1 ACID CLEANING OF SOME DESAL UNITS AT AL-JUBAIL PLANT A. U. Malik , Shahreer Ahmad, Ismail Andijani, Nausha Asrar Research And Development Center Saline Water Conversion Corporation P. O. Box 8328, Al-Jubail 31951, Kingdom of Saudi Arabia Abdul Salam Al-Mobayed , Abdullah M. Al-Jardan, Anwar Ahsan Al-Jubail Desalination & Power Plants, Al-Jubail SUMMARY The scaling or deposition on the heat exchanger tubes of an MSF desalination plant is a phenomenon of common occurring. If the proper cleaning or descaling is not carried out then it would greatly affect the efficiency of the plant. Acid cleaning is so far the most reliable way to clean the heat exchanger tubes. In SWCC desalination plants, IBIT 570 inhibited sulfamic acid has been used frequently for cleaning, demisters and other components due to its efficient and safe cleaning. However, due to high cost and slow action of this combination, the application of some low cost and fast reacting inhibitor - acid combination was considered. The main concern was that the inhibited acid should have corrosive action within allowable limits. The Corrosion Department of RDC with collaboration of Al-Jubail plant carried out a systematic study on testing of CP-20 inhibited H 2 SO 4 and HCl on plant component materials, namely carbon steel (flash chamber), stainless steel (flash chamber, demister), 70-30 and 90-10 cupronickel (heat transfer tubes) and Ni resist D2 (recycle brine pump). The interaction behavior of materials with inhibited acids was studied by carrying out laboratory dynamic testing in open atmosphere and deaerated conditions. The results of the testing indicate that inhibited H 2 SO 4 or HCl could be used for cleaning cupronickel (70-30 and 90-10), Ni-Resist D2 and carbon steel without any 1 Issued as Technical Report No. TR3804/APP95007 in February 1997, A paper from this report entitled " Corrosion Behavior of some MSF Plant Materials in Inhibited Sulfuric Acid and Hydrochloric Acid” was presented in VIII - IDA Conference , Madrid, Oct.6-9, 1997. Another paper entitled " Effectiveness of inhibited H2SO4 and HCl in Cleaning Some Desal Plants at Al-Jubail Plant” was presented in Second Acquired Experience Symposium on Desalination Plants O&M, SWCC, Al-Jubail, Sept.29 - Oct.3, 1997. 1575
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
1ACID CLEANING OF SOME DESAL UNITS AT AL-JUBAIL PLANT
A. U. Malik , Shahreer Ahmad, Ismail Andijani, Nausha Asrar Research And Development Center
Saline Water Conversion Corporation P. O. Box 8328, Al-Jubail 31951, Kingdom of Saudi Arabia
Abdul Salam Al-Mobayed , Abdullah M. Al-Jardan, Anwar Ahsan Al-Jubail Desalination & Power Plants, Al-Jubail
SUMMARY The scaling or deposition on the heat exchanger tubes of an MSF desalination plant is a
phenomenon of common occurring. If the proper cleaning or descaling is not carried
out then it would greatly affect the efficiency of the plant. Acid cleaning is so far the
most reliable way to clean the heat exchanger tubes. In SWCC desalination plants,
IBIT 570 inhibited sulfamic acid has been used frequently for cleaning, demisters and
other components due to its efficient and safe cleaning. However, due to high cost and
slow action of this combination, the application of some low cost and fast reacting
inhibitor - acid combination was considered. The main concern was that the inhibited
acid should have corrosive action within allowable limits.
The Corrosion Department of RDC with collaboration of Al-Jubail plant carried out a
systematic study on testing of CP-20 inhibited H2SO4 and HCl on plant component
demister), 70-30 and 90-10 cupronickel (heat transfer tubes) and Ni resist D2 (recycle
brine pump). The interaction behavior of materials with inhibited acids was studied by
carrying out laboratory dynamic testing in open atmosphere and deaerated conditions.
The results of the testing indicate that inhibited H2SO4 or HCl could be used for
cleaning cupronickel (70-30 and 90-10), Ni-Resist D2 and carbon steel without any
1 Issued as Technical Report No. TR3804/APP95007 in February 1997, A paper from this report entitled " Corrosion Behavior of some MSF Plant Materials in Inhibited Sulfuric Acid and Hydrochloric Acid” was presented in VIII - IDA Conference , Madrid, Oct.6-9, 1997. Another paper entitled " Effectiveness of inhibited H2SO4 and HCl in Cleaning Some Desal Plants at Al-Jubail Plant” was presented in Second Acquired Experience Symposium on Desalination Plants O&M, SWCC, Al-Jubail, Sept.29 - Oct.3, 1997.
