Hydraulic Principles of Centrifugal Pumps · 2019. 11. 25. · • Vortex pump • Self-priming pump Pumps for transport of water and ... Types of Pumps –Special Designs • Liquid

Technische Universität BerlinDepartment of Fluid System Dynamics

Prof. Dr.-Ing. P.U. Thamsen

Technical University Berlin

[email protected]

Hydraulic Principles of

Centrifugal Pumps

mailto:[email protected]


Lets talk about pumps…


Types of Pumps – Positive Displacement Pumps

• Rotary lobe pump• Progressive cavity pump• Gear pump• Piston pump• Diaghragm pump• Vane pump• Peristaltic pump

Pumps for transport of water and wastewater


Types of pumps – Centrifugal pumps

• Radial flow pumps• Axial flow pumps• Mixed flow pumps

Special designs, e.g. • Vortex pump• Self-priming pump


Pic

ture

s:K

SB

Pic

ture

s:K

SB

Pic

ture

s:K

SB

Pic

ture

s:K

SB


Types of Pumps – Special Designs

• Liquid ring pump• Side channel pump• Jet pump• Airlift pump• Hydraulic ram



21.05.2019 Pumping systems in the wastewater infrastructure 6

Operation Areas of Different Pumps‘

head [m]

10

10 000

1000

100

10

100 1000 10 000 100 000

capacity [m3/h]

axial pump

diagonal pump

multistage centrifugal pump

Single stage centrifugal pump

rotating displacement pump

displacement

pumps

single stage,

double flow pump

side channel

pump,

vane pump


Picture: After [TW90]

Characteristics: centrifugal/positive displacement pump

7


Picture: After [TW90]

Work principle

positive displacement flow lifting lift (force)

design piston pump diaphragm

pump progressive cavity pump

centrifugal pump screw pump chain pump airlift pump

single stage

multi-stage

sketch

H-Q- curve

M- n- curve

-

H head

Q flow rate

−

−

M torque

n rotation speed

−

−

Overview of Different Pump Designs

8

n nn n n

n


Definition of a Fluid Flow Machine

Mechanical power P is transfered into hydraulic power Pu by an impeller which

deflects the fluid‘s flow direction.

Mechanic: P = M w

Hydraulic: Pu = g H Q

𝑷 = 𝑷𝒖 + 𝑷𝒋

U

i

M torque

angular speed

P required power

P hydraulic power

P losses of power

density of the fluid

g gravity acceleration

Q flow rate

H head

−

w −

−

−

−

−

−

−

−


Seal

Shaft

Bearing

housing

Impeller

Discharge

flange

Spiral

casing

Suction

flange

Basic Components of a centrifugal Pump

Picture: Flowserve


Impeller with Nomenclature

Meridional Section Plan view

Picture: After [GU10] p.40


How does a centrifugal pump work?


How does the pump work?


How does the pump work?

𝐻𝐸 =1

𝑔𝑢2. 𝑣3𝑢 − 𝑢1. 𝑣0𝑢

𝐻𝐸 =1

𝑔𝑢2 ⋅ 𝑣3𝑢 (𝑓𝑜𝑟 𝛼0 = 90° → 𝑣0𝑢 = 0 𝑚

𝑠)

Picture: After [PF05] p.25


Picture: After [PF05] p.25

Centrifugal partDeceleration of rel. velocity

Acceleration of abs. velocity

𝐻𝐸 =1

2𝑔𝑢22 − 𝑢1

2 + 𝑤02 − 𝑤3

2 + 𝑣32 − 𝑣0

2

𝐻𝐸 =1

𝑔𝑢2. 𝑣3𝑢 − 𝑢1. 𝑣0𝑢

𝐻𝐸 - Euler Head [m]𝑔 - Gravity acceleration [m/s2]𝑢 - circumferential speed [m/s]𝑣 - Velocity [m/s]

The Euler Turbine Equation (Euler's equation

of fluid machines)


Theoretical Head Curve


Impellers‘ and Pumps‘ Design

Picture: above [KSB89] p.171

Radial centrifugal pump Diagonal centrifugal pump Axial centrifugal pump

Radial impeller Diagonal impeller Axial impeller


Performance Curves for Different Designs


Multistage


Multistage

Picture: Sulzer


Multiflow


Double Suction Pump

Pictures: Flowserve


Multistage - Multiflow

Quelle: KSB


Designs of Pumps- Examples

single stage,

single flow

centrifugal pump

single stage, double

flow centrifugal pump

Single flow, multi-

stage centrifugal

pump

Pictures: Flowserve


Designs of Pumps- Examples

Pictures: Flowserve

axial centrifugal pump

diagonal

centrifugal pump

submersible pump

Vertical turbine

pump


Pump & System


System‘s Head Requirement

𝐻𝐴 =𝑝𝐷 − 𝑝𝑆𝜌. 𝑔

+𝑣𝐷

2− 𝑣𝑆

2

2 ∙ 𝑔+ ∆𝑍

HA – System Head [m]p – pressure [N/m²]g – gravity acceleration [m/s²] – density [kg/m³]v – velocity [m/s]Dz – difference of height [m]

