© Copyright KSB SE & Co.

KGaA
Online Training A14: Water Hammer in Pump Systems

Georg Flade , November 2022

© Copyright KSB SE & Co. KGaA 2022

 Agenda

1. Introduction
- Water hammer problem in pumping systems
- Risks
- Calculation

2. Safety measures in detail

- Air valves
- Surge vessels
- Excursus: Check valves
- Flywheels

© Copyright KSB SE & Co. KGaA 24.11.2

- Exotics

022
3. Reverse Speed Calculation
Agenda- 4 Quadrant operation of pumps
- Modelling Details
- Results

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 © Copyright KSB SE & Co. KGaA 24.11.2
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3 I Druckstoß Wasserforum I September 2022
Water Hammer Introduction

Sudden Valve Closure in a Pipeline

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Introduction
Joukowsky – Equation Sudden Valve Closure

v0

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© Copyright KSB SE & Co. KGaA
-v0
Model: Pipe with neglected friction

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Introduction
Joukowsky – Equation Sudden Valve Closure

v0

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-v0

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Introduction
Joukowsky – Equation Sudden Valve Closure

v0

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-v0

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Introduction
Joukowsky – Equation Sudden Valve Closure

v0

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-v0

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Introduction
Joukowsky – Equation Sudden Valve Closure
Water hammer: Positive Pressure corresponds with negative pressure due to wave reflection!

Joukowsky equation:

Steel Pipe:
[bar] 3𝑚 𝑠

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© Copyright KSB SE & Co. KGaA
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Water Hammer Introduction

What Happens After Pump Trip?

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 Introduction
What happens during pump trip?

v=v0
Head

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Check Valve

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 Introduction
What happens during pump trip?

Head

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Check Valve

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 Introduction
What happens during pump trip?

Head

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Check Valve

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 Introduction
What happens during pump trip?

Head

v=0

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Check Valve

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Introduction
Conclusion
 High pressure and low pressure occur together due to wave reflection
 Phase shift at reflection points
• Pipeline end points
• Partial reflections at cross-section jumps or material changes of the tubes
 Propagation at the speed of sound (wave speed)
 Steel pipes: 1000…1300 m/s
• Plastic pipes: 250…450 m/s

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© Copyright KSB SE & Co. KGaA
15
Water Hammer Introduction

Risks of Water Hammer in Pipelines

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Introduction
Risks
- Both negative pressure and positive pressure can be critical

- Overpressure: limit is the permissible pipeline pressure (usually nominal pressure stage).

© Copyright KSB SE & Co. KGaA 24.11.2

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Introduction
Risks
Negative pressure:

- Cavitation! (with the consequence of uncontrollable pressure surges when the steam bubbles collapse).

- Permissible negative pressure of the pipes

- Permissible negative pressure of seals, fittings and vessels

- If necessary, external pressure influences must also be taken into account (e.g. road loads or external pressure for
submersed pipes)

© Copyright KSB SE & Co. KGaA 24.11.2

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Source Fa. Airvalve Source Fa. Airvalve
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Water Hammer Introduction

Calculation of Water Hammer

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Introduction
Calculation Methods
Graphical methods (obsolete) - here example pump failure

Pump curve
H
2. Phase: pressure increase

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C+ Q
C-

Plant curve

Charakteristics dH/ dQ 1. Phase: pressure drop

from Joukowski equation

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 Introduction
Calculation Methods
Now: Calculation using method of characteristics
 Commercial programs available. KSB uses Wanda (Deltares) and Flowmaster (Siemens)
 Plant is modeled (“digital twin”) and various scenarios are calculated without or with safety measures

Example for calculation

model

© Copyright KSB SE & Co. KGaA 24.11.2

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Boundary Conditions
Szenarios
 Calculation must always include the entire plant!
 Usually pump failure most critical
• Power failure...common failure of all pumps
• Individual failure of one pump - other pumps continue to run
 Calculation pump start or other scenarios if specified
 If necessary: valve closure or opening / rpm change / …

