Acid Cleaning of Some Desal Units

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

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 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.

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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.

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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.

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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)

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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.

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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.

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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.

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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

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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].

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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

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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.

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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.:

First Charge Second Charge Third Charge Total

Consumed Day 1 Day 2 Day 3

Sulfuric acid (tons) 28.8 28.8 29 86.6 Acid inhibitor (kg) 200 200 400 800 Anti-Foam (kg) 10 10 10 30

Figures 6 to 9 show plots computed from the data obtained from Ca2+, Cu2+, acid

concentration and pH determinations during acid cleaning of the evaporators.

Variation in Ca2+ ion during cleaning in all the three charges vs time is shown in Fig. 6

Figures 7-9 show the variation in Cu2+ ion, acid % and brine pH vs time respectively.

Figures 10 and 11 show the variation in Cu2+ ion and pH of the recirculating acidified

brine in BHO samples during rinsing.

In first 3-5 hrs of acid injection in each charge, an increase in Ca2+ concentration is

observed. After 3-5 hrs, the Ca2+ concentration becomes almost constant, which shows

that the dissolution of scales became negligible. This represents the stage where it

would be advisable to blow-down the brine and add acid to fresh seawater. A similar

trend has been for Cu2+ ion also. After 5 hrs of dosing in the I charge, the Cu2+ ion

concentration reached to its maximum value of 32 ppm, which is an indication that the

heat exchanger tubes (with or without protective oxide film) are almost free from scales

and are in direct contact with the acid. The value of Cu2+ ions in following II and III

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13

charges reached to higher levels than charge I. In charge III, the concentration of Cu2+

ion is less as compared to charge II due to the use of double amount of inhibitor, In Fig.8

a sharp decrease in acid percentage is also observed after 3-5 hrs of injection

indicating that all the acid injected has been consumed within this period. An increase

in pH (Fig. 9) of the brine supports this viewpoint. However, in the subsequent II and

III charges, the pH of the brine does not rise to the same level as that was noted in the

Charge I and the rate of increase in pH is also very slow as compared to charge I.

During flushing (rinsing) decrease in Cu2+ ions to a minimum level (1.0 ppm) in 8 hrs

and increase in pH up to a maximum level of 8.05 in 15 hrs has been observed (Fig. 10-

11). Therefore, a minimum of 15 hrs to flushing should be carried out before putting the

system on load. However, a minimum of 8 hours of flushing is necessary while

changing over from one charge to another.

During the acid cleaning process, laboratory analysis of the sample was carried out as

follows:

S. No. Operation Kind of Analysis Sample Collection Point

1 Acid charging pH, Cu RB pump discharge and brine heater outlet

2 Acid circulation pH, Acid % and Cu Brine heater outlet 3 Rinsing pH, Ca and Cu Brine blow down

When the evaporator was back to production after inspection, monitoring of copper

values both in the blow down and product water was continued until the values became

normal.

The results of the post cleaning observations and the performance data for the tubes

cleaned with H2SO4 solutions containing inhibitor CP-20 are given in Tables 9 to 10.

The results indicate much better performance of heat exchanger tubes after acid

cleaning.

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4.2.2 Acid Cleaning of Desal # 27

The visual observations of the desal unit # 27 before and after acid cleaning operation

are presented in Table 11. The different sections of the unit were also photographed

before and after acid cleaning. A brief description of the photographs is as follows:

Figures 12A and 12B show water box wall, tubes and tube sheet of brine heater inlet

before and after acid cleaning, respectively. Uniform thin scales were found on the

walls of the water box, tube sheet and tubes in the inlet of brine heater (Fig. 13A).

Similarly, slightly thicker scales were observed in the outlet of the brine heater (Fig.

14A). Figures 12B, 13B and 14B show the water box wall, tube sheet and close up

view of the tubes in the inlet and outlet of the water box of brine heater after acid

cleaning. Some small pieces of scales can be seen scattered on the tube sheet in the

outlet of the brine heater. All sections of the brine heater were satisfactorily cleaned

after acid cleaning.

Figures 15A and 15B show a close up view of the tubes of water box stage#1 before and

after the acid cleaning, respectively. Figure 16 represents the level of thickness (in %)

of scales in the tubes of heat recovery section and brine heater before the acid cleaning.

