Control System Instrumentation C h a p t e r 9
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Control Loop
Typical Control Loop
Transducers and Transmitters
element com"ined #ith a dri$ing element %transmitter&.
C h a p
Sensors
The repeata"ility of the sensor
temperature that is to "e measured is 3/0F
and the minimum temperature is 1//0F.
Then the )ero is 1//0F and the span is 2/0F
In addition if the measured temperature is
4no#n at t#o different sensor output le$els
%i.e. ma5s& the span and )ero can "e
calculated directly.
num"er of diagnostic capa"ilities.
trigger a #ash cycle due to "uildup on the electrode
surface.
for plugging of the lines to the D' cell.
Smart temperature sensors use redundant sensors to
identify drift and estimate e-pected life "efore failure.
more precise "ut are less rugged and cost more than
thermocouples %TC5s&.
seconds for *TDs thermistors and TC5s.
Additional thermal resistance on inside or on the
outside of the thermal #ell can result in an
e-cessi$ely slo# responding temperature
<ery fast measurement dynamics
*epeata"ility $aries =/.1> =1>
@ood repeata"ility and fast dynamic response.
:agnetic flo# meters and $orte- shedding
flo# meters are also used in certain situations.
They are more e-pensi$e "ut more relia"le
and re7uire less maintenance.
accuracy for all flo# meters.
Indicator in a Flo# Control Loop
FC
FT
'ressure Flo# Indicator
β is the ratio of the orifice diameter to the pipe diameter.
/.2 β /.B
'ressure drop at minimum flo# should "e greater than /. psi.
<orte- Shedding :eters
A "lunt o"ect is placed in the flo# path and
the fre7uency of tur"ulent oscillations
correlates #ith the flo# rate.
;seful for clean lo# $iscosity li7uids and
gases.
measuring )one.
:eter
:agnetic Flo# :eters
ased on measuring the current generated "y the flo# a conducting fluid through a magnetic field.
6a$e lo# pressure drop associated #ith them.
Are applied to conducti$e fluid %tap #ater is conducti$e enough&
Deposition on the electrodes is a limitation.
See picture in te-t.
:eter
:agnetic flo# meters and $orte- shedding
flo# meters re7uire less maintenance and
are generally more relia"le than orifice plate
flo# meters ;T they are much more
e-pensi$e.
$essel measured "y the differential pressure.
6as a repeata"ility of a"out =1> #ith a
time constant less than 1 second.
Le$el measurements "ased upon a float or
-rays are also used in special situations.
Analy)er Sensor Systems
@C most common composition analy)er. ased on plug flo# of a $olatile sample through a pac4ed "ed"eha$es as deadtime. Deadtime and repeata"ility depend on the particular components "eing measured.
#idespread use.
The "oo4 "riefly discusses commonly used sensors for the most
important process $aria"les. %See te-t.&
Transmitters
A transmitter usually con$erts the sensor output to a signal le$el
appropriate for input to a controller such as to 2/ mA.
Transmitters are generally designed to "e direct acting .
In addition most commercial transmitters ha$e an adusta"le
input range %or span&.
For e-ample a temperature transmitter might "e adusted so that
the input range of a platinum resistance element %the sensor& is
/ to 1/ C.
9
%"& dead"and.
thermowell)
time delays for electronic lines) less common today
compared to electronic transmissions!
(%) &inear 'rim
() qual *ercentage
actual valve performance
l fraction open of the $al$e %/ 1& f l l = = ≤ ≤
f l =
1 slopeKf
Leads to an e7ual > change in flo# for an e7ual > change in lift3l
2/9./
from *iggs .. Chemical 'rocess Control Gum"ers in ta"le
a"o$e correspond to C v f(l) d p in psi q in gal+min and g s is
specific gra$ityH
increased! f p through exchanger did not change,
valve would behave linearly (true for low flow rates),
since it takes most of p ! or higher flow rates, p
through exchanger will be important, changing effective
valve characteristics (valve must open more than
expected ⇒ nonlinear behavior)!
valves need to operate between 1. and 31.,
$al$eM'Kflo#
valve should be specified for the following manipulated
variables6
(b) low rate of reactants into a polymeriation reactor!
(c) low of effluent from a wastewater treatment
holding tank into a river!