D – discharge

S – suction


Bezugs-niveau

𝑝𝑎𝑚𝑏

𝑝1

1 𝐷𝑆 2

𝑌𝐽1𝑆

𝑔 ⋅ 𝑧1

𝑝1 + 𝑝𝑎𝑚𝑏 /𝜌

𝑣12/2

𝑔 ⋅ 𝑧𝑠

(𝑝𝑠 + 𝑝𝑎𝑚𝑏)/𝜌

𝑣𝑆2/2

𝑌 = 𝑔.𝐻

𝑔 ⋅ 𝑧𝐷

𝑣𝐷2/2

(𝑝𝐷 + 𝑝𝑎𝑚𝑏)/𝜌

𝑌𝐽𝐷2

𝑔 ⋅ 𝑧2

𝑣22/2

(𝑝2 + 𝑝𝑎𝑚𝑏)/𝜌

𝑝2

Energy

Pump

System‘s Head Requirement


Q

H

No static head

Circulation pump

Q

H

~ Q2

No dynamic head

Fire pump

v2

Variable curve

Filling a tank

H

Q

Curve at the

end of pumping

Curve at the

beginning of

pumping

start

stop

stop

start

HA

HA

Examples of System Characteristic Curves


Choosing the Pumph

ead

H

capacity Q

HA

Speed n=const.

Hs

tati

cH

dyn

amic

H

→ No tolerances !→ No margins !

The required pump head must be equal to the system resistance at that flow rate where the pump reaches its best efficiency.

JgeoCP Hg

vv

g

ppHH +

−+

−+=

2

2

1

2

212

AoptCP HQH =)(


Qmin 0.5 * QBep

Qmax 1.2 * QBep

Q

H

QmaxQBEPQmin

H

hWorking outside of the

operation area causes:

• vibrations

• cavitation

• recirculation

• …

21.05.2019

Recommended operation area

BEP

Shutoff

Choosing the Right Pump


Pump Curve Sensitivity & Reliability of Centrifugal Pumps

32

Picture: After Judy Hodgson,Du Pont


Choosing the Right Pump

33

1. Mark the calculated Q, H-values in the performance characteristics for the pump series (above)

2. Find the right pump size (here: 65-250)3. Mark the calculated Q,H-values in the performance characteristics

for the right pump size and find the correct pump diameter (right top, here 255 mm), the NPSHr (right middle, here 2.8 m), and the shaft power (right bottom, here 28 kW)


Pump Control – Adjusting pump performance

Sometimes it is not possible to select a pump that works at the optimum duty point because the requirements of the system. Therefore, it can be necessary to adjust the pump performance

The most common methods for controlling flow through a pumping system:• Throttle control• Bypass control• Speed control• Modifying impeller diameter

The selection of the most appropriate pump flow control method depends on:• Fluid properties,• System layout• System flow/pressure requirements over time• System size

can be carried out continuously during operation


Pump Control – Control by modifying impeller diameter

Reducing the impeller diameter cannot be done while the pump is operating.

Reducing the diameter D2

The relation between the impeller diameter and the pump performance:

𝑸𝒏

𝑸𝒙=

𝑫𝒏

𝑫𝒙

𝒙

𝑯𝒏

𝑯𝒙=

𝑫𝒏

𝑫𝒙

𝒙

X 2


Pump Control – By Throttling

Hp=Hs+Hv


Pump Control – By Bypass

Qp=Qs+QBP


38

Capacity - When the rotational speed is changed:

𝑸 = 𝒗 ∙ 𝑨𝑸 ~ 𝒗 ~ 𝒏

Head - When the rotational speed is changed:

𝒖 ~ 𝒏 𝒗 ~ 𝒏 𝒗𝒖~ 𝒏

𝑨 = 𝒄𝒐𝒏𝒔𝒕.

𝑯𝑬 =𝟏

𝒈∙ 𝒖𝟐 ∙ 𝒗𝒖𝟑

𝒖𝟐 ~ 𝒏

𝒗𝒖𝟑 ~ 𝒏𝑯 ~𝑯 𝑬~𝒏𝟐

Pump Control – By speed control

Speed control by means of a frequency converter is the most efficient way of adjusting pump performance exposed to variable flow requirements.

