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1
MSF EVAPORATOR START UP: FROM MANUAL TO FULLY AUTOMATIC
OPERATION
Authors:
E. Ghiazza, G. Chiola (Fisia Italimpianti)
ABSTRACT:
The start up of a MSF distiller is one of the most complex operations that can be carried out on a
desalination plant, far more complicated than distiller normal operation (at constant production) or
even than distiller load increase or decrease.
In the earliest times start up operation were performed completely manually acting directly on the
single plant equipment (pumps, valves, etc.) in the field, but nowadays the trend is towards start up
operations with higher and higher level of automation.
The paper presents an extensive overview of the various types of evaporator start up, differentiating
among manual, remote, semi-automatic and fully automatic operation, focusing on the necessary
plant requirements and operational sequences to made the desired kind of start up possible.
The difference between the cases of cold evaporator start up (after a long shut down) and of warm
evaporator start up (after a short shut down) is also analysed.
A final discussion about future possibilities of further automation of this operation is presented.
INDEX
INDEX 1
FIGURES INDEX 1
1. INTRODUCTION 2
2. PRELIMINARY PREPARATION OF THE PLANT 3
3. START UP SEQUENCES 3
4. DIFFERENT MODES OF START-UP 5
4.1. MANUAL MODE AND REMOTE MODE 6
4.2. SEMI AUTOMATIC MODE AND FULLY AUTOMATIC MODE 6
4.2.1. From minimum to nominal load 7
5. CONCLUSIONS 8
FIGURES INDEX
Fig. 1 Desalination unit systems and general start up sequence 2
Fig. 2 Time diagram for desalination unit cold and warm start up 4
Fig. 3 Time diagram for desalination unit warm start up after a short trip 4
Fig. 4 Flow diagram for make up system start up 5
Fig. 5 Main variables time diagram for plant loading 7
2
1. INTRODUCTION
The start up of a desalination unit has to be carried out according to some pre-defined sequences.
Each sequence corresponds to a single system in the plant, where different equipment (pumps,
valves, etc.) are involved. With the lowest degree of automation, the equipment belonging to each
system can be started only from field, but in most of the modern plants most of the equipment can
be started automatically from CCR (Central Control Room). With an higher level of automation a
whole system can be automatically started from CCR, and as a limit situation, with the highest
degree of automation, the whole plant could be in principle automatically started from CCR.
According to these scenarios, the following operating mode have been considered:
a) equipment local start-up
b) equipment start-up from CCR
c) single system start up from CCR
d) plant start-up from CCR
e) automatic plant loading from CCR
The last operating mode e) corresponds to the loading of the plant from minimum load (at the end
of start up operation) to nominal or maximum load (according to the water demand).
There are several systems for each desalination unit as detailed in Fig. 1.
Fig. 1 Desalination unit systems and general start up sequence
PRE- CONDITIONS TO
START-UP FULLFILLEDHYPOCHLORITE PLANT
SEA WATER SUPPLY AND
SEA WATER TO UTILITIES1
HYPOCHLORITE
DOSING2
MAKE-UP4
BLOWDOWN5
ANTIFOAM DOSING6
VACUUM SYSTEM7
BRINE
RECIRCULATION8
ANTISCALE DOSING9
SODIUM SULPHITE
DOSING10
CONDENSATE FROM
BRINE HEATER11
STEAM TO BRINE
HEATER12
PRODUCT WATER
STORAGE14
DISTILLATE
SYSTEM13
SEA WATER
RECIRCULATION3
ON LOAD TUBE CLEANING16
PLANT LOADING15
system or plantMAKE-UP4
start-up sequence
LEGEND
system
3
Not depending on which kind of start up will be performed, there are some pre-conditions to be
fulfilled in order to allow the start up sequences to be initiated. To fulfil these pre-conditions a
preliminary preparation of the plant must be completed before the start up.
It must be pointed out that even in case of a fully automatic start up, almost all the preliminary
operations have to be performed manually by the operator on the field, or in some cases from LCR
(Local Control Room).
2. PRELIMINARY PREPARATION OF THE PLANT
A certain number of manual operations and visual inspections have to be carried out by operator
and field attendants before initiating the start-up sequence, and several release conditions must be
verified to ensure that each sequence proceeds safely.
