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ELSEVIER Desalination 120 (1998) 61-72
DESALINATION
Rehabilitation of a long-tube parallel flow evaporator A1-Ghubrah power plant and desalination station
A.R. Abu Dayyeh*, Ribhi Hamdan, P.K. Mukerjee, P.A. Vijay Kumar Sogex Oman Co., LLC, Ghubrah Power and Desalination Station, PO Box 1170, Ruwi-112, Sultanate of Oman
Tel. +968 564636 Fax +968 564608
Abs t rac t
The No. 1 desalination plant of the A1-Ghubrah power and desalination plant, Sultanate of Oman, is a multi-stage flash long-tube parallel flow type with 18 stages of heat recovery and two stages of heat reject in five vessels. The plant was commissioned in the year 1976. It had experienced severe corrosion in the vapour space, and more in the high temperature stages. The support plate, inter-stage walls, and top roof plate were badly corroded. Tube bundles began to sag and some collapsed in vessel No. 1. Tube leaks were encountered frequently. It was analyzed that corrosion was due to build-up of non-condensable gases in these stages. The plant venting system was designed to have series venting (cascaded) from stage No. 1 to 20. The plant was rehabilitated in 1990. The paper describes the study of corrosion, modifications of the venting system and preventive measures being carried out to prevent the accumulation of non-condensable gases. After modification and tube bundle replacement in vessel No. 1, no significant corrosion has been observed in this vessel even after 5 years of operation.
Keywords: Multi-stage flash; Long-tube evaporator; A1-Ghubrah; Oman
1. Introduction
The desalination plant consists of five rectang- ular vessels. The first vessel is comprised of six stages (i.e., Stages No. 1-6), and the second, third and fourth vessels have four stages each and
*Corresponding author.
the fifth vessel has two stages (heat reject stages). The evaporator has a separate deareator and decarbonator. The decarbonator is no longer used as the plant is running on a polymer-based anti- scale additive. Caustic soda is dosed in the stage No. 1 distillate tray to maintain a final pH of greater than 8.5 in the distillate. The plant venting system was designed to have cascaded venting
Presented at The Third Gulf Water Conference, Muscat, Sultanate of Oman, 8-13 March 1997.
0011-9164/98/$ - see front matter © 1998 Elsevier Science B.V. All rights reserved PII S0011-9164(98)00202- l
62 A.R. Abu Dayyeh et al. / Desalination 120 (1998) 61-72
HI:kicK - ~ I
HEADER
Fig. 1. Venting arrangement, before modification.
I I
t ) BRINE RECIRCULATION PUMP
..T H[A lira I
HEADER
I 1 i
1 Fig. 2. Venting arrangement, after modification.
from stage No.1 to Stage No. 20 (see Fig. 1). The tube bundles of the recovery and reject sections are divided into two, each bundle consisting of 5036 tubes. There is one metallic expansion joint installed in the middle of each side between bundle tubes of each vessel 1, 2, 3 and 4.
The materials used in the construction of the plant were:
Tube stages: Nos. 1-18 Nos. 19,20
Evaporator shell
Material Thickness, I T I m
CuNi 10 Fe 1.0 Titanium 0.5 mm Steel St 37-2 Bottom plate (floor) 25 Top plate (roof) 20 East and west wall 30
(partition wall) North and south wall 25 Distillate tray and 15
condensing shell
The vessels were constructed from bare
-n= Lt~BRINE RECIRCULATION PUMP carbon steel without painting. The design technical data are:
Type Make Year of manufacture
Plant capacity:
Economy ratio Concentration factor Quality of distillate Seawater temp. (design) Cooling water flow to
heat reject Brine recycle flow Heat recovery section:
No. of stages No. of tubes in each
stage Outside dia. of tubes Tube material
MSF long tube SOGEX/DEMAG 1975/76 18,200 M3/d (4 migpd) at TBT--90°C, using low temp. chemical scale control. 27,600 M3/d (6 migpd) at TBT=113 °C, using high temp. improved chemical scale control or acid (H2SO4) 6.8 kg of distillateJkg of steam 1.5 100 ppm TDS 35°C
11,800 M3/h 10,000 M3/h
18
10,072 16mm CuNi 10 Fe
A.R. Abu Dayyeh et al. / Desalination 120 (1998) 61-72 63
Heat rejection section: No. of stages 2 No. of tubes in
each stage 10,072 Outside dia. of tubes 16 mm Tube material Titanium
2. History of corrosion
The plant was commissioned in 1976 and initially it was run on polyphosphate producing design and guaranteed values o f 4migpd. The 24 h performance test of desalination plant was carried out on 17/18 March 1979, after the plant had produced water for almost 3 years. The contractual and actual values are tabulated in Table 1.