1575
possibility of significant corrosion if applied under deaerated conditions. Stainless
steel AISI 316L could be used both under open atmospheric and deaerated condition.
The second phase of the study was to apply the results of lab test studies in cleaning the
desal tubes of some of the units of Al-Jubail desalination plant. On the basis of
laboratory results, Al-Jubail plant authorities agreed to carry out the cleaning of some
of phase-II desal units by using inhibited H2SO4.
Acid cleaning of the desal units of C4, #27 and 29 was carried out by applying Brine
Recycle Pump procedure using CP 20 inhibited deaerated H2SO4 (prepared in
seawater). The operation was consisted of recirlculating the acid through evaporator
and heat exchangers including water boxes and brine heater. For whole operation 3
charges were used and after every charge, the system was flushed thoroughly. The
Ca++, Cu++, pH and acid concentration was monitored. The post cleaning inspection of
the desal units and performance data (GOR, PW, HTC) indicate that acid cleaning has
enhanced the efficiency of the plant. No significant corrosion was observed during
cleaning.
1. INTRODUCTION
1.1 BACKGROUND
The inhibitor IBIT 570S and sulfamic acid have commonly been used for descaling heat
transfer tubes in desalination plants and also for cleaning demisters. The main
advantage of this combination is the safe cleaning operation of the tubes. The principle
disadvantages are its slow action and high cost. In order to assess the capability of a
new acid inhibitor CP-20 which is less expensive, the Manager, Al-Jubail plant, in
January 1995, asked R&D Center to test the efficiency of the chemical and its
performance vis-a-vis sulfuric acid cleaning IBIT 570S. Subsequently, studies based on
dynamic testings were carried out on CP-20 inhibited H2SO4 or HCl on cupronickel and
titanium metals. The results of the test studies are given in Table 1 and 2 and are
illustrated in Fig. 1. The main conclusion of the studies was that although CP-20 was
inferior to IBIT for inhibiting corrosion of cupronickel and titanium in HCl and H2SO4
but it could be used as an alternative to IBIT due to its allowable corrosion rates.
1576
To consider the evaporator cleaning in the Al-Jubail Plant, a committee named as
Evaporator Tube Acid Cleaning Committee (ETACC) was formed by the Manger, Al-
Jubail Plant and corrosion department of RDC was asked to provide its expertise in this
matter.
In its meeting on May 1, 1995, the ETACC requested R&D Center to carryout Phase -2
Evaporator Flash Chamber Materials Corrosion Evaluation by using CP-20 inhibited
HCl and H2SO4. Subsequently, R&D Center Al-Jubail submitted a project entitled "The
Corrosion Behavior of Flash Chamber and Recycle Pump Materials During Acid
Cleaning of the MSF heat Transfer Tubes". The project was approved by the Director
General, R&D Riyadh in June 1995. The work was carried out in the Corrosion
Department and a report was submitted in the last week of August 1995. The report
was reviewed by the R&D Department, Riyadh and the Director General in his letter
dated September 16, 1995 advised Manager, R&D Center that the report could be sent
to the Al-Jubail plant and they may carryout acid cleaning of some units of the plant as
per the recommendation of the report. The Director General further advised that after
obtaining the acid cleaning results from the plant, a comprehensive report from R&D
Center could be issued.
The ETACC agreed to accept the recommendation of R&D Center aforementioned
report and decided to test the inhibited sulfuric acid in some of the desal units of Al-
Jubail plant. The project of acid cleaning of desal tubes in Al-Jubail plant was started in
April 1996. The acid cleaning of several desal units of C4 was completed by the end of
December 1996.
This report presents the results of acid cleaning project-using CP-20 as an acid inhibitor,
for cleaning desal tubes of Al-Jubail Plant. The report is divided into 2 parts : (i)
Laboratory Studies and (ii) Plant Studies.
Laboratory studies are comprised of testing the corrosion behavior of flash chamber and
recycle pump materials in 2% HCl and H2SO4 solutions prepared in natural Gulf
Seawater with 0.7% Nevamin CP-20 under deaeration or open atmospheric conditions.
