CAPITULO I. LA ACTIVIDAD EXPORTADORA DE LA EMPRESA COMO DECISION
I.1. LA ESTRATEGIA DE LA EMPRESA
A suitable chemical treatment program is determined according to the make-up and cooling water qualities, and the operational conditions, such as the cycles of concentration and water temperature, in an open recirculating cooling water system. Es-pecially, corrosion and scale control methods are varied in accordance with the cooling water quality.
Case studies of chemical treatments for typical types of cooling water qualities are described below.
(1) High hardness water
In the case of high calcium hardness (over 150 mg CaCO3/l ), high M-alkalinity (over 150 mg CaCO3/l ) and high pH (over 8) water, phosphate based corrosion inhibitors easily form the protec-tive films on metal surfaces. Accordingly, the phosphates of low concentration, such as 3 to 6 mg T-PO4/l, show the sufficient corrosion inhibitions in high hardness water as shown in Figures 3.17(p.3-16) and 3.19(p.3-17). However, the sufficient amount of scale inhibitors (polymers) should be used together with corrosion inhibitors to prevent the
Pi = 0.06 degrees Pm = 5 degrees Pf = 10 degrees
E = 1.5% of circulating water quantity N = cycles of concentration
N = 6
N = 3
0 1 2 3 4 5
0 10 15 40
35
30
25
20
Filtration rate (% against circulating water quantity)
Fig. 3.67 Relationship between the turbidity of cooling water and the side stream filtration rate
Turbidity of circulating water (degree)
where
P = turbidity of circulating water under the application of side stream filtration (degree)
R = circulating water quantity (m3/h)
Pi = turbidity increased in the cooling water by the microorganism growth and fouling ma-terials from air during the one circulation (degree)
M = make-up water quantity (m3/h) Pm = turbidity of make-up water (degree) F = filtration rate (m3/h)
Pf = turbidity of filtered water (degree) B = blowdown water quantity (m3/h) W = windage loss (m3/h)
Figure 3.67 shows the relationship between fil-tration rate and cooling water turbidity calculated by using the equation (3.59). The cooling water turbidity is kept at 15 degrees or less by applying the side stream filtration of above 2% against the circulating water quantity.
(2) Control of the operational conditions of heat exchanger
The influences of water flow rate on slime adhe-sion and sludge accumulation are shown in Figures
Circulation rate (m3/h): 20,000 Holding water volume (m3): 13,000 Temperature difference (°C): 8 Cycles of concentration: 2.5 Corrosion and scale inhibitor
(phosphonate-polymer) (mg/l ): 40
Chlorination (mg Cl2/l ): 0.5 –1.0, 3 h/day Non-oxidizing biocide (mg/l ): 50/month 3% against circulation rate
Turbidity (degree) pH
Electrical conductivity (µS/cm) M-alkalinity (mg CaCO3/l ) Ca-hardness (mg CaCO3/l ) Chloride ion (mg/l ) Sulfate ion (mg/l ) Silica (mg SiO2/l )
16 years
Carbon steel: 3 – 4 mg/dm2·day (0.014–0.019 mm/year) No corrosion, no scaling and no biofouling are found in most heat exchangers.
Small amount of scale and sludge is found in few heat exchangers with the low water flow rate. Operational conditions of
system
Chemical treatment
Side stream filtration
Water analysis
Period of the chemical treatment
Corrosion rate
Inspection results of heat exchangers
Table 3.26 Example of a high hardness water treatment (alkaline treatment) scaling of calcium phosphate and calcium carbonate
in the heat exchangers in the case of high hardness water.
The water treatment program which uses a low concentration of corrosion inhibitors with a rela-tively high concentration of scale inhibitors for high hardness, high alkalinity and high pH water is called
“alkaline treatment”. The good application result of an alkaline treatment is shown in Table 3.26 and the treatment result of a heat exchanger is shown in Photograph 3.13 (p.3-75).
The alkaline treatment is widely applied for high hardness cooling water in refineries, petrochemical plants, iron and steel works, electric power plants and so on.