A list of minimum requirements that shall be met prior to start-up the desalination units is as
follows:
All instruments must be put in service (drained, vented, etc.) and visually inspected
All compressed air supply valves to instruments, cooling and other users must be opened
and visually inspected
All water pipes must be filled up and vented
Evaporator and brine heater tube bundles must be filled and vented
Pump suction lines up to the first delivery shut off valve must be filled and vented
Brine heater shell must be filled with water up to a nominal level
All manual valves must be open or closed according to their duty
Steam pipework must be heated-up
Vacuum must be raised in the evaporator and deaerator chamber by the hogging ejector
and phase ejectors must be put in operation.
The following systems which are not belonging to the desalination unit but are strictly connected to
it shall be monitored as well:
Sea water intake pumps
Chlorination system
Steam headers up to battery limit
Condensate pipework from battery limit to boiler deaerator
Distillate collecting header and tanks
3. START UP SEQUENCES
The desalination unit start up operation is performed according to a predefined general sequence
involving all the systems in the plant, as shown in Fig. 1. For transients and duration of each phase
of start up reference can be made to the diagram reported in Fig. 2.
The two different kinds of start up (cold start up and warm start up) can be distinguished on the
diagram according to the curve of last stage pressure. A cold start up ( for instance first start up or
start up after long maintenance period) includes the rebuilding of the vacuum into evaporator,
including in the diagram the time from 0 to 150 minutes. A warm start up ( for instance after a short
trip) is possible whenever the vacuum into evaporator is not lost or is only partially decreased, and
everything can be started up again as represented in the diagram for the time from 150 minutes on.
As a further reference , the preconditions for a warm start up are represented in diagram in Fig. 3,
for a plant trip not overcoming the situation of time 120 minutes (no distillate production and last
stage pressure about to increase ).
To carry out the desalination unit start up, both in manual, remote, semi-automatic and fully
automatic mode, detailed sequences for all the main systems are followed. An example is reported
in the flow diagram of Fig. 4 for the activation sequence of the sea water make up system.
The example flow diagram has to be intended valid for first start up, cold start up and warm start
up, since tests and conditions to be checked are included in the sequence to account for each
possible case.
4
Fig. 2 Time diagram for desalination unit cold and warm start up
Fig. 3 Time diagram for desalination unit warm start up after a short trip
MSF EVAPORATOR START UP DIAGRAM
distillate
last stage pressure
make up
steam to b.h.
steam to ejectors
brine recycle
cooling water
0 60 120 180 240 300 360
minutes
coolin
g w
ate
r
t/h
6700
6000
5000
4000
3000
2000
1000
0
bar
1.0
0
t/h
0
last sta
ge p
ress.
ste
am
to e
jecto
rs
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
12
make u
p
t/h
t/h
ste
am
to b
.h.
brine r
ecycle
6250
6000
5000
3000
2000
1000
4000
t/h
dis
tilla
te
620
600
500
300
200
100
0
400
100
90
80
70
60
50
30
20
40
10
t/h
0
2000
1200
800
1600
400
REMARK:The diagram provides the start up times and sequences of main operations to bring the desalination plant from cold to 105° TBT with 35°C SW
The diagram is only for informastion and should be read together with the start up procedures
00
MSF EVAPORATOR SHUT DOWN DIAGRAM
distillate production
last stage pressure
make up
steam to b.h.
steam to ejectors
brine recycle
cooling water
-60 0 60 120 180 240minutes
co
olin
g w
ate
r
t/h
6700
6000
5000
4000
3000
2000
1000
0
bar
1.0
0
t/h
0
last
sta
ge
pre
ss.
ste
am
to
eje
cto
rs
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
12
make
up
t/h
t/h
ste
am
to
b.h
.
brine
re
cycle
6250
6000
5000
3000
2000
1000
4000
t/h
dis
tilla
te
620
600
500
300
200
100
0
400
100
90
80
70
60
50
30
20
40
10
t/h
0
2000
1200
800
1600
400
REMARK:The diagram provides the shut down times and sequences of main operations to bring the desalination plant from 105° TBT to cold
The diagram is only for informastion and should be read together with the shut down procedures
00
5
Fig. 4 Flow diagram for make up system start up
4. DIFFERENT MODES OF START-UP
The different modes of start up are strictly connected with the automation degree present on the
plant.