1577
Plant studies are consisted of testing the performance of acid inhibitor CP-20 in
acidified brine (pH 1.5-2) as a cleaning agent in desal units #27 and 29 of C4 in Al-
Jubail desalination plant.
1.2 LITERATURE REVIEW
Scale deposition in heat transfer tubes is a unique feature in multistage flash (MSF)
evaporators of seawater desalination plant. The scales are mainly consisted of calcium
carbonate and magnesium hydroxide and occasionally of CaSO4. The scaling induces
insulation in heat exchanger tubes resulting in lowering of heat transfer efficiency and
in consequence, significant losses in production and high-energy consumption.
Mechanical cleaning by balls does not remove the entire scales and some residual scales
are always present in the tubes and there is a progressive build up of the scales with
time which make it essential for the plants to clean the heat transfer tube periodically by
chemical means. Acid cleaning is by far the most effective way to descale the tubes
periodically by chemical means. The application of dilute aqueous solutions of
sulfamic acid, sulfuric acid or hydrochloric acid containing some inhibitor, has been a
standard practice to clean the tubes in desalination plants [Hanbury, et. al., 1993]. 3%
HCl with 0.5% corrosion inhibitor was used successfully as a descalant in heat transfer
tubes of a 100 tons/day flash evaporation plant [Noguchi, Hamada 1969]. Japan
titanium society [1994] reported that the H2SO4 solution with IBIT 570S and HCl
solution with IBIT 2S prevent the corrosion and hydrogen absorption of titanium and
copper alloys. Sulfamic acid solution with IBIT 570S has been used for descaling of
tubes in MSF plants. Inhibited sulfamic acid and sulfuric acid have been used since
long as a descalant in heat exchanger tubes in Al-Khafji and Al-Khobar desalination
plants, respectively.
The function of an acid inhibitor is to diminish the corrosive action of acid on metals
[Metal Hand Book 1987]. The organic inhibitors show its inhibiting action by surface
adsorption forming a thin film on the metal surface with no significant reaction with the
substrate [Nathan 1973]. It has been observed that one particular type of inhibitor
effective in one system may not be effective in another. ARMOHIB 28 inhibitor has
been effectively used with HCl for cleaning fouled demisters (SS 316L with mesh pads)
1578
without deterioration of demister metal whilst the same inhibitor has been found
corrosive for cupronickel alloys [Malik et. al. 1994].
Rocchini [1994] discussed at length the application of polarization resistance technique
for corrosion rate monitoring during acid cleaning. A reliable evaluation of the true
corrosion rate of tubular specimen can be made using appropriate calibration charts for
the given environment and geometry of the electrochemical probe.
2. OBJECTIVES
(i) To determine the corrosion rates of SS 316 L, 90-10 Cu-Ni, 70-30 Cu-Ni,
Carbon steel and Ni Resist D2 in 2% (HCl and H2SO4) inhibited with 0.5%
Nevamine CP20 in deaerated or normal (non deaerated) seawater, in
laboratory.
(ii) To establish the acid cleaning process for desal units by recirculation pump
method.
(iii) To determine the pH, Ca++, Cu++ and acid concentration during acid
charging, circulation and flushing for some of the selected desal units.
(iv) To determine the evaporator performance data after acid cleaning and
comparing with pre-cleaning data.
(v) To provide recommendation regarding acid cleaning of the heat transfer
tubes in the plant.
1579
3. EXPERIMENTAL
3.1 CORROSION BEHAVIOR OF SOME MSF PLANT MATERIALS IN
INHIBITED H2SO4 AND HCl - A Laboratory Study
3.1.1 Summary
Effects of addition of inhibitor to the acid solution under deaerated and open
atmospheric conditions were studied in order to control the corrosion of alloys used in
flash chambers, recycle pumps and MSF heat transfer tubes during acid cleaning
Corrosion behavior of carbon steel, Ni-resist, SS 316L and cupronickel alloys was
studied in 2% HCl and H2SO4 aqueous solutions (prepared in Gulf Seawater) with and
without inhibitor Nevamin CP-20 under deaeration and open atmospheric condition.
Immersion and polarization resistance tests show that corrosion rates of carbon steel and
Ni-Resist alloy in 2% HCl and H2SO4 solutions with inhibitor are higher in open
atmosphere as compared to the deaerated condition. However, inhibited acid solutions
in both deaerated and normal atmospheric conditions show good corrosion resistance
towards stainless steel 316L and cupronickel alloys. Results of this study indicate that
the addition of 0.5% inhibitor along with the deaeration of the acid solution (dissolved
oxygen < 20 ppb) is the best condition for acid cleaning with negligible corrosion of
heat exchanger tubes, evaporator and recycle pump materials.
