from a valve plughammering against a valve seat.

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1. Why is it important to know about cavitation? 2

2. Who should know about cavitation? 43. What is cavitation? 54. What is NPSH? 75. Cavitation in theory 96. Cavitation in practise 11

7. Short-term effects of cavitation 138. Long-term effects of cavitation 159. How to stop cavitation once it has occurred 1710. How to avoid cavitation when designing processes 19

Glossary 21

Other handbooks in this series 23

Contents

Cavitation

1


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Understanding aspects of cavitation makes it easier to optimiseprocess/system designs and to select and handle flow equipment correctly.

1. Why is important to know about cavitation?

Cavitation in processes affects the performance of flow equipmentand may damage it. Cavitation should always be avoided.

2


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Examples: Effects of cavitation

1. It is very important to have correct inlet conditions (suction head,pressure drops, fluid temperature etc.) to avoid cavitation.

2. Cavitation may reduce pump performance to nearly zero.3. Cavitation may cause noise and vibration in the system and it may

damage flow equipment in the long term.4. Correct process design and correct selection of flow equipment can

limit the risk of cavitation.

Fig.1.1. CavitationFig.1.2. Reduced pump

performance

Fig.1.3. Damaged steel part Fig.1.4. Inlet conditions/NPSHa

Q

HPump curve

During

cavitation

Pvp

Pvp= Vapour pressure

Plocal= Local suction pressure

Plocal

Flow equipment

Fluid (liquid form)

Source: Burgmann

3


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Knowing the effects of cavitation, how to identify and how to preventcavitation helps to ensure that processes are optimised and that errors,damage and personal injuries are avoided. The target groups may include:

1. Process designers, who should design inlet conditions so that the

available suction pressure is as high as possible to minimise the risk ofcavitation.

2. Pump designers, who should optimise pump inlet designs to minimiseNPSH and the risk of cavitation.

3. Field operators, who should be able to identify cavitation and know howto change operating conditions to prevent cavitation.

4. Beginners in the flow industry, who should have a basic understandingto work efficiently.

2. Who should know about cavitation?

All people who work with flow equipment during its life cycle shouldknow about cavitation in flow systems.

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Cavitation is caused by difficult suction conditions.

Pressure drop/decrease: The pressure at inlet of flow equipment locally drops, e.g. if a valve is

throttled in the suction line.

The local pressure is now below the vapour pressure. The fluid boils (evaporates) and generates small vapour bubbles.

Temperature increase: The vapour pressure increases above the local suction/inlet pressure.

The fluid boils (evaporates) and generates small vapour bubbles.

Cavitation occurs:

The vapour bubbles are carried along with the fluid and collapse(implode) instantly when they enter areas of higher pressure, e.g.a pump casing.

This collapsing of vapour bubbles is registered as noise. Cavitation has

occured.

3. What is cavitation?

Cavitation occurs when the pressure in flow systems and flowequipment locally drops below the vapour pressure of the fluid atthe current temperature.

5

Fig.3.1. Vapour bubbles collapseand cavitation occurs


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Examples of causes of cavitation

1. Pressure drop/decrease in suction/inlet line:Local inlet pressure will drop below vapour pressure.

2. Increase of fluid temperature:Vapour pressure will increase above local suction/inlet pressure.

3. Suction lift or vacuum:This will reduce available suction/inlet pressure, maybe to belowthe vapour pressure.

4. Too high capacity in system:

The flow equipment requires higher suction/inlet pressure (NPSHr)than is currently present in the suction/inlet line (Plocal).

Fig.3.2. Effect of valve throttling

Fig.3.4. Effect of suction lift orvacuum

Q

NPSHr

NPSHr> Plocal

Cavitation risk !

Fig.3.3. Effect of temperatureincrease

Fig.3.5. NPSHr

6


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NPSHr: Required minimum pressure that must be present in the suction line to

prevent cavitation.

Result of equipment design (e.g. pumps).

Should be as low as possible. Normally specified in pump flow charts.

Increases with increasing capacity.

NPSHa: Available suction pressure at current process conditions.

Should be as high as possible.

Result of suction conditions - can be calculated.

Formula:

Where:Pa= Pressure absolute above fluid level (bar a or psia)hs= Static suction head (m or ft)hfs= Pressure drop in suction line (m or ft)Pvp= Vapour pressure (bar a or psia)

4. What is NPSH?

NPSH (Net Positive Suction Head) specifies the suction pressurerequired (NPSHr) or available (NPSHa) in the suction line of aprocess. Normal unit: m or ft

Pressureon fluid (Pa)

Static head(hs)

-Pressuredrop (hfs)

+ -

-Vapourpressure (Pvp)

NPSHa=

NPSHa= Pa hs hfs Pvp

Fig.4.1. Calculation of NPSHa

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Examples: NPSH calculations/cavitation check

Process:

Water at 45 C (113 F)Pa= 1 bar a = 10m = 33ft (open tank)hs= - 3m = - 10ft (lift)

hfs= 1.5m = 5ftPvp= 0.096 bar a = 1m = 3.3ft

Pump selected: NPSHr= 3.5m = 11.5ft

NPSHa(m) = 10-3-1.5-1 = 4.5m > 3.5mNo cavitation!

NPSHa(ft) = 33-10-5-3.3 = 14.7ft > 11.5ft No cavitation!

Fig.4.2. NPSHa


General:

Pa= 1 for open tank

Pa> 1 for pressure

Pa< 1 for vacuum

hs> 0 for flooded inlet hs< 0 for suction lift

8


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9

5. Cavitation in theory

Cavitation check:NPSHa > NPSHr No cavitation!

Cavitation can be identified in theory by calculations. Cavitationwill occur if the available suction/inlet pressure is lower than thepressure required from flow equipment in the flow system.

In other words:

General:The available suction/inlet pressure (NPSHa) should always be identified ifthere is indication of difficult suction/inlet conditions or other critical processconditions.


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10

Examples of identification of cavitation calculations

Process:

Sugar solution at 50 C (122 F)Pa= 1 bar a = 10m = 33ft (open tank)hs= 1.5m = 5fthfs= 9m = 30ftPvp= 0 (assumed)

Pump selected: NPSHr= 3.5m = 11.5ft

NPSHa(m) = 10+1.5-9-0 = 2.5m < 3.5mCavitation will occur!

NPSHa(ft) = 33+5-30-0 = 8ft < 11.5ftCavitation will occur!

Fig.5. Cavitation check


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Cavitation can be identified in practise as:

Noisy operation.

Vibration in system and flow equipment.

Damage of processed fluid. Reduced or no performance of flow equipment.

Visual leakages in flow equipment.

Visual mechanical damage in flow equipment, including corrosion anderosion.

6. Cavitation in practise

Cavitation will affect performance of the process and the flowequipment. Cavitation may damage the fluid and the flowequipment.