To better understand the needs for each mode the automation of the plant can be divided in the
following four Vertical jerarchical levels:
drive level: operates on the single drive (motor, valve, control device). Single open and
closed control loops, interlocking functions of single drivers are performed at this level.
sub-group level: concerns grouped drives, as for example groups of pumps and their suction
and isolating valves. At this level take place necessary co-ordinator functions for automatic
machinery start-stop and where required stand-by function handling.
group level: concerns the functional groups and define interconnections between subgroups.
In this level subgroups are co-ordinated corresponding to the process plant requirements
during start-up or shut down.
For plants with high degree of automation a “co-ordinator level” can be foreseen, where functional
groups and subgroups are co-ordinated and automatic set points are actuated, to manage various
functional areas during plant loading.
For each of these levels a plant interaction capacity is provided generated by :
a manual command (command / control instrument on desk or panel) by means of command
push button / led station with numerical step indicator.
FIRST MAKE UP STRAINER INLET VALVE OPEN
SECOND MAKE UP STRAINER INLET VALVE OPEN
MAKE UP READY TO START
ALARM
STEP 1
STEP 2
STEP 3 STEP 4
STEP 5
STEP 6
STEP 7END
AND
OR
FIRST MAKE UP STRAINER
INLET VALVE
OPEN
START MAKE UP
AND
SWITCH MAKE UP FLOW CONTROLLER
TO MANUAL AND CLOSE MAKE UP FLOW
CONTROL VALVE
OPEN FIRST MAKE UP STRAINER
OUTLET VALVE
OPEN SECOND MAKE UP STRAINER
OUTLET VALVE
OR
ADJUST THE SET POINT TO
MAKE UP MINIMUM AND OPEN FLOW CONTROL
VALVE TO REACH THE VALUE
SWITCH THE MAKE UP / DISTILLATE RATIO
CONTROLLER TO AUTO
6
an automatic command request (on/off program) that comes in execution on request of
programmable logic sequence defined in the hardware that is provided for sub-group/group
level on the base of process requirements.
For plants with a high level of automation a Horizontal exchange of information between other
distiller units and other related systems (e.g. distillate town water, reminalization-chlorination,
boiler or turbine) is also possible.
4.1. MANUAL MODE AND REMOTE MODE
The MANUAL MODE does not involve any automatic operation from CCR, and only the "Drive"
level can be used by the operator directly on the equipment themselves. This kind of start up was
used in the past when the capabilities of automation systems and of remote controlled devices were
far below the level reached today, or even totally absent. In modern plants the manual start up of a
desalination unit is almost never used anymore.
The start up in REMOTE MODE constitutes an improvement with respect to the previous one.
Again the operator has to start up each single equipment, but some of them are motorized and
remote controlled. This means that the start up of most devices can be carried out from CCR
without the need to go and act on the field. During this “operator managed” plant start-up each step
of the sequence for each system can in this way be initiated by the operator acting on the
instrumentation and control system at the "Sub-group" level (remote controlled devices) and/or at
the "Drive" level (locally controlled devices).
4.2. SEMI AUTOMATIC MODE AND FULLY AUTOMATIC MODE
A further improvement is represented by the SEMI AUTOMATIC MODE of start up during which
the operator will initiate the start-up sequence acting on the instrumentation and control system at
the "Group" level.
The semi-automatic mode may be used for the plant start-up from cold condition, as well as to
resume the operation from hot conditions.
When the semi-automatic start-up is initiated, the instrumentation and control system, according to
the permissive signals from process or from equipment protections, performs the various steps of
the start-up sequence and gives to the operator information relevant to the sequence position and to
the criteria to be fulfilled to proceed to the next step
The basic philosophy for the semi-automatic plant start-up is to keep all the time the plant operator
fully aware of the sequential step under progress. The sequence for each systems is automatically
performed, except in case manual overriding should become necessary. At the end of the sequence a
confirmation is requested to the operator before going on with next sequence. Moreover, within the
sequence, some manual actions (e.g. filling or warming-up of systems, priming of pumps, etc.) may
be required in order to guarantee the safety of the plant or of single equipment. In such a case the
sequence is stopped until the missing criteria relevant to the manual action is fulfilled.
