WATER DISTRIBUTION SYSTEM ANALYSIS
I.
PROPOSED DISTRIBUTION SYSTEM ANALYSIS
A.
Problem
The current Eagle Pass Distribution System is experiencing summer peak
demands that are exceeding its pumping capacity. Due to the growth,
especially to the North and South of the City, the existing distribution
lines are reaching flow velocities in excess of 6 fps. This is causing
excessive head losses and corresponding low system pressures.
As evidenced by the recent TNRCC enforcement action taken on El Indio
Water Supply Corporation for their low system pressures (below 20 psi),
the Eagle Pass Water Works System is working daily to meet the
increased demand and keep system pressures up to avoid violation of
TNRCC regulations.
B.
Analysis Overview
1.
Piping – General: Review of a system model prepared using the
EPA Net computer program, along with system readings of
pressure and review of SCADA system tank Status logs, a central
theme emerged regarding system deficiencies. The central
problem involves several bottle necks in the system.
2.
Growth: Additionally, in looking at the 20-year panning horizon,
the system is projected to provide 20-mgd ultimately. A simplified
division of the flows is with (a) 5-mgd flow to the north, (b) 5-mgd
flow to the south, (d) 10-mgd flow to the city center and east.
This is the basic flow pattern for the system and will continue to be
the pattern until some time in the future, when the proposed
TXDOT outer loop becomes a reality. At that time, a more
complete looped distribution system would circle the city and its
E.T.J.
The central location of the existing (and proposed) water treatment
plant is well suited for the distribution system. The existing pipe
network has been simplified and is attached as Appendix B.
Comparing this to the existing land use map shows the central
spine of the system running north and south. New growth is seen
in the future land use map which extends along the three main
transportation corridors: (a) Del Rio – Highway 277 to the north,
(b) Highway 57 / Highway 277 to the east, and (c) Highway 1021
south to El Indio. As mentioned earlier, the future outer loop will
be a further growth corridor.
3-90
3.
Demands
Current peak demands on the surface water treatment plant are at
9.5 mgd, which is reaching its maximum capacity. The proposed
new 20 mgd plant will double the water production capacity.
Demands for the computer simulation are based on approved
population projections which extend to 2025 for this study. This
allows Eagle Pass Water Works System to meet over 20 years of
growth (assuming a 2003 completion date for construction).
Detailed population and water consumption figures were compiled
as part of the Eagle Pass Regional Water and Wastewater Facility
Plan. The results of the population projections were approved by
the Rio Grande Regional Water Planning Group.
The plant design is based on the following engineering data
provided as part of the Facility Plan.
a.
b.
c.
d.
Average usage per connection = 532 gpd
Peak factor of 1.66 peak day vs. average
Consumption per person based on 3.86persons/connection
– 138 gppd.
Connections
Population
(1)
Year 2000
11,834
45,642
(2)
Projected 2025
22,577
87,148
For line sizing and head loss calculations the following was used:
a.
b.
12 hour Peak = 2 x Average Day
Peak Hour = 4 x Average Day
In conclusion, the detailed analysis of the Eagle Pass Regional
Water and Wastewater Facility Plan shows a doubling of water
demand by 2025. The distribution of this demand is shown in the
previously referenced future land use map.
C.
System Model
1.
General
The Eagle Pass Water Treatment Facility currently has a 10.4 mgd
production capacity. The highest demand of record is 9.5 mgd (on
five consecutive days in July, 2001). The current average
consumption is 5.5 mgd.
3-91
The average consumption in 1985 was 3.3 with a peak of 7.8 mgd.
The distribution system is currently able to handle average daily
demands; however, problems in maintaining adequate pressures
and tank levels have been experienced during peak demand
periods.
Portions of the existing system (See Existing Hydraulic Schematic)
would be unable to maintain adequate pressure during the existing
peak day if a fire demand of 3,000 gpm was imposed for the high
service plain.
Although the water plant is currently capable of 10.4-mgd
production, the distribution system is currently not capable of
delivering it. To meet current demands, and future demands, the
distribution system requires further expansion.
The future distribution simulation improvements are attached.
Fire flows were simulated for the future demands and the results
show that the proposed storage tanks are necessary to maintain
adequate fire flow and pressure.
There is some question as to the possible connection of distribution
lines between the high and low service plains. These connections
will be eliminated or control valves installed in order to prevent
possible problems in system control. Lastly, all booster stations
which pump to the high service plain should have bypass lines
which would allow emergency flow back to the low service plain.
II.
SCOPE AND OBJECTIVES
The scope of work included in the analysis was a review of the existing water
distribution system and current demands, along with a review of historical
demands. The first step was to model this system and recommend immediate
improvements. The area of review is illustrated on the accompanying Water
Distribution Map and includes the existing limits of the system.
Detailed review was required in order to accurately model the pipe network and
assign appropriate demands at the junction nodes of the pipes. High users were
located on an individual basis.
Improvements were recommended to insure that current peak demands could be
safely met using firm pumping capacity.
The pattern for growth and corresponding demand increases were approximated
in order to make expansion recommendations, in line with the plant’s proposed 20
million gallons per day (mgd) production capacity.