Note!The first and most common signs of cavitation are noise, vibration

and reduced performance of flow equipment.

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Q

HNormal pump

curve

Reduced

capacity!

Vibration!

Source: Burgmann

12

Fig.6.1. Noise Fig.6.2. Vibration

Fig.6.3. Reduced pump capacity Fig.6.4. Leaking seal (principle)

Fig.6.5. Damaged steel parts


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13

Noise and vibration in system: Normally easy to hear, feel and see.

Normally a good indication of cavitation if the noise and vibration can beremoved, e.g. by reducing capacity in the system.

Reduced performance of centrifugal pump: Cavitation is defined as a pressure reduction of 3% of normal head.

Cavitation may be identified by the pump flow chart.

Current system pressure and capacity should be measured andcompared with the pump flow chart.

Cavitation has occured if the head has been reduced by more than 3%.

7. Short-term effects of cavitation

The short-term effects of cavitation are noise, vibration and reducedperformance in the system and in flow equipment.


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Fig.7.1. Noise

Fig.7.2. Vibration

Fig.7.3. Reduced pump performance

Vibration!

Q

H

Normal pump

curve

During

cavitation

Cavitation

point!

3%of H

Pay attentiontoexcessive noise!

Pay attention to

excessive vibration!

Pay attention topump performance!

14


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15

Leaking seals:May be caused by vibration or beginning material/seal surface damage.

Damaged flow equipment:

Cavitation will cause mechanical damage as well as increased localcorrosion and erosion.

Cavitation will actually eat away the material. The first sign will be anuneven surface finish.

The material will disappear into the processed fluid.

Damaged processed fluids:Shear sensitive fluids or similar may be damaged by the vapour bubbles andthe collapsing of the bubbles (very high local pressure).

8. Long-term effects of cavitation

The long-term effects of cavitation are leaking flow equipment,damage to system and flow equipment and damage to processedfluid.


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Pay attention toleaking equipment!

Pay attention todamaged parts!

Source: Burgmann

16

Fig.8.1. Leaking seal (principle)

Fig.8.2. Damaged steel part


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17

Parameters involved:

NPSHa: NPSHa= Pahs hfs PvpStatic head, pressure drops, temperature

NPSHr: System capacity

Static head:If possible, maximise the static head, either by moving a pump to a lowerpoint or by increasing the fluid level in the tank.

Pressure drops:If possible, open any valves in the suction line to minimise pressure drops.

Temperature:If possible, reduce the fluid temperature so that the vapour pressure will bebelow the available suction pressure.

System capacity:

If possible, reduce the system capacity so that the required suction pressure(NPSHr) is lower than the available suction pressure (NPSHa).

This should stop cavitation and minimise risk of future cavitation.

9. How to stop cavitation once it has occurred

Cavitation can be removed by optimising the parameters thatadversely affect the suction conditions and the requirements of theflow equipment.



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Fig.9.1. Maximise static head Fig.9.2. Open valves in suctionline

Fig.9.3. Reduce fluidtemperature Fig.9.4. Reduce capacity insystem

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Fig.10.1. Max. static head Fig.10.2. Few components

Fig.10.3. Large diameter Fig.10.4. Large bend radii

Fig.10.5. Correct routing Fig.10.6. Vacuum and temperature

20

Fig.10.7. Minimise NPSHr


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Capacity The maximum volume of fluid that can pass acertain area per unit of time.

Cavitation The formation and collapse of regions of lowpressure in a flowing liquid. Occurs when fluidpressure in flow equipment locally drops belowthe vapour pressure of the fluid at the current

temperature.

Flooded inlet Positive inlet pressure/head.


Fluid Liquids/media (non-solid and non-gas) processed in

flow systems.


Inlet/suction Pressure at which a fluid enters flow equipment.



NPSHa Available suction pressure at current processconditions.

Pressure Force per unit area.

Pressure drop Result of frictional losses in piping and flow

equipment.

Glossary

21


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Pressure drop Result of frictional losses in piping and flowequipment.

Shearing Motion between fluid layers in laminar flow.

Static suction Difference in height between fluid/tank level (suctionside) and center, e.g. of a pump.

Suction lift Negative inlet head/pressure.

Vacuum Pressure below atmospheric pressure.


22


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1. What is rheology? 2

2. Who should know about rheology? 3

3. Viscosity 4

4. Density 6

5. Specific gravity 8

6. The effects of temperature 9

7. Other fluid data 11

8. Types of flow 12

9. Vapour pressure 14

10. Particles in fluids 15

Glossary 17Other handbooks in this series 19

Contents

1

Fluid properties


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Understanding the various aspects and principles of rheology makes iteasier to select and handle flow equipment correctly for various processconditions.

1. What is rheology?

Rheology is the science of fluid properties.

2

Examples of the effects of rheology

1. Change in shear rate from flow equipment may change the viscosityof a fluid and therefore change the performance, e.g. of a pump or avalve.

2. Incorrect sizing of tubing will affect the system flow characteristicsand therefore affect pressure losses in the flow system.

Fig.1.1. Shear rate/viscosity Fig.1.2. Temperature

Fig.1.3. Flow type Fig.1.4. Vapour pressure

Fluid (liquid form)

Pvp


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3

Knowing about the relationship between fluid behaviour and the flowequipment in the process one is working with helps to ensure that processesare optimised and that errors, damage and injuries are avoided. The targetgroups may include:

1. Process designers, who should know how the fluids will affect the flowequipment to be used and how the equipment will affect the fluids.

2. Sales people, who should know possibilities and limitations of flowequipment related to the fluids to be processed.

3. Beginners in the flow industry, who should have a basic understanding

to work efficiently.

2. Who should know about rheology?


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Relation between absolute and kinematic viscosities:

, where = fluid density (see chapter 4)

General: Viscosity generally decreases with increasing temperature.

Viscosity is normally lower in flow systems than specified fromviscometer readings (disc type viscometers).

3. Viscosity

The viscosity of a fluid specifies how thick or thin it is, i.e. itsflow properties.

Absolute (dynamic) viscosity is often used when specifying a fluid.

Symbol:Unit normally used: Centipoise (cP or mPas).

Kinematic viscosity is often used when selecting flow equipment.

Symbol:Unit normally used: Centistoke (cSt).

Temperature

Viscosity

Fig.3.1. Viscosity/temperature Fig.3.2. Viscosity/shear rate

4

Shear rate

Viscosity

Viscometer

readingActual inflow system


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5

Examples of fluid behaviour types

Newtonian: Viscosity constant regardless of shear rate.Typical fluids: Water, beer, milk, mineral oils, syrups.

Pseudoplastic: Viscosity decreases with increasing shear rate.Typical fluids: Blood, lotions, soap, toothpaste, yeast.

Dilatant: Viscosity increases with increasing shear rate.