A further step on this way which is sometimes advised or requested is the FULLY AUTOMATIC
MODE of start up. In this way all the sequences are automatically performed, but no confirmation
from the operator is requested. The start up will automatically proceed from a sequence to the next
one providing that the operating conditions in the preceding sequence have attained acceptable
stability conditions. These checks are made relying on permissive signals coming from the plant.
To allow this mode in acceptably safe way the number of signals needed can be very huge, and also
a very high amount of motorized and remote controlled devices is requested. This has a strong
impact on the costs of the plant, but on the other hand the corresponding benefits are not of the
same magnitude. In fact, for safety reasons some steps should be anyway left to human decision,
and the time requested for start up would not be reduced significantly. In addition to that, the
systematic use of the automatic mode would result, in the long run, to loss of expertise from the
operator.
7
4.2.1. From minimum to nominal load
When the start up operations have been completed for all the systems, the plant is at minimum load.
From this point it is possible to increase automatically the load up to a preset value. To achieve the
desired production, the difference between the actual production level and the requested one has to
be evaluated, and the new plant set points are calculated.
According to the steam availability and to the sea water temperature a limitation of the maximum
permissible load should be provided. This in order to avoid troubles when attempting to take from
the boilers a quantity of steam higher than the one available.
According to the sea water temperature a limitation of the minimum permissible load should be
provided. This in order to avoid that at very low load with very low sea water temperature the unit
cools down, the level increases in the stages, the distillate is polluted by brine sprayed on the
demisters and eventually the control of the process is lost.
During the load variation the brine recycle flow set point should follow the TBT variation. Since an
increase / decrease in the TBT value at constant brine recycle flow would result in a change of the
evaporator stages brine levels.
In order not to exceed the maximum design recycle flow, for every temperature of sea water at
reject inlet, the brine recycle flow control should prevent evaporator system trouble in case brine
recycle flow is greater than the maximum pumps capacity or in tube velocity results in too high
values, to avoid erosion problems.
In order not to exceed the minimum design recycle flow, for every temperature of sea water at reject
inlet, the brine recycle flow control should also prevent evaporator system trouble in case brine
recycle flow is lower than the minimum pumps capacity or in tube velocity results in too low
values, to avoid fouling problems.
During the load variation the sea water make up flow should be automatically adjusted by a pre-
selected ratio with the distillate flow.
During the load variation the antiscale solution flow should be automatically adjusted by a pre-
selected curve with the top brine temperature.
The variation of main process variables versus time during a load change is reported in Fig. 5.
Fig. 5 Main variables time diagram for plant loading
Min. winter
Max. winter
80
85
90
95
100
105
110
115
-20.0 0.0 20.0 40.0 60.0 80.0 100.0
time [min.]
To
p b
rin
e t
em
p. [°C
]
350
400
450
500
550
600
650
700
750
800
Dis
tilla
te p
ro
du
ctio
n [
t/h
]
TBT
Production - summer
Production - winter
Max. summer
Min. summer
Min. winter
Max. winter
5400
5600
5800
6000
6200
6400
-20.0 0.0 20.0 40.0 60.0 80.0 100.0
time [min.]
Brin
e r
ec
ycle
flo
w
[t/h
]
350
400
450
500
550
600
650
700
750
800
Dis
til
late p
ro
du
cti
on
[
t/h
]
Brine recycle - summer
Brine recycle - winter
Production - summer
Production - winter
8
5. CONCLUSIONS
From the review of the various modes for desalination unit start up, the following conclusions can
be summarised:
1. Considering the high ratio between hardware capabilities and costs for standard automation
components, the use of manual or remote start up in a modern desalination plant is not
advisable since time consuming and exposed to faults by human error. The system can be
cheaper as far as investment costs are concerned, but more expensive throughout the overall
life of the plant.
2. The use of fully automatic start up mode involves non standard high cost automation
components and increases dramatically the number of signals exchanged between the field and
the control system. Being the human factor in any case still important for some basic choices,
the advantage in start up time reduction is not comparable to the increase of cost, complexity
and personnel skill required.