The plant is also capable o f producing 6 migpd (non-guaranteed) with acid dosing at a TBT of 113°C. It is a well known fact that plants on acid dosing will suffer more corrosion; hence in 1979, the plant was run with the high-temperature anti- scale additive Belgard, and a performance test run was carried out on 2 March 1981 (see performance data, Table 1.) During the test run, it was observed that the pressure in Stage No. 1 was at 1.15 bar (Abs) and therefore the first stage vent to atmosphere was opened which was emitting a great deal o f steam, making the surrounding area near stage No. 1 wet and corrosive. Afterwards the plant was run around 5 migpd capacity. Typical values are tabulated in Table 1.
Table 1 Plant performance test data
Description I a, contractual Actual II b lip value value
Distillate production, ma/h 757 d Cooling water to heat reject, ma/h I 1,800 Brine flow to brine heater, ma/h 10,000 Brine inlet temp. to brine heater, °C 85 Brine outlet temp. from brine heater, °C 91 Steam flow to brine heater, t/h 113.3 Steam pressure to brine heater, bar (abs) 1.0 Steam temp. to brine heater, °C 100 Economy ratio, (kg distillate)/(kg condensate) 6.8 Heat consumption, (K cal)/(kg distillate) 79.4 Purity of distillate, ppm 100 Distillate temp., °C 41 pH value of distillate after caustic dosing 8-9 pH value of brine recycle 8.4-8.6 Brine recycle concentration, ppm Seawater temp., °C Seawater TDS, ppm
797 e 1,130 970 9504 8,900 11,796 8521 9,900 9,300 82 97.6 96.4 90 108.3 104 119.3 186.5 f 139 h 0.795 1.46 1.2 93 113 106.5 6.7 6.05 6.98 82.0 - - - - 6.0 138 92.7 33 39.1 40.8 8.8 8.5 8.6 8.5 8.6 8.6 56,949 60,649 59,040 23 23.9 31.1 39,575 g g
aPerformance test data with polyphosphate on 17/18 March 1979. bperformance test data with Belgard EV on 2 March 1981. CGeneral performance of plant with Belgard EV on 4 June 1983; production approx. 5 migpd, non-guaranteed. d 4.0 migpd. e4.21 migpd. f 3 194 m/h condensate. gSeawater TDS constant between 39,000-40,000. h114.4 m3/h condensate.
64 A.R. Abu Dayyeh et al. / Desalination 120 (1998.) 61-72
The plant is shut down annually for 1 month for maintenance and inspection. During the annual inspection and shutdown from 08/04/83 to 05/05/83, it was found that tube support plate of stage No. 6 had severe corrosion at the bottom. As there were no manholes to enter the distillate tray, the inspection was made with the help of a boroscope.
The first tube leak was observed on 24 October 1983. As this was the first one, various tests (e.g., change in brine flow and pressure, change in cascade venting) were carried out. Ultimately the plant was shut down on 18 December 1983. To detect the tube leak, a vacuum test was carried out subsequently on two tubes of stage Nos. 4-6 South, which were plugged.
During the next annual shutdown from 19/02/1984 to 24/03/1984, the plant was inspected by M/s. Incon Anlagentechnik GmbH, Hamburg, Germany [1]. It was ascertained that the support plates of tube bundles in the region of Stages No. 5 and 6 in vessel No. 1 were badly corroded. In the passage of non-condensable gases of stages No. 5 and 6, an approximately 1 mm thick deposit was found adhering to CuNi tubes. This material also partly covered the support plates and inner surface of the distillate duct. The colour of the deposit was red and lightly adhered to the material surface. The chemical analysis undertaken in Germany showed the compositions of CuO, 36.9%; Fe304, 61.2%.
The CuNi 10Fe tubes under the deposits showed no visible signs of corrosion while the supports, distance holding pipes and distillate duct were attacked by corrosion. Some of the supports were totally corroded up to the first few rows of tubes.
From 1983 onwards, tube failure occurred frequently, and 1988 alone, 644 tubes were plugged in vessel No. 1. Before rehabilitation, the following tubes were plugged.