© Copyright KSB SE & Co. KGaA 24.11.2

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 Scenarios different water levels (esp. in inlet)

 Calculation (simulation over time) until pressure waves have subsided and stable condition has been
reached (also BEV)

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Boundary Conditions
Szenarios

Example for scenario table

Scenario Data Results

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Water Hammer Introduction

Safety Measures - Overview

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Introduction
Water Hammer – Overview safety measures
Surge Vessels

Airvalves
Limitations for
operation

Flywheels

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Chek valves

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Safety measures in detail
Airvalves

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Safety measures in detail
Airvalves
High capacity air release Low capacity air release
Function
Air inlet (filling of pipe) (pipe operation)

© Copyright KSB SE & Co. KGaA 24.11.2

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Air flows into the line at Air flows out of the line Operating vent
negative pressure  Filling the line  Air collects in the line at high points
 Vacuum breaker function  After pressure reversal at pressure surge Operational venting also works when the line
the air is let out again is pressurized
 Important safety measure
in case of pressure surge!  Not relevant to pressure surge

Line start and distributed on  High points

the route

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 Safety measures in detail
Airvalves
1. Without safety Measures:

Envelope maximum head

Reservoir
Station

Head

© Copyright KSB SE & Co. KGaA 24.11.2

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Profile

Envelope minimum head

28 I Druckstoß Wasserforum I September 2022

 Safety measures in detail
Airvalves
1. With airvalves as safety measures:

Station
Envelope maximum head Reservoir

Head

Envelope minimum head

© Copyright KSB SE & Co. KGaA 24.11.2

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Profile

3 Airvalves

29 I Druckstoß Wasserforum I September 2022

Safety measures in detail
Airvalves
Comparison: Negative pressure / No safety measures vs. Airvalves as safety measures

Head

No safety measures Airvalves

© Copyright KSB SE & Co. KGaA 24.11.2

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Profile

Profile

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Safety measures in detail
Airvalves
Comparison: Negative pressure / No safety measures vs. Airvalves as safety measures

Pressure

No safety measures Airvalves

© Copyright KSB SE & Co. KGaA 24.11.2

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Safety measures in detail
Airvalves
 Designs for drinking water  Designs for waste water

Source Fa. KSB

Source Fa. Airvalve Source Fa. KSB

© Copyright KSB SE & Co. KGaA 24.11.2

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• Entire valve filled with water

• Water in contact with sealing surfaces • Pollution resistant design

 Sensitive to contamination • Sealings in contact with air

• Protective sieves prevent foreign particles / insects

etc. from being sucked in
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Safety measures in detail
Airvalve –Example sectional drawing

 Design for waste water

High capacity valve plate Buffer air volume for

pollution resistance

Low capacity valve

Ca. Water Level

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Floating ball

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Pipe
Safety measures in detail
Airvalves
 Limitations / Disadvantages

 Effect as pressure surge protection only with negative pressure

 Pipe enters negative pressure area

• Pipe is filled with air
• Contamination can occur

 When using pressure vessels or flywheels, the system can rather be kept in the
overpressure range

© Copyright KSB SE & Co. KGaA 24.11.2

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 Running maintenance costs

34 Source Fa. Airvalve

Safety measures in detail
Airvalves
Air inlet vs. Air release Air Valve

Smash

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 Fast air release may lead to water hammer!
 Air outlet need to be limited in some cases
 Soft Closing Airvalve type

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Safety measures in detail
Airvalves– Normal Closing vs. Soft Closing

Air Intrance Air Outlet

Normal Closing Soft Closing

© Copyright KSB SE & Co. KGaA 24.11.2

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Soft Closing

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Normal Closing
Safety measures in detail
Airvalves
Necessary data in the report of the pressure surge calculation

• Position in the pipeline

• Necessary capacity

 Sample Capacity Curves:

© Copyright KSB SE & Co. KGaA 24.11.2

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 Soft Closing or Standard Closing

 Reference: German Standard DVGW 334 („Ventilation of drinking water pipelines“)