3-92
Specific proposed improvements were modeled in order to evaluate their
efficiency and determine resultant production increases. Final recommendations
were based on budget requirements and expected needs.
III.
EXISTING SYSTEM MODEL
A.
Distribution System
1.
Pipe System
The distribution system is modeled as a network of equivalent
pipes. Since each pipe cannot be input into the computer
simulation, a model is based on the major distribution network that
uses an individual pipe of calculated diameter to represent several
pipes. This network is assigned node numbers at each junction and
each pipe is assigned a number, diameter length and friction factor.
The system demands are assigned in each node based on the area it
serves. Elevations are approximated at each node from
topographic maps in order to determine system pressure at each
node.
2.
Pump Station
Pumps are input along individual lines to model an individual
booster station. The pump is given an efficiency and an effective
horsepower. The required kilowatts are determined based on the
efficiency of the pump. The pump is assigned pressure-switching
valves which are read at assigned storage tanks. See Existing
Hydraulic Schematic of the existing Pump & Storage Network.
3.
Storage System
Tanks are modeled with height, diameter and overflow elevations
to allow a real time model of the tank levels during an extended
period simulation. As tanks reach their overflow level, an
automatic altitude switch should close the tank until pressure
demands cause the valve to open.
4.
Distribution System Control
The current system of booster stations has a centralized control and
can automatically respond to demand changes and also by manual
control. This has solved the problems with overflows at numerous
storage tanks, especially at the Industrial Park Tank. The overflow
problem was a result of differential demands and pressures in the
system which would allow one tank to fill while another was not.
3-93
Several problems resulted from this situation, beyond the obvious
problem of waste.
First, if the overflow of one tank requires the reduction of booster
pump power, then other tanks in the system may not fill and
adequate suction pressure may not be provided to the higher level
service pumps. This can become critical on the system when
maximum storage capacity is required to meet peak day requests.
Secondly, without means of automatic control and monitoring, the
system cannot respond to sudden changes in system requests,
whether from fire flow demands or loss of a booster pump, in a
particular area.
B.
System Demands
1.
Historical
The historical demands of the network were based on data from the
Eagle Pass Water Works System. The record peak day, current
peak day along with various average flows were used in the
analysis. The number of connections was reviewed with particular
attention to the corresponding flows for residential, commercial,
industrial and governmental uses. This was updated to reflect
current demands using total connections combined with analysis of
the updated land use maps (using similar density assumptions for
connections per acre of improved property).
The figures were reviewed for normal historical demands and for
high consumption. The high 1978 figures can be attributed to past
industrial consumption (especially by the ALCO Company which
has since ceased operation).
2.
Allocation in Network Model
The first step in allocating the demands throughout the system at
the network nodes was to determine the breakdown of connections,
both by class and location in the network.
Using data provided by the meter records, the distribution system
was divided into geographical areas and the number of connections
was determined for each area and listed as residential, apartment,
commercial, governmental and industrial connections.
The next step was to locate the individual high users (copy of
current peak users follows the end of this Section) in the network.
A careful review of meter records resulted in the allocation of high
3-94
use among schools, parks, apartment complexes and housing
projects, along with commercial and industrial users.
The average gallons per connection were used to determine the
demand per area and the high users were added.
The final process in allocation of network demands was to divide
each of the demands of the areas between the network nodes.
3.
Daily Pattern
The hourly distribution that was used is shown in the following
graph. The percentages shown were converted to demand factors
and shown with the times they correspond. Note that the Extended
Period Simulation (E.P.S.) hour 0 corresponds to a real time of
6:00 a.m. This time is used since it begins with a minimum
demand and tanks near full capacity.
3-95
Insert graph
3-96
C.
Results for Existing System
As a preface to this section, it is noted that all analysis of the Eagle Pass
Distribution System was performed using the latest version of “EPANET”.
The analysis was made using an E.P.S. of twenty-four hour duration.
Using an E.P.S., the system can be accurately modeled for effects on tank
draw down and replenishment as demands change hourly. Due to the
large amount of output resulting from each simulation, only a portion of
the final runs for each particular model are included in this report for
reference. Each simulation included in this report will consist of an input
data summary, selected output at particular intervals (normally peak
demand periods) and a short evaluation of the results.
1.
Existing Average Day Demands
Using the existing average daily demand and the existing network,
a twenty-four hour simulation showed few system problems. The
simulation considered each station to only be operating at its firm
capacity (i.e. the largest pump out of service.) This is a
conservative assumption.
At the E.P.S. peak demand time of 4 hours (10:00 a.m.), the results
show the Leona Tank at low levels.
At the peak demand time the distribution system pressures ranged
from a low of 28 psi to a high of 90 psi. These are acceptable
results and correspond with current experience. See Appendix B
for partial data printout of existing average day simulation.
2.
Existing Peak Day Demands
The factor for maximum day versus average day of 1.66, this was
applied to all junction demands. As in the average day simulation,
the pump stations were input as firm horsepower.
The central problem that became obvious from this simulation was
the inability to maintain adequate tank levels.
Although adequate pressure was maintained, there would not be
adequate pressure to maintain fire flows in the upper service level.
