Real-time monitoring and control of mechanical face-seal dynamic behaviour

Red-ti me monitoring and control of mechanical face-seal dynamic behatiour By Professor Itzhak Green, The George W Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia, USA

The monitoring of

the

rotor

time,

Seal failure in critical applications can have severe implications, which is why it is important to detect problems and correct them before they develop further. This article looks at monitoring and controlling the dynamic behaviour of a noncontacting mechanical face seal, having a flexibly-mounted rotor in a seal test rig. In particular, it focuses on detecting and controlling the contact between the rotor and stator that may cause severe face wear and imminent seal failure. Noncontacting

mechanical

extensively powered

vessels

rotating

machinery.

and

may

Environmental

other

research

work

failure

have

severe

in

critical

implications.

and maintenance

reliable

that and

prevention

can be applied

that critically

cost

monitoring, of catastrophic

and reduce

contact

that

basis,

technology applied

has

equally

to

on an ad hoc

This

described

or article

in

can seals

be for

summarizes

between

dynamic

detail

The work

described

seal failure

typically

harmonic

contact

(that

relative

measures control,

are caused

by the

the rotor

and the

are

the

techniques

in

alter

have

been

a flexibly-mounted 1). In particular,

the rotor

and controlling and

stator

severe face wear and imminent

that

the

The monitoring

corrective

through

active

dynamics

behaviour. identification developed

by

stator

is

angular

A dominant

is the presence which

system

operation

of higher

are

integer

shaft.

and

orbit

suppress

an active control strategy

the rotor

seal

clearance.r2] pneumatic

chamber,

which

film with

in the clearance

axial modes.

is achieved

force. The coefficients

which causes

responses

by

in the rotor

damping

the clearance,

contact

by adjusting

the closing

and

change

incor-

is to eliminate

This

change

rotor-dynamic

by

air pressure

rotor

may

and signal

contact strategy.

governs

stiffness

the contact

seal contact

plots

and the stator

the work

cause

can detect

using

controlling

fluid

mech-

in a seal test rig (Figure

between

and

Control strategy

porating

means changes

in both

that a in the

angular

and

seal failure.[‘]

Pattern recognition

leads to face

occurring,

the system

damaging

and control

of a noncontacting

anical face seal, having

focuses on detecting

to be

of relative

of the speed of the rotating

the

Condition monitoring system

implemented

of monitoring, that

by an excessive

This

seal failure.

this from

which

eliminate

w

These

is, they are caused

To prevent

that

itself in higher

between

face misalignment).

wear and eventual

control

manifests

oscillations.

intermittent stator

here has determined

rotor

oscillations,

multiples

the

here is to monitor

the

chosen

and seal clearance.

The control

behaviour

and

between

been

the

of contact

processing,

Objective The objective

have

an Key

such as seal clearance

by a combination

between

Higher harmonic oscillations

behaviour.

misalignment

the stator

Contact detected

the

references.[‘-‘I

failure are very important.

real

monitored.

during

incompressible

much

in gives

the

developed

mechanical

(gas/air)

fluids.

work

been

and

harmonic

costs.

phenomenologically,

is, that

compressible (liquid)

is determined

parameters, angular

indication

to increase

maintenance

dynamic

the orbit

of the orbit

of the seal dynamic

relative rotor

can display

misalignment

the shape

misalignment

to applications

need to be monitored

their reliability

the cost of just the seal itself,

means

identification

compressors,

high-performance

seal

concerns

outweigh

which

pumps,

Because

Unpredictable applications

often

face seals are used

in centrifugal

where

indication

system

angular

The

condition

three

monitoring

eddy-current

and

universal

controller

The

personal

and

the noncontacting

this

on-line.

computer.

system

of

response

and the instantaneous

are determined

from

probes

and

a

eddy-current

proximity

to

a

determined

phenomenologically

The dynamic mechanical

consists

connected

proximity board

The seal clearance

behaviour

seal is monitored

of

process

of pattern

to the trum

probe

densities,

probes.

recognition

signals

and

that their

as well as angular

rotor

signals

of three

Contact

is

using

a

is applied

power

spec-

misalignment

Sealing Technology

No. 96

pressurized water

pressurized air spindle

shaft

\

I

stator

I

lsealing

dam

lip seal /

-\rotof cham br\\ \ carbon ring contacting seal \ \ \ Part Ill Part II Part I

orbit

plots,

all calculated

and

displayed

in

The

contact

tigated

elimination

experimentally

stator

experimental

is invesvalues

initial

seal pressure

rotor

of mis-

contact

can

sealed

rotor

chamber

a pressure

be

eliminated

pressure,

provide

regulator

automatically.

