Hydrocarbon Processing - March 2022 - 85

Water Management
changers or by the pressure required to deliver cooling water to
the heat exchanger located at the highest elevation above grade.
Setting the supply pressure to satisfy the second criterion results
in a higher differential pressure to all other services, resulting in
an increased flow of cooling water.
Hydraulic analysis. As an initial starting point, all cooling
water services should be specified for the same-or nearly the
same-pressure drops to a practical extent. Subsequently, the
cooling water rate should be adjusted by performing a hydraulic
analysis of the entire network. Such an analysis should be
plot sensitive and account for the take-off sequences and actual
equipment elevations referenced from grade level. A comparison
of the cooling water " initial balance " -determined from process
datasheets with the " Base Case hydraulic balance " derived from
the hydraulic analysis of the network-is summarized in TABLE 1.
The actual flow of cooling water differs from the datasheet
values to satisfy the available differential pressure between the
CWS and cooling water return (CWR) headers. Notably, heat
exchangers E-5 and E-6 are located 40 m above grade and require
an increase in supply pressure from 3.7 kg/cm2
kg/cm2
g to 4.4
g. Since the 2.8 kg/cm2
g back pressure in the CWR
header is the same for all services, the increase in the available
differential pressure resulted in additional flow of cooling water
to the other services.
Auxiliary supply network. In cases where only a few services
govern the required supply pressure, it may be beneficial to consider
implementing auxiliary supply headers at two (or more)
pressure levels. The pressure in each header can then be tailored
for the services supplied by that header. Implementing supply
headers at multiple pressure levels can be accomplished by either
throttling the pressure to an auxiliary header, or by boosting
the pressure by using auxiliary pumps. Jin Sun2
, et al. have
proposed an algorithm for determining an optimal number of
auxiliary pumps and their installation locations.
Either of these two concepts can be applied to reduce the
Base Case cooling water balance (TABLE 1). Segregation of the
heat exchangers located at higher elevations above grade (E-5
and E-6) allows cooling water to be supplied at two different
pressures, 4.4 kg/cm2
g for E-5 and E-6 and 3.7 kg/cm2
g for all
others. The Base Case balance modified with an auxiliary network
at the higher pressure is summarized in TABLE 2.
Supplying cooling water via an auxiliary header to heat exTABLE
2. Initial vs. modifi ed Base Case cooling water balances
Base Case
Item No.
E-1
E-2
E-3
E-4
E-5
E-6
E-7
E-8
E-9
FIG. 6. Preliminary cascaded cooling water network.
FIG. 7. Workflow schematic.
E-10
E-11
E-12
E-13
E-14
E-15
E-16
E-17
E-18
E-19
E-20
E-21
E-22
E-23
E-24
E-25
E-26
E-27
E-28
E-29
Total
Initial balance
(data sheet),
kg/hr
1,890,289
1,890,289
1,890,289
1,890,289
1,263,162
825,089
43,808
8,672
36,069
169,841
51,104
285,585
69,413
311
6,592
19,088
5,041
4,862
372,010
556,487
1,552,247
2,336,021
2,336,021
494,842
494,842
12,131
12,580
596,164
6,851
19,119,989
balance modifi ed
network, kg/hr
2,245,776
2,235,694
2,204,780
2,197,184
1,296,808
847,067
43,950
8,672
49,237
216,025
57,026
304,756
79,626
4,369
95,391
38,334
12,645
24,947
476,684
613,308
2,685,105
2,922,791
2,922,791
762,315
762,315
42,937
133,528
609,393
39,534
23,982,989
Duty,
10°kcal/hr ∆T, °C
3.2
3.2
3.2
3.2
1.91
1.25
0.25
0.05
0.23
1.12
0.61
4
0.4
0.004
0.09
0.08
0.06
0.06
1.69
2.54
28.8
9.61
9.61
6.03
6.03
0.16
0.09
2.47
0.08
90
1.4
1.4
1.5
1.5
1.5
1.5
5.7
5.8
4.4
5.2
10.8
13.1
5
1
0.9
2.1
4.9
2.4
3.6
4.1
10.7
3.3
3.3
7.9
7.9
3.5
0.7
4.1
2.1
3.8
Hydrocarbon Processing | MARCH 2022 85
Hydrocarbon Processing - March 2022

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