Typical fluids: Clay slurries, paper coatings.

Thixotropic: Viscosity decreases with time under shear. It returns tooriginal value after shear stop.Typical fluids: Creams, greases, yoghurt.

Anti-thixotropic:Opposite behaviour to thixotropic.Typical fluids: Vanadium pentoxide solution.

Rheomalactic: Viscosity decreases with time, does not recover.Typical fluids: Natural rubber latex, natural yoghurt.

Plastic: Need certain stress to overcome solid-like structure.Typical fluids: Chocolate, tomato ketchup.

Fig.3.3. Newtonian Fig.3.5. DilatantFig.3.4. Pseudoplastic

Fig.3.6. Thixotropic Fig.3.8. RheomalacticFig.3.7. Anti-thixotropicc

Fig.3.9. Plastic


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General: Density in generally regarded constant in fluids.

Density in gases varies with pressure and temperature.

Effect on power: required power increases proportionally to density.

Pfluid= Pwaterx fluid/ water

4. Density

Density of a fluid specifies the mass per unit of volume.Symbol: Unit normally used: kg/m

3or lb/ft

3

6


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7

Examples of density of fluids

1. 1 m3of water has a mass of 1000 kg.

This means that the density is 1000 kg/m3.

1 ft3of water has a mass of 62.4 lb.

This means that the density is 62.4 lb/ft3.

2. 1 m3of alcohol has a mass of 789 kg.

This means that the density is 789 kg/m3.

1 ft3of alcohol has a mass of 49.2 lb.

This means that the density is 49.2 lb/ft3.

Fig.4.1. Density of water

Fig.4.2. Density of alcohol


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In other words:

waterofDensity

fluidofDensitySG =

5. Specific gravity

The specific gravity (SG) of a fluid is the ratio of its density to thedensity of water.Specific gravity has no unit.

8

Examples of specific gravity of fluids

Density:Water: 1000 kg/m

3or 62.4 lb/ft

3

Alcohol: 789 kg/m3or 49.2 lb/ft

3

Specific gravity (SG):

Water: 110001000SG ==

Water: 14.62

4.62SG ==

Alcohol: 789.01000

789SG ==

Alcohol: 789.04.62

2.49SG ==

Fig.5. Specific gravity


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9

Relationship between C and F:C = (F 32) x 0.5556

F = (C x 1.8) + 32

It is very important to consider the temperature of a fluid when selecting floequipment, as:

Vapour pressure increases with increasing fluid temperature (seechapter 9);

Viscosity decreases with increasing fluid temperature (see chapter 3);

Fluid temperature has a great effect on the selection of suitableelastomer grades.

6. The effects of temperature

Temperature specifies internal energy in a fluid.Symbol: T

Unit normally used: C or F


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10

Examples of effects of temperature

1. Increased vapour pressure due to temperature increase will reduce

the system inlet pressure, e.g. before pumps and valves. This mayresult in cavitation, reduced system performance and damage ofequipment.

2. A viscosity change due to a temperature change will changeperformance, e.g. of valves and pumps, as they were originallyselected for another viscosity value.

3. The elastomer grade NBR has a temperature limit of approx. 100 C

(212 F). Another elastomer grade should be selected for highertemperatures.

Fig.6.1. Temperature effect on vapour pressure

Fig.6.2. Temperature effect on viscosity

Fluid (liquid form)

Pvp

T Pvp

T Viscosity

(See also chapter 9)

(See also chapter 3)


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7. Other fluid data

There are other important fluid data that must be considered whendealing with flow equipment.

Examples of considerations based on other fluid data

1. Yoghurt is shear-sensitive and should be pumped with a low-speedpump.

2. Too high concentrations, e.g. of lye or acid, may damage elastomersand stainless steel. Suitable equipment materials should be used.

3. Certain pharmaceutical fluids may be damaged due to heat-up in apump. A pump cooling system can solve this.

4. Some cleaning agents become more aggressive with increasingtemperature.

Fig.7.1. Fluid type Fig.7.2. Concentration

Fig.7.3. Combination of fluid type, concentration and temperature

??

Identify fluid!

Pay attention toconcentrations!

??

+ +

11

These data are:

Type of fluid

Concentration

Combination of fluid temperature and type

Combination of fluid temperature ands concentration


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Fluid can flow in two different ways.

Laminar flow:Normally appears at fluids with viscosity >5 mPas (cP).

Turbulent flow:Normally appears at fluids with viscosity 4000 : Turbulent flow

12


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13

Fig.8.1. Laminar flow Fig.8.2. Turbulent flow

Examples of flow types formulas for Re

1. , where

D = tube diameter (mm), V = fluid velocity (m/s),

= density (kg/m3), = absolute viscosity (cP)

2. , where

D = tube diameter (mm), Q = Capacity (l/min),= absolute viscosity (cP)

3. , where

D = tube diameter (in), Q = Capacity (US gal/min),

= kinematic viscosity (cSt)

4. , where

D = tube diameter (in), Q = Capacity (UK gal/min),

= kinematic viscosity (cSt)


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In other words:

General for Pvp: Depends upon the type of fluid.

Increases at higher temperature.

Is very important to inlet conditions of flow equipment.

Should be determined from relevant fluid tables.

9. Vapour pressure

Vapour pressure is the minimum required external pressure toprevent boiling of a fluid at a given temperature.

Fluid is maintained in liquid form by means of the external pressure,which is absolute pressure.Symbol: Pvp

Unit normally used: KPa (a), bar (a) or psia

14

Examples: Vapour pressure

Water will boil at temperatures of:

0 C (32 F) if Pvp= 0.006 bar a (0.087 psia)



Fluid (liquid form)

Pvp

Fig.9. Vapour pressure


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15

Concentration of particles in fluids is specified as:

Percentage by weight (W/W) or

Percentage by volume (V/V) or

Combination of weight and volume (W/V)

Particles in fluids may influence: Selection of design and materials for flow equipment.

Selection of suitable types of valves, pumps etc.

Selection of suitable pump shaft seals.

Operation of flow equipment.

Lifetime and service of flow equipment.

10. Particles in fluids

Particles in fluids and size and concentration of particles affectselection, function and performance of flow equipment.


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16

Fig.10.1. Shutter valve (top viewprinciple)

Fig.10.2. Rotary-lobe pump (frontview principle)

Fig.10.3. Water-flushed shaft seal (principle)

Valve bodyParticles

Rotatingshutter

Fruit partsPumprotor

Flushing outletwith particles

Seal face areawith particles

Pumped mediawith particles

Shaft withimpeller

Examples of particles in fluids

1. A seat valve may not be able to seal correctly against the valve seatdue to particles. A shutter valve with a scraping plug may be a goodalternative.

