M14Cent Pump Vib

8/13/2019 M14Cent Pump Vib

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Mechanical, Module 14-Centrifugal Pump Vibration

EPRI TR-1010792

MECHANICAL SERIES

MODULE 14

CENTRIFUGAL PUMP VIBRATION


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MODULE OBJECTIVES

Describe types of pumps and their

typical applications

Define/Describe:

-Pump Curves

-Best Efficiency Point

-Head vs. Resistance

-How to Use Performance TestingData to Monitor Pump InternalClearance Degradation


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MODULE OBJECTIVES

Present Basic Vibration Theory

Present Simple Vibration Calculations

Define/Describe Typical Pump Failure

Modes

Describe Standard Vibration

Monitoring, Analysis, and Diagnostics


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SINGLE VOLUTE PUMP


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DOUBLE VOLUTE PUMP


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DIFFUSER PUMP


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IMPELLER CONFIGURATIONS

Enclosed Semi-Enclosed Open


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SINGLE SUCTION IMPELLER

ImpellerDiameter

Wear Ring Hub

Suction Vane Edge

Shroud

Discharge Vane

Edge or Tip

EyeDiameter

Suction Eye


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DOUBLE SUCTION IMPELLER

ImpellerDiameter

EyeDiameter

Wear Ring Hub

Suction Eye

Suction Vane Edge

Shroud

Discharge Vane

Edge or Tip


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PUMP AND HEAD TERMINOLOGY

Head - used as a measure of energy and has the units of feet.

Friction head(hf)- the energy required to overcome resistance

to flow in the pipe, fittings, valves, entrances and exits.

Velocity head(hv)- the energy of a fluid as a result of its

kinetic energy.

Pressure head(hp)- the pressure of the fluid being pumped.


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Static suction head(hs)- the vertical distance in feet above thecenterline of the pump inlet to the free level of the fluid source. If

the free level of the fluid source is below the pump inlet, hswill be

negative and is referred to asstatic suction lift.

Static discharge head(hd)- the vertical distance in feet above thepump centerline to the free level of the discharge tank.

Net suction head(Hs)- the total energy of the fluid entering the

pump inlet. It includes thestatic suction head (hs), plus the

pressure head(if any) in the suction tank (hp

), plus the suction

velocity head (hv), minus the friction head (hf)in the suction piping.


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NET SUCTION HEAD

Net suction head(Hs)- the total energy of the fluid entering thepump inlet. It includes thestatic suction head (hs), plus the

pressure head(if any) in the suction tank (hp), plus the suction

velocity head (hv), minus the friction head (hf)in the suction piping.

p, hp

hs

hf

h


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Net discharge head(Hd)- the total energy of the fluid leaving the

pump. It includes the static discharge head (hd), plus the dischargevelocity head (hv), plus the friction head in the discharge piping

(hv), plus the pressure head (if any) in the discharge tank (hp).

p, hp

hdhf

h

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Total dynamic head(H) - the net discharge head minus

the net suction head. The total amount of energy added

to the fluid by the pump.

H = Hd- Hs

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Net positive suction head required(NPSHR)- the minimum fluid

energy required at the inlet to the pump for satisfactory operation.

Net positive suction head available(NPSHA)- the fluid energy at

the inlet to the pump above the fluids vapor pressure.

Cavitation- the vaporization of the

fluid within the casing or suction line.

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INTEGRATED CURVE

4000

HEA

D(ft)

3500

3000

20002500

FLOW (gpm)

0 100 200 300 400 500 600 700

100

90

80

70

60

50

40

30

20

10

0

Eff. %60

4020

0

600

400

200

0

PSHR(ft)

BHP

PSHR

Head

EFF %

BHP@ 1.0 Specific Gravity

Typical Auxiliary Feedwater Pump Curves

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ESTIMATING PUMP INTERNAL

CLEARANCE DEGRADATION

1.Using Performance Test Data to Calculate % Head Loss at the

Operating Conditions

2.Using a Balance L ine DP Correlation to I nternal Clearance Status

3. Performing a Static Rotor L if t Check and Measur ing Clearances as

Close as Possible to the Wear Rings

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BASIC VIBRATION THEORY

m

a

F

maFNewtons Second Law = Dynamic Equation of Motion

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SINGLE DEGREE OF FREEDOM SYSTEM

kc

mx(t)

F(t)

kx

F(t)

m x

xmxckxtFF )( )(tFkxxcxm

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FREE VIBRATIONS

Time

Amplitude

e n t

sin( ) d t

)sin()( tAetx d

t

cn

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FREE VIBRATIONS

m

kn

21 nd

km

c

c

c

cr 2

Natural Frequency of

Undamped System in rad/sec

Damping Factor

Natural Frequency of Damped

System in rad/sec

Period of Oscillations in sec

f = Frequency in HzfT

d

12

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APPLICATION

x0

x1

x2x3

x4

d t

d t0 d t1 d t2 d t3 d t4

pi

i

x

x

p ln

2

1

ipi

dtt

p

2

21

dn

2

nmk

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FORCED VIBRATIONS

)cos()( tXtx

)()sin()( txtXdtdxtx

)()cos()( 22 txtXdtxdtx

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FORCED VIBRATIONS

fT

12

)cos()( tXtx

0RXX

kFX 00

222 )2()1(

1)(

rr

rR

2

1

1

2tan)(

r

rr

n

r

with

Time t

x(t)

X

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FORCED VIBRATIONS

0

0 1,

0 2,

0 3,

0 6,

1

0 1.0 2.0

0.1

1.0

10.