Tube failures were confined to the first vessel. Failures tended to increase in Stages No. 1-3 compared to Nos. 4-6. They were more frequent
Stages Tubes Total Percent age plugged tubes
1-3 1135 10,072 11.25 4-6 72 10,072 - - 7-8 2 10,072 - - 9-18 0 10,072 - - 19-20 13 10,072 - -
in lower parts of the bundle, near the non- condensable gases entrance.
Galvanic corrosion was also observed between tube and tube support plates due to displacement of insulation material between them. Conse- quently, the hole in the tube support plate was enlarged causing rattling of tubes. Tube support plates and holding pipes were badly corroded and missing in some plates causing tubes to sag (Fig. 3).
There was no provision to enter the distillate duct. Hence manholes were made in distillate ducts of Stage No. 6 in 1986, Stage Nos. 1-5 in 1987, and for all other stages in 1988 for close inspection of the condensing chamber. Extra saddle supports were also installed with Teflon sheets on top of saddles to prevent sagging in the same year of making the manholes. Severe corrosion was also observed in the vapour space of the distillate duct. Black flakes of iron oxide were found all over the vapour space. It was also noted that corrosion product formed on the steel broke offand fell on tubes. Corrosion was usually most severe in the inter-stage walls.
3. Analysis of corrosion
In 1987, 25 tube samples were extracted from vessel No. 1 and delivered to the Nickel Develop- ment Institute to examine them [2]. The analysis showed that corrosion was confined to the exterior surface. The inside (brine side) surface was in good condition and covered with light scale with no evidence of serious corrosion.
The analysis of deposits on tubes showed mainly oxides of Cu/Fe. The deposit on tubes
A.R. Abu Dayyeh et al. / Desalination 120 (1998) 61-72 65
i . i~ i
Fig. 3. Corroded tube support plate/extra saddle support provided (before rehabilitation).
exterior was probably caused by soluble corrosion product dripping through the bundle and depositing as their oxide. The evidence in the tube sample and distribution of failure indicated vapour side corrosion as the cause of the problem. It was clear that corrosion must be caused by non- condensable gases. The main component of these gases is carbon dioxide, which is created by thermal decomposition of bicarbonate in sea- water.
2 HCO~ ~* CO 2 I + CO~- + H20
The second most likely gas is oxygen. As a MSF plant is operated at reduced pressure air ingress will occur through gasket joints, level gauges, and manhole covers, any leakage in the upper section of the plant is unlikely to enter the brine which is flashing. It will, however, be swept into the vapour space by the flow ofvapours. It is therefore evident that quite a high oxygen level could exist in the vapour space even when the oxygen level in the brine recycle is at an acceptable limit of 50 ppb.
Oxygen scavengers such as hydrazine are often added to the boiler waters. These decompose to form ammonia, which in the presence of oxygen is extremely corrosive to copper base alloys. In MSF plants the steam to brine heater will contain ammonia if hydrazine is used for boiler water treatment. If the brine heater is vented to stage No. 1 of the evaporator, then very severe vapour side corrosion can occur as oxygen is usually present.
Corrosion of copper alloy in vapour space requires the presence of both carbon dioxide and oxygen [2]. The presence of carbon dioxide makes condensate acidic. The developed acidity does not cause copper alloys to corrode because these alloys are more noble than the hydrogen release reaction, and in the absence of oxygen corrosion would be negligible [3]. The acidity, however, removes the protective film and in the presence of the oxygen corrosion can proceed. This corrosion normally takes place in the form of uniform thinning of tubes. This is the type of corrosion observed in the samples.
66 A.R. ,4bu Dayyeh et al. / Desalination 120 (1998) 61-72
4. Rehabilitation
The plant was rehabilitated in 1990. An Eddy current test and ultrasonic thickness measurement were carried out during rehabilitation in order to determine the plant life [4].
The Eddy current test was carried out for 10% of 5036 tubes in each bundle for Cu-Ni tubes of Stages No. 7-18 and titanium tubes of Stage Nos. 19 and 20. The thickness reduction ratio in almost all the tubes tested was within the range of 20-30%, and there was no tube more than 30% in reduction ratio of tube thickness. On the other hand, the allowable thickness reduction ratio in structural strength (no requirement of corrosion allowance) is 75% in 90-10 Cu/Ni tubes and 50% in titanium tubes. Accordingly, the tube life was assessed to be durable for another 10 years. During the Eddy current test, conditions of the tube support plate were also checked. Defects/ damage on the tube support plates were found in vessels Nos. 2 and 3.