3.1.2 Experimental
3.1.2.1 Materials
Commercially available sheets of carbon steel, 316L stainless steel and 70-30, 90-10
cupronickel alloys were used for making test specimens. Specimens of Ni-Resist D2
were cut from blocks obtained from casings of used brine recycle pumps. Hydrochloric
acid and sulfuric acid of commercial grades and corrosion inhibitor Nevamin CP-20
were obtained from Al-Jubail desalination plant. 2% solutions of HCl and H2SO4 were
prepared in natural seawater and 0.5% inhibitor was added to these acid solutions.
1580
3.1.2.2 Immersion Tests
Corrosion rates of materials by immersion test were determined following ASTM
method (G 31-72; reapproved 1990).
Polished coupons of carbon steel, 316L stainless, Ni-resist and 70-30, and 90-10
cupronickel alloys of 60x40x2 mm dimensions were immersed in one liter each of 2%
hydrochloric and sulfuric acid solutions after taking their initial weight. Recommended
amount (0.5%) of Nevamin CP-20 inhibitor (5 ml inhibitor/liter of acid solution) was
added to each of the aerated and deaerated acid solutions kept at room temperature
under dynamic condition. The deaeration in the test solution was done by purging
nitrogen gas (99.99%) in an air tight experimental set up. The traces of oxygen present
in the nitrogen gas were removed by bubbling through an alkaline solution of
pyrogallol. In these aqueous acid solutions, 2 coupons were immersed for 6 hrs. After
taking out the coupons from the acid solution, they were cleaned, dried and weighed and
weight loss was calculated. This experiment was repeated three times for each
condition and means of corrosion rates were calculated.
3.1.2.3 Electrochemical Polarization Studies
Liner polarization resistance experiments were carried out using an EG&G
Electrochemical Corrosion measurement System following procedure ASTM-G59-78
(Reaproved 1984). The Corrosion System is consisted of Model 273
potentiostat/galvanostat; model 342-softcorr software and model 30 IBM PS/2
computer. All the experiments were carried out using a corrosion cell with saturated
calomel as reference and graphite as counter electrodes. Flat circular test specimens
were used. The test specimens were screwed in the sample holder and the exposed area
was 1 cm2.
The polarization resistance measurements were conducted at a scan rate of 0.1 mV/s
with starting and final potentials corresponding to -20 mV and +20 mV as open circuit
corrosion potential (OCP), respectively. Before starting the experiments, the specimens
were left in the acid solution for about 1 hour to attain a steady state, which was shown
by a constant potential.
1581
3.1.2.4 Measurements of Dissolved Oxygen
Dissolved oxygen in the deaerated test solution was measured by Dissolved Oxygen
Measurement-System model 273 of Orbisphere Laboratories, Geneva. The sensitivity
of this system is one microgram per liter (1 ppb).
3.2 ACID CLEANING OF SOME DESAL UNITS AT AL-JUBAIL PLANT
Al-Jubail desal and power plant Phase II A & B combined has forty (40) evaporators
with base design distillate production capacity of 5.25 MGPD each unit. One set of acid
cleaning system has been provided for twenty (20) desalination units, i.e., each phase
has one set of acid cleaning facility. Location of the acid cleaning system in each phase
is in the center of the area covering ten evaporators in each side. Facilities for pumping
and transportation of acid solution have been provided at the suction of each recycle
brine pump. The designed acid cleaning procedure requires circulation of acidified
brine at pH 2-2.5 through the recovery, brine heater tubes and the flash chambers. Al-
Jubail Phase-II evaporators do not have 'tubes only cleaning facility' as some other
plants do have as designed and built in components.