This graph has been drawn on the basis of visual observations of scale thickness. Heavy

scale deposition was found in the tubes of water box # 7, 8, 9, 10 and 11 (Figures 17

and 21). In general, the deposition of scales on the tubes of water boxes # 12-17 was

higher than the tubes of the water boxes # 1-6. Figure 19A shows a close up view of the

tubes and detached scales in the outlet of the water box # 9 before acid cleaning,

appeared in the center of the photograph. Figure 19B shows the presence of residual

scales in the same section after acid cleaning. Partial cleaning of the tubes of the water

box # 7, 8, 10 and 11 can be seen in Figures 17, 18, 20 and 21. Figure 22A shows the

presence of sludge, debris (scales) and tubes plugged with balls at 6-9 O'clock position

in water box # 14 and 16 also. Figures 22B shows a overall view of the water box # 14

after acid cleaning.

Figure 23 shows partial cleaning of tube sheet of the water box # 15 and Figure 24

shows satisfactory cleaning of the tubes and tube sheet in the water box # 17. Bluish

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color stains are seen on the tubes of the water box # 15 and 17. It was observed that

cleaned tubes exposed to atmosphere for long periods (for inspection) develop blue

stains on the inner surface for the tube .The bluish stains would have formed as a result

of dissolution of copper oxide protective film on the inner surface of tubes and

subsequent attack of acid on bare metal. Such stains are likely to disappear after the

formation of a fresh copper oxide protective film. Figures 25A and B show the acid

injection site (drain of flash chamber # 18) before and after the acid injection. Partial

removal of scales in the vicinity of the hole and a uniform iron hydroxide thin layer can

be seen (Fig. 14B).

The physical inspection and study of the photographs of desal # 27 before acid cleaning

showed:

(a) Very thin scales were present in the tubes of brine heater inlet.

(b) Thin scales were found in the tubes of water box # 1-9 and brine heater outlet.

From the post acid cleaning inspection, the following important observations were

emerged:

(a) Scales were removed satisfactorily from the tubes, tube sheet and walls of the

water box of brine heater inlet and outlet and water boxes # 1-6 and # 12-17.

(b) Partial cleaning of scales in the tubes, tube sheet and walls of the water box # 7-

11was observed.

(c) In general, brine heater inlet and outlet tubes and most of the heat recovery stage

tubes were cleaned satisfactorily. The results of pre- and post acid cleaning

performance for desal Unit # 27 are summarized in Table 12.

Tables 10 provides plant analysis data for desal cleaning samples collected at certain

intervals at 3 locations:

(i) During Acid Charging: Brine recycle pump inlet and brine heater outlet

(pH)

(ii) During Acid Circulation: Brine heater outlet (pH, % H2SO4)

(iii) During Flushing: Brine blowdown (pH, Ca2+)

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A general pattern emerges from analytical data, which is summarized as follows:

(a) During acid charging, pH was maintained around 1.5-2.0 in inlet (RB). In

outlet (BHO), pH goes down slowly in the beginning and rises sharply at the

later stages showing consumption of acid.

(b) During circulation, pH in the brine heater outlet increases gradually with

increase in time and H2SO4 concentration decreases showing the process of

acid cleaning is under completion.

(c) During flushing, in blow down, pH increases and Ca2+ concentration

decreases with time showing the process of flushing is under completion.

The cleaning procedure was first evolved by carrying out test cleaning of desal # 29

unit. Besides pH, acid concentration and Ca2+, Cu2+ concentration was determind in the

brine heater outlet during the three charges. During first charge, amount of copper

leached out during the first 2 hrs or so is quite low (< 2 ppm) but after this period, there

is tremendous increase in copper concentration (22 ppm) which increases gradually with

time reaching a value of 32 ppm after 7 hrs circulation. The amount of the copper

leached out during second charge (> 50 ppm) was much higher than that in the first

charge. This indicates that the acid has now in direct access to descaled or undeposited

desal surfaces leaching out significant amount of copper. Lower values of leached

copper in the third charge might be attributed to the formation of passive Cu2O film

formed during post charge II flushing.

