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MSF (Multi Stage Flash) desalination plants: Copper Alloy Tubes, their economic aspects for overall cost reduction
Desalination,
55 (1985) 307--317 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
MSF (Multi Stage Flash) Desalination Plants: Copper Alloy Tubes, Their Economic Aspects for Overall Cost Reduction
by: Dr. Vittorio Canetta, Chairman, Quality Label Association for Copper Alloy Tubes for Desalination (Q.L.A.) - Zurich Dr. N.M. Valota, Technical Committee Member, Quality Label Association for Copper Alloy Tubes for Desalination (Q.L.A.) - Zurich
i. FOREWORD Only a few and concise words are needed in front of a qualified audience of desalination experts to emphasize how much desalination plants have contributed,
and are likely to do so for the next decades to the year
2000, to the economical and welfare growth in such arid areas as the Arabian Gulf, the Red Sea and North African coasts. Desalination plants for sea water are a fundamental
aspect of infrastruc-
ture to sustain the growing industrialization and population in these countries, where rainfall is poor and rivers not available, where - as in Tripoli and Bahrain - the water acquifer decreases yearly or where huge industrial towns like
Jubail and Yanbu are erected in a previously
desert landscape. While during the '70s, however, desalination plants mushroomed in these mostly Arabic and oil-rich countries,
the rate of investment in desalina-
tion during the last five years has declined sharply, as a consequence of oil price decreases and growing current account deficit in petroleum countries.
0011-9164/85/$03.30
Elsevier Science Publishers B.V.
307
308 But the need for them is still there, and - as an example - H. M. Kin K Fahd of Saudi Arabia, for instance, has announced in his 4th five-year Plant (1985-1990) that water resources in his country should increase by some 400 MIGPD until 1990
(mostly by seawater desalination) 3 as
against present installed capacity of about 500 MIGPD. Budget constraints,
however, require careful consideration and analysis
of overall plant costs, much more than in the booming years after the oil crisis: and cost reductions nowadays are only likely to come from technical rather than commercial considerations,
in view of recent dra-
matic plant price decreases which have entirely wiped out any profit margin and brought the situation to a bone-dry condition. Being involved since two decades in desalination as suppliers of heat exchange tubes, and of ancillary non ferrous metal semis like tubeplates and sheets, we would like to enlarge on this subject as a significant item of possible cost reductions.
2.
COPPER ALLOY TUBES IN MSF DESALINATION
The MSF (multi stage flash distillation) process is the world leader for seawater desalination,
especially where it is coupled with availabi-
lity of cheap energy and/or steam. Statistics show that in 1980 MSF repre sented some three fourths of total installed capacity world-wide (and some 90% in the Arabian Gulf), and no significant change is likely to have occurred in
these figures since.
309 The reasons of its predominace are clear-cut: ce, ruggedness and ease of operation,
long-standing experien-
independence from excessively
sophisticated and exotic materials. It is true that in recent years, more and more attention has been paid to RO technologies,
and - as in Bahrain - some very interesting
large-scale experiments are made to advance and test their economic performance and service reliability; but questions have still to be answered definitely,
e.g. about
memhrsne
durability and availability.
So, the MSF process will certainly retain a fairly constant share of the market in the years ahead; this share will, of course, be easier to defen
if techno-economical
improvements are fostered and introdu-
ced. Copper alloy (brass
and cupro-nickel ) tubes are almost exclusively
chosen when the plant is of the MSF type, and have over three decades of good service behind them. Their economical impact on plant cost is quite apparent when remembering that in gross detail a MSF desalinator's cost would be broken down as follows:
810
- Civil works
20 - 25 %
- Mecanical construction and erection
35 - 50 %
- Engineering
-
Tubes and related semis
5 -
8 %
20 - 25 %
Thus, between one fourth and one fifth of equipment cost is made of copper alloy materials. Notwithstanding, however, their economical importance, copper alloys have received proportionately little attention hitherto, and tradition has often prevailed over techno-economical considerations. For instance, designs and specifications established some 15 or even 20 years ago have only been revised slowly, without any substantial consideration of new knowledge coming forward from metallurgists, corrosion experts, semi-manufacturers and plant designers alike: short-term price reasoning has too often prevailed. In realisation of this situation, a group of leading tube manufacturers have considered the establishment of a technical association world-wide, comprising customers and consultants as well, as a central organization entrusted with the widespread dissemination of present technical information to the desalination industry. In July 1984, the Quality Label Association for Copper Alloy Tubes for Desalination - Q.L.A. for short - was established in Zurich, Switzerland, grouping the main tubemakers together with desalination end-users and consultants.