3.2.1 Acid Cleaning With Sulfuric Acid
For evaporator acid cleaning, acid to the circulating brine was injected directly from
acid tanker through an acid resistant flexible pipe connected to the drain valve of the
last stage flash chamber. The tanker was fully vented to atmosphere and connected to
the drain valve of last stage. Acid inhibitor to the circulation brine was charged through
a sample collection point was approximately 0.7% of the acid charged. A higher charge
of the inhibitor than the recommended one (0.5%) was used to take care of the possible
fluctuations in the dozing system and to ensure homogeneity of mixing under plant
operation conditions. Acid injection was regulated to maintain circulating brine pH 1.5-
2.0 at the discharge of recycle brine pump. Circulating acidified brine temperature was
maintained at 48-50 oC and brine velocity at about 0.7 m/sec. The acid cleaning of
evaporators was conducted in three charges. After completion of the acid cleaning
process, rinsing and draining of the evaporators were carried out. During the acid
1582
cleaning process laboratory analysis of the brine samples from recycle brine (RB) pump
discharge, brine heater outlet (BHO) and brine blow down (BB) pump were carried out
for pH, % acid, Ca2+ and Cu2+.
After the acid cleaning operation was completed, brine heater inlet and outlets, water
boxes manhole, heat recovery north-side water box manholes (stage 1-17) and flash
chamber stages # 1-4 and 16-19 were opened for inspection.
4. RESULTS AND DISCUSSION
4.1 IMMERSION TESTS
4.1.1 Immersion Test Under Atmospheric Conditions
Table 3 summarizes the mean corrosion rates of 6 hrs duration immersion tests for
carbon steel, SS 316L, Ni-resist and cupronickel alloys in 2% of HCl and H2SO4 with
and without inhibitor under dynamic condition. In HCl acid solution, 316L stainless
steel showed low corrosion rate (< 1 MPY) in presence and absence of inhibitor.
However, high corrosion rates were found for cupronickel (53-66 MPY), carbon steel
(101 MPY) and Ni-resist (46 MPY) in absence of inhibitor. Although, the addition of
inhibitor decreases the corrosion rates of the alloys, viz., cupronickel (1.5-2.5 MPY),
carbon steel (46 MPY) and Ni-resist (33 MPY) but the corrosion rates of the carbon
steel and Ni-Resist remain high. Similar trend was found in inhibited H2SO4 acid
solution but this acid solution was found less corrosive than HCl (Fig. 2 and 3).
Cupronickel 90-10 and 70-30 alloys have shown good corrosion resistance in 2%
H2SO4 acid environment (5 MPY) and poor corrosion resistance in 2% HCl acid
environment (53-66 MPY). This is because of the fact that in the presence of an
oxidizing agent (e.g., dissolved oxygen) HCl becomes more corrosive than H2SO4
[ASM Metal Handbook 1987]. The results also indicate that 70-30 cupronickel is less
corrosion resistant than 90-10 cupronickel. Nickel is more active than copper in the
electrochemical series. The passive film of nickel in not particularly stable and, hence,
nickel cannot generally be used in oxidizing environments such as nitric acid [Sridhar
and Hodge, 1989].
1583
Table 4 summarizes the inhibitor efficiency in open atmospheric condition. The
efficiency of inhibitor can be calculated from the following equation.
IRo Ri
Roxeff =
−100
Ieff = Inhibitor efficiency
Ro = Corrosion rate without inhibitor
Ri = Corrosion rate with inhibitor
Tables 4 shows that in both the acid media, inhibitor Nevamin CP-20 is very effective
in suppressing the corrosion of cupronickel alloys (efficiency ≥ 80%) while its
efficiency is comparatively low (28 to 54%) for carbon steel and Ni-resist alloys. This
may be due to the fact that the mode of adsorption of an inhibitor depends on chemical
structure of the molecule, chemical composition of the solution, nature of the metal
surface and the electrochemical potential at the metal-solution interface.
4.1.2 Immersion Test Under Deaeration Condition
Table 5 lists the corrosion rates of alloys in acid solution with and without inhibitor
under deaeration and dynamic conditions. The dynamic conditions were simulated by
shaking vigorously the solution through a magnetic device. For acid solution without
inhibitor, results show high corrosion rates for carbon steel (10-17 MPY), and low
corrosion rates for cupronickel alloys (2-4 MPY) while Ni-resist alloys occupy the
middle position (4.5-6.8 MPY). No perceptible weight change was recorded for 316L.
The overall corrosion rates of alloys are in the allowable range. Therefore, deaeration
appears to be more effective than addition of inhibitor in lowering the corrosion rate in
2% acid solution environment.
