4BiggestMistakesInInstrumentationAndHowToAvoidThem

8/12/2019 4BiggestMistakesInInstrumentationAndHowToAvoidThem

1/7

This w hite paper is brought to you by:

IN IN S T R UM EN TA TIO N A N D H O W TO A V O ID TH EM


2/7

THE FO U R B IG G ES T M IS TA KES IN I N S T R U M EN TA T IO

Desp ite o n go in g ad van cem en ts in m easu re -

m ent and com m unicat ions te chnology, instru -

m ent ing a process for fee dback contro l

rem ains a te chnical challenge . Today s sensors

are cer t a in ly m ore sophist icate d than ever

bef ore, an d f ieldbus te chnology ha s sim plif ied

m any insta llat ion issues considerably.

N o n e th e less , m u ch can s t ill g o w ro ng w ith an

instrum ent at ion project .

The problem lies in the stra ight forw ard nat ure

of instrum enta t ion pro jects: each va r iab le to be

m e a s u re d m u s t b e m a t c he d w it h t h e m o s t

appropr ia t e sensor ; the se nsor must be

insta l led , ca l ibrate d , and in ter fa ced t o the con-

t ro ller ; and the in form at ion generat ed by t he

sensor m ust be f i lte red , fact ored , and f i led in

order to g ive the contro ller a n accurate p ic ture

of w hat s go ing on in the process. Th is appa r -

ent s im plic ity is w hat o f te n lea ds to a fa lse

sense of se cur ity and m issteps in a m inef ie ld

of pote nt ia l prob lem s.

W ith th a t in m in d , he re a re fo u r c ircu m stan ces

yo u d e f in ite ly w an t to av o id .

>> M is t a k e # 1 :

Se lec t in g th e w ro n g

sensor

Technology m ism atch

A lthough its generally obvious

w hat quantity needs to b e m eas-

ured in a flow , tem perature, or

pressure control application, its notalw ays ob vious w hat kind of flow

m eter, tem perature sensor, or pres-

sure gauge is best suited to the

job. A m ism atch b etw een the sens-

ing technology and the m aterial to

be sensed can lead to skew ed

m easurem ents and severely

deg rad ed control.

This is especially true w hen

m easuring flow rates. A ll flow

m eters are d esigned to m easure

the rate at w hich a gas or liquid

has been p assing through a partic-

ular section of pipe, but not all flow

m eters can m easure all flow s. A

m ag netic flow m eter or m agm eter,

for exam ple, can only detect the

flow of electrically conductive

m aterials by m eans of m ag netic

induction. N on-conductive fluidslike pure w ater w ill pass through a

m agm eter undetected.

M ag m eters also have trouble

distinguishing air bubbles from the

fluid in the pipe. A s a result, a

m agm eter w ill alw ays yield an artifi-

cially high reading w hen b ub bles

pass throug h because it cannot

sense the decrease in fluid volum e

caused by the p resence of the

bub bles. In a feedback loop , this

occurrrence w ould cause the con-

troller to throttle back the flow rate

m ore than necessary, preventing

the required volum e of fluid from

reaching the dow nstream process.

The problem gets even w orse if

the pipe is so full of air that it is

only partially filled w ith liquid, a

cond ition know n as open channel.A lthough recent technolog ical inno-

vations allow certain m agm eters to

w ork in such a challenging environ

m ent, m echanical sensors such as

turbines yield artificially high read-

ings, since a trickle of fluid w ill

m ove the m eters m echanism just

as m uch as a full-pipe flow travel-

ing at the sam e speed . O n the

other hand , m echanical sensors

are not affected by the conductivity


3/7

N D H O W T O AV O I D T H EM 3

2

of the fluid, so they w ill som etim es

w ork w here m ag m eters fail.

A n even m ore challenging ap pli-

cation is the m easurem ent of pH in

a caustic liquid such as the slurries

found in p aper m ills. A general-pur-

pose pH probe m ade of corrodible

m aterials m ight not only generate

inaccurate data, it m ight die alto-

gether, som etim es w ithin a m atter

of days. Som e probes, such as

those offered by A B B , are specifi-

cally designed for such tough envi-

ronm ents. They can double, triple,

and even q uadrup le p rob e life in

m any ap plications.

The trick is to find the right tech-

nology for the application, or to

choose instrum ents that span a

broad er range of solutions. For

exam ple, new digital technolog ies

allow som e flow m eters to solve

m any m ore flow problem s than

their pred ecessors.