The designated position of the end plate, partition plates, side plate, top plate and
condensing chamber plate were measured by an ultrasonic thickness indicator. In vessel No. 1 between stage Nos. 2 and 3, a thickness reduction ratio of the partition plate (30mm thick) was highest at 13.7%, i.e., 4.1 mm of wall plate was corroded against 10 mm corrosion allowance.
The reduction ratio of the top plate (20mm thick) was highest in Stage No. 18 at 15%, i.e., 3 mm of ceiling plate was corroded against 5 mm corrosion allowance. The reduction ratio of the condensing chamber plate was highest in Stage No. 12 at 16%, i.e., 1.2mm of chamber plate was corroded against 5 mm corrosion allowance.
From the above it was concluded that the body shell itself of the evaporator will be durable for another 10 years. Hence it was decided to replace only the badly corroded tube bundles and the condensing section of vessel No.1 only. All the tube bundles of vessel No. 1 and condensing section were cut and dismantled (Figs. 4 and 5).
A new tube bundle and condensing section was erected. Fabrication of the condensing section and insertion/fitting of the tubes were completed at the supplier's factory.
i I 1 . , I
;-L!-.,II II I i , L3 1 it, 2 + . . . . ~ ' - - ~ ~'~ i ,l, ,], ;----" \ ~ . / I ~ , ~ o,: woR,, I B-BLOCK. A-BLOCK
BRINE MANIFOld. L~F_.XPANSION JOINT<-~J
L i i .L ,I. ,L .L t I J. I JL J.. J. ,L J,
Fig. 4. Evaporator block, stages 1-6. View A.
A.R. Abu Dayyeh et al. / Desalination 120 (1998) 61-72 67
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Fig. 6. Evaporator (A&B block), detail of condensing tank.
The material of construction of new bundles was as follows:
Name of part Material
Shell for condensing section
Tube
Tube plate Tube support plate Partition plate Distillate tray
Carbon steel + SS 316L cladding
CuNi 10 Fe 1 Mn (16 mm OD, 1.0 mm thick)
CuNi 10Fe 1Mn SS 316 L SS 316 L Carbon steel + SS 316 L
cladding
Baffles were also provided in the new tube bundle to promote cross flow so that non- condensable gases are not accumulated at the bottom of the bundle. No baffles were provided in old bundle. Manholes were also provided in condensing chamber.
The plant venting system was designed to have series venting (cascaded) from stage No. 1 to Stage No. 20. The practice of cascading non- condensable gases from stage to stage with venting at just a few stages in the mid and low temperature sections of the plant means that gas concentration will increase until the vented stage is reached. For this reason the stage No. 1, 2, 3
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A.R. Abu Dayyeh et al. / Desalination 120 (1998) 61-72 69
venting was modified. Separate direct vent line to deaerator was provided to remove non- condensable gasses quickly and completely from the process as possible from stages 1, 2, 3. For cascaded vent line from stage No. 4 to stage No. 5, stage No. 5 to stage No. 6, vent line angle was changed. Earlier the vent line was connected at the center of the tube bundle from side of vessel. This was done to avoid pocketing of non- condensable gases. Water hammering was observed in brine inlet line to vessel No.1 after modification. Direct atmospheric vents with remote control valve for stages No. 1 and 2 was provided on both the bundles side to suppress the hammering/flashing in the brine outlet of brine heater/header in the case of the trip of the brine recycle pump at an elevated temperature. An earlier brine heater vent was connected to stage # 1. In order to avoid any corrosion due to the presence of ammonia in steam supplied to the brine heater after rehabilitation, the brine heater was directly vented to the atmosphere (see Figs. 2 and 6).
5. Performance test prior to rehabilitation
A performance test at a steady condition of 860m'/h production at TBT 99.2°C was carried out between 30 December 1989 and 1 January 1990. The distillate product of vessel No. 1 was monitored by using an ultrasonic meter [5].
The result of the performance test showed the product flow in close agreement with calculated production based on flash down. The maximum output test was carried out on 2 January 1990 prior to the shutdown of EVAP # 1 on 3 January 1990 for rehabilitation vessel No. 1. The plant output was gradually increased to the data for maximum attainable output with the existing vacuum unit and venting all non-condensables to the vacuum system (atmospheric vent not opened). The distillate conductivity shot up more than full scale (+1000 #s/cm) when distillate production reached 990m3/h at 104°C and
dumped to waste. The analysis of the distillate samples for the stages showed pick-up of the brine from vessel No. 1 (due to a tube leak). The production was further increased up to 1120- l130m3/h by increasing the TBT to 106°C. Performance test results are tabulated in Table 2.