After passing N2 at the rate of 150 ml/min for 30 minutes in the acid solution, < 10 ppb
dissolved oxygen was found. For acid solution with inhibitor, results show no
perceptible weight loss and no change in the surface appearance of the coupons of SS
316L, Ni-resist, 90-10 and 70-30 cupronickel alloys. Even the carbon steel specimens
show a low corrosion rate of 3.5 MPY without any remarkable change in the surface
1584
appearance of the coupons. The efficiency of inhibitor increases under deaerated
condition (Table 6); for all the alloys except carbon steel have efficiency approaching to
100% even the latter has a good efficiency performance of 65% and 79% in HCl and
H2SO4 solutions, respectively. This indicates that the addition of inhibitor along with
the removal of oxygen provide good corrosion protection to the above alloys in both the
2% HCl and 2% H2SO4 acid medium.
4.1.3 Polarization Resistance Tests
Table 7 lists the corrosion rates of alloys in 2% HCl and H2SO4 acid solutions (prepared
in seawater) with 0.5% CP-20 inhibitor in open atmosphere and deaeration under
dynamic condition. The corrosion rates are computed from the Liner Polarization
Resistance (LPR) plots. These results show that in HCl acid solution in atmospheric
condition, the corrosion rates of carbon steel (50 MPY) are higher as compared to Ni-
resist (31 MPY) and much higher than that of cupronickel alloys (< 0.50 MPY). AISI
316L occupies the middle position (5 MPY). Same trend was found in the case of
H2SO4 acid solution but in general all the alloys show comparatively low corrosion rate.
Corrosion rates of alloys in inhibited acid solution under deaeration condition are lower
than that found under open atmospheric condition. It was interesting to note that the
corrosivity of HCl acid solution is higher than H2SO4 acid solution for carbon steel and
Ni-resist alloy in open atmospheric condition but in deaerated condition this behavior is
just opposite.
Figure 4 shows the anodic and cathodic polarization's curves (Tafel plots) of carbon
steel in inhibited acid solution at open atmosphere and deaeration under dynamic
conditions. Icorr (corrosion rate) is determined by the extrapolation of the liner region
of the Tafel plots. In case of inhibited 2% HCl acid solution under open atmospheric
condition, the corrosion current is 100 µA/cm2, however, the deaeration effectively
reduced the corrosion current by around two orders of magnitude. 2% H2SO4 acid
solution results show similar trends as those observed in HCl acid solution. Figure 5
shows a typical LPR plot for carbon steel in inhibited sulfuric acid deaerated condition.
The slope of the plot represents the corrosion rate of the alloy.
1585
4.2 Acid Cleaning of Desal Units # 29 and 27
4.2.1 Acid Cleaning of Desal # 29
The test cleaning of this unit was done to evaluate the cleaning procedure. A cleaning
procedure which has been detailed in the above section 3.2.1, was followed.
The acid cleaning of heat exchanger tubes in evaporators of desal #29 was conducted in
three charges. During acid cleaning the brine flow was kept at 5000-6000 m3/hr. The
temperature of the acidified brine was maintained 47-49 oC at the brine heater outlet.
The amounts of acid, inhibitor and antifoam are given below.:
(i) Addition of 0.5% CP-20 inhibitor along with deaeration of 2% HCl or H2SO4 solution provide good corrosion control of the evaporator and brine cycle pump
materials.
(ii) The corrosion rates of MSF plant materials namely carbon steel, AISI 316L,
Ni-Resist D2 and cupronickel (90-10 and 70-30) in CP-20 inhibited HCl or
H2SO4 solutions under dynamic and deaerated conditions are much below the
allowable range. Therefore, a 2% HCl or H2SO4 solution prepared in seawater
and inhibited with 0.5% Nevamin CP-20 under deaerated (O2 ~ 10 ppb) and
dynamic conditions can be utilized for cleaning heat exchanger tubes of the
plants. Table 8 provides a comparative study of the corrosion rates.
(iii) H2SO4 appears to be a better choice provided the CaSO4 scales formed by
H2SO4 could easily be removed during cleaning otherwise HCl can be used as
a cleaning agent.
1591
(iv) For acid cleaning following steps must be followed :
• Acid solution should be free from oxidizing contaminants.
• Full deaeration of the acid solution.
• Inhibited acid solutions should be under dynamic condition.
• Post acid cleaning procedures should be strictly followed.
5.2 Plant Studies The analysis of pre-and post cleaning operations data obtained from C4 desal units #27
and 29 provides the following information: (i) In general, the acid cleaning of the desal tubes and water boxes in different units
appears to be effective in the sense that the scales are removed without producing
any significant corrosion attack.
(ii) In all the units, acid cleaning resulted in satisfactory improvement in evaporator
performance.