2. Yoghurt with fruit parts requires a slow-speed pump that treats thefruit parts gently so that they are not destroyed. A rotary-lobe pump isa suitable solution.

3. Particles may cause leakage in a pump shaft seal. A water-flushedshaft seal is a suitable solution.


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17


Cavitation The formation and collapse of regions of lowpressure in a flowing liquid. Occurs when fluidpressure in flow equipment locally drops below thevapour pressure of the fluid at current temperature.

Density Mass per unit of volume of a fluid.

Elastomer Non-metallic sealing part with elastic properties, e.g.natural or synthetic rubber.

Flow characteristics Behaviour of fluids that flow through piping and flowequipment.


Fluid Liquids/media (non-solid and non-gas) processed inflow systems.

Flushed shaft seal Pump shaft seal which is cleaned and/or cooled byeg. water.

Inlet pressure Fluid pressure entering the pump inlet. Will affectthe seal faces.

Laminar flow Non-turbulent flow in which parallel layers havedifferent velocities relative to each other. Appears atrelatively low velocity and/or relatively high viscosity.

Pressure losses Result of frictional losses in piping and flow

equipment.

Reynolds number Calculated value to determine flow type (laminar,

turbulent or transitional). Expressed as Re.

Glossary


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18

Rheology The science of fluid flow.

Rotary-lobe pump Type of positive-displacement pump.


Specific gravity Ratio of a fluids density to the density of water.

Transitional flow Combination of laminar and turbulent flowbehaviour.

Turbulent flow Flow in which the velocity varies rapidly and in anirregular way. The most common flow type in

practise due to relatively high process velocities.


Viscosity Specifies how thick or thin a fluid is.

Absolute (dynamic) Generally used when specifying a fluid.

Anti-thixotropic Viscosity has opposite behaviour to thixotropic.

Dilatant Viscosity increases with increasing shear rate.

Kinematic Generally used when selecting flow equipment.

Newtonian Viscosity constant regardless of shear rate.

Plastic Needs certain stress to overcome solid-likestructure.

Pseudoplastic Viscosity decreases with increasing shear rate.

Rheomalactic Viscosity decreases with time, does not recover.

Thixotropic Viscosity decreases with time under shear. It returnsto original value after shear stop.


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1. Why is it important to know about sanitary pumps? 2

2. Who should know about sanitary pumps? 4

3. What is a sanitary pump? 5

4. What is a centrifugal pump? 7

5. What is a liquid-ring pump? 9

6. What is a rotary-lobe pump? 11

7. What is a screw pump? 13

8. What are piston and diaphragm pumps? 15

9. Pump selection 17

10. How are pumps controlled? 19

Glossary 21Other handbooks in this series 23

1

Pumps for sanitary processes

Contents


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Understanding aspects of sanitary pumps makes it easier to select pumps

correctly for various sanitary process conditions.

A sanitary pump has a design and material usage that fulfils givenstandards and accepted practises within the food industry.

Many different types are available.

2

1. Why is it important to know aboutsanitary pumps?


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Examples of sanitary pumps

1. A pump is used to transfer fluids and/or gases from one place toanother within a given time at a certain pressure to overcome systemlosses.

2. A centrifugal pump is the most common sanitary pump type, thanksto its relatively simple design and low price.

3. A pump can be selected from a pump curve that is based onperformance tests of the pump.

4. A pump can be controlled in various ways. Speed control by means

of a frequency inverter is a common and economical solution.

Fig.1.1. Pump process(principle)

Fig.1.2. Centrifugal pump(principle)

Fig.1.3. Centrifugal pumpcurve (principle)

Fig.1.4. Pump control(principle)

3

Q

H

Pump curve

Process curve

D1D2D3

QD

HD

FlowmeterFrequency

inverter


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Knowing about the various aspects of sanitary pumps e.g. principle anddesign, available types, suitability and limitations related to given processes helps to ensure that processes are optimised and that errors, damagesand personal injuries are avoided. The target group may include:

1. Process designers, who should know what pump types andconfigurations to select so that they will operate correctly in the process.

2. Sales and sales support people, who should know possibilities andlimitations of various pumps related to applications/processes inquestion.

3. Field operators, who should know possible causes of pump malfunctionand know how to solve them.

4. Beginners in the flow industry, who should have a basic understanding


2. Who should know about sanitary pumps?

All people who work with flow equipment during its life cycle shouldknow about sanitary pumps.

4


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Pump function: A pump transfers fluids and/or gases from one place to another within a

given time. Pump capacity is typically specified as m3/h or GPM.

A pump transfers fluids and/or gases from one place to another at acertain pressure to overcome losses in the system (piping, valves etc).

The typical unit of pressure is bar or psi.

Typical pump types:Pumps are generally split into two main categories: rotodynamic pumps andpositive displacement pumps. Typical sanitary pump types are:

Single-stage and multi-stage centrifugal pumps (rotodynamic).

Liquid-ring pump (rotodynamic)

Rotary-lobe pump (positive displacement) Screw pump (positive displacement)

Piston pump (positive displacement)

Diaphragm pump (positive displacement)

Pump selection: The pump type and configuration to select for a given process very

much depends on the type of application (fluid type, viscosity,temperature, solids, pressure, vacuum etc) and on customer preference.

A pump is typically selected (sized) from a pump curve or from a PCselection program.

A pump is typically configured (materials, shaft seal, surface finish,connections, motor, options etc) based on experience data or customer

preference.

3. What is a sanitary pump?

Sanitary pumps are used in almost any sanitary processes totransfer quantities of fluids and/or gases from one place to another.

5


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6

Fig.3.1. Pump function/principle Fig.3.2. System losses (principle)

Fig.3.3. Sanitary pump types (principle)

Size/configure pump!Customer demands?

Process type?Fluid data?

Performance data?

Multi-stage

Pumps

Rotodynamic Positive displacement

Single-stage ReciprocatingRotor

Typical sanitary pumps

Centrifugal

Liquid-ring

Centrifugal Rotary lobe

Screw

Piston

Diaphragm

Fig.3.4. Pump selection process

(principle)


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7

Principle/design: Fluid enters the pump casing and impeller center and is forced into a

circular movement by the impeller vanes and centrifugal force. The fluidthus leaves the casing with increased pressure and velocity.

The pump casing should be filled with fluid to ensure correct pumpoperation.

A standard centrifugal pump typically consists of casing, impeller, shaft,shaft seal, motor adapter and motor.

Steel parts are typically 316L (fluid contact) or 304 stainless steel.

Elastomers are typically NBR, EPDM, FPM or PTFE.

Typical pump types: A standard pump for most applications typically has a max. system

pressure of 10 bar (147 psi). High-pressure pumps, e.g. for filter applications, typically have a max.

system pressure of approx. 40 bar (588 psi).

Multi-stage pumps, working in series. Typically a booster pump for highpressure at relatively low capacity.