311 Its main initial task is to promote the adoption by desalination plant designers of a uniform specification covering the main alloys, namely:
-
aluminium brass
- 90/10 cupro-nickel - 70/30 cupro-nickel - 66/30/2/2 cupro-nicke~ thus overcoming present puzzling and mostly historical differences between national standards, tube manufacture,
improving the overall quality and accuracy of
and requiring the enforcement of well-defined quali-
ty control and assurance procedures in tube manufacture. It is needless to say that this will only be the first step in the Q.L.A. activity, whose purpose also encompasses such fields as alloy promotion, customer and consultant assistance, operator training and occasional trouble-shooting. But the basic aim remains to supply the end-users with tubes having longer life in service, free from failures and misuse; an aim to which the tubemaking members are fully committed, having so far supplied some 99% of the tonnages assembled in the Arabian Gulf countries since 1975, for a total of about 120,000 metric tons.
312 3. ARE COPPER ALLOYS TRULY ECONOMICAL IN DESALINATION ? The fact that copper alloy tubes are economical indeed is implicitly prooven by their prevaling by far over any other material when selecting materials for heat exchange surfaces in distillers and brine heaters, too. Some other competitive materials,
such as titanium or super-
stainless steels, have been advocated in view of their superior resistance to corrosion, but technical papers on the subject have generally avoided a specific economical ccnparision. We have, therefore, deemed it appropriate to draft a rough calculation, based on present prices and on assumed interest and inflation rates, of 8% and 4% a year respectively. The initial tube cost, related to 31.75 mm O.D. tubes, has been escalated at the capital interest rate of 8% until the end of the design life (25 years), and the present scrap value (an important factor, as shall be seen) at the inflation rate of 4% yearly.
313
Size
Tube Material
25 Years Cost USD/M2
Present Price USD/M2
Al-brass tube
31.75 OD
33.4
scrap value
18 BWG
i0.0
(100)
I00.2
Overall Fina~ Cost USD/M2
t 80.2
(lOO)
20.0
(1.245) 90/10
CuNi
31.75 OD
41.4 (124)
scrap
value
18 BWG
15.3
124.2
(
93.6 (117)
30.6 L
(1.245) 66/30/2/2
CuNi'
scrap value
31.75 OD
56.0 (168)
18 BWG
21.5
168.0
f
t 125.0 (166)
43.0
(1.245)
Titanium
31.75 OD
71.0 (213)
213.0
scrap value
22
15.0
30.0
63.8 (191)
191.4
BWG
t
183.0 (228) P
(0.711)
Super ferritic steel scrap value
31.75 OD 22 BWG
3.0
6.0
t 185.4 (231)
(0.711) Super austenitic steel scrap value
31.75 OD 22 BWG
91.9 (275) 5.0
275.7
t
265.7 (331)
i0.0
(0.711)
This table (expressed in USD per M2 of heat exchange surface) clearly shows the economical advantages of copper-alloy tubes from a two-fold aspect:
/
•
314 -
they
are
cheaper
to
buy,
between
two and three
times,
in
compari-
s o n w i t h t i t a n i u m and stainless steel; this means that titanium or stainless steel tubed desalinators w o u l d cost some i0 to 20% more. - their r e c o v e r y
value in scrap form is large enough to give them
a further edge over p r o p o s e d substitutes,
e s p e c i a l l y the steels who-
se cost differential is increased by another 40-50%. The d i f f e r e n c e s are thus so huge that any "minor" service troubles c o u l d be overruled from the start; but r e c e n t advantages in e v a p o r a tor start-up and service
p r o c e d u r e s - c o u p l e d w i t h r e c o u r s e to top-
quality tubes - have proven that stoppages can be reduced to a minimum, well w i t h i n the p r o g r a m m e d m a i n t e n a n c e allowances. A s t r i k i n g example in this e o n n o e t i o n are the data g a t h e r e d by ENEL - the Italian E l e c t r i c i t y B o a r d - on its fully copper alloy tubed pow e r s t a t i o n s over the last 20 years,
since the adoption of more care-
ful tube p r e s e r v a t i o n and m o n i t o r i n g techniques: UNAVAILABILITY
(% OF HOURS) OF ENEL P O W E R S T A T I O N S DUE TO STEAM C O N D E N S E R S
Cause
1974
1980
Leakage
0.210%
0.040%
Fouling
0.421%
0.236%
Total
0.631%
0.276%
315 It is seen that, by appropriate means estimated to cost at most some 5-6% of initial tube value, copper alloy tube leakage had a five-fold reduction, and their fouling was halved. This is just one example of how good heat exchanger husbandry can, with expert advice, bring stoppages to practically nil, thus avoid i n g recourse to so-called6fool-proo~Imaterials
which cost twice
or three times more, and may give cause for trouble if unduly attended.