(2) Low hardness water
The critical concentration of phosphate based cor-rosion inhibitors required for corcor-rosion inhibition becomes higher in low hardness water comparing with high hardness water as shown in Figures 3.17 and 3.19. The critical concentration is 10 to 15 mg T-PO4/l for the water of calcium hardness around 100 mg CaCO3/l, and 15 to 20 mg T-PO4/l for the water of calcium hardness around 50 mg CaCO3/l.
When the reduction of critical concentration is required, zinc salts are applied together with
phosphates as shown in Figures 3.18 and 3.20(p.3-17 and 3-18).
Therefore, the relatively high concentration of phosphate based or phosphate-zinc based corro-sion inhibitors are usually applied for low hardness water. Scale inhibitors are used to prevent scale problems at the high temperature areas of heat exchangers even in the case of low hardness water.
The typical application result of a zinc-phosphate-polymer treatment for a low hardness water is shown in Table 3.27. Almost no corrosion, no scal-ing and no biofoulscal-ing are observed in all heat exchangers inspected during the scheduled turn-around of the plant.
(3) High salinity water
Chloride and sulfate ions are the typical aggres-sive ions, and the increase of their concentrations generally lowers the corrosion inhibition provided by inhibitors. The influence of chloride and sulfate ion concentration on the effects of various corro-sion inhibitors is shown in Figure 3.27(p.3-20).
Generally, zinc-phosphate based inhibitors provide the better corrosion inhibitions than phosphate based ones in the water of a high chloride and sulfate ion concentration.
Table 3.28 shows the treatment result of a
zinc-Circulation rate (m3/h): 2,000 Holding water volume (m3): 250 Temperature difference (°C): 10 Cycles of concentration: 5 Corrosion and scale inhibitor (mg/l )
(zinc-phosphonate): 45
Scale inhibitor (polymer)(mg/l ): 20
Chlorination (mg Cl2/l ): 0.5 –1.0, 3h/day Non-oxidizing biocide (mg/l ): 50/week pH control (H2SO4): pH 7.5 – 8.5
Turbidity (degree) pH
Electrical conductivity (µS/cm) M-alkalinity (mg CaCO3/l ) Ca-hardness (mg CaCO3/l ) Chloride ion (mg/l ) Sulfate ion (mg/l ) Silica (mg SiO2/l )
12 years
Almost no corrosion, no scaling and no biofouling are found in all heat exchangers.
Table 3.28 Example of a high salinity water treatment (zinc-phosphonate-polymer treatment)
Make-up water Operational conditions of
system
Chemical treatment
Water analysis
Period of the chemical treatment
Inspection results of heat exchangers
phosphonate-polymer program for a water of high chloride and sulfate ion concentration. In this factory, as the intake point of raw water is located near the mouth of a river, the water quality is widely
fluctuated by the influence of the ebb and flow tides.
Accordingly, the maximum chloride and sulfate ion concentrations of cooling water reach 1,394 mg/l and 780 mg/l respectively. Even though the cooling
Circulation rate (m3/h): 5,000 Holding water volume (m3): 24,000 Temperature difference (°C): 12 Cycles of concentration: 3 Corrosion and scale inhibitor (mg/l )
(zinc-phosphonate): 50
Scale inhibitor (polymer)(mg/l ): 30
Chlorination (mg Cl2/l ): 0.3 –1.0, 3h/day
Turbidity (degree) pH
Electrical conductivity (µS/cm) M-alkalinity (mg CaCO3/l ) Ca-hardness (mg CaCO3/l ) Chloride ion (mg/l ) Sulfate ion (mg/l ) Silica (mg SiO2/l )
17 years
Carbon steel: 3 –5 mg/dm2·day (0.014 – 0.024 mm/year)
Almost no corrosion, no scaling and no biofouling are observed in all heat exchangers.
Operational conditions of system
Chemical treatment
Water analysis
Period of the chemical treatment
Corrosion rate Inspection results of heat exchangers
Table 3.27 Example of a low hardness water treatment (zinc-phosphonate-phosphate-polymer treatment)
Circulation rate (m3/h): 2,000 Holding water volume (m3): 700 Temperature difference (°C): 11 Cycles of concentration: 3 Corrosion and scale inhibitor (mg/l )
(zinc-polymer): 120
Chlorination (mg Cl2/l ): 0.5 –1.0, 3h/day
pH
Electrical conductivity (µS/cm) M-alkalinity (mg CaCO3/l ) Ca-hardness (mg CaCO3/l ) Chloride ion (mg/l ) Silica (mg SiO2/l )
Carbon steel: 3.0 –3.6 mg/dm2·day (0.014 – 0.017 mm/year)
15 years
Almost no corrosion, no scaling and no biofouling are observed in all heat exchangers.