An

the

mounted is

to

study

because

they

various

operating

A mechanical to

operate

face

abnormal,

oscillations,

thus

of rubbing

contact

It is this

wear which

opening

have

seal

that

detected

is designed mode,

may

harmonic the

the

that

presence rotor

leads

and

to face

must be avoided.

operates and

under the

on the end of the housing,

was not

electro-

force.

proximity

dynamic

probes,

response

proximity

probes

about

10 kHz. They can measure distances

mounted

are used to detect the

These

dynamic

adjusted

in the rotor the closing

instantaneous

by

controlled

an

eddy-current

of the rotor.

have a bandwidth

of

the static and

between

their

tips and the

with

a cut-off

frequency

rotor end surface.

chamber,

A low-pass

force acting

rotor.

filter

of 1 kHz

is used to eliminate

crosstalk

noise

across

the

high-frequency probes

that

are

being used, and also to serve as an anti-aliasing

probes

The monitoring proximity

filter.

system,

based on eddy-current

probes and a flowmeter

to monitor

the rotor-dynamic

the

presence

behaviour,

of

The

has been used

higher

and

harmonic

The

outputs

proportional

to

a control

algorithm

was

incorporated.

ted,

the

control to

ment between reduction

the

The

flowmeter

the

a voltage

flow on

rate. the

that

is

Ultimately

closing

force

reduced

universal computer. signal

system

altered

the

system

components,

harmonic

oscillations.

are

is installed

then

of

signals

The

such

processor

sent

converters.

into

a

digital has

been

peripheral

as analogue-to-digital,

are obtained

analogue-to-digital

the

in a personal

by a set of on-board

digital-to-analogue probe

voltages

The board has a floating-point

supplemented

the rotor and stator for maximum

proximity

is used to drop

from -24 V to -10 V

that

processor.

detec-

misalign-

output

voltages

board

was

relative

of each divider

amplified

probes’

contact

the

output

maximum

Once

reduce

of higher

maximum

probe is -24 V. A voltage

proximity

oscillations.

dynamics

the balance

closing

Three

in the force.

automatically

pressure

Proximity

detect

higher

contact

Sealing Technology No. 96

rig

harmonic

been

closing

Monitoring system

initial

to

conditions.

between

force

built

face seal. This higher

indicating

obviously

The seal system of the

been

of a flexibly-

in a noncontacting

experience

stator.

rig has response

mechanical

used

oscillations under

test

dynamic rotor

being

the

on the flexibly-mounted

experimental

study

the

Subsequently transducer

the pneumatic

while

was set manually

and

and in this was adjusted

Test rig

the opening

interface,

and the air pressure

the air pressure

pneumatic

reduction.[‘~‘]

provides

sealing

hydraulic

compression,

study,13-51

the

drop

the

spring

Initially,

results show that for the seal under

clearance

the

across

but compliant

to intuition,

of a parametric

consideration, through

and

Contrary

the results

strategy

for various

misalignment

alignment. with

The force

real time.

and

The proximity-

through

the board’s

converter.

0

A

8

cr6jlrm

flowmeter

the leakage signals

and

processed

to

on-board

the

recording.

results

are

as rotor

then signal

sent

to

display

the

or for

parameters

are

misalignment,

precession

angle,

orbit,

and clearance.

response

are

digital

Key dynamic

such

measure

proximity-probe

in real time for on-line

monitored, rotor

used

measurement

the

and

computer

also

leakage by

processor,

data

is

of the seal. The

the

the

rotor

angular

Real-time monitoring The sealed water pressure 0.2

0 1

0.0

0.3

0.4 Time

05

06

07

0.8

the stator

(sec.)

is set to 345 kPa, and

the shaft speed is 28 Hz. The graphite

coning

assembly

is deformed

in a

angle of 1 mrad.

Experimental

results

clearances

are presented

procedure

for taking

setting

l

stator

to provide

for

four

in Figures

experimental

the air pressure

different 2a-2e.

The

data involves:

in the rotor chamber

to

7.6 kPa; running

l

the shaft at 28 Hz and recording

monitored repeating

l

the

data; and this procedure

air pressure

by incrementing

(by approximately

the

14 kPa) to 41.4

kPa, then 55.2 kPa and lastly 69.0 kPa.