100.

R

r

Dynamic amplification factor R r( ) as a function of

(Logarithmic scale)

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FORCED VIBRATIONS

( )r

0

0 01,

0 1,

0 2, 0 6,

1

0.0 1.0 2.0

0

40

80

120

160

r

180

Phase angle ( )r as a function of

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PRACTICAL APPLICATIONS

Following is a presentation of practical methods

for the determination of system dynamic characteristics

from vibration plots. We are interested in calculating:

Damping factors

Natural Frequencies

Critical speeds

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Rmax

Rmax

2

r1

r2

0. 1. 2. 3.r

R

r

dB

n

r

2

1

2

1

Plot of Pump Bearing Vibration Amplitude

as a Function of Excitation Frequency

nFor r = 1

Critical Speed fNcr 60 , RPM

2

nf , Hz

, rad/sec

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X3

X2

X1

N1 N2 N3N rpm

X Vibration Amplitudes Provide an Easy and Fairly Accurate

Method of Determining Critical Speeds from Bode Plots

n

Damping Factor

Natural Frequency

2

n

f

, rad/sec

, Hz


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Using Phase Angle Measurements(Not Very Ac cu rate due to Uncertaint ies o f

An gle Measurements)

n

Damping Factor

Natural Frequency

( )r

2

1

N1 N20.

0

40

80

120

160

N rpm

180

2

nf

, rad/sec

, Hz


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SIMPLE ROTOR HAVING 2 CRITICAL SPEEDS

DUE TO DISSIMILAR SUPPORT STIFFNESS

PO G x

y

z

t

O

P

G

e

u

y

xx

y

e1

e2

MKxnx

M

Kyny

nx

xr

ny

yr

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SIMPLE ROTOR

X

e,Y

e 0 180 360

rx 1 ry 1 r rx y,

0.

1.

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PUMP FAILURE MODES

Loading ResponseStatic forces and moments on the casing:

- Impeller radial thrust (primary static load of

the rotor)

- Weight of the rotating assembly

- Static loading varying with pump flow rate

- Static bearing and seal forces

- Static shaft deflection

- Rotor axial position movement

Dynamic forces and moments fixed on therotor:

- Impeller hydraulic unbalance (primary

dynamic load of the rotor)

- Rotor mechanical unbalance (secondary

dynamic load of the rotor)

- Dynamic bearing and seal forces

expressed as vibrations

Dynamic instability mechanisms:

- Pump bearing whirl

- Rubs

- Impeller vane pass

- Rotor eccentricity

Multiples and sub-multiples of running

speed from analysis:

- Sub-synchronous frequencies

- Impact

- Multiples of running speed

Pump Loading and Associated Response

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d l if l ib i


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PUMP FAILURE MODES

Loading Response

Static:

- Nozzle loads

- Foot loads

Static structural response is not currently

measuredDynamic:

- Seismic nozzle loading- Seismic foot loading

Dynamic structural response:

- Seismic response not recorded- Natural frequencies usually

accounted for

- Running speed components

- Sub-harmonics analyzed

Structural L oading and Associated Response

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PUMP FAILURE MODES

Loading Response

- Bearing and seal static and dynamic

loading- Flow induced loading

Mostly pressure, temperature and flow

measurements and performance trending

Operational Loading and Associated Response

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BASIC VIBRATION DIAGNOSTIC TECHNIQUES

Vibration Measurements

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Vibration Measurements

Shaft

Bearing

Casing

MotorVertical

Accelerometer

Axial

Accelerometer

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Vibration Sensors

Moving Object: (Shaft)

Fixed Object

(Bearing)

Displacement

Probe

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Mechanical Module 14 Centrifugal Pump Vibration


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Vibration Sensors

Transducer Case

Vibrating Mass

Piezoelectric Element

(Spring)

Transducer Mount

Transducer

Wiring

Integrated

Electronics

Magnetic Base

Support Element

Piezoelectric Accelerometer

Transducer Case

Coil

Damper (oil)

Vibrating Mass

Stiffness Element

(Spring)

Permanent Magnet

Transducer Mount

Transducer Wiring

Seismic Velocity Transducer

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Vibration Acceptance Criter ia