Instrum entation vend ors can be

of help in avoiding the technolog y

m ism atch m istake. The best ven-

dors train their sales people to

assist w ith sensor selection and

provide clients w ith easy-to-use

selection guides. Som e even offer

extensive look-up tab les b ased on

prod uct num ber, ap plication, and

serial num bers of past installationsan especially useful service w hen

replacing older prod ucts.

Finding all the right parts can

also b e a challenge. Som e instru-

m ents require sp ecific housings,

m ounting hardw are, and transm it-

ters to forw ard the sensors data to

the controller. The right vendor can

m ake all the difference by provid-

ing the entire assem bly und er a

single catalog num ber. W hen it

com es to tem perature instrum enta-

tion, for exam ple, training costs

and purchasing effort are red uced

w hen then vend or offers com pati-

ble p rob es and transm itters togeth-

er as a package.

Paying too m uch (or too little)

C orrect sensor selection is also a

m atter of balancing cost ag ainst

perform ance. W hen theres a

choice of equally effective tech-

nologies, the right choice is gener-

ally the cheap est one that gets the

job done.

Tem perature instrum entation is a

classic exam ple. The tw o dom inant

technologies are resistance tem -

perature detectors(RTD s) and

therm ocouples. A n R TD consists of

a m etal plate or rod through w hich

a current is passed. The resistance

that the current encounters varies

w ith the tem perature of the m etal. A

therm ocouple consists of tw o dis-

sim ilar m etal w ires joined together

at one end. The voltag e betw een

the unjoined ends varies w ith the

tem perature of the joint. B oth yield

voltages that can be electronically

interpreted to indicate the tem pera-

ture of the surroundings.

Therm ocouples are generally

cheaper, thoug h less accurate thanRTD s. If the ap plication does not

require particularly tight tem pera-

ture control, an inexpensive therm o-

coup le and a w ell-tuned PID loop

should do the trick. But for process-

es that w ill only w ork correctly at

very specific tem peratures, it w ould

be a m istake not to p ay for the

greater accuracy that an RTD

affords. The cost of scrap ping a

batch of under-cooked or scorched

prod ucts w ould eventually dw arf

any savings in eq uipm ent costs.

A fast sensor can also b e w orth

the extra cost. If the process

req uires a rap id succession of

heating and cooling cycles, the

tem perature sensor m ust be able

to generate a reading before its too

late to be of any use. D espite their

cheaper ped igree, therm ocouples

tend to respond faster than R TD s

so if speed is the only im portant

perform ance issue, choose a

therm ocouple.

>> M is t a k e # 2 :

Insta ll ing se nsors

incorrectly

Placem ent

The best sensor can yield disap-

pointing results if not installed cor-

rectly. M agm eters, for exam ple,

tend to generate noisy signals if the

flow theyre m easuring is turbulent.

B end s, junctions, and valves in a

pipe can all cause turbulence, thus

m agm eters w ork best w hen

installed in sections of straight

pipe.

Tem perature sensors are also

sensitive to p lacem ent. Even a

highly accurate R TD tucked in the

corner of a m ixing cham ber w illonly be able to d etect the tem pera-

ture of its im m ediate vicinity. If the

m ixing of the m aterial in the cham -

ber is incom plete, that local tem -

perature m ay or m ay not represent

the tem perature of the m aterial

elsew here in the cham ber.

Local tem perature issues are

the classic m istake that hom e heat-

ing contractors often m ake w hen

installing household therm ostats. A


4/7

THE FO U R B IG G ES T M IS TA KES IN IN S T RU M EN TA T IO

m ounting location closest to the

furnace m ay be convenient for

w iring purposes, but if that spot

happens to b e in a hallw ay or other

dead air space, the therm ostat w ill

not be able to d eterm ine the aver-

ag e tem perature elsew here in the

house. It w ill only be able to m ain-

tain the desired tem perature in its

im m ediate vicinity. The rest of the

house m ay end up roasting or

freezing.

Controller perform ance

Poor control also results w hen a

sensor is installed too far aw ay

from the associated actuator. A dis-

tant sensor m ay not be able to

m easure the effects of the actua-

tors last m ove in tim e for the con-

troller to m ake an ed ucated deci-

sion ab out w hat to do next.

For exam ple, consider the

process of flattening hot steel into

uniform sheets by m eans of tw o

opposing rollers (see Figure 1). A

thickness sensor dow nstream from

the rollers gauges the sheet and

causes the controller to apply

either m ore or less pressure to

com pensate for any out-of-spec

thickness.

Ideally, the thickness sensor

should be located ad jacent to the

rollers to m inim ize the tim e

betw een a change in roller pres-

sure and the resulting change in

the thickness m easurem ent.