Self-priming pump, working as a liquid-ring pump for aerated fluids, suchas CIP return.

High purity pump for pharmaceutical or similar applications with highdemands on cleanability. Typical design features are 45 casing outlet,casing drain, flushed shaft seal, polished surface finish and materialtraceability.

Selection/operation: Typically suitable for low-viscosity, non-particulate and non-aerated

fluids, e.g. beer, CIP, cream, juice, milk, soft drinks, water etc.

Typically selected from pump curve or PC selection program.

Typically operated on/off or controlled by simple valve throttling or by

frequency inverter (speed control). The most common pump failures are leaking shaft seal, low flow and

pressure, noise, high power usage and rapid pump wear.

4. What is a centrifugal pump?

A centrifugal pump is a rotodynamic pump type, the most commonsanitary pump type. Benefits include the relatively low purchase cost,availability of many types, simple design and easy maintenance.


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8

Fig.4.1. Pump principle Fig.4.2. Standard design

(principle)

Fig.4.3. Multi-stage pump(principle)

Fig.4.4. High purity pump(principle)

Fig.4.5. Pump selection

(principle)

Fig.4.6. Valve throttling

(principle)

Flow/pressure

1

2

3

Rotating

impeller

Casing

Casing

Impeller

Shaft

Shaftseal

AdapterMotor

Several stages(casings/impellers) Flushedseal

Casingdrain

45ocasingoutlet

Flowmeter

Throttlingvalve

Flow

Q

H

Pump curve

Process curve

D1D2D3

QD

HD


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10


(principle)

Fig.5.3. Impeller design(principle)

Fig.5.4. Power usage(principle)

Fig.5.5. Pump installation

(principle)

Rotatingimpeller

CasingCasingoutlet

Ringchannel

Casing

Impeller

Shaft

Shaftseal

AdapterMotor

Ringchannel

Impellervanes

Fixeddiameter

Ring

Impellerhub

Capacity

Power

Centrifugal

pump

Liquid-ring

pump


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Principle/design: Two rotors counter-rotate within the pump casing and create a partial

vacuum at the inlet, which sucks in fluid.

The fluid is transferred (displaced) through the pump in the cavitiesformed between rotors and pump casing.

A rotary-lobe pump is normally self-priming once there is fluid betweenthe rotors, due to very close tolerances between rotors and casing.

A rotary-lobe pump typically consists of pump casing, rotors, shafts, shaftseals, gearbox, coupling, geared motor and base.

Rotors are typically available in various shapes (2-lobe, 3-lobe),materials and temperature ranges (tolerances), depending upon usage.

A rotary-lobe pump is typically available with both mechanical and non-

mechanical shaft seals. A rotary-lobe pump is typically available with both horizontal and vertical

port orientation.

Steel parts are typically 316L (fluid contact) or 304 stainless steel.

Elastomers are typically NBR, EPDM, FPM or PTFE.

Typical pump types: Standard pump for most duties.

High-purity pump with design to ensure increased cleanability.

Selection/operation: Typically suitable for viscous, particulate and shear-sensitive fluids, such

as blood, chocolate, cosmetics, greases, ketchup, yeast, yoghurt etc.


Typically operated on/off or controlled by frequency inverter (speedcontrol).

A safety valve should be installed on the pump discharge side to avoid

possible over-pressure damage if the discharge line is blocked. The most common pump failures are leaking shaft seal, low flow and

pressure, noise, high power usage and rapid pump wear.

6. What is a rotary-lobe pump?

A rotary-lobe pump is a positive displacement pump type, typicallyused where a centrifugal pump is not suitable. Benefits includegentle transfer of sensitive fluids with solids, bi-directionaloperation and high efficiency.

11


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12


(principle)

Fig.6.3. Rotor design(principle)

Fig.6.4. Port orientation(principle)

Fig.6.5. Pump selection(principle)

Base

Geared

motor Coupling Gearbox

Casing, rotors,shafts, shaft seals

Flowin

1

Flowout

2Casing

RotorCavity

2-lobe 3-lobe


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Principle/design: A steel rotor (circular section single-threaded) rotates inside a rubber

(vulcanized steel pipe) stator, which has a double-threaded screw-

shaped hollow core. The fluid is transferred (displaced) through the pump in the cavities

formed between rotor and stator during rotor rotation.

A screw pump is normally fully self-priming, as the closed sealing designof rotor and stator ensures no back-flow (slip) of the pumped fluid. Thisfeature also ensures accurate dosing capability.

A screw pump typically consists of stator, rotor, drive shaft, shaft seal,bearings/bushes, adaptor, frame and geared motor.

Rotor and stator are typically available in various lengths (stages),

depending on the required pressure. Steel parts are typically 316L (fluid contact) or 304 stainless steel.

Elastomers are typically NBR, EPDM or FPM.

Typical pump types: Single-stage pumps.

Multi-stage pumps (longer rotor and stator).

Selection/operation: Typically suitable for viscous, particulate and shear-sensitive fluids, suchas cream, chocolate, cosmetics, eggs, toothpaste, yeast etc.


Typically operated on/off or controlled by frequency inverter (speedcontrol) or by an adjustable by-pass valve between inlet and outlet.

A safety valve should be installed on the pump discharge side to avoidpossible over-pressure damage if the discharge line is blocked.Alternatively install an overload switch.

Most common pump failures are leaking shaft seal, low flow andpressure and rapid pump wear.

7. What is a screw pump?

A screw pump is a positive-displacement pump type, typically usedwhere centrifugal and rotary-lobe pumps are not suitable. Benefitsinclude good solids handling, good metering ability, bi-directionaloperation and high efficiency.

13


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14

Fig.7.1. Pump principle Fig.7.2. Standard design, withoutmotor (principle)

Fig.7.3. Rotor/stator design(principle)

Fig.7.4. Pump selection (principle)

Flow in

Rotor

StatorBushing

Frame

Bearing

Shaftseal

Driveshaft

Flowout

Rotor

Stator

Frame/base

By-passarrangement

Flow in

Flowout

Screwpump

Gearedmotor Coupling

Automatic

by-pass valve

Fig.7.5. Pump control with by-pass valve (principle)


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Piston pump principle/design: A piston inside a cylinder moves forwards and backwards, driven by a

double-action reciprocating air motor.

The piston pulls fluid in at the inlet when moving back from the outlet

and displaces fluid to the outlet when moving forwards. Intake and displacement are controlled by an inlet/outlet valve system.

A piston pump typically consists of cylinder with inlet and outlet, piston,inlet/outlet valve system, seals and air motor.

Diaphragm pump principle/design: Two diaphragms inside two casings (chambers) move left and right,

driven by a double-acting reciprocating air piston system.