6. RECOMMENDATION
The usefulness of inhibited H2SO4 solution (under deaerated condition) in cleaning heat
transfer tubes of desal units #27 and 29, C4 in Al-Jubail phase-II is firmly established
and the cleaning operation may be extended to other units of the plant.
7. FUTURE WORK In order to have better understanding of the nature of meal dissolution (corrosion)
process involved during acid cleaning, it is recommended to carry out further work
stated as follows :
(a) Corrosion monitoring of closely spaced segments of inside of the entire length
of desal tube. This would provide information about the segments which
contribute most to carryover copper and nickel.
1592
(b) Effect of cleaning agent (acid) velocity on the corrosion of desal tubes and
other components.
(c) Effect of dissolved oxygen level in acid solution on the dissolution of scales
and corrosion of components.
(e) Investigation to the cause of much heavier scaling in the tubes of later (lower
temperature) heat recovery stages.
REFERENCES
1. Fontana M. G., (1987) "Corrosion Engineering, Mc Graw Hill International, 3rd Edition,
p. 172. 2. Hanbury, Hodgkiess and Moris, (1993) "Desalination Technology '93', Porthan,
Glasgow, UK. 3. Japan Titanium Society (1994) "Countermeasures Against Deposit of Scale Oceanic
Lives - Light Gauge Titanium Tubes for Seawater Desalination Plants - 3 Q&A Practical Application", p. 14.
4. Malik A. U., Mayan Kutty P. C., Nadeem Siddiqi A., Ismaeel Andijani N. and Shahreer
Ahmed (1992) Corrosion Science, 33, 1810. 5. Malik A. U., Andijani, I. N., Nadeem Siddiqi A., Shahreer Ahmed and Al-Mobayaed A.
S., (1994) "Studies on the Role of Sulfamic Acid as a Descalant in Desalination Plant. Proc. VI Middle East Corrosion Conference, Bahrain, 24-26. Jan., pp. 65-78.
6. Metal Hand Book (1987) Vol. 13, Corrosion, Specific Industries and Environments,
American Society of Metals, p. 1140. 7. Nathan, C. C. (1973) "Corrosion Inhibitor", Betz Laboratories, Inc. National
Association of Corrosion Engineers, Houston, Texas, NACE Publication. 8. Noguchi N., T. Hamada (1969) "Prevention of Scale and Corrosion of Flash
Evaporators", Nuclear Desalination, International Atomic Energy Agency Vienna, p. 879.
9. Rocchini G., (1994) Corrosion Prevention and Control, 41, 108. 10.Uhlig. H. and R. Revie, (1985) "Corrosion and Control", John Wiley and Sons, 3rd
Edition (1985) p. 13.
1593
Table 1. Corrosion Rate (MPY) of Heat Exchanger Tube Materials in 2% Inhibited Acid Solutions under Dynamic Condition
Table 4. Efficiency (%) of Inhibitor Nevamin CP-20 in Controlling the Corrosion
of Different Alloys in Acid Solutions Under Dynamic and Open Atmospheric Conditions
2% Acid Alloys Solution C. steel SS 316 L Ni-Resist
D-2 Cupronickel
90-10 Cupronickel
70-30
HCl 54 ~ 100 28 97 96 H2SO4 50 ~ 100 51 80 80
Table 5. Corrosion Rate (MPY) of Different Alloys Immersed in Deaerated inhibited (CP-20-0.5%) Acid solutions (Prepared in Gulf Seawater) Under Dynamic Condition for 6 Hrs.
2% Acid Alloys Solution C. steel SS 316 L Ni-Resist
Table 6. Efficiency (%) of Inhibitor Nevamin CP-20 in Controlling the Corrosion of Different Alloys in Deaerated Acid Solutions Under Dynamic Condition
2% Acid Alloys Solution C. Steel SS 316 L Ni-Resist
Figure1. Corrosion rate (mpy) of Titanium and Cupronickel alloys in 2% Sulfuric
(H2SO4) acid solution inhibited with IBIT and CP-20 inhibitors.
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
6 24 72
Immersion Time (Hours)
Cor
rosi
on R
ate
(mpy
)
90-10: IBIT
70-30: IBIT
Ti: IBIT
90-10:CP-20
70-30: CP-20
Ti: CP-20
1600
Fig.2 Corrosion rate (mpy) of different alloys in 2% Hydrochloric (HCl) acid
solution (prepared in Gulf seawater) inhibited with 0.5% CP-20 inhibitor under dynamic condition by immersion test method.