From

the

leakage

the clearances

0 1

0.0

0.2

0.3

0.4 Time

0.5

0.6

0.7

08

measurement

are calculated

pm,

1.5 pm,

from

the three

and

2d),

the

stator

calculated

to

0.5

Figures changes

0.5

2a and

cleafanl%! = 0.5p /

large

0.0001 -

-o.oooo-

-0.0001

respectively,

the

0.0004

O.ClOCE 0.0006

and

as

of the

the

are

and the Both the

the

rotor

and

their

seal

clearance

by the large

initial

respect

the

to

amplitude,

operation,

about

As explained,

clearance

decreases,

precession

angle

misalignment misalignment

the rotor

as the

both

rotor

the

and misalignment The

precession

approaches

angle

mean

misalignment

gradually

misalignment

and

are also reduced.

at

is brought

rotor stator

itself to the stator

and the rotor

amplitudes

value of the rotor the stator

angle,

approaches

that of

the stator. The Figure response

same 2c.

the orbits

phenomenon This

orbits

experiments

v

rad,

periodically

with

adjusts

0.0003 us @ad)

0.9

angle

peak-to-peak

beginning

(0.5 mrad).

00002-

0.0002

angle

decreases.

the

0.0003

angle

decreases

The

clearan 0.0004 -

0.0001

and and

mrad

with the clearance.

vary

amplitude

o.ooc6

-0.0000

(Figure

precession

precession

misalignment

-0.0001

Also,

signals

2b depict,

in the rotor

rotor

-0.0002

respectively.

misalignment

rotor misalignment

_.____

pm,

proximity-probe

be

2e) 2.8

respectively.

(sec.)

0.0005

(Figure to be 6 pm,

is

shows

the

for different

and approach

clearances

in simulation. circular

observed rotor

in

angular in both

As expected

shapes. The smaller

Sealing Technology

No. 96

the clearance the

orbit

polar

is the smaller

centres

coordinates

and the stator

of

of

i Probe0

initial

1.5 mrad)

misalignment in

the

the

$

mrad). at

initial

230

2

is not

(0.5

clearance

test,

g Y

220

the

d

rotor

drops.

FFT

analyses

the experimental very

rotor

each

misalignment The

for the

decrease

begmnmg

240

-

the

pm,

(presumably

the

.

whose

misalignment

of the experiment,

6

to the stator

With

size, and

point

angle.

misalignment close

the

are the stator

At the beginning clearance

the orbit

approach

minor

that

were

sets shown

second

higher

ponents

in the eddy-current

signals

for

performed

in Figure

on

2d reveal

harmonic

com-

proximity-probe

0.1

0.0

indicating

all

of

the

tested

0.2

0.3

0.4

0.5

0.6

0.7

0.8

clearances,

that there is no contact

between

Time

the

(sec.)

seal faces.

Clearance control A more advanced

and proactive

dynamics

is to control

behaviour

and prolong

A control action monitoring

that

on

the

and contact

described.

Seal

advancement

detection

because

a desired

in

5

an

E

face-

d

variation,

i

may cause either leakage,

d

has

3

2

the seal clearance

been

accomplished,

disturbances,

shaft

4

each of

as seal failure.

operation

variations

is

mechanical

of controlling value

overcoming

results is now

seal clearance

or excessive

is regarded

6

dynamic

control

by process disturbances,

The objective at

real-time

clearance

severe face contact which

can take meaningful

in noncontacting

seal operation caused

its life.

system

based

step in face-seal

the seal rotor-dynamic

speed

I

including

and

sealed

fluid

0 0.3

pressure. The the

“clearance

closing

mounted has

control

force rotor.

anti-windup.

is to adjust

on

been

the

The

has

point

seal maintains,

clearance

disturbances (Figure

shown

changes

then

3). The controlled

to respond

constant)

quickly

that

approach

set-

parametric

for contact study

without

seal dynamics. Through reduce

seal has been

(having

with a small control

the

and shaft

and

pressure

a small time

Contact elimination through clearance adjustment feasibility

noncontacting anical

face

of

eliminating

flexibly-mounted seal

has

SealingTechnology No. 96

been

in

rotor

mech-

studied.i6)

The

a

seal face contact.

experimental

phenomenologically of probe

signals

densities,

as

orbit

plots

investigated stator

misalignment.

elimination experimentally

misalignment Contrary

through

contact.

faces

causes

the

Controlling

power-

which

can be damaging.

as

angular

that

includes

feedback inner

strategy

has been

for various

values

initial

rotor

intuition,

but

versatile

loop

maintains Clearance

control

the air pressure The

outer

clearance

loop

loops

face the

face contact controller

has

been

used.

control

loop

clearance

is accomplished

set

The that point.

by adjusting

in the rotor chamber adjusts

is

proportional-integral

is a clearance desired

seal

A cascade

two

control

between

intermittent

way of eliminating

a

the

can be eliminated

misalignment

seal

their

and

study,(3-5)

that for the seal

reduction.