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Safety measures in detail
Surge Vessels

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 Safety measures in detail
Surge Vessels

Classic pressure surge protection in the water sector

- Buffer gas volume is compressed by pressure of the
Surge vessel
pump
- In case of pump trip - expansion of the gas and
compensation of the missing water volume
- Buffering of pos. pressure waves

© Copyright KSB SE & Co. KGaA 24.11.2

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Check valve

Pump

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 Safety measures in detail
Surge Vessels

Head without vs. with surge vessel

Pumping Head with surge vessel Reservoir

Station

© Copyright KSB SE & Co. KGaA 24.11.2

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Head without surge vessel
Profile

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Safety measures in detail
Surge Vessels
Fluid Level in surge vessel

Fluid Level

Pump speed

© Copyright KSB SE & Co. KGaA 24.11.2

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Head

Profile

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 Safety measures in detail Source: Fa. Orelltec Source: Fa. Charlatte

Surge Vessels

Pressure surge vessel with bladder:

- Bladder with preset (nitrogen) pressure
- No compressor required, nitrogen bottle for maintenance if
necessary
- Variants with forced flushing of the bladder

© Copyright KSB SE & Co. KGaA 24.11.2

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Without bladder, direct contact drinking water - gas (air)
Water filled bladder Gas filled bladder

- Compressor necessary, because air goes into solution

 Oil free for drinking water
- Level or pressure control

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Safety measures in detail
Surge Vessels

Example: big surge

vessel with bladder (ca.
20 000 Liters)

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 Safety measures in detail
Surge Vessels – Important Details

What quantities should a surge calculation yield?

• Vessel volume
Depending on type:
• Filling pressure if bladdered
• Water level in standard vessel

• Geodetic position of the vessel

© Copyright KSB SE & Co. KGaA 24.11.2

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?

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Safety measures in detail
Surge Vessels – Important Details

• Geodetic position of the

vessel much higher than in
simulation
 Negative pressure leads to
implosion

© Copyright KSB SE & Co. KGaA 24.11.2

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Safety measures in detail
Surge Vessels – Important Details
Influence of the nominal connection size
Water Level in Surge Vessel
If the connection line of the surge
vessel is too small – the function
of the vessel will be impaired!

Nominal Diameter DN65

DN100
DN200

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Minimum Head

022
© Copyright KSB SE & Co. KGaA
Nominal Diameter DN200
DN100
DN65

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Safety measures in detail
Surge Vessels – Important Details
Source: Fa. Charlatte

• Nominal connection width needs to be considered!

• Length, nominal width and internals of the connection line
• Connection can be additionally narrowed (e.g. by bubble protection screens)

 Caution when using pressure compensation vessels for pressure surge

damping Source: Fa. Orelltec

• Usually very small nominal size of connection pipe

© Copyright KSB SE & Co. KGaA 24.11.2

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Use of a pressure surge damper / air vessel may require a fast check
valve or flap. This should be explained in the report!

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Safety measures in detail
Check Valves

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 Safety measures in detail
Check Valve Dynamics
Swing Valve - slow Source: Fa. Erhard
Reverse flow 𝑟
velocity 0

Nozzle valve - fast

𝑑𝑣
Flow deceleration
𝑑𝑡

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Steep profile

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Flat profile

 A slow swing check valve is "slammed" at high flow deceleration.

 Steep profile > Slam!
 A pressure hammer damper or air vessel works the same way!
 Vessel presses water and shuts the valve with "slam"
 Dynamic requirements for backflow preventers need to be considered

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Safety measures in detail
Flywheels

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Safety measures in detail
Flywheels
- Delay pump standstill and flattening of transients

- Very reliable, no additional maintenance

- System can be kept in overpressure similar to a surge vessel

- For certain pump types only feasible (e.g. not for submersible pumps)

- Design limitations (envelope and bearing loads)

- For flywheel on motor shaft: Check and, if necessary, reinforce bearings

KSB Omega KSB Sewatec (with Beltdrive)