3. A well designed semi-automatic start up system, with an optimal compromise between
automatic sequences and human interventions, seems to be at the moment the best solution,
achieving a reduction of operational costs in terms of time and safety with a increase in
investment costs quite reasonable.
FISIA ITALIMPIANTI 1
IDA WORLD CONGRESS Desalination & Water Reuse
San Diego, California USA August 29th – September 3rd, 1999
MSF Evaporator Start up:
From Manual to Fully Automatic Operation
E.Ghiazza - Fisia Italimpianti G.Chiola - Fisia Italimpianti
FISIA ITALIMPIANTI 2
MSF Evaporator start up: from Manual to Fully Automatic Operation
San Diego, California USA August 29th – September 3rd, 1999
FULLY AUTOMATIC:
start from CCR
from
MANUALLY:
direct action on equipment in field
to
FISIA ITALIMPIANTI 3
MSF Evaporator start up: from Manual to Fully Automatic Operation
San Diego, California USA August 29th – September 3rd, 1999
Desalination Unit
SYSTEMS:
PRE- CONDITIONS TOSTART-UP FULLFILLED
HYPOCHLORITE PLANT
SEA WATER SUPPLY AND
1
HYPOCHLORITE DOSING
2
MAKE-UP4
BLOWDOWN5
ANTIFOAM DOSING6
VACUUM SYSTEM7
BRINE RECIRCULATION
8
ANTISCALE DOSING9
SODIUM SULPHITEDOSING
10
CONDENSATE FROMBRINE HEATER
11
STEAM TO BRINE HEATER
12
PRODUCT WATER STORAGE
14
DISTILLATESYSTEM
13
SEA WATERRECIRCULATION
3
ON LOAD TUBE CLEANING16
PLANT LOADING15
FISIA ITALIMPIANTI 4
MSF Evaporator start up: from Manual to Fully Automatic Operation
San Diego, California USA August 29th – September 3rd, 1999
Plant Preliminary Preparation: Instruments in service and visually inspected Compressed air supply valves opened All water pipes filled up and vented Evaporator and brine heater tube bundles filled and vented Pump suction lines filled up and vented Brine heater shell filled with water up to a nominal level Manual valves in correct position Steam pipework heated-up Vacuum raised in evaporator and deaerator
Sea water intake pumps / Chlorination system Steam / condensate headers Distillate collecting header and tanks
FISIA ITALIMPIANTI 5
MSF Evaporator start up: from Manual to Fully Automatic Operation
San Diego, California USA August 29th – September 3rd, 1999
MSF EVAPORATOR START UP DIAGRAM
distillate
last stage pressure
make up
steam to b.h.
steam to ejectors
brine recycle
cooling water
0 60 120 180 240 300 360
minutes
cool
ing
wat
ert/h
6700
6000
5000
4000
3000
2000
1000
0
bar1.0
0
t/h
0
last
sta
ge p
ress
.
stea
m to
eje
ctor
s
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
12
mak
e up
t/h
t/h
stea
m to
b.h
.
brin
e re
cycl
e
62506000
5000
3000
2000
1000
4000
t/h
dist
illat
e
620600
500
300
200
100
0
400
100
90
80
70
60
50
30
20
40
10
t/h
0
2000
1200
800
1600
400
00
FISIA ITALIMPIANTI 6
MSF Evaporator start up: from Manual to Fully Automatic Operation
San Diego, California USA August 29th – September 3rd, 1999
MSF EVAPORATOR SHUT DOWN DIAGRAM
distillate productionlast stage pressure
make up
steam to b.h.
steam to ejectors
brine recycle
cooling water
-60 0 60 120 180 240
minutes
cool
ing
wat
ert/h
6700
6000
5000
4000
3000
2000
1000
0
bar1.0
0
t/h
0
last
sta
ge p
ress
.
stea
m t
o ej
ecto
rs
0.9
0.8
0.7
0.6
0 5
0.4
0.3
0.2
0.112
mak
e up
t/h
t/h
stea
m t
o b.
h.