relative

most

- all calculated

to

large,

contact

pattern

well

and

the parametric

results have shown

clearance

The

has been

in real time. contact

with

consideration,

seal

from and

to

therefore,

Contact

spectrum

of

compliant

relative

seal faces and,

recognition

The

on a rotor

it is possible

normalized

between

misalignment

contact

adjustment

maximum

misalignment

displayed

is based

flexibly-mounted

under

the

eliminate

effort.

elimination

using

clearance

determined

The

0.6

been

or follows,

with

in sealed water

0.7

with

to the test seal. Results have shown

the controlled

0.6

model for

controller

controller

0.5

flexibly-

determined

of a proportional-integral

applied

speed

acts

The seal axial dynamic

experimentally

design

concept”

that

0.4

Time (sec.)

the desired

of the seal. clearance

@

when

-

Deairedclearaoe8

I

i

0.10

:

contact

variances

is detected,

of the probe

Experiments cascade 6

T P .o 5 E a4 2 E m 3 d 2

is

is, when

have been conducted

controller.

presented

that

the

signals are different.

The

to test the

results

which

are

here show that when the coning

small

(and

reduction

contrary

in

seal

to

angle

intuition)

clearance

can

the

eliminate

contact. Experiments different

have

stator

sealed water

.,,,,.,.,... .,.‘...................................,............

i

entire

2

1

3

4

5

pressures,

cascade

conducted

angles,

and whether

controller

under

shaft speeds and or not the

is able to eliminate

..I....

6

7

face

0.01

_

U

been

coning

8

contact.

The

experiments

0.00

water

Time (Min.)

pressure

is 344.8

Hz and

stator

plotted

in

when

(Figure

of

the

speed

is 28

is 2 mrad) The

the rotor

the

one

angle is 1 mrad,

kPa, shaft

4-6.

between

smaller

of

misalignment

Figures

alignment

are

relative

mis-

and the stator

control

is marked

is

“on”

4).

The the

results

(where the coning

rotor

misalignment

magnitude

positioned

of

at

its

angle, is plotted more

defined

rotor

indicating

misalignment

instantaneous

in Figure

circular

its centre

orbit,

the

precession

5. The orbit becomes

for the “control

moves

towards

by the stator

on” case,

the

point

misalignment

and

that

is

and stator

angle. When

the cascade

control

loop drives the system 0.0019

(eliminating from

0.0018

the contact)

Figure

clearance. test

0.2

0.0

0.4

0.6

0.8

1.0

1.2

is on, the variance

toward

better

and,

6, it automatically

reduces

This is an indication

conditions,

reducing

indeed

reduce

shown

analytically.

alignment

as can be seen

the

clearance

and

with

in

the

good

agreement

contrel

controller

in the rotor

The

control

inner

desired

7~

@ad)

0.0022

clearance, and dictates

and

measured

while

are signal

oscillations

variance

loop,

measuring

loou

based

on

contact

of abnormal in the signal

by

of

output

probes).

processor,

differences,

outer

The

(the

detected

for the seal. Once

control

the

result.

proximity

the digital

maintains

the set point,

clearance

oscillations

control

loop

by the appearance

harmonic

eddy-current

orbit

0

control

contact-detecting

the 0.0021

small,

by a cascade

The

is determined

J

is realized

loops.

higher

0.0020

is very

in (air

is well tuned

using two proportional-integral

the

0.0019

chamber)

that the control

active

calculates

0.0018

is required

scheme

the

0.0017

that

(showing

The change

quite effective.

\i

3’ 16

are adequate).

output

demonstrating

off;

L

clearances

from leakage measurements

pressure ---

calculated

are well correlated

that both methods

on. 1st time off, 2nd time on. 2nd time off, 3rd time

as was

from the probe measurements

calculated

control control control control

the does

the relative misalignment,

Figure 6 also shows that clearances Time (sec.)

the

that under

of These

parameters

of

and misalignment detected, the

determines

a feedback probe the

signal new

Sealing Technology No. 96

Tribofqy at

140 clearance from probes clearance mean value from probes ._..__. clearance from flowmeter - 120 Air pressure -

the

Transactions 42(3) 535-540 ASME-STLE

Toronto,

Canada,

Tribology

1998).

3. Zou, M., Dayan, for

Analysis

J. and Green,

Contact

Noncontacting

of Vibration,

Dynamics”,

I. Parametric

Control

of

a

Face

Seal.

In

Mechanical

“Proceedings

(presented Conference,

Noise

Venice, Italy, 28-30

& Structural April 1999, pp

493-499. 4. Zou,

M.,

Dynamic

Dayan,

J. and

Simulation

Noncontacting Mechanical

Face

5. Dayan,

M. and for

Proc.