When the rotational speed is changed the velocity triangles remain geometrically similar:


Can be written as:

39

𝑸 ~ 𝒏 𝑸 = 𝑲𝟏 ∙ 𝒏 𝑯 ~ 𝒏𝟐 𝑯 = 𝑲𝟐 ∙ 𝒏𝟐

Substituting the rotational speed n in the equation of H:

𝑯 = 𝑲𝟐 ∙𝑸

𝑲𝟏

𝟐

=𝑲𝟐

𝑲𝟏𝟐∙ 𝑸𝟐 𝑯 = 𝑲 ∙ 𝑸𝟐

Pump Control – By speed Control

𝑸𝟏

𝑸𝟐=𝒏𝟏𝒏𝟐

𝑯𝟏

𝑯𝟐=

𝒏𝟏𝒏𝟐

𝟐


40

• Rotational Speed changes → operational point moves along the parabola• If similarity rule applies multiple times → head capacity curve for new rotational

speed is found

n1

n2

n3

n1n2n3

capacitycapacity

efficiencyhead

H=K’ Q²

H=K’’ Q²

H=K’’’ Q²

Pump Control – By speed Control


Head

H

Capacity Q

h = 75%

h = 50%

h = 50%

h = 30%

h = 30%

Qmin

Hst

atic

Hd

ynam

ic

system curve

Head- capacity curve:

H = f ( Q, b2, hhydr, HShock, ….)

System characteristic curve:

21

2 2

static dynamic

A static dynamic

A

H H

H H H

p vH z

g g

= +

D= + D +

2

AH const. v= +

Shell – Curve of a Pump


Operation of Pumps

Q1=Q2

Q

H

H1+H2

Q1=Q2

H1,H2

Q2Q1

Q1+Q2 H

QQ1+Q2

H1=H2Parallel Operation of Pumps

Series Operation of Pumps


Parallel Operation of Pumps


Cavitation


Cavitation

Detailed view on

cavitation damages

Impeller with

cavitation damages


Cavitation process:

Pressure drop below vapour pressureplus

increase of pressure=> Collapse of Vapour Bubbles

Cavitation


Pump‘s Spots of increased Cavitation Risk

Leading edge of

the impeller

gap

gap

Leading edge of the diffuser


NPSHA = Net Positive Suction Head Available

Cavitation - NPSHA

𝑁𝑃𝑆𝐻𝐴 = 𝑍𝑠 +𝑃𝑆 − 𝑃𝐷𝜌 ∙ 𝑔

+𝑉𝑠

2

2 ∙ 𝑔

𝑁𝑃𝑆𝐻𝐴 = 𝑍𝑒 +𝑃𝑒 − 𝑃𝐷𝜌 ∙ 𝑔

+𝑉𝑒

2

2 ∙ 𝑔−𝐻𝑉𝑆


A Pump System‘s Head

g H

2

2

2

v

2 bp p+

IIg z

II bp p+

2

2

IIv

I bp p+

2g z1g z

Ig z

2,J IIg H

1,J Ig H

1 b Dp p p+ −

2

1

2

vI bp p+

2

1

2

v

1 2 III

energy line

pressure line

netto energy line

reference line

2

2

Iv

Picture: After [KSB89] p.289

pV

NPSHA


NPSHR = Net Positive Suction Head Required,minimum required netto suction head

Cavitation - NPSHR

1

2

λ 1.1 1.2

λ 0.2 0.3

0av absolute velocity just before

the inlet edge of the impeller

−

0aw relative velocity just before

the inlet edge of the impeller

−

𝑁𝑃𝑆𝐻𝑅 = 𝜆1𝑉0𝑎

2

2 ∙ 𝑔+ 𝜆2

𝑊0𝑎

2

2 ∙ 𝑔≈ 0.3 … . 0.5 ∙ 𝑛 ∙ 𝑄


Cavitation - NPSH


1 Cooling or heating coils

2 Stilling screens

3 To vacuum or pressure control

4 Spray nozzle for liquid de-aeration

5 Flowmeter

6 Flow control valve

7 Isolating valve

8 Measuring point for gas content

9 Test pump

[ISO 9906]

H100%

NPSHANPSH3%

DH

Q = constant

n = constant

H

Cavitation – Measurement of NPSH3%


Cavitation – Measurement of NPSH



B=1

Q/Qopt=1,0




NPSHA

Recirculation flow,

Pre rotationIndex “St”-

for

shockless

entrance



Head- capacity-

curve

NPSHANPSH

HNormal trend of the

head capacity curve if

NPSHA ≥ NPSHR

the head- capacity-

curve drops off if

NPSHA < NPSHR

NPSHR

Cavitation - NPSH

To avoid cavitation: NPSHA ≥ NPSHR

57


1. Know the pumping system’s cavitation behaviour well (NPSHA)

2. Know the pump’s suction behaviour well (NPSHR)

3. Use cavitation resistant materials

4. Supply suction nozzle with a bit of air (mechanic and noise damping)

5. Reduce rotation speed

6. Optimize the angle on the blade’s leading edge

7. Unify the velocity in the suction nozzle

Actions to actively avoid Cavitation


Prof. Dr.-Ing. P.U. Thamsen

Technical University Berlin

[email protected]

Thank you for your attention !

mailto:[email protected]

Documents

Hydraulic Principles of Centrifugal Pumps · 2019. 11. 25. · • Vortex pump • Self-priming pump Pumps for transport of water and ... Types of Pumps –Special Designs • Liquid