4. SCOPE FOR MUTUAL CO-OPERATION The aim of true cost reduction of MSF plants - today's main objective as earlier stated - can only be pursued and achieved by close mutual co-operation between tubemakers and designers/consultants
in
view of optimum performance to be guaranteed to constructors and endusers. From the substantial amount of practical experience gathered over the last decade, we feel that several subjects could be most usefully discussed between consultants and the Q.L.A. technical expert members in order to reduce costs of MSF plants at equal or improved performance/ cost effectiveness.
816 Among these, we would like to name but a few, of some importance: a) wider recourse to high-temperature operation (120-135°C) to reduce heat exchange surfaces and increase efficiency:
there are
now several examples of well-performing plants of this type, and copper alloy tubes there gave excellent service b) a definite emphasis on uniformisation and rationalisation of material specifications,
including quality control and assurance; and
the already existing Q.L.A. specification - which was established with expert advice from both consultants and manufacturers - is certainly one very big step in the right direction c) a better understanding of practical alloy performance as a function of plant site location and service required (brine heater, heat reject, etc); there would clearly be a significant cost advantage in using Al-brass more widely than today's practice dictates d) a reconsideration of presently adopted sizes, especially as far as thickness is concerned, where the present 18 BWG (or 18 SWG) gauges may prove to be over-cautio~s considering presently available anticorrosion remedies and procedures: by this simple change, a red/cticn of about 10% on tube cost can be achieved, corresponding to 2-2.5% saving on the desalinator as a whole; similarly, should process considerations allow it, shorter tube lenghts of 12-15 metres would also afford tube manufacturing cost savings
317 e) a very detailed and well-informed drafting procedures the tube bundles, techniques,
Clearly,
attention should be paid in
for the assembly,
start-up and service of
together with pre-treatment
(or conditioning)
and corrosion monitoring on line.
more subjects will arise in due course during meetings
and discussions with the interested parties, known to the desalination
which will be made
community during planned Q.L.A. conferen-
ces or in Q.L.A. documents and reports.
55 (1985) 307--317 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
MSF (Multi Stage Flash) Desalination Plants: Copper Alloy Tubes, Their Economic Aspects for Overall Cost Reduction
by: Dr. Vittorio Canetta, Chairman, Quality Label Association for Copper Alloy Tubes for Desalination (Q.L.A.) - Zurich Dr. N.M. Valota, Technical Committee Member, Quality Label Association for Copper Alloy Tubes for Desalination (Q.L.A.) - Zurich
i. FOREWORD Only a few and concise words are needed in front of a qualified audience of desalination experts to emphasize how much desalination plants have contributed,
and are likely to do so for the next decades to the year
2000, to the economical and welfare growth in such arid areas as the Arabian Gulf, the Red Sea and North African coasts. Desalination plants for sea water are a fundamental
aspect of infrastruc-
ture to sustain the growing industrialization and population in these countries, where rainfall is poor and rivers not available, where - as in Tripoli and Bahrain - the water acquifer decreases yearly or where huge industrial towns like
Jubail and Yanbu are erected in a previously
desert landscape. While during the '70s, however, desalination plants mushroomed in these mostly Arabic and oil-rich countries,
the rate of investment in desalina-
tion during the last five years has declined sharply, as a consequence of oil price decreases and growing current account deficit in petroleum countries.
0011-9164/85/$03.30
Elsevier Science Publishers B.V.
307
308 But the need for them is still there, and - as an example - H. M. Kin K Fahd of Saudi Arabia, for instance, has announced in his 4th five-year Plant (1985-1990) that water resources in his country should increase by some 400 MIGPD until 1990
(mostly by seawater desalination) 3 as
against present installed capacity of about 500 MIGPD. Budget constraints,
however, require careful consideration and analysis
of overall plant costs, much more than in the booming years after the oil crisis: and cost reductions nowadays are only likely to come from technical rather than commercial considerations,
in view of recent dra-
matic plant price decreases which have entirely wiped out any profit margin and brought the situation to a bone-dry condition. Being involved since two decades in desalination as suppliers of heat exchange tubes, and of ancillary non ferrous metal semis like tubeplates and sheets, we would like to enlarge on this subject as a significant item of possible cost reductions.