© Copyright KSB SE & Co. KGaA 24.11.2

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Safety measures in detail
Flywheels
Flywheel in pump head diagram
Pump Speed

With flywheel

No flywheel

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Minimum Head

© Copyright KSB SE & Co. KGaA

With flywheel

No flywheel

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Safety measures in detail
Further Safety Measures

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Further safety measures
Surge Towers

© Copyright KSB SE & Co. KGaA 24.11.2

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Pumped storage power station

 Function comparable to surge vessel. Works with geodetic height

instead of gas compression

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Further safety measures
Pressure relief valves

 Applicable in case of problem regarding overpressure

(E.g. quick closing of valves)

 Damping of pressure peaks

 In case of pump failure mostly protection of the 2nd step

© Copyright KSB SE & Co. KGaA 24.11.2

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Further safety measures
Bypass Line

 Typical for "pump as turbine"

application

 Turbine idling with high speed when

disconnected from the mains

© Copyright KSB SE & Co. KGaA 24.11.2

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 Turbine acts like a fast closing
valve

 Water can pass through bypass line

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Water Hammer Calculation
Summary

 Necessary Input
 Conclusion

© Copyright KSB SE & Co. KGaA

Surge Analysis
Necessary Input Data
- Pipeline profile
- Pipeline data (wall thickness , (inner) diameter, roughness, nominal pressure, additional flow resistances)
 Complete system is required
- Data of pump
- System operation data
- Data of check valves, air valves or surge vessels (if already planned or installed)

© Copyright KSB SE & Co. KGaA 24.11.2

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- Water levels inlet, outlet
- Station drawing, P&I scheme

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Surge Analysis
Conclusion
 Positive and negative pressure correspond due to wave reflection

 Air valves
• Diminishing of negative pressure
• Correct position and size needs to be determined
• Soft closing as an option

 Air vessels
• Size matters

© Copyright KSB SE & Co. KGaA 24.11.2

• Level, fluid level, filling pressure is important as well

022
• Connection pipe needs sufficient nominal diameter
• Fast closing check valve required

 Other options:
• Flywheels, surge towers, pressure relief valves, bypass pipes are other options for protection

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Transient Flow Analysis

Reverse Speed Calculation

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Reverse Speed Calculation
Introduction
 Example for Pumping Station

 No check valve

 In case of pump trip

• Reverse flow through pump
• Pump acts as turbine

 2 Typical questions
• Reverse speed…rpm within acceptable

© Copyright KSB SE & Co. KGaA 24.11.2

limits?

022
• Duration of reverse running phase…blocking
time before restart

 Occasionally other questions (pressure

transients, airvalve opening etc.)

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Reverse Speed Calculation

Four Quadrant Operation of the Pump

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 Reverse Speed Calculation
Introduction – 4 Quadrant Operation

1 2 3 Flow vs. Speed Diagramm

1
1 Head
2
Horizontal pipe section

© Copyright KSB SE & Co. KGaA 24.11.2

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022
0 rpm
0 flow
Speed rpm

Flow m³/min 2

Q1Q2 Q3

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 Reverse Speed Calculation
4 Q – Operation

Flow > 0 Normal pump quadrant

Rpm > 0  Data available

 Torque and Head can be

calculated from existing data!

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© Copyright KSB SE & Co. KGaA
Flow < 0 No (useful) steady operation
Turbine operation
Rpm < 0 possible
 Data for some pumps
available (e.g. some Omega  Transient quadrant
were PAT is documented) • Pump still running forward
due to inertia
 Momentum and head can be • Flow already revers
calculated from existing data Flow < 0  Normally no data directly available
in some cases! Rpm > 0
Reverse Speed Calculation
4 Quadrant Operation
 For transient reverse flow calculation, specific data are required which are not available in the specific
pump documentation!

Example for Q – n field in

Amacan – Know How Booklet

(taken from Stepanoff- Book)

Not specially valid for Amacan!