Intake and displacement in each chamber are controlled by a ball valve

system in each chamber. Piston movement to one side will push this diaphragm, empty the

chamber and displace fluid to the outlet. The other diaphragm willsimultaneously be pulled and fill the other chamber with fluid that issucked in from the inlet. The operation then reverses.

A diaphragm pump typically consists of casings/chambers, diaphragms,ball valve system, seals and air piston system.

General design: Steel parts are typically 316L (fluid contact) or 304 stainless steel.

Elastomers are typically NBR, EPDM, FPM and PTFE.

Selection/operation: Typically suitable for powders and high-viscous, particulate and abrasive

fluids, such as chocolate, egg, melted cheese, shampoo, toothpaste etc.


Typically controlled by regulating air pressure supply to the air motor.

The pumps cause flow pulsation use pulsation damper if possible. The most common pump failure is worn seals and diaphragms.

8. What are piston and diaphragm pumps?

Piston and diaphragm pumps are positive-displacement pump types.The benefits include good dry running, handling of particulate andabrasive fluids and suitability for hazardous environments.

15


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16

Fig.8.1. Piston pump principle Fig.8.2. Diaphragm pumpprinciple

Fig.8.3. Piston pump design(principle)

Fig.8.4. Diaphragm pump design(principle)

Fig.8.5. Piston pump selection(principle)

Flow

in

Fluid intake

Fluiddisplacement

Piston

Flowout

Air in

Air outAirin

Ballvalve

AirAir

motor

Piston Cylinder

InletvalveStem

sealOutletvalve

Flowout

Ballvalve

Airchamber

Diaphragm Air piston system

Tube/frame

Fluidchamber

Fig.8.6. Diaphragm pump selection(principle)

Flow in

Flow out

Fluid intake

Fluid displacement

Pistonmovement

Pistonmovement

Open

inlet valve

Closed

outlet valve

Closedinlet valve

Openoutlet valve

Fluid

Fluid


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Determine process type: Standard or high-pressure application?

CIP return process?

Viscous and/or shear-sensitive processing?

High purity process? Influences the selection of pump type (single- or multi-stage centrifugal,

liquid-ring, rotary lobe, screw etc.).

Collect fluid data: Such as type, temperature, viscosity, density, abrasives, solids etc.

Influences the selection of pump type/design and use of shaft seal type.

Influences selection of elastomers and surface finish (often preference).

Collect performance/site data: Such as capacity, inlet pressure, head, NPSHa, possibility of cavitation

and pressure shocks.

Such as voltage supply and frequency, installation conditions (space forinstallation), flame/explosion hazard and similar.

Influences the selection of pump/motor type/design and pump/motorsize.

Influences the use of shaft seal type and materials.

Pump selection/configuration: Select suitable pump configuration in accordance with these guidelines

and pump suppliers recommendations.

Ensure correct pump type, size, porting, surface finish and elastomergrade.

Ensure correct shaft seal type and seal face combination.

Ensure correct motor type and size.

Always check compatibility of pump materials.

9. Pump selection

Successful pump operation very much depends on correct pumpselection. It is important to get all necessary process and site data,as well as customer preferences, to ensure correct pump selection.

17


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18

Fig.9. Pump selection process

(guidelines only)

SELECTION:

Check process andcustomer demands

Get porting/connection data

3. Size, orientation,standard?


4. Machined, polished,electro-polished, other?

Get surface finish

6. Get elastomergrade

- Check compatibility- Check with seal

suppliers

NBR, HNBR, EPDM,FPM, PTFE, other

Centrifugal pump:- Single-stage- Multi-stage (for

high pressure)

- Low viscosity?- Non-particulate?- Non-aerated- General/simple pump

duty?

1. Liquid-ring pump

Positive displacementpump:- Rotary lobe- Screw- Piston/diaphragm

Aerated?Get process/fluid data:

- Fluid type?- Viscosity?- Density?- Temperature?- Particles?- Concentration?- Other?

- Viscous (higherthan 500-1000 cP)?

- Shear- sensitive?- Particulate/powder?

Get performance/site data:

- Capacity?- Head?- NPSHa?- Voltage?- Frequency?- Air pressure?- Other?

2. High inlet pressure?- Pump with internal

shaft seal (ifpossible)

- Shaft seal withhard face material

- Motor with specialbearing design- Special combination

of voltage/frequency?- Flame/explosion

hazard?- Special el. motor- Air/oil motor

5. Get shaft sealconfiguration

- Check compatibility

- Check with shaftseal suppliers

- Non-mechanical/

mechanical seal?- External/internal seal?- Single seal?- Single flushed seal?- Double flushed seal?


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On/off operation: For simple (start/stop) pump duty with no need to change pump

performance during operation.

Possible for both rotodynamic and positive-displacement pumps.

May not be suitable for large pumps/motors, as they pull out highstarting current when started, overloading the site supply system.

Changing impeller diameter: Possible for single-/multi-stage centrifugal pumps.

Reducing impeller diameter reduces the pump capacity andpressure/head. Increasing diameter increases pump performance.

This pump control cannot be done continuously during operation, as thepump should be disassembled to change impellers or impeller diameter.

Valve throttling: Possible for rotodynamic pumps. Should be on the discharge/outlet side

of the pump.

Throttling reduces capacity and increases system loss (changes processcurve/characteristic). The increase in system loss is waste of energy.

Simple and continuous pump control during operation.

Speed control: Possible for both rotodynamic and positive-displacement pumps.

Reducing pump speed reduces the pump capacity and pressure/head.Increasing speed increases pump performance.

Typically done with frequency inverter which changes motor speed.

Common, economical and continuous pump control during operation.

Other control: Centrifugal pumps coupled in parallel. The second pump is typically

used as a backup or to give additional capacity, e.g. during CIP.

A screw pump can be controlled by an adjustable by-pass valvebetween the inlet and outlet.

Piston and diaphragm pumps can be controlled by changing supplied airpressure.

10. How are pumps controlled?

Pumps are typically operated on/off, or they are controlled by valvethrottling, valve by-pass, air pressure control or speed control.

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20

Fig.10.1. Reducing impellerdiameter, centrifugalpump (principle)

Fig.10.2. Valve throttling, centrifugalpump (principle)

Fig.10.3. Speed control,centrifugal pump

(principle)

Fig.10.4. Centrifugal pumpscoupled in parallel(principle)

Q

H

Pumpcurve

Processcurve

D1D2

Q2

H2

H1

Q1

D1 D2

Q

H

Pumpcurve

Processcurve

n1n2

Q2

H2

H1

Q1

n1 n2>

Q

H

Pumpcurve

1+21,2

Q3

H

Q1,2

Q3= Q1+ Q2 H = Constant

Q

Q1

Q2

Q3

1

2Q1= Q2


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Abrasives Particulates in the fluid that give it a polishing effecton surfaces and may wear down materials andsurfaces.

Air motor Air pressurised unit to operate piston anddiaphragm pumps.