Post acid cleaning results of the different desal units of C-4 indicate satisfactory

cleaning of units under study. For example, there is a significant improvement in water

production. Similarly, in gain output ratio (GOR) value there is an immense

enhancement on acid cleaning in different units. This is shown below:

1590

Unit # Water Production (m3/hr.) Gain Output Ratio (GOR)

Pre Cleaning

Post cleaning

Improvement %

Pre Cleaning

Post cleaning

Improvement %

27 920 1120 20.7 6.3 8.6 36.5

29 820 1010 23.1 6.0 8.8 47.1

The values of heat transfer coefficient (HTC) of brine heater and heat recovery section

were determined for desal unit # 29 before and after acid cleaning, an improvement of

53.3 and 62.3%, respectively was noted as shown below:

Heat Transfer Coefficient Brine Heater Heat Recovery

HTC, W/m2/oC (Before Acid Cleaning) 1908 1663 HTC, W/m2/oC (After Acid Cleaning) 2925 2700 % Improvement 53.3 62.3

5. CONCLUSION

5.1 Laboratory Studies

(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.

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(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

Acids Duration (hrs) 90-10

Cupronickel 70-30

Cupronickel Ti

Hydrochloric 6 0.09 0.09 0.07 + 24 0.03 0.05 0.03

0.3% IBIT 72 0.04 0.02 0.02 Sulfamic 6 0.06 0.06 0.02

+ 24 0.04 0.03 0.03 0.3% IBIT 72 0.03 0.03 0.01

Sulfuric 6 0.09 0.1 0.02 + 24 0.09 0.1 0.02

0.3% IBIT 72 0.04 0.05 0.06

Table 2. Corrosion Rate (MPY) of Heat Exchanger Tube Materials in 2%

Inhibited Acid Solutions Under Dynamic Condition

Acids Duration (hrs) 90-10 Cupronickel

70-30 Cupronickel

Ti

Hydrochloric 6 2.58 3.25 0.37 + 24 1.85 2.23 0.28

0.3% CP-20 72 1.37 3.66 0.06 Sulfamic 6 0.55 0.41 0.02

+ 24 0.31 0.73 0.02 0.3% CP-20 72 0.5 0.2 0.02

Sulfuric 6 0.66 0.73 0.06 + 24 0.6 0.5 0.08

0.3% CP-20 72 0.6 0.6 0 .03

Table 3. Corrosion Rates (MPY) of Different Alloys Immersed in inhibited (CP20-0.5%)

Acid Solutions (Prepared in Gulf Seawater) Under Dynamic and Open Atmospheric Condition for 6 hr.

2% Acid Alloys Solution C. Steel SS 316 L Ni-Resist

D-2 Cupronickel

90-10 Cupronickel

70-30

HCl 101 < 1 46 53 66 H2SO4 81 < 1 45 5 5 HCl + CP-20 46 < 1 33 1.5 2.5 H2SO4 + CP-20 40 < 1 22 1 1

1594

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

D-2 Cupronickel

90-10 Cupronickel

70-30

HCl 10 < 1 4.5 2 4 H2SO4 17 < 1 6.8 4 4 HCl + CP-20 3.5 << 1 < 1 < 1 < 1 H2SO4 + CP-20 3.5 << 1 < 1 < 1 < 1

Table 6. Efficiency (%) of Inhibitor Nevamin CP-20 in Controlling the Corrosion of Different Alloys in Deaerated Acid Solutions Under Dynamic Condition


D-2 Cupronickel

90-10 Cupronickel

70-30

HCl 65 ~ 100 ~ 100 ~ 100 ~ 100 H2SO4 79 ~ 100 ~ 100 ~ 100 ~ 100

1595

Table 7. Corrosion Rate (MPY) of Different Alloys Immersed inInhibited Acid

Solutions (Prepared in Gulf Seawater) Under Dynamic Condition by LPR Technique


D-2 Cupronickel

90-10 Cupronickel

70-30

HCl + atmospheric 50 5 31 0.4 0.5 H2SO4 + atmospheric 32 4 21 0.1 0.2 HCl + deaeration 1 4 0.2 0.2 0.3 H2SO4 + deaeration 3 2 2 0.1 0.03

Table 8. Corrosion Rate (MPY) of Different Alloys in Inhibited Acid Solutions Under Atmospheric or Deaerated Condition

0.5 % CP - 20 Inhibited 2% HCl 2% H2SO4 Corrosion Rates, MPY

Test Condition Alloy Found Max. Allowance* Found Max. Allowance*

Atmospheric Deaerated

C. S 46 3.5

20-50 Fair 1-5 Excellent

40 3.5

20-50 Fair 1-5 Excellent


SS 316 L < 1 << 1

< 1 outstanding < 1 outstanding

< 1 << 1

1-5 Excellent 1-5 Excellent


Ni-Resist 33 < 1

20-50 Fair < 1 Excellent

22 < 1

20-50 Fair < 1 outstanding


90 Cu 10 Ni 1.5 < 1

1-5 Excellent < 1 outstanding

1 < 1



70 Cu 30 Ni 2.5 < 1


1 < 1

1 Excellent < 1 outstanding

* Represent maximum allowable corrosion rates in corrosive environments [Fontana 1987, Uhlig and Revie 1985]

1596

Table 9. Post Cleaning Observations Desal # 29

S. No Components subjected

to cleaning State of

Cleaning Remarks

1 Brine heater inlet Very good Cleaned to base metal. 2 Brine heater outlet Good Center tubes were not as clean

as the lower tubes. Upper row tubes cleaned to bare metal.