(d) low of cooling water to a distillation condenser!
ailure philosophy6 7eep process pressure low,
protect environment (equipment and engineers)
4-$ / 5
Typical Control Loop
Transducers and Transmitters
element com"ined #ith a dri$ing element %transmitter&.
C h a p
Sensors
The repeata"ility of the sensor
temperature that is to "e measured is 3/0F
and the minimum temperature is 1//0F.
Then the )ero is 1//0F and the span is 2/0F
In addition if the measured temperature is
4no#n at t#o different sensor output le$els
%i.e. ma5s& the span and )ero can "e
calculated directly.
num"er of diagnostic capa"ilities.
trigger a #ash cycle due to "uildup on the electrode
surface.
for plugging of the lines to the D' cell.
Smart temperature sensors use redundant sensors to
identify drift and estimate e-pected life "efore failure.
more precise "ut are less rugged and cost more than
thermocouples %TC5s&.
seconds for *TDs thermistors and TC5s.
Additional thermal resistance on inside or on the
outside of the thermal #ell can result in an
e-cessi$ely slo# responding temperature
<ery fast measurement dynamics
*epeata"ility $aries =/.1> =1>
@ood repeata"ility and fast dynamic response.
:agnetic flo# meters and $orte- shedding
flo# meters are also used in certain situations.
They are more e-pensi$e "ut more relia"le
and re7uire less maintenance.
accuracy for all flo# meters.
Indicator in a Flo# Control Loop
FC
FT
'ressure Flo# Indicator
β is the ratio of the orifice diameter to the pipe diameter.
/.2 β /.B
'ressure drop at minimum flo# should "e greater than /. psi.
<orte- Shedding :eters
A "lunt o"ect is placed in the flo# path and
the fre7uency of tur"ulent oscillations
correlates #ith the flo# rate.
;seful for clean lo# $iscosity li7uids and
gases.
measuring )one.
:eter
:agnetic Flo# :eters
ased on measuring the current generated "y the flo# a conducting fluid through a magnetic field.
6a$e lo# pressure drop associated #ith them.
Are applied to conducti$e fluid %tap #ater is conducti$e enough&
Deposition on the electrodes is a limitation.
See picture in te-t.
:eter
:agnetic flo# meters and $orte- shedding
flo# meters re7uire less maintenance and
are generally more relia"le than orifice plate
flo# meters ;T they are much more
e-pensi$e.
$essel measured "y the differential pressure.
6as a repeata"ility of a"out =1> #ith a
time constant less than 1 second.
Le$el measurements "ased upon a float or
-rays are also used in special situations.
Analy)er Sensor Systems
@C most common composition analy)er. ased on plug flo# of a $olatile sample through a pac4ed "ed"eha$es as deadtime. Deadtime and repeata"ility depend on the particular components "eing measured.
#idespread use.
The "oo4 "riefly discusses commonly used sensors for the most
important process $aria"les. %See te-t.&
Transmitters
A transmitter usually con$erts the sensor output to a signal le$el
appropriate for input to a controller such as to 2/ mA.
Transmitters are generally designed to "e direct acting .
In addition most commercial transmitters ha$e an adusta"le
input range %or span&.
For e-ample a temperature transmitter might "e adusted so that
the input range of a platinum resistance element %the sensor& is
/ to 1/ C.
9
%"& dead"and.
thermowell)
time delays for electronic lines) less common today
compared to electronic transmissions!
(%) &inear 'rim
() qual *ercentage
actual valve performance
l fraction open of the $al$e %/ 1& f l l = = ≤ ≤
f l =
1 slopeKf
Leads to an e7ual > change in flo# for an e7ual > change in lift3l
2/9./
from *iggs .. Chemical 'rocess Control Gum"ers in ta"le
a"o$e correspond to C v f(l) d p in psi q in gal+min and g s is
specific gra$ityH
increased! f p through exchanger did not change,
valve would behave linearly (true for low flow rates),
since it takes most of p ! or higher flow rates, p
through exchanger will be important, changing effective
valve characteristics (valve must open more than
expected ⇒ nonlinear behavior)!
valves need to operate between 1. and 31.,
$al$eM'Kflo#
valve should be specified for the following manipulated
variables6
(b) low rate of reactants into a polymeriation reactor!
(c) low of effluent from a wastewater treatment
holding tank into a river!
(d) low of cooling water to a distillation condenser!
ailure philosophy6 7eep process pressure low,
protect environment (equipment and engineers)
4-$ / 5
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