Recommended Limi ts for Overall Casing Velocity

Peak Velocity Acceptance Class

Less than 0.15 ips (3.8 mm/sec) Acceptable

0.15 to 0.25 ips (3.8 6.3 mm/sec) Tolerable

0.25 to 0.4 ips (6.3 10 mm/sec)May be tolerable for moderate periods of

time. Monitor closely to warn of changes0.4 to 0.6 ips (10 15 mm/sec)

Impending failure; watch closely for changes

and be prepared to shut down for repairs

Above 0.6 ips (15 mm/sec) Danger of immediate failure

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Vibration Acceptance Criter ia

Recommended Limits for Shaft Vibration

- API and AGMA specify that, for the purpose of acceptance, maximum shaft

amplitude peak-to-peak expressed in mils shall not exceedRPM

000,12 or 2 mils

whichever is less.

- In this criterion, shaft motion includes runout, which can be no more than25% of the allowable displacement.

- This guideline is established for new machinery; operating machinery cantolerate higher levels.

- Consult OEM

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Mechanical Module 14-Centrifugal Pump Vibration


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I nterpretation of Vibration Spectra

Cause Frequency Amplitude Phase NotesUnbalance 1 x RPM Proportional to

unbalance Radial

- steady

1 Reference

mark - steady

Most common cause of vibration, no phase

change

Misalign-

ment

(1, 2, 3, ) x

RPM

Axial high 1, 2 or 3

referencemarks

Second most common cause of vibration.

Axial amplitude may be twice the vertical orhorizontal.

Eccentricity 1 x RPM Varies 0 or 180o

between

Horizontal

and Vertical

Balancing may reduce vibration in one

direction but increase it in the other

Bent shaft (1 to 2) x

RPM

Axial - high 180oout of

phase axially

Same radial phase on both bearings Orbit

and phase are good parameters to monitor

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Mechanical, Module 14 Centrifugal Pump Vibration



Cause Frequency Amplitude Phase Notes

Thermal

bow

1 x RPM Varies 1 Reference

mark - steady

Increasing vibration during load variations

and startup from a cold condition

Looseness (1, 1.5, 2, 2.5,

3, ) x RPM

Proportional to

load

2 reference

marks,

slightly

erratic

Frequently coupled with misalignment

Strobe may help. Amplitude depends on

load

Soft foot 1 to 2 x RPM Proportional to

load

Check mountings for variations in amplitude

Electrical 1 x RPM or 1

to 2 x line

frequency

Large Erratic When power is turned off vibrations

disappears instantly

Sleeve

bearings

wear and

clearance

(1, 2, 3, 4, )

x RPM

May be higher in

Vertical than

Horizontal

Erratic Compare shaft to bearing displacement

readings. Oil analysis best monitor for wear

Oil whip .5 x RPM Radial

unsteady,

excessive

Erratic Frequency is near one-half running speed

(machine speed is nearly 2x critical speed)

Oil temperature is a good indicator

Oil whirl (.42 to .48) x

RPM

Radial

unsteady,

sometimes severe

Erratic Caused by unloading of bearing. Tangential

destabilizing force due to lube film in the

direction of rotation adds energy to vibration

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Cause Frequency Amplitude Phase NotesAnti-friction

bearings

BPFI, BPFO,

BSF, FTF and

Harmonics

Radial - low Erratic Use velocity, acceleration or spike energy

Rubbing (0-0.5)x, 1x,

and higher

harmonics

Erratic Erratic Similar to impact, may excite many system

frequencies

Gears GMF=Z xRPM Radial - low Erratic Use velocity or acceleration. Tooth wear isbetter indicated by side-bands around GMF

and excitation of tooth natural frequency.

Higher tooth load will increase amplitude at

GMF. Backlash is characterized by

decreasing amplitude at GMF when load is

increased. Gear misalignment shows with

higher 2x and 3x GMF. A cracked or broken

tooth is best seen on the time signal. Ahunting tooth problem shows at very low

frequencies

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Cause Frequency Amplitude Phase NotesFoundation Unsteady Erratic Unstable

reference

Strobe may help

Resonance System

specific

High Erratic Increased levels at resonant frequency. Often

appears on old machines pedestals

Cracks 1x, 2x RPM, Variable during

transients. Drop

in higherharmonics

Phase shift Increased levels at resonant frequency

Phase is a good indicator. 2x RPM

excitation of critical speed during coastdown.

Hydraulic

Forces

Vane Pass =

Z x RPM and

harmonics

High radial and

axial

NA Use velocity or acceleration. Due to uneven

internal gap between rotating vanes and

diffuser. May excite natural frequencies.

Flow obstructions are common causes.

Cavitation Random high

frequency +Vane Pass

High radial and

axial

NA Due mainly to insufficient suction pressure

and the presence of vapor and air in theliquid.

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VIBRATION SIGNALS EXAMPLES

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, g p

Unbalance

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, g p

Misalignment

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, g p

Vane Pass

5x1x

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M14Cent Pump Vib