O therw ise, the controller w ill not be

ab le to detect any m istakes it m ay

have been m aking soon enough to

prevent even m ore of the sheet

from turning out too thick or thin.

W orse still, an appreciable dead

tim e betw een the controllers

actions and the resulting effects on

the steel can cause the controller

to b ecom e im patient. It w ill see no

results from an initial control m ove,

so it w ill m ake another and another

until som e chang e b eg ins to

appear in the m easurem ents

rep orted by the sensor. B y that

tim e, the controllers cum ulative

efforts w ill have alread y overcom -

pensated for the original error,

causing an error in the opposite

direction. The result w ill be a con-

stant series of up and dow n sw ings

in the roller pressure and a lot of

steel ruined by lateral corrugations

O f course, overall process per-

form ance considerations arent lim -

ited to how w ell the sensor feed s

data to the controller during opera-

tion. O ther factors to consider

include ease of installation and

tim e spent on the selection

process, set up routines, and any

lab or-intensive m aintenance.

Fortunately, som e instrum entation

vend ors design their sensors to

accom m odate such challeng es,

thereb y im proving perform ance

before the system even goes on-

line. A B B , for instance, offers a

sw irl flow m eter that significantly

reduces the need to install special

upstream and dow nstream devices

to accurately m easure the flow

through a p ipe.

Protection

A steel m ill is also a classic exam -ple of a harsh environm ent that can

destroy inadequately p rotected

sensors. Fortunately, the hazards

posed by a m anufacturing process

are generally ob vious and can often

be overcom e by installing a shield

or choosing a rug ged instrum ent.

O ften overlooked , how ever, are

the effects of w eather. O utdoor

instrum ents can take quite a beat-

ing from rain, snow , hail, and falling

CONTROLLER

D

PISTON

ADJUSTABLEROLLER

FIXEDROLLER

OPTICALTHICKNESSGAUGE

FI G U R E 1 . P O O R S E N S O R P L A C E M E N T

In this steel rolling exam ple, D is the distance betw een the steel rollers and the thickness

gauge dow nstream . If D is too large, the controller w ill take too long to correct thickness

errors and m ay even m ake m atters w orse by becom ing im patient.


5/7

N D H O W T O AV O I D T H EM 5

Instrum ents m ust be

ground ed to provide

a reference voltage

for the d ata signals

they g enerate.

R elying on earthground is risky since

not all of the earth

shares the sam e

electrical potential.

The resulting currents

w ill interfere w ith the

sensorssignals.

ice. O ver tim e, outdoor instrum ents

can fail slow ly unless enclosed in

ap prop riate housing s.

B ut even the housings them -

selves can cause problem s for the

enclosed instrum ents, particularly

tem perature sensors. If an RTD or

therm ocoup le is m ounted on the

sam e piece of m etal that supports

the housing, the housing w ill w ork

like a heat sink w hen the am bient

tem perature drops low enough. It

w ill tend to draw heat out of the

sensor and artificially low er its

reading. The heat-sink effect w ill

also tend to reduce the benefits of

any internal heat that has been

ap plied to p revent an instrum ent

from freezing.

C onversely, if a housing is

equipped w ith fins intended todraw heat out of the enclosed sen-

sor during w arm w eather, the fins

m ust be m ounted vertically.

O therw ise, the w arm air around the

fins w ill not be able to rise aw ay

from the housing (see Figure 2).

G round loops

W hile its g enerally a good practice

to insulate a sensor from the ther-

m od ynam ic effects of its surround -

ings, its absolutely critical to estab-

lish electrical isolation. The m ost

com m on electrical prob lem s due to

poor installation are ground loop s.

G round loop s occur w hen an

extraneous current flow s through

the instrum entation w iring betw een

tw o p oints that are supposed to b e

at the sam e voltage, but arent (see

Figure 3). The resulting electrical

interference can cause random

fluctuations in the sensorsoutput

and m ay even dam age the sensors

them selves.

A s the nam e im plies, ground

loops m ostoften occur w hen instru-

m ents and theircab les are ground-

ed im properly or not at all.

Interestingly, the best w ay to isolate

a plants instrum ents from ground

loop currents is to connect them

tog ether at one m aster ground ing

point.

If thats not possible, a grid of

ground ing points m ust be spread

throug hout the plant, m aking sure

that all points on the grid are at

the sam e electrical potential.

Insecure connections and inade-

quate w ires can cause a voltag e

im balance in the grid and ground

loops betw een the instrum ents

connected to it.

Even the orientation of an instrum ent can affect its p erform ance. H ere, the sensor

is enclosed in a housing designed to d issipate the heat it generates. The fins m ust

be m ounted vertically to allow w arm air to escape.