0
20
40
60
80
100
120
C.S Ni-Resist 90-10 70-30
Alloys
Cor
rosi
on R
ate
(mpy
)
HCl
HCl + Inhib.
HCl + Inhib. + Deaeration
1601
Fig.3 Corrosion rate (mpy) of different alloys in 2% Sulfuric (H2SO4) acid
solution (prepared in Gulf seawater) inhibited with 0.5% CP-20 inhibitor under dynamic condition by immersion test method.
0
10
20
30
40
50
60
70
80
90
C.S Ni-Resist 90-10 70-30
Alloys
Cor
rosi
on R
ate
(mpy
)
H2SO4
H2SO4 + Inhib.
H2SO4 + Inhib. + Deaeration
1602
1603
-5 15
- 5 2 0
-525
- 5 3 0
-535
E( CQRR)
I ( UA/CMAZ)
Fig. 5 Typical polarization resistance curve for carbon steel in inhibited 2%sulfuric acid solution under deaerated condition.
1604
1st. Fig 6 Plots of Ca++ Vs Time in BRP and BHO samples for all charges.
Fig. 7 Plot of CU++ Vs Time in BHO samples for all charges; BHO: Brine Heater
Outlet
0
500
1000
1500
2000
2500
3000
0 5 10 15 20 25 30 35 40 45
Time (Hrs.)
Ca+
+
(ppm
)
BHOBRP
I - Charge II - Charge III - Charge
0
10
20
30
40
50
60
0 5 10 15 20 25 30 35 40 45
Time (Hrs.)
Cu+
+ (
ppm
) BHO
I - Charge II - Charge III - Charge
1605
Fig.8 Plot of Acid% Vs Time in BRP and BHO samples for all charges.
Fig. 9 Plot of pH Vs Time in BRP and BHO samples for all charges. BRP:
Brine Recycle Pump; BHO: Brine Heater Outlet.
0
1
2
3
4
5
6
0 5 10 15 20 25 30 35 40 45
Time (Hrs.)
Brin
e (p
H)
BRPBHO
0 .0 0
0 .0 5
0 .1 0
0 .1 5
0 .2 0
0 .2 5
0 .3 0
0 .3 5
0 5 10 15 20 25 30 35 40 45
T im e (H rs.)
Ac
id
(%
B R P B H O
I - C ha rge II - C h arg e III - C h arg e
1606
Fig. 10 Plots showing the variation in Cu++ ion in BH outlet samples versus
time for each Charge during flushing.
Fig. 11 Plots showing the variation in pH in BH outlet samples versus time
for each Charge during flushing.
Variation in pH during Flushing
0
1
2
3
4
5
6
7
8
9
0 4 8 12 16
Time (Hrs.)
pH
I-charge
II-charge
III-charge
0
5
10
15
20
25
0 4 8 12 16
Time (Hrs.)
Cu+
+ (
ppm
)
I-charge
II-charge
III-charge
1607
[A]
[B] Fig. 12 box wall, tubes and tube sheet showing scale deposition in the inlet of
brine heater of desal #27, C4, phase-II, Aljubail plant: A-before acid cleaning and B-after acid cleaning.
1608
[A]
[B] Fig.13 A close-up view of the tubes and tube sheet in the outlet of the brine heater
of desal #27, C4, Phase-II, Al-Jubail Plant : A-before acid cleaning and B-after acid cleaning.
1609
[A]
[B]
Fig.14 A close-up view of the tube and tube sheet in in the outlet of the brine heater of desal #27, C4, Phase-II, Al-Jubail Plant: A-before acid cleaning and B-after acid cleaning.
1610
1611
Fig. 16 Reltive thickness of scale deposition in the tube of heae recovery section
and brine heater of desal #27, C4 area Al-Jubail desalination and power plants, SWCC, - Evaluated on the basis of visual examination only. (Maximum scale deposition in the tubes of stage #08 = 100%)
1612
Fig. 17 A close-up view of the tubes and tube sheet in the outlet of the water box#07 of desal #27, C4, Phase-II, Al-Jubail, after acid cleaning.
Fig. 18 A close-up view of the tubes and tube sheet in the outlet of the water box#OS of desal#27, C4, Phase-II, Al-Jubail, after acid cleaning.
1613
1614
40
1615
[B]Fig. 22 A general view of the tubes and tube sheet in the outlet of the water box