See Appendix B for a partial data printout of existing maximum
day simulations.
3-97
3.
IV.
There were two areas which have been previously identified and
show up in the analysis.
a.
The supply line to Hillcrest is undersized and has high
velocities and head loss.
b.
The Deer Run area experiences low pressure (28 psi),
especially at areas of high terrain due to low tank levels at
Seco Mines Tank.
c.
Inadequate supply pressure to feed the College Hills Tank.
GROWTH PROJECTION
In order to make reasonable assumptions as to the required improvements, it is
necessary to make growth projections. Population and water demand projects
are expected to double by 2025 as described in Section 3.a.1).
V.
PROPOSED SYSTEM EXPANSION
A.
Distribution System Control
With the construction of a new water treatment plant utilizing the latest
computer technology available, the distribution SCADA control center
will be relocated and upgraded. All proposed facilities will be controllable
either automatically or manually as currently operated.
B.
Future Demand Models
As a preface to this section, several important criteria must be detailed.
One key assumption is the total available pumping horsepower available
for the system. The other assumption is the effective separation of the
high and low service plains.
The total effective pumping power for the following models assumes that
an effective remote monitoring and control system is in place for reliable
pump control.
C.
Proposed Water Transmission Facilities
North
1.
15,000 l.f. of 24” dedicated water transmission line from the new
water treatment plant to the Seco Mines Elevated Storage Tank.
This line will follow an existing utility easement from the water
3-98
plant along the Rio Grande high bank, and the high bank of Seco
Creek, to Highway 277, crossing Seco Creek on the bridge and
proceeding north along Highway 277 to Seco Street to Sanchez to
the Seco Mines Tank. This pipeline will have a capacity of 5
million gallons per day.
East
2.
800 l.f. of 24” and 18” pipe to connect the new water treatment
plant to the existing 24” feeder and 18” feeder for Leona Pump
Station and extend the 24” feeder 1,400 l.f. to include the Hillcrest
Pump Station. This construction will be on the existing water plant
site and within the right of way of Leona St. from Nueces to the
Hillcrest Pump Station. These pipelines will have a capacity of 10
million gallons per day from the plant to the Leona Tank location
(it is proposed to eliminate the Leona Pump and Tank) and 5
million gallons per day to Hillcrest. Future extension of this line to
intersect the proposed outer loop transmission line will provide the
supply to the central area of the City and to the loop.
South
3.
14,000 l.f. of 24” dedicated water transmission line from the new
water treatment plant to the Industrial Ground Storage Tank and
Pumping Station. This line will follow an existing utility easement
from the water plant along the Rio Grande to the new international
bridge, then follow the proposed truck routing under the bridge
into the industrial park to the intersection of the truck route and
Calle Merida, then along Calle Merida to the Industrial Tank and
Pump Station. This pipeline will have a capacity of 5 million
gallons per day.
4.
17,200 l.f. of 18” transmission line from Seco Mines Elevated
Tank to the Deer Run/Thompson Road Elevated Tank. This line
will provide supply to the northern colonias and the surrounding
area.
5.
6,000 l.f. of 18” transmission line from the Industrial Pump Station
to the Callejon Teran Elevated Tank.
6.
23,000 l.f. of 18” transmission line from Callejon Teran to the El
Indio/Chula Vista Elevated Storage Tank. These lines will provide
supply to the southern colonias, the El Indio Distribution System
and the Kickapoo Nation.
7.
Future 18” loop transmission line from Deer Run to El Indio will
close the loop around the entire City.
3-99
D.
Line Sizing
Based on normal standards for distribution systems, the ideal flow rates
are 3 fps for normal flows with peak flows not to exceed 6 fps.
The following lists proposed line size, flow, lengths and velocities for
average and peak demands.
Table 1. Pipeline Characteristics.
Size
(in.)
Length
(ft)
Avg.
Flow
MGD
hL/100
(total
head) ft.
Avg Vel
fps
Peak
Flow Peak vel.
MGD
Fps
hL/100
North Water
1) Transmission Main
24 15,000
4.0
1.9
0.045
5
2.5
0.07
East Water Transmission
2) Main
24
1,400
4.0
1.9
0.045
6.8
3.25
0.12
South Water
3) Transmission Main
24 14,000
4.0
1.9
0.045
5
2.5
0.07
Seco Mines to Deer Run
4) Transmission Main
16 17,200
2.0
2.8
0.1
3
3.3
0.22
Industrial Pump Station
5) to Callejon Teran Tank
18
6,000
4.0
3.4
0.19
5
4.3
0.32
Callejon Teran Tank to El
6) Indio Storage Tank
18 23,000
3.5
3
0.19
5
4.3
0.32
7) Rosita Valley Pipelin
14 16,000
2
3
0.2
3
4.4
0.45
8) Outer Loop Pipeline
18 46,000
3.5
3
0.15
5
4.3
0.32
E.
Elevated Storage Tanks.
1.
The colonia areas to the north of the City of Eagle Pass have
experienced significant problems with water supply pressures and
volumes. The proposed new one million gallon storage tank on
Thompson Road at