FI G U R E 2 . P O O R M O U N T I N G

} Data signalsto I/O panel

} Earth grounds atunequal voltages

} Instruments

Ground loop currents

FI G U R E 3 . P O O R G R O U N D I N G


6/73

THE FO U R B IG G ES T M IS TA KES IN IN S T RU M EN TA T IO

>> M is t a k e # 3 :

Ge nerat ing g ibber ish

N oise

G round loops are not the only

source of noise that can distort a

sensors readings.R ad io frequency

interference(R FI) is even m ore

com m on in plants that use w alkie-

talkies, pagers, and w ireless net-

w orks extensively. R FI also results

w henever a current changes, such

as w hen an electrom echanical con-

tact or a static discharge generates

a sp ark.

The sources of R FI noise m ust

be elim inated or at least kept aw ay

from the plants instrum entation if at

all possible. R eplacing electro-

m echanical eq uipm ent w ith solid-

state devices w ill elim inate arc-gen-

erated R FI. O r, it m ay be sufficient

to sim ply relocate sw itch boxes

and relays to instrum ent-free areas

of the plant. If all else fails, it m ay

be p ossible to p assively shield the

source of the interference or the

instrum ents being subjected to it.

Ignoring the prob lem is not an

option, especially w hen the source

of the noise is ordinary house cur-

rent. A t 60 H z, house current oscil-

lates slow ly enough to have an

ap preciab le effect on som eprocesses.

C onsider the steel rolling appli-

cation again. A 60 H z noise super-

im posed on the output of the thick-

ness gauge w ill pass through the

controller and ind uce a 60 H z oscil-

lation in the roller pressure. If the

sheet exits the rollers w ith a veloci-

ty of six feet per second, those

oscillations w ill ap pear as b um ps in

the sheet ap pearing every tenth of

an inch. W hether those flaw s are

ap preciab le or not w ill dep end on

the am plitude of the original noise

signal, the inertia of the rollers, and

the tuning of the controller.

PID controllers tuned to p rovide

appreciable derivative action are

particularly susceptible to the

effects of m easurem ent noise. They

tend to react aggressively to every

blip in the m easurem ent signal to

quickly suppress deviations from

the setpoint. If a blip turns out to

be nothing but noise, the controller

w ill take unw arranted corrective

actions and m ake m atters w orse.

Filtering

U nfortunately, it is not alw ays pos-

sible to elim inate noise sources

altogether. It is often necessary to

filter the raw sensor data by aver-

ag ing several sam ples tog ether or

by ignoring any changes less than

som e sm all percentage. M any digi-

tal instrum ents, like A B Bs FSM

4000 flow m eter, com e eq uipped

w ith built-in filters.

H ow ever, it is a m istake to think

that num ber crunching alone can

fix all m easurem ent noise prob -

lem s. Filtering tends to increase the

tim e req uired to detect a change in

the m easured value and can evenintroduce spurious inform ation into

the signal. W orse still, it can m ask

the actual behavior of the p rocess

if it is overdone.

It is generally m ore cost-effec-

tive in the long run to install sen-

sors correctly and m inim ize the

sources of interference than to rely

strictly on m athem atics to separate

the data from the noise. W hen con-

structing a control loop, data filters

should be applied in the final

stages of the project, just before

loop tuning .

M is t a k e # 4 :Quit t ing too soon

Even w hen the data filters are in

place and the last loop has been

tuned, the project isnt over. There

are som e com m only neg lected

chores that should continue as

long as the instrum entation system

is in place.

C alibration

M ost instrum entation engineers

know that a sensor m ust be cali-

brated in order to associate a

num erical value w ith the electrical

signal com ing out of the transm itter

Yet all too often, the instrum ents are

calibrated just once during installa-

tion then left to operate unattended

for years.

The result is an insidious prob-

lem known as drift. A sensors out-

put tends to creep higher and high

er (or low er and low er), even if the

m easured variab le hasnt changed

D ep osition on the sensing sur-

faces, corrosion in the w iring, and

long term w ear on m oving parts

can all cause an instrum ent to

begin generating artificially high (orlow ) read ings. A s a result, the con-

troller w ill gradually increase or

decrease its control efforts to com -

pensate for a non-existent error.

A nalog instrum ents are particu-

larly susceptible to drift, m uch like

old FM rad ios. The slightest nudge

on the dial could cause the radio to

lose its signal. W ith m odern digital

radios, the one true frequency for

each station is digitally encoded at


7/7

N D H O W T O AV O I D T H EM

a fixed value. Sim ilarly, m odern

instrum ents that em ploy digital sig-

nal processing cant be nudged.

