Problem:

Mechanical deaeration equipment of seawater injection system will be out of operation for 3

months.

Seawater injection system operating condition:

Kinetic viscosity, = .

Volume flow rate, = = .

Diameter of pipe, = = .

Pipe area, = = (.

) = .

Sherwood number, = . .. =

Where, = =

and =

Corrosion implication of operating the system with mechanical deaeration and oxygen

scavenger injection:

Concentration of oxygen in seawater, C = 0.01ppm = 3.125 x 10-4 molm-3

= . .

. . = .

= .

= .

= . .. =

= . . (. ). .

. = .

=

= . . = . = .

Corrosion implication of operating the system without mechanical deaeration:

Concentration of oxygen, C = 7ppm = 0.21875 molm-3

Corrosion rate without mechanical deaeration is 27.5mmy-1 = 2.29mm/m

Amount of corrosion without mechanical deaeration equipment for 3 months is 6.87mm.

Corrosion control assessment:

In order to reduce corrosion risk during repair of mechanical deaeration equipment, 2

possible changes to mode of operation to reduce corrosion risk which are as following:

1. Increase amount of sodium sulphide oxygen scavenger to seawater

From inspection data indicates that acceptable corrosion rate is < 0.1mmy-1:

. = . =

Therefore, m

Chemical reaction is as below:

Na2SO3 + 0.5 O2 -> Na2SO4

Theoretically, 8 ppm of sodium sulphide is needed to reduce 1 ppm of dissolved oxygen.

Initial concentration of oxygen scavenger (Cos) reduce dissolved oxygen in seawater from 0.1

ppm to 0.01 ppm:

= .

= .

Table 1: Change of corrosion rate of system corresponding to the amount of oxygen

scavenger added to seawater injection system.

Concentration of oxygen

scavenger, Cos (ppm)

Oxygen concentration,

Co2 (ppm)

Corrosion rate,

ilim (mmy-1)

0.72 6.91 27.16

4 6.5 25.40

8 6 23.63

12 5.5 21.62

16 5 19.62

20 4.5 17.72

24 4 15.72

28 3.5 13.76

32 3 11.80

36 2.5 9.81

40 2 7.85

44 1.5 5.89

48 1 3.93

52 0.5 1.96

55.8 0.025 0.12

Efficiency of sodium sulfite oxygen scavenger is higher when it injected with

continuous batch at suitable point in the feed water system. However, oxygen corrosion

cannot inhibited by oxygen scavenger completely and increasing amount of oxygen

scavenger with continuous batch injection will increase cost. In additional, sodium sulfide

oxygen scavenger can act as nutrient for SRB bacteria from seawater.

From table 1, amount of oxygen scavenger required to reduce acceptable corrosion

rate of 0.1 mmy-1 is 55.8 ppm. High concentration of sodium sulfite will increase pH of

seawater but this also increase solid to the pipe leading erosion corrosion and possibility of

pipe blockage. However, increasing amount of oxygen scavenger will not increase

effectiveness of deoxygenate due to low reaction rate of oxygen scavenger with oxygen at

room temperature. Catalyze is require to increase reaction rate of oxygen scavenger with

oxygen.

Amount oxygen scavengers increased will not effectively substitute the function of

the deaerator. For example, if the oxygen content of the feedwater of boiler system is greater

than 50ppb then oxygen corrosion will occur although oxygen scavenger added into

feedwater. Sulfite does not passivate metal surface.

2. Decrease flow rate of water

Initial reynold number of seawater flow is 1.46 x 106 which indicate turbulent flow inside

pipeline. Flow rate of water will cause change in reynold number.

From =

, Re directly proportional to u.

Initial flow rate of water, = . = 0.184 m3s-1

Velocity of seawater flow in pipe, = .

..= .

Theoretically,

Critical velocity for erosion, =

r

Density of water r = 1030 kgm-3

Empirical constant c is between 180 and 245 for continuous service or up to 300 for

intermittent service. Therefore, Vc is ranging from 5.61 ms-1 to 9.35 ms-1.

Table 2: Change of corrosion rate of system corresponding to flow rate of water and reynold

number.

Flow rate of water, u

(m3s-1)

Reynold number, Re Corrosion rate, ilim

(mmy-1)

0.184 1.46 x 106 27.16

0.160 1.27 x 106 23.79

0.140 1.11 x 106 21.46

0.120 0.955 x 106 18.86

0.100 0.796 x 106 16.15

0.080 0.637 x 106 13.34

0.060 0.477 x 106 10.35

0.040 0.318 x 106 7.35

0.020 0.159 x 106 4.06

Initial velocity of seawater flow in pipe is 10.08 exceed critical velocity for

erosion will cause erosion corrosion and enhance corrosion by mass transport controlled with

increase oxygen delivery to pipe internal surface. From table 1, reducing flow rate of water

from 0.184 m3s-1 to 0.020 m3s-1 will decrease the corrosion rate from 23.41 mmy-1 to 3.5

mmy-1. This shown that decrease flow rate of water without change in initial amount of

oxygen scavenger (0.72 ppm) is less effective to reduce corrosion rate compare to effect of

increase amount of oxygen scavenger.

In conclusion, amount of oxygen scavenger is the major factor to reduce corrosion

risk in seawater injection system without mechanical deaeration equipment for 3 months. In

order to achieve a cost effective plan, both amount of oxygen scavenger and flow rate of

water operation condition can be changed to compensate loss of mechanical deaeration

equipment.