Hydrocarbon Processing - March 2022 - 84

Water Management
of heat exchanger services. Such a complex hydraulic analysis
can be performed using commercially available softwarea
capable
of modeling the entire network in which each node in the
network is balanced based on flow conservation, and then corrected
with a calculated pressure drop until the mass and energy
balances are solved. The Newton-Raphson1
technique is used
to solve the matrices developed by a multi-nodal network system.
Each node is iteratively solved for flow until the overall system
pressure drop is actualized to satisfy the given constraints.
Heat exchanger pressure drops are modeled as resistance
curves proportional to the square of the volumetric flow. Consequently,
the hydraulic model of the network self-adjusts the
flow of cooling water into each heat exchanger to maintain a
realistic pressure profile across the entire network. Each heat
exchanger is modeled with its elevation above grade and with
either a fixed duty specification or a fixed temperature rise.
TABLE 1. Initial vs. Base Case cooling water balances
Item No.
E-1
E-2
E-3
E-4
E-5
E-6
E-7
E-8
E-9
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
Base Case
hydraulic
balance, kg/hr
2,600,975
2,587,740
2,547,429
2,537,358
1,265,290
1,038,988
50,187
9,906
56,282
247,123
65,121
348,685
90,974
5,006
10,754
44,174
14,558
28,740
548,217
700,814
3,080,104
3,356,801
3,356,801
874,907
874,907
49,055
152,637
696,213
45,175
27,284,921
84 MARCH 2022 | HydrocarbonProcessing.com
Duty,
10°kcal/hr
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
∆T, °C
1.2
1.2
1.3
1.3
1.5
1.2
5
5
3.8
4.5
9.4
11.4
4.4
0.9
8.3
1.8
4.3
2.1
3.1
3.6
9.3
2.9
2.9
6.9
6.9
3.1
0.6
3.6
1.9
3.3
FIG. 5. Base Case: Auxiliary pump network.
PROCESS DESIGN CONSIDERATIONS
The process design of a cooling water network involves balancing
the combination of the following process design considerations
to reduce the cooling water circulation:
* Line sizing
* Cooling water supply pressure
* Hydraulic analysis
* Auxiliary supply networks
* Cascaded network.
Line sizing and cooling water supply pressure are the only
variables that can be manipulated to control the flow and velocity
of cooling water to each exchanger. The other three design
considerations require a more in-depth discussion and are more
readily understood by an analysis of a real-world case study.
Line sizing. The main criterion for sizing cooling water lines
is maintaining a velocity high enough to avoid sedimentation
and low enough to prevent erosion. Typical limits are between
1.5 m/sec and 4.5 m/sec, with the high end also being limited
by pipeline pressure drop.
Cooling water supply pressure. The pressure required in the
cooling water supply (CWS) header is set either by the greater
of the sum of frictional resistances in the piping and heat exFIG.
4. Base Case: Pressure control network.
http://www.HydrocarbonProcessing.com
Hydrocarbon Processing - March 2022

Table of Contents for the Digital Edition of Hydrocarbon Processing - March 2022