They m aintain the sam e calibration

in the field as in the lab.

D rift can also be red uced by the

choice of sensing technolog y.

Tem perature sensors w ith m ineral

insolated cables, for exam ple, are

less prone to drift. D rift due to w ear

can be elim inated entirely b y

choosing instrum ents w ith no m ov-

ing parts, like A B Bs sw irl and vor-

tex m eters.

A nd even w hen drift cannot be

elim inated, recalibrating every sen-

sor in the plant at intervals recom -

m ended by their m anufacturers

can accom m od ate it. U nfortunately,

project eng ineers are often so anx-

ious to finish a job and get on w ith

operating the p rocess that they

neglect such basic m aintenance.

A rguably the m ost challenging

sources of drift are those that vary

over tim e. D eteriorating prob es and

m oving parts beginning to w ear out

can slow ly change an instrum ents

accuracy. So m aintenance calibra-

tion is req uired periodically even if

there are no know n issues w ith the

instrum ent.

Som e m anufacturers are recog-

nizing the tim e and efforts involvedin traditional recalibration exercises

and are designing instrum entation

products to sim plify m atters. For

exam ple, the C alM aster portab le

calibrator from A B B provides in-situ

calibration verification and certifica-

tion of A B Bs M agM aster electro-

m agnetic flow m eters w ithout req uir-

ing access to the flow m eter or

op ening the pipe.

Instead, the operator sim ply

connects a C alM aster to the

flow m eters transm itter and a PC . A

W indow s interface guides the op er-

ator through a series of tests to

evaluate the status of the transm it-

ter, sensor, and interconnecting

cables. The tests are com plex, but

so autom ated that the w hole cali-

bration routine can b e accom -

plished in 20 m inutes.

O nce the tests are com plete,

C alM aster will evaluate the m eas-

urem ents taken. If all satisfy the cali-

bration req uirem ents, then a calibra-

tion certificate can be printed either

at that tim e or later. These certifi-

cates can then b e catalog ued in

order to m eet auditing and reg ulato-

ry requirem ents such as ISO 9001.

A n added benefit of C alM aster

is that it can be used as a d iag -

nostic and condition m onitoring

tool. It autom atically stores all

m easured values and calibration

inform ation in its ow n d atabase

files for each m eter, thus m aintain-

ing a calibration history log and

m aking it easier to undertake long-

term trend analysis. D etailed

ob servation can give early w arning

of possible system failure,

enab ling the m aintenance engi-

neer to anticipate p rob lem s and

take proactive rem edial action.Such autom ated system s m ake

routine verification of flow m eter cal-

ibration and the traceability of infor-

m ation m uch less cum bersom e

and costly than in the past. In the

w ater industry, for exam ple, such

tasks form erly entailed m echanical

excavation of the flow m eter result-

ing in a disruption of the w ater sup-

ply and a substantial investm ent in

m anpow er and equipm ent.

Planning for the road

a h e a d

A ll too often, an exp ansion proj-

ect beg ins w ith w eeks of w onder-

ing w hy the existing instrum enta-

tion system w as constructed the

w ay it w as an d w hy it doesnt

m atch the projects original plans.

To avoid this, future planning

should be a part of your im ple-

m entation process and also

includ e thorough d ocum entation

of w hats been done before.

Som eone w ill eventually w ant to

exp and the project and w ill need

to know exactly w hich instru-

m ents have been placed w here,

w hat the instrum ents w ere sup-

posed to b e accom plishing , and

how they w ere installed and con-

figured.

Even if the instrum entation sys-

tem is never expanded , it w ill even-

tually have to b e repaired. W ires

break and sensors w ear out. A

good inventory of the system com -

ponents w ill indicate w hat need s to

be replaced, but thats only half the

battle. R ep lacem ent parts m ust be

acquired along w ith the technical

specs necessary to install them

correctly.

A n ong oing replacem ent parts

program is a m ust. Either the origi-nal vendor m ust m ake provisions

for stocking rep lacem ents (or

upgrades) for all the instrum ents

theyve provided to date, or the

project engineers m ust continue to

m onitor their suppliers to m ake

sure that spare p arts rem ain avail-

able. For hard-to-find instrum ents, it

m ay even b e necessary to m aintain

an in-house supply of rep lacem ent

parts, just in case.

7

4BiggestMistakesInInstrumentationAndHowToAvoidThem

Documents

Transcript of 4BiggestMistakesInInstrumentationAndHowToAvoidThem