Operational conditions of system
Chemical treatment
Water analysis
Corrosion rate Period of the chemical treatment
Inspection results of heat exchangers
Table 3.29 Example of a non-phosphorous corrosion inhibitor treatment (1) (zinc-polymer treatment)
Make-up water 8.0 116
44 32 7 12
Cooling water 8.8 387 138 108 20 32
with good performances in open recirculating cool-ing water systems. However, the development of non-phosphorous corrosion inhibitors has been strongly required in Japan and the extensive researches have been carried out from the view point of environmental protection, such as the prevention of nutrification in closed water areas.
Among non-phosphorous corrosion inhibitor treatment programs, zinc-polymer treatments and all polymer treatments have been applied for many
Circulation rate (m3/h): 470
Holding water volume (m3): 5
Temperature difference (°C): 5
Cycles of concentration: 8
Corrosion and scale inhibitor (mg/l ) (polymers): 50
Non-oxidizing biocide (mg/l ): 7
Turbidity (degree) pH
Electrical conductivity (µS/cm) M-alkalinity (mg CaCO3/l ) Ca-hardness (mg CaCO3/l ) Chloride ion (mg/l ) Silica (mg SiO2/l )
11 years
No thermal efficiency drop of refrigerating machines is observed during their operation period.
No corrosion, no scaling and no biofouling are found in all heat exchangers of the refrigerating machines.
Operational conditions of system
Chemical treatment
Water analysis
Period of the chemical treatment
Treatment result
Table 3.30 Example of a non-phosphorous corrosion inhibitor treatment (2) (all polymer treatment)
Make-up water Below 1
7.4 180
50 50 8 20
Cooling water 2 8.7 1,400
330 380 60 180
water quality is very corrosive, this program pro-vides the good performance. Almost no corrosion, no scaling and no biofouling have been found in the heat exchangers at the scheduled turnarounds of the system.
(4) Non-phosphorous corrosion inhibitor treatment
At present, various kinds of organic and inorganic phosphates are widely used as corrosion inhibitors
Table 3.31 Outline of chemical treatments for closed recirculating cooling water systems Applicable operational
conditions of system
Make-up water; FW*1, SW*2, DW*3
Retention time*4; No limitation Water temperature; No limitation
Make-up water; FW, SW, DW Retention time; Within 50 days Water temperature; Below 60°C
Make-up water; FW
Retention time; Within 10 days Water temperature; Below 50°C
Target corrosion rate
Carbon steel; Below 1 mg/dm2·day (Below 0.005 mm/year)
Copper and its alloys; Below 1 mg/
dm2·day (Below 0.005 mm/year)
Carbon steel; Below 10 mg/dm2·day (Below 0.05 mm/year)
Copper and its alloys; Below 1 mg/
dm2·day (Below 0.005 mm/year) Carbon steel; Below 10 mg/dm2·day (Below 0.05 mm/year)
Copper and its alloys; Below 1 mg/
dm2·day (Below 0.005 mm/year)
Remarks Biocides for nitrification bacteria should be used in the case of cooling water temperature of 10 to 40°C where the bacteria rapidly grow.
Polymers are used as corrosion inhibitor.
Polymers are used for preventing calcium phosphate scale.
Type of corrosion inhibitor
*1 FW: fresh water (tap water, well water, industrial water, etc.)
*2 SW: softened water
*3 DW: demineralized water
*4 Retention time (day) =Holding water volume (m3) Make-up water quantity (m3/day)
cooling water systems in Japan. Table 3.29 shows the example of a zinc-polymer treatment. Table 3.30 shows the typical application conditions of an all polymer treatment.
Both non-phosphorous treatments show the suffi-cient corrosion, scale and biofouling control effects.
3.5.2 Closed Recirculating Cooling Water