brin
e re
cycl
e
62506000
5000
3000
2000
1000
4000
t/h
dist
illat
e
620600
500
300
200
100
0
400
100
90
80
70
60
50
30
20
40
10
t/h
0
2000
1200
800
1600
400
00
FISIA ITALIMPIANTI 7
MSF Evaporator start up: from Manual to Fully Automatic Operation
San Diego, California USA August 29th – September 3rd, 1999
SYSTEM START UP:
MAKE-UP
STRAINER A INLET VALVE OPEN
STRAINER A INLET VALVE OPEN
READY TO START
ALARM
STEP 1
STEP 2
STEP 3 STEP 4
STEP 5
STEP 6
STEP 7END
AND
OR
STRAINER AINLET VALVE
OPEN
START MAKE UP
AND
SWITCH FIC TO MANUAL AND CLOSE VALVE FV
OPEN STRAINER A OUTLET VALVE
OPEN STRAINER B OUTLET VALVE
OR
ADJUST THE SET POINT TO 700 m3/h AND OPEN FLOW CONTROL VALVE TO REACH 700 m3/h
SWITCH THE CONTROLLER 4235 FIC 101 TO AUTO
FISIA ITALIMPIANTI 8
GROUP LEVEL: sub group coordination functions
SUB GROUP LEVEL: coordination functions for machinery start / stop / standby
DRIVE LEVEL: Single Control Loops / Interlocks on single drivers
MSF Evaporator start up: from Manual to Fully Automatic Operation
San Diego, California USA August 29th – September 3rd, 1999
FISIA ITALIMPIANTI 9
MANUAL: by push button /led station with numerical step indicator
Interactions generated by COMMANDS:
AUTOMATIC: from execution on request on process requirements basis of programmable logic sequence
MSF Evaporator start up: from Manual to Fully Automatic Operation
San Diego, California USA August 29th – September 3rd, 1999
FISIA ITALIMPIANTI 10
MANUAL: operator direct action on single drivers in the field
DIFFERENT START UP MODES:
MSF Evaporator start up: from Manual to Fully Automatic Operation
San Diego, California USA August 29th – September 3rd, 1999
REMOTE: operator direct action on systems from field / CCR
SEMI AUTOMATIC: operator initiates each start up sequence at group level
FULLY AUTOMATIC: all sequences are automatically performed (no confirmations)
FISIA ITALIMPIANTI 11
from min to nominal / max distillate production PLANT LOAD VARIATION:
MSF Evaporator start up: from Manual to Fully Automatic Operation
San Diego, California USA August 29th – September 3rd, 1999
TBT
RECYCLE FLOW
CHEMICALS
MAKE - UP FLOW
Constraints: - steam availability - SWT
FISIA ITALIMPIANTI 12
Recycle time diagram during loading:
MSF Evaporator start up: from Manual to Fully Automatic Operation
San Diego, California USA August 29th – September 3rd, 1999
Min. summer
Max. summer
Min. winter
Max. winter
5400
5600
5800
6000
6200
6400
-20.0 0.0 20.0 40.0 60.0 80.0 100.0
time [min.]
Bri
ne r
ecyc
le fl
ow
[t/h]
350
400
450
500
550
600
650
700
750
800
Dis
tilla
te p
rodu
ctio
n [
t/h]
Brine recycle - summerBrine recycle - winterProduction - summerProduction - winter
FISIA ITALIMPIANTI 13
TBT time diagram during loading:
MSF Evaporator start up: from Manual to Fully Automatic Operation
San Diego, California USA August 29th – September 3rd, 1999
80
85
90
95
100
105
110
115
-20.0 0.0 20.0 40.0 60.0 80.0 100.0time [min.]
Top
brin
e te
mp.
[°C
]
350
400
450
500
550
600
650
700
750
800
Dis
tilla
te p
rodu
ctio
n [
t/h]
TBTProduction - summerProduction - winter
FISIA ITALIMPIANTI 14
MSF Evaporator start up: from Manual to Fully Automatic Operation
San Diego, California USA August 29th – September 3rd, 1999
CONCLUSIONS:
ADVANTAGES DISADVANTAGES
MANUAL Low investments costs Time consumingHuman error
SEMIAUTOMATIC
Considerable start up timereduction Human skill required
FULLYAUTOMATIC
Strong start timereduction
High investments costsComplexity(Human skill required)