MSF EVAPORATOR START UP: FROM MANUAL TO FULLY … · MSF EVAPORATOR START UP DIAGRAM distillate last stage pressure make up steam to b.h. steam to ejectors brine recycle cooling water

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


MSF Evaporator start up: from Manual to Fully Automatic Operation


FULLY AUTOMATIC:

start from CCR

from

MANUALLY:

direct action on equipment in field

to




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




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




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




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




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


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




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




MANUAL: operator direct action on single drivers in the field

DIFFERENT START UP MODES:



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)


from min to nominal / max distillate production PLANT LOAD VARIATION:



TBT

RECYCLE FLOW

CHEMICALS

MAKE - UP FLOW

Constraints: - steam availability - SWT


Recycle time diagram during loading:



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


TBT time diagram during loading:



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




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)

Documents

MSF EVAPORATOR START UP: FROM MANUAL TO FULLY … · MSF EVAPORATOR START UP DIAGRAM distillate last stage pressure make up steam to b.h. steam to ejectors brine recycle cooling water