3 Heat recovery tubes Stage 1-6 Fully cleaned

Some tubes were found plugged

4 Stage 7-12 Good Scale debris were found at the bottom of the tube sheet which blocked tube opening.

5 Stage 13-17 Good Some tubes were found plugged with scale debris and/or cleaning ball blockage at tube inlet/outlet.

6 Flash chambers Uniform minor descaling

7 Acid injection point #18 flash chamber drain point

Cleaned to base metal

No visible sign of corrosion

Table 10: Post Acid Cleaning Performance Data for Desal # 29

Description Design Before After Improvement

TBT oC 90.6 87.5 89.9 2.4 - Dist. Prod. M3/hr 988 820 1010 190 23% LP steam consumption M3/hr 129.4 142 118.5 23.5 16.5%

(reduced) P. R. Kg.Dist./1000 KJ 3.44 2.64 3.88 1.24 47% G. O. R. KgDist../KgSteam 7.9 5.98 8.8 2.82 47% HTC brine heater W/M2/ 0 C 2467 1908 2925 1017 - HTC recovery W/M2/ 0 C - 1663 2700 1037 - Recycle brine flow at max. v/v opening M3/hr.

12480 10500 11300 800 -

1597

Table 11A. Before Acid Cleaning Observations Deasl#27

Location Scale Presence Remarks

Brine Heater Inlet & Outlet Tubes Tube sheet Water box

Very Thin Scales Very Thin Scales Very Thin Scales

At outlet the scaling was slightly more than the inlet

Heat Recovery Section Water Box #1 Outlet Tubes Tube sheet Water box

Very Thin Scales Very Thin Scales Very Thin Scales

Similar scales were found as were in brine heater outlet.

Water Box #3 - 7 Tubes Tube sheet Water box

Thin Scales Thin Scales Thin Scales

Few tubes were found blocked either by balls or debris (scales).

Water Box #8-11 Tubes Tube sheet Water box

Heavy Scales Heavy Scales Heavy Scales



Thick Scales Thick Scales Thick Scales


1598

Table 11B. Post Acid Cleaning Observations Desal #27

Location Scale Presence Remarks

Brine Heater Inlet & Outlet Tubes Tube sheet Water box

Satisfactorily Cleaned Satisfactorily Cleaned Satisfactorily Cleaned

No sign of corrosion was observed.



No sign of corrosion was observed.

Water Box #6 Tubes Tube sheet Water box

Satisfactorily Cleaned Partially Clean Satisfactorily Cleaned

Few tubes were blocked.

Water Box #7,8,9,10 &11 Tubes Tube sheet Water box

Partially Clean Partially Clean Partially Clean

Water Box # 12, 15 Tubes Tube sheet Water box

Satisfactorily Cleaned Partially Clean Partially Clean

Water Box #13, 14 Tubes Tube sheet Water box

Satisfactorily Cleaned Satisfactorily Cleaned Partially Clean

Water Box #16, 17 Tubes Tube sheet Water box


Few tubes were blocked.

Table 12. Evaporator Performance Summary After Acid Cleaning

Unit Acid Evap Pre Cleaning Values Post Cleaning Values Improvement No. Used

(Tons) Off.

Prod. Hr. P. W. M3/hr

Cond M3/hr

GOR

P. W. M3/hr

Cond M3/hr

GOR

P. W.

GOR

D-27 91 - 928 147 6.31 1120 130 8.61 192 2.3 D-29 87 116 820 142 5.98 1010 118.5 8.8 190 2.82

1599

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

-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

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

40

1615

[B]Fig. 22 A general view of the tubes and tube sheet in the outlet of the water box

#14 of desal #27, C4, Phase-II, Al-Jubail Plant : A-before acid cleaningand B-after acid cleaning.

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Acid Cleaning of Some Desal Units

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