Cavitation The formation and collapse of regions of lowpressure in a flowing liquid. Occurs when fluid

pressure in flow equipment locally drops belowthe vapour pressure of the fluid at the currenttemperature.

CIP Cleaning In Place (cleaning without dismantlingequipment first).

Elastomer Non-metallic sealing part with elastic properties, e.g.natural or synthetic rubber.


Fluid Liquid/medium (non-solid and non-gas) processedin flow systems.

Flushed seal External (water or similar) flushing arrangement,

typically used to cool or clean the seal faces.

Frequency inverter Electronic device to regulate speed of an electricmotor. Common and economical way to controlpump performance.


High-purity Process with special cleanability demands. The termis typically used within the pharmaceutical industry.

Glossary

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22

Impeller Mechanical pump part fitted on the pump/motorshaft. The rotating impeller (vanes) converts fluidvelocity to fluid pressure.

Mechanical seal Shaft seal where seal faces (sealing interface)consist of a stationary and a rotating mechanicalseal part.

Non-mechanical seal Such as packed gland, typically used for industrial(non-sanitary) types of applications.



NPSHa Available suction pressure at current processconditions.


Differential Difference between inlet and outlet pressures.

Inlet Pressure at which a fluid enters flow equipment.

Losses Result of frictional losses in piping and flowequipment.

Outlet Pressure at which a fluid leaves flow equipment.

Shock Results of change in fluid velocity.

Rotor Mechanical part in a pump that is fitted on apump/motor shaft. Transfers fluid and builds up fluidpressure during rotation.


Stator Stationary part (casing) of a screw pump. Is acounterpart to a rotor which transfers fluid andbuilds up fluid pressure.

Viscosity Specifies how thick or thin a fluid is.


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1. Why is it important to know about centrifugal pumps? 2

2. Who should know about centrifugal pumps? 4

3. The principle of a centrifugal pump 5

4. Typical parts in a centrifugal pump 7

5. Sanitary centrifugal pumps 9

6. Typical range of centrifugal pumps 11

7. Centrifugal pump selection 13

8. Installation 15

9. Operation and service 17

10. Troubleshooting 19

Glossary 21

Other handbooks in this series 23

Contents

1

Centrifugal pumps


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The various aspects of centrifugal pumps are very important to considerwhen dealing with flow technology and flow equipment.

Understanding the aspects of centrifugal pumps makes it easier to select

correct pumps, optimize processes and minimize costs.

1. Why is it important to know aboutcentrifugal pumps?

2

Fig.1.1. Process with centrifugal pump (principle)

CIP

Milk

CIP

Milk

A centrifugal pump is typically the most common sanitary pumptype used in sanitary processes. Benefots include a relatively lowpurchase cost, wide selection, simple design and easymaintenance.


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Examples of centrifugal pumps

1. A centrifugal pump is used in processes with non-viscous and non-particulate fluids, e.g. beer, CIP, cream, milk, soft drink and purifiedwater.

2. There are typically many types of centrifugal pumps available forvarious types of applications.

3. The main parts of a centrifugal pump are motor, shaft, adapter, shaftseal, impeller, casing and seals.

4. A centrifugal pump is typically selected from a pump curve or a pump

selection program.

Fig.1.2. Centrifugal pump types (principle)

Fig.1.3. Centrifugal pump design (principle)

Fig.1.4. Centrifugal pumpselection (principle)

3

Standard High-cleanMulti-stage

Casing

Impeller

Shaft

Shaftseal

AdapterMotor


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It is important that the target group knows the various aspects of centrifugalpumps, including principle and design, available types, selection, suitabilityand limitations related to given processes.

This ensures that processes are optimised and that errors, damage and

personal injuries are avoided. The target group includes:

1. Process designers, who should know what centrifugal pump types andconfigurations to select so that the process is optimised related toquality and costs.

2. Sales and sales support people, who should know the possibilities andlimitations of centrifugal pumps related to applications/processes inquestion.

3. Field operators, who should know the possible causes of centrifugal

pump malfunction and know how to solve them.4. Beginners in the flow industry, who should have a basic understanding


2. Who should know about centrifugal pumps?

All people in touch with flow equipment during its life cycle shouldknow about centrifugal pumps.

4


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General principle: Fluid enters the pump casing and impeller center and is forced into a

circular movement by the impeller vanes and the centrifugal force. Thefluid thus leaves the casing with increased pressure and velocity.

Typically suitable for low viscous, non-particulate and non-aerated fluidssuch as beer, CIP, cream, juice, milk, soft drinks, water etc.

Single-stage principle:The fluid inlet, the built-up of velocity and pressure and the fluid outlet allhappens in one stage (one casing and one impeller).

Multi-stage principle: Fluid enters the pump casing and impeller center, and fluid pressure and

velocity are built up in the first stage (casing and impeller) similar to thesingle-stage pump.

Fluid with increased pressure and velocity is directed to the secondstage (casing and impeller), where the fluid pressure and velocity isfurther increased.

The result is a pressure increase (boost) in each stage, where the totalpressure increase depends on the number of stages in the pump.

Typically available with 2-4 stages.

Priming of a centrifugal pump: The pump casing should always be filled with fluid before starting the

pump to ensure correct operation.

The pump can operate with a positive inlet pressure (flooded inlet) orwith a negative inlet pressure (suction lift).

For suction lift, fluid can remain in the pump casing by using a non-

return valve in the suction line.

3. The Principle of a centrifugal pump

The centrifugal pump transfers fluid at a certain capacity from onepoint to another in a process. The pump builds up fluid pressure toovercome losses in the process. Capacity and pressure are createdby the rotating impeller inside the pump casing.

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Fig.3.5. Priming of pump

(principle)

Fig.3.1. Single-stage centrifugal

pump (principle)

Fig.3.3. Multi-stage centrifugalpump (principle)

Several stages(casings/impellers)

Fluidin

Fluidout Stages

2 1

Fully fluidfilled casing!

Pinlet

Pinlet> 0 = Flooded inlet

Pinlet< 0 = Suction lift

Flow/pressure

1

2

3

Rotatingimpeller

Casing

Fig.3.6. Priming of pump for

suction lift (principle)

Fig.3.2. Single-stage centrifugal

pump (principle)

Fig.3.4. Multi-stage centrifugalpump (principle)

Casing

Fluidin

Fluidout

Rotatingimpeller

Fluidin

Fluidout

Stage/casing2 1

Pressure increaseduring stages!

Rotatingimpellers

6


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4. Typical parts in a centrifugal pump

Typical main pump parts:

Main pump part Description/functionCasing/backplate Contains impeller where fluid is transferred

from inlet to outlet. Includes inlet and outlet ports.

Typically flexible port orientation.

Typically fitted to an adapter.

Shaft Rotates impeller which is fixed to it.