2.
COPPER ALLOY TUBES IN MSF DESALINATION
The MSF (multi stage flash distillation) process is the world leader for seawater desalination,
especially where it is coupled with availabi-
lity of cheap energy and/or steam. Statistics show that in 1980 MSF repre sented some three fourths of total installed capacity world-wide (and some 90% in the Arabian Gulf), and no significant change is likely to have occurred in
these figures since.
309 The reasons of its predominace are clear-cut: ce, ruggedness and ease of operation,
long-standing experien-
independence from excessively
sophisticated and exotic materials. It is true that in recent years, more and more attention has been paid to RO technologies,
and - as in Bahrain - some very interesting
large-scale experiments are made to advance and test their economic performance and service reliability; but questions have still to be answered definitely,
e.g. about
memhrsne
durability and availability.
So, the MSF process will certainly retain a fairly constant share of the market in the years ahead; this share will, of course, be easier to defen
if techno-economical
improvements are fostered and introdu-
ced. Copper alloy (brass
and cupro-nickel ) tubes are almost exclusively
chosen when the plant is of the MSF type, and have over three decades of good service behind them. Their economical impact on plant cost is quite apparent when remembering that in gross detail a MSF desalinator's cost would be broken down as follows:
810
- Civil works
20 - 25 %
- Mecanical construction and erection
35 - 50 %
- Engineering
-
Tubes and related semis
5 -
8 %
20 - 25 %
Thus, between one fourth and one fifth of equipment cost is made of copper alloy materials. Notwithstanding, however, their economical importance, copper alloys have received proportionately little attention hitherto, and tradition has often prevailed over techno-economical considerations. For instance, designs and specifications established some 15 or even 20 years ago have only been revised slowly, without any substantial consideration of new knowledge coming forward from metallurgists, corrosion experts, semi-manufacturers and plant designers alike: short-term price reasoning has too often prevailed. In realisation of this situation, a group of leading tube manufacturers have considered the establishment of a technical association world-wide, comprising customers and consultants as well, as a central organization entrusted with the widespread dissemination of present technical information to the desalination industry. In July 1984, the Quality Label Association for Copper Alloy Tubes for Desalination - Q.L.A. for short - was established in Zurich, Switzerland, grouping the main tubemakers together with desalination end-users and consultants.
311 Its main initial task is to promote the adoption by desalination plant designers of a uniform specification covering the main alloys, namely:
-
aluminium brass
- 90/10 cupro-nickel - 70/30 cupro-nickel - 66/30/2/2 cupro-nicke~ thus overcoming present puzzling and mostly historical differences between national standards, tube manufacture,
improving the overall quality and accuracy of
and requiring the enforcement of well-defined quali-
ty control and assurance procedures in tube manufacture. It is needless to say that this will only be the first step in the Q.L.A. activity, whose purpose also encompasses such fields as alloy promotion, customer and consultant assistance, operator training and occasional trouble-shooting. But the basic aim remains to supply the end-users with tubes having longer life in service, free from failures and misuse; an aim to which the tubemaking members are fully committed, having so far supplied some 99% of the tonnages assembled in the Arabian Gulf countries since 1975, for a total of about 120,000 metric tons.
312 3. ARE COPPER ALLOYS TRULY ECONOMICAL IN DESALINATION ? The fact that copper alloy tubes are economical indeed is implicitly prooven by their prevaling by far over any other material when selecting materials for heat exchange surfaces in distillers and brine heaters, too. Some other competitive materials,
such as titanium or super-
stainless steels, have been advocated in view of their superior resistance to corrosion, but technical papers on the subject have generally avoided a specific economical ccnparision. We have, therefore, deemed it appropriate to draft a rough calculation, based on present prices and on assumed interest and inflation rates, of 8% and 4% a year respectively. The initial tube cost, related to 31.75 mm O.D. tubes, has been escalated at the capital interest rate of 8% until the end of the design life (25 years), and the present scrap value (an important factor, as shall be seen) at the inflation rate of 4% yearly.
