Considerthe reactor systemshown in Figure 1G6.The reaction is exothermic.A cooling systemis provided to remove the excessenergyof reaction. In the event the cooling function is lost, the temperatureof the reactor would increase.This would leadto
an increasein reaction rate leading to additional energyrclease.The rcsult could be a
runaway reactionwith pressuresexceedingthe bursting pressureof the reactor.
The temperaturewithin the reactor is measuredand is usedto control the cooling water flow rate by a valve.
Perform aHAZOP on this unit to improve the safety of the process.
Solution The guide words are applied to the cooling coil system.The designintention is cooling. The results of the investigationare shown in Table 10-4.
Reac t or
M onom er F e e d
Cooling
Co o l i n g
Nat er
Cooling
to
Sewer
l. { at e r
Co i ls
In
T h e rmocoup l e
Flgure 1G.6 Temperature control of an exothermic reactor,
It
d
i
TABLE10.'I HAZOPSTUDYON DEVIATIoNS
FBoMcooLING FLow. APPLIED
To
REACTOR
OF FIGURE10.6
Guide
word
Deviation
Possiblecauses
Consequences
NO
No cooling
l. Control valve fails
closcd
2. Pluggedcooling
line
3. Cooling water
servicefailure
4. Controller fails
and closesvalve.
5. Air pressureto
drivc valve fails,
closingvalve
l. Temperature
increasein reactor
2. Possiblethermal
funaway
l. Installback-up
control valves,or
manual bypass
valve.
2. Installfilters to
preventdebris.
from entering line
3. Installback-up
coolingwater
source
4. Install back-up
controller
5, Installcontrol
valvethat fails
open
6. Installhigh
temperaturealarm
to alert operator
7. Install high
temperature
emefgency
shutdown
8. Installcooling
water flow meter
and low flow
alarm
MORE
More
cooling
f low
L Control valve fails
to open
2. Controller fails
and opensvalve
l. Reactorcools,
reactantbuilds-up,
possiblerunaway
on heating
l. Instruct operators
on procedure
LESS
Less
cooling
flow
l. Control valve fails
to respond
2. Partially plugged
coolingIine
3. Partialwater
sourcefailure
1. Coveredunder
"NO"
l. Coveredunder
"NO"
AS \T/ELL
AS
Cooling
watcr in
rcactor
Leak in cooling
coils, pressurein
reactorlessthan
prcssurc in coils
l. Dilution of
contents
2. Product ruined
3. Ovcrfilling of
reactor
l. Install high level
and/or pressure
alarm
2. Install proper
rclief
3. Check
maintenance
procedurcand
schedulc
AS WELL
AS
Reactor
product
in coils
Leak in coils with
reactof pfessure
greaterthan coil
l. Productlost thru
coils
2. Loss of product
yield
3. Reductionin
cooling function
4. Possible
contaminationof
water
l. Check
maintenance
procedureand
schedules
2. Install upstream
checkvalve in
cooling water
soufce
Pressufe
PART OF
Partial
cooling
flow
Coveredunder
"LESSCOOLING
FLOW"
REVERSE
Reverse
cooling
flow
l. Failureof water
sourceresultingin
backwardflow
2. Backflow due to
backpressure
l. Impropercooling,
possiblerunaway
l. Installcheckvalve
in coolingwater
line
2. Install high
temperaturealarm
to alert operator
OTHER
THAN
Another
material
besides
cooling
\4rater
L Water source
contaminated
2. Backflow from
sewer
l. Possiblelossof
coolingwith
possiblerunaway
L lsolation of
cooling water
soufce
2. lnstall checkvalve
to preventfeverse
f low
3. Install high
temperaturealarm
.
lng.
The potential processmodificationsresultingfrom this study are the
follow-
' Installationof a high temperaturealarm to alert the operatorin the eventof
cooling function lòss.
' Installationof a high temperatureshutdownsystem.This systemwould automatically shutdownthe processin the event of a high reíctor temperature.
The shutdown temperaturewould be higher than thé alarm temperatureto
;
provide the operator with the opportunity to restorecooling befoie
the reac- :
tor is shutdown.
j
lnstallation
of a check valve in the cooling line to prevent reverseflow. A
'
I
checkvalvecould be installedboth beforeand afte. tùe reactorto
f.euent ttei
reactorcontentsfrom flowing upstreamand to prevent the backilow the'
in
eventof a leak in the coils.
.t,
. Periodicallyjnspect the cooling
coil to ingure its integrìty.
._
' Study of the coolingwater sourceto considerpossiblecontaminationand interruption of supply.
. Installation of a cooling water
flow meter and low flow alarm. This wil
provide an immediateindicationof cooling loss.
ln the eventthat the coolingwatersystemfails (regardless
of the sourceof the
failure), the high temperaturealarm and emergencyshutdownsysrem
preventsa
runaway.The reviewcommitteeperformingthe HAZOP decided
that tÉe installation of a backupcontrollerand control valve was not essential.
The high temperature alarm and shutdownsystempreventsa runawayin this event.
simiarly, a loss
of coolant water sourceor a pluggedcooling line would be detected
by either the
alarm or emergencyshutdownsystem.ThJ review committee
suggestedthat all
coolantwater failures
!e-.ploperlyreported.In the eventthat a particular causeoccurs repeatedlythen additionalprocessmodificationrur" *u.rànt"d.
that the number of processchangessuggestedis
. This exampledemonstrates
quite numerousalthoughonly a singleprocessintentiàn is considered.
Th" advantageto this approachis that it providesa more completeidentification of the hazards,includinginformationon hòw hazardscan develòp
as a resultof
operatingproceduresand.operationalupsetsin the process.The disadvantages
are
that the HAzoP appr:?cl is tediou-sto-appry,requiies considerabre
manpolier and
-ttur"io,
time, and can potentially identify all of thà
independenfof the risk.