Green, the

of a Noncontacting

Seal, ZMechE, hoc. Time (sec.)

ZMechE,

Rotor Instn.

Part C 1195-1206.

Analysis

Operation

of a

Mounted

Seal,

J., Zou,

Sensitivity

I. (2000)

Monitoring

Flexibly

Me&. Engrs. 214(C9)

2

Green,

and

I. (2000)

Design

and

Mechanical

Face

Znsn. Mech. Engrs. 214(C9)

Part C 1207-1218. 6. Zou,

M.,

Feasibility

Dayan, of

Mechanical

J. and

Contact

Face

Seal

I (2000)

through

Trans.

Adjustment,

Green,

Elimination

of

a

Clearance

ASME,

Journal

of

Engineering for Gas Turbines and Power 122(3) target

gap,

which

and resume Also, because clearance leakage

will eliminate

the contact

noncontacting

operations.

normal

the leakage

cubed,

when

is significantly

is proportional the control

flexibly-mounted eliminates

rotor

to adjust

is on, the

reduced.

Summary In

summary,

References

loop,

and

harmonics applying

the

interpreting

control

London,

existence

of high

423-430.

the

or noncircular the

in a cascade

necessary

orbits

as contact,

closing

force

on

the

Control

of

Many

cells

is spirally uration

wound

inserted

that

a sealed

in which

the various

occur

USA

STLE

cells,

nickel-metal

of flexible

chemical

pertains

to rechargeable

cells, but in particular

roved seal (and method the electrolyte

within

electro-

it covers an imp-

of sealing)

for retaining

the casing of the cell.

Sealing Technology No. 96

1999,

It is important

incorporate

this

the seal associated

a cylindrical

config-

undesirable

conditions

or other

evaporation

of the

into

reduced

a “can”,

and surrounding

the the

The cell also includes with

the

container

within

electrochemical and release

experiences reason

leaking may

reactions of electrical

the service

electrolyte

an

captured

with

function

may

a number occur. may

or failure (typically and

of

Firstly, result

in

of the cell, and

a corrosive

damage

agent)

components

to the cell. of seal designs

attempt

electrolyte

considerable

seal material

seal

life of the cell. If the

electrolyte

performance

A variety in

sealing

is not maintained,

contaminate

exterior

the cell

it is necessary

wall to help

an effective

throughout

is introduced

an insulating

that

sealing function

environment

often

For

the container

the cell maintains

to

leaking

incorporate

container

pressure.

and

by a thin

together

for the storage

the cover

assembly

into

and separator.

USA.Tel: +1 404 894 6779, Fax: +l

404 894 8336, Email: [email protected].

electrode

Because of the nature of the processes involved, the

and

June

against leakage of electrolyte.

energy. This invention

Control

28-30

Green, The George W. Woodruff School of Mechanical

the

and

within

provides

Florida,

Seal,

are separated

Electrolyte

can and is retained

Inc, Alachua,

and

into

a cover

Systems

Face

separator,

Date: 14 August

Power

Israel,

pp 618419.

GA 20332-0405,

Clearance

electrochemical

The electrodes

electrodes

Assignee: Moltech

Haifa,

of the IEEE 7th on

For more information, contact: Professor ltzhak

pp

an electrochemically-active

Patent number: US 6274267 200 1

In

College, 1997,

I. (1999)

are made with

and

container.

I?E. Pate

Conference

Automation”,

Seal.

Imperial

Green,

rechargeable

loaded

material.

Inventor:

In “Proceedings

Leeds-Lyon

September

a Mechanical

nonconductive

cell

Face

24th

as nickel-cadmium

hydride

Seal aims to prevent electrolyte leakage

Condition

I. Contact

Face Seals Using

Engineering, Georgia Institute of Technology, Atlanta, M. and

plates

Title: Seal for electrochemical

4-6

2. Zou,

Patents

the

on Tribology”, UK,

and

such

I. Real-Time

M. and Green,

in Mechanical

Mediterranean

of

Symposium

proximity-probe

J., Zou,

Active Control.

of a Mechanical

“Proceedings using

7. Dayan, Elimination

Monitoring

as feedback

478-484.

to the

1. Zou, M. and Green,

measurements

the clearance,

seal-face contact.

have been employed

resolve from

a deformable and compressed

the

cells.

problem Many

seal material between

of

designs that

is

a rigid disk-

like cover and the cell container-walls. to

between

This patent

describes

that uses amorphous

an improved

polymers

seal design

as a seal-element

0