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022
© Copyright KSB SE & Co. KGaA
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Reverse Speed Calculation

4 Q - Data Acquisition

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Reverse Speed Calculation
4 Q – Data Acquisition
Typical Torque and Head curves are available in the literature for special impeller shapes.
Specific pump speed is a criterion for impeller shape.

© Copyright KSB SE & Co. KGaA 24.11.2

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Semi-axial
nq ca. 10 20 35 60 120 200

67
Indicates were data for H and M are available
 Reverse Speed Calculation
4 Q – Data Acquisition

Sample Curves for Head (H)

and Torque (M) for several
specific speeds

(Source: Thorley; Fluid

Transients in Pipeline
Systems)

Several ways to make curves

dimensionless

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© Copyright KSB SE & Co. KGaA
Here: Suter transformation

68
Reverse Speed Calculation
4 Q – Data Acquisition – Suter Transformation

Head Torque Flow Speed

Reference: Best Efficiency Point of Pump (BEP)

Head description Torque description

flow

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X=1,5

022
© Copyright KSB SE & Co. KGaA
BEP: x=1,25 WB=0,5 WH=0,5

X=0
speed
X=1

X=0,5

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Reverse Speed Calculation
4 Q – Data Acquisition – Suter Transformation
Example: KRT K 600-710 Q: 5850m³/h H=25m, specific speed nq = 78 rpm in BEP
Suter curve available for nq =76 rpm

WH curve Literature nq=76

WH curve calculated from KSB curves

WB curve Literature nq=76

WH curve calculated from KSB curves > perfect match!

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Reference BEP x=1,25 WH=0,5 WB=0,5

© Copyright KSB SE & Co. KGaA

Pump operation
Turbine idling

Result Curves put into the model

documented

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Reverse Speed Calculation
4 Q – Data Acquisition – Conclusion
- Transient reverse speed calculation is set up using curves from literature for similar impeller shapes

- BEP is used as reference point

11/24/2
 Curve can be adjusted, were data are available

022
(Pump or Turbine quadrant)

© Copyright KSB SE & Co. KGaA

Required Pump  Calculation can be done for several Nq‘s (sensitivity
analysis)
Calculation 1

Calculation 2

 Calculation can be made for „standard hydraulics

(Amacan, Omega, RDLO, K, (E) )

 Calculation of raw waste water hydraulics (F, D,S) will

71
not lead to reliable results
Reverse Speed Calculation

Modelling the complete system

© Copyright KSB SE & Co. KGaA

Reverse Speed Calculation
System Modelling  Pump Discharge
• Volume (Head dependent area)

 Inlet boundary condition

• Head

 Weir
• Level

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© Copyright KSB SE & Co. KGaA
 Pump
• BEP (Q, H, Torque, Speed, Momentum
of Inertia)
• Suter curve

73
Reverse Speed Calculation
Model of Discharge Pipe

Discharge Pipe Reservoir with

height dependent
area to represent

© Copyright KSB SE & Co. KGaA 24.11.2

022
the volume of the
discharge pipe
Suction reservoir Suction reservoir

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Reverse Speed Calculation
Further Model Details
- Momentum of inertia for Pump incl. Drive Drain

- Flow resistance (e.g. Duck Foot Bend, Pipe

sections)
- Different water level scenarios if necessary
- Water hammer can be modeled with some
limitations if necessary

© Copyright KSB SE & Co. KGaA 24.11.2

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75
Reverse Speed Calculation
Results
 Pump Speed and Head over time
 Speed: Comparison with documented unit speeds or approval by R&D department
 Duration of reverse rotation: There is no exact standstill in the calculation results! – Estimation
necessary!

Speed rpm

© Copyright KSB SE & Co. KGaA 24.11.2

022
Pump head in m

76
 Thank You!

For transient simulations like water hammer or resverse speed calculation please contact:

KSB SE & Co. KGaA

LAC Water Halle

Technical Support

© Copyright KSB SE & Co. KGaA

Dr.-Ing. Georg Flade

Project Manager

Tel.: +49 345 4826 4790

E-Mail: georg.flade@ksb.com