Based on the results of the test, it was recommended that the unit can be operated only at a maximum TBT of 104°C and a recycle flow of 10,000 M3/h. Operation at a TBT in excess of 104°C could lead to corrosion of vessel No. 2 tubes and its support plate. Also venting of all the non-condensables at the vacuum unit at a temperature above 104°C will cause excessive carry-over and build-up of salt on the nozzle periphery and diffuser which will restrict the vapour flow, causing the ejector to choke; secondly, it will create problems of stress cracking of nozzles.
6. Optimization test after rehabilitation
The test was conducted in three parts: the first on the relation between TBT and brine recycle flow (BRF), the second on the relation between TBT, BRF and seawater flow, and the third on the relation and effect of venting and vent valve adjustment to temperature profile [6].
The maximum production in the relations of BRF and TBT was obtained at the conditions of 113 o C/9000 m3/h and 1030 m3/h production and 109 °C/9500 m3/h production of 1000 m3/h using atmospheric venting pipes.
At 113 °C operation requires severe control to prevent scale trouble and at 109 °C the operation required the atmospheric vent valves of stage No. 1 to be opened. Also operation of 107°C/ 9500 m3/h required the operation of two units of the vacuum system. At this condition the safe operation was found to be 104 o C/9500 m3/h. The limitation of brine recycle pump capacity restricted the production capacity in a certain level at high TBT. The reason was that the maximum flow rate of brine recycle pump
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A.R. Abu Dayyeh et al. / Desalination 120 (1998) 61-72 71
decreases corresponding to higher TBT, where the delivery pressure had to be increased to match with an increase of the first stage pressure and pressure drop of flow line including stage tubes.
7. Conclusions and recommendations
The effect of varying seawater flow rate to the production was small. It was recommended that seawater flow rate be within the flow range of 8500-9500m3/h. The variation of seawater temperature was observed, and it is expected due to the tidal condition and to stabilize the operation variation, and is to control the seawater flow rate as the plant does not have a tempering system.
At the end of the test operation, eight modes of vent valves were tested when the plant was operated at 104°C TBT and 9500m3/h BRF. It was found that the best position of the vent valve is mode No. 6 in considering the balance of venting from each stage by judging the temperature profile (see Fig. 7).
For the steady operation condition, the conditions of 104°C TBT and 9500ma/h BRF producing around 980m3/h of distillate were found optimum.
A special vacuum test is carried out at each start-up to ensure all oxygen leaks are arrested.
No significant corrosion is observed in vessel No. 1 even after 5 years of operation (Fig. 8).
110 1105.S
100
90
O0
50
50
40
O(SALn4Jf ZON PLANT TEMP. PFI ~Vl[I
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~ - ~ msrzcc. Ftow - m N
VENT VALVE : MOOE&
&l,O
FOSlTIm 2 2 ~ 4 K~50% "~. O.S (P~ (~$E}
Fig. 7. Temperature profile.
Fig. 8. No corrosion observed in new bundle and condensing section after 5 years of operation (after rehabilitation).
72 A.R. Abu Dayyeh et al. /Desalination 120 (1998) 61-72
References
[I] Incon Anlagentechnik GmbH, Inspection report of Desalination Plant No. 1, Ghubrah, Oman, 1984.
[2] Nickel Development Institute, Report on corroded tubing from No. 1 distiller, Ghubrah, Oman, 1987.
[3] L.L. Shreir, ed., Corrosion, Vol. 1, Metal/ Environmental Reactions, Newnes Butterworth, 1963.
[4] Hitachi Zosen Corp., Ghubrah Power & Desali- nation Plant, Plant No. 1, Replacement of Tube
Bundles of Stage 1 to 6. Report of eddy current test and ultrasonic thickness test, HZ Letter L-171 1 SO/CS/280,27/6/1990.
[5] Hitachi Zosen Corp., Performance test prior to shut- down of desalination plant No. 1, January 1990. HZ letter L-RT-SO/C%028,4/l/1990.
[6] Hitachi Zosen Corp., Desalination Plant No. 1, replacement of the tube bundles for Stage 1 to 6, Report of Optimisation Test: July 1990. HZ letter L-1711-SO/CS-296, 16/7/1990.