Is fixed to the motor and rotates with it.

Impeller Transfers fluid from inlet to outlet withincreased capacity and pressure.

Is fixed on the shaft and rotates with it.

Typical types are open, semi-open orclosed.

Shaft seal Seals between rotating shaft and stationarycasing.

Typically a mechanical seal, external orinternal.

Typically available as single, single flushedand double flushed seal.

Adapter Fixes pump casing to the motor.Motor Rotates shaft (impeller) which is fixed to it.

Typically a 3-phase electrical motor.

Typically available for various electrical sitesupplies (voltage and frequency).

Typically available in various protectionclasses (flameproof etc.).

Other parts Seals, motor cover, seal flushing, coupling/base (base-mounted pump).

Typical materials Steel parts of 316L or 304 stainless steel. Elastomers of NBR, EPDM, FPM, PTFE.

A centrifugal pump is a relatively simple pump. Design, types andnumbers of parts vary depending on centrifugal pump brand, typeand configuration.

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Fig.4.1. Main pump parts(principle)

Fig.4.3. Impeller types/design(principle)

Fig.4.5. Mechanical shaft seal(flushed seal principle)

Casing

Impeller

Shaft

Shaftseal

AdapterMotor

Fig.4.2. Pump casing/portorientation (principle)

Fig.4.4. Mechanical shaft seal(single seal principle)

Fig.4.6. Base-mounted pump(principle)

CouplingMotor

Base

Casing

Motor shaft/pump shaft

8


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5. Sanitary centrifugal pumps

Large radii and clearances: Use large radii on corners to ensure easy cleaning.

Use large clearances to ensure good fluid and cleaning flow.

Critical areas are welds and connections in general, porting and shaft

seal areas.

Drainage: Drainage ensures that the pump can be emptied completely so that

there are no remains (no sump) of processed fluid or cleaning agents.

Drainage is typically achieved through a drain fitted on the bottom of thepump casing or by rotating the casing outlet so that fluid can drain fromit.

The critical area is the bottom of the pump casing.

Minimum elastomer usage: Elastomers wear down over time and can cause contamination.

Therefore, elastomer usage and elastomer surface exposed to fluidsshould be minimised.

If possible, seals should be designed with fixed compression. Sealcontraction/swelling due to fluids/temperature should be minimised.

Correct materials and surface finishes: Typical materials for fluid-contact parts are 316L stainless steel and

various elastomer grades. Elastomers are often FDA-compliant.

Typical surface finishes of fluid-contact parts are 1.6m (64Ra) or 0.8m(32Ra), normally machined or polished.

A sanitary centrifugal pump is designed according to given hygienicstandards. This includes easy cleanability and use of correctmaterials for internal pump parts.

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Fig.5.1. Large radiis (principle) Fig.5.2. Large clearances(shaft seal principle)

Fig.5.3. Drainage (principle) Fig.5.4. Minimum elastomerusage (principle)

Fig.5.5. Correct materials and surfacefinishes (shaft seal principle)

Wrong! Correct!

Impeller

Fluidarea

Seal

Impellernut

10


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Standard pump: For most applications, typically with max. system pressure of 10 bar

(147 psi).

Typically available in many sizes to cover a wide range of duties.

High-pressure pump: For example for filter applications, typically with max. system pressure of

approx. 40 bar (588 psi).

Typically with a heavy casing/backplate design, with internal shaft sealand special motor (bearings) to withstand high inlet/system pressures.

Multi-stage pump: Working as pumps coupled in series. Typically a booster pump for high

pressure at relatively low capacity.

Typically with a heavy casing/backplate design, with internal shaft sealand special motor (bearings) to withstand high inlet/system pressures.

Self-priming pump:Working as a liquid-ring pump for aerated fluids, such as CIP return.

High-clean pump: For pharmaceutical or similar applications with demand of increased

cleanability.

Typical design features are 45 casing outlet, casing drain, flushed shaftseal, polished surface finish and material traceability.

Pumps with typical optional equipment: Pump with heating/cooling jacket, fitted to pump casing to either heat

viscous fluids or to cool heat-sensitive fluids.

Pump with inducer fitted to the impeller to increase suction capability.

6. Typical range of centrifugal pumps

A centrifugal pump is typically available in many types andconfigurations to fulfil most process demands. This includesstandard pump, high-pressure pump and high-clean pump.The available types depend on the pump brand.

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Fig.6.1. Standard pump

(principle)

Fig.6.3. Multi-stage pump(principle)

Fig.6.4. High-clean pump(principle)

Flushedseal

Casingdrain

45ocasingoutlet

Heavy casing/backplate

Internalshaft seal

Specialmotor High

pressure!

Fig.6.2. High-pressure pump

(principle)

Highpressure!

Specialmotor

Internalshaft seal

Heavy casing/backplate

Fig.6.5. Pump with heating/coolingjacket (principle)

CasingHeating/coolingjacket

Heating/coolingsupply

Heating/coolingsupply

12


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Determine process type: Standard sanitary or high-clean process?

High inlet pressure or booster process?

Influences the selection of pump type (standard, high-pressure, high-clean etc.).

Collect fluid data: Such as type, temperature, viscosity, abrasives, solids etc.

Influences the use of shaft seal type and seal face material.

Influences selection of elastomers and surface finish (often preference).

Collect performance/site data: Such as capacity, inlet pressure, head, NPSHa, and risk of cavitation.

Such as voltage supply, frequency, and installation conditions.

Influences the pump and motor size, shaft seal and motor type.

Pump selection/configuration: Select suitable pump configuration according to guidance shown and

according to recommendations of pump suppliers.

Select/size suitable pump from pump curve or pump selection program.

Ensure correct pump/motor type, size, porting, shaft seal, surface finishand elastomer grade.

7. Centrifugal pump selection

A successful centrifugal pump operation very much depends oncorrect pump selection. It is important to get all necessary processand site data, as well as customer preferences to ensure correctpump selection.

Q

H

Pump curve

Process curve

D1D2D3

QD

HD

Fig.7.1. (Selection principle)The pump is sized by using the pumpduty (QD,HD) in the pump curve. Correctpump/impeller size is the intersection

point between the process and pumpcurves.

13


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Fig.7.2. Centrifugal pump selection process

(guidance only)

SELECTION:


Get porting/connection data

3. Size, orientation,standard?


4. Machined, polished,electro-polished, other?

Get surface finish

6. Get elastomergrade

- Check compatibility- Check with pump

suppliers

NBR, HNBR, EPDM,FPM, PTFE, other

Get performance/site data:- Capacity?- Head?- NPSHa?- Voltage?- Frequency?- Other?

2. High inlet pressure?- Pump with internal

shaft seal (ifpossible)

- Shaft seal withhard face material

- Motor with specialbea

process control handbooks of knowledge

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