313
Size
Tube Material
25 Years Cost USD/M2
Present Price USD/M2
Al-brass tube
31.75 OD
33.4
scrap value
18 BWG
i0.0
(100)
I00.2
Overall Fina~ Cost USD/M2
t 80.2
(lOO)
20.0
(1.245) 90/10
CuNi
31.75 OD
41.4 (124)
scrap
value
18 BWG
15.3
124.2
(
93.6 (117)
30.6 L
(1.245) 66/30/2/2
CuNi'
scrap value
31.75 OD
56.0 (168)
18 BWG
21.5
168.0
f
t 125.0 (166)
43.0
(1.245)
Titanium
31.75 OD
71.0 (213)
213.0
scrap value
22
15.0
30.0
63.8 (191)
191.4
BWG
t
183.0 (228) P
(0.711)
Super ferritic steel scrap value
31.75 OD 22 BWG
3.0
6.0
t 185.4 (231)
(0.711) Super austenitic steel scrap value
31.75 OD 22 BWG
91.9 (275) 5.0
275.7
t
265.7 (331)
i0.0
(0.711)
This table (expressed in USD per M2 of heat exchange surface) clearly shows the economical advantages of copper-alloy tubes from a two-fold aspect:
/
•
314 -
they
are
cheaper
to
buy,
between
two and three
times,
in
compari-
s o n w i t h t i t a n i u m and stainless steel; this means that titanium or stainless steel tubed desalinators w o u l d cost some i0 to 20% more. - their r e c o v e r y
value in scrap form is large enough to give them
a further edge over p r o p o s e d substitutes,
e s p e c i a l l y the steels who-
se cost differential is increased by another 40-50%. The d i f f e r e n c e s are thus so huge that any "minor" service troubles c o u l d be overruled from the start; but r e c e n t advantages in e v a p o r a tor start-up and service
p r o c e d u r e s - c o u p l e d w i t h r e c o u r s e to top-
quality tubes - have proven that stoppages can be reduced to a minimum, well w i t h i n the p r o g r a m m e d m a i n t e n a n c e allowances. A s t r i k i n g example in this e o n n o e t i o n are the data g a t h e r e d by ENEL - the Italian E l e c t r i c i t y B o a r d - on its fully copper alloy tubed pow e r s t a t i o n s over the last 20 years,
since the adoption of more care-
ful tube p r e s e r v a t i o n and m o n i t o r i n g techniques: UNAVAILABILITY
(% OF HOURS) OF ENEL P O W E R S T A T I O N S DUE TO STEAM C O N D E N S E R S
Cause
1974
1980
Leakage
0.210%
0.040%
Fouling
0.421%
0.236%
Total
0.631%
0.276%
315 It is seen that, by appropriate means estimated to cost at most some 5-6% of initial tube value, copper alloy tube leakage had a five-fold reduction, and their fouling was halved. This is just one example of how good heat exchanger husbandry can, with expert advice, bring stoppages to practically nil, thus avoid i n g recourse to so-called6fool-proo~Imaterials
which cost twice
or three times more, and may give cause for trouble if unduly attended.
4. SCOPE FOR MUTUAL CO-OPERATION The aim of true cost reduction of MSF plants - today's main objective as earlier stated - can only be pursued and achieved by close mutual co-operation between tubemakers and designers/consultants
in
view of optimum performance to be guaranteed to constructors and endusers. From the substantial amount of practical experience gathered over the last decade, we feel that several subjects could be most usefully discussed between consultants and the Q.L.A. technical expert members in order to reduce costs of MSF plants at equal or improved performance/ cost effectiveness.
816 Among these, we would like to name but a few, of some importance: a) wider recourse to high-temperature operation (120-135°C) to reduce heat exchange surfaces and increase efficiency:
there are
now several examples of well-performing plants of this type, and copper alloy tubes there gave excellent service b) a definite emphasis on uniformisation and rationalisation of material specifications,
including quality control and assurance; and
the already existing Q.L.A. specification - which was established with expert advice from both consultants and manufacturers - is certainly one very big step in the right direction c) a better understanding of practical alloy performance as a function of plant site location and service required (brine heater, heat reject, etc); there would clearly be a significant cost advantage in using Al-brass more widely than today's practice dictates d) a reconsideration of presently adopted sizes, especially as far as thickness is concerned, where the present 18 BWG (or 18 SWG) gauges may prove to be over-cautio~s considering presently available anticorrosion remedies and procedures: by this simple change, a red/cticn of about 10% on tube cost can be achieved, corresponding to 2-2.5% saving on the desalinator as a whole; similarly, should process considerations allow it, shorter tube lenghts of 12-15 metres would also afford tube manufacturing cost savings
317 e) a very detailed and well-informed drafting procedures the tube bundles, techniques,
Clearly,
attention should be paid in
for the assembly,
start-up and service of
together with pre-treatment
(or conditioning)
and corrosion monitoring on line.
more subjects will arise in due course during meetings
and discussions with the interested parties, known to the desalination
which will be made
community during planned Q.L.A. conferen-
ces or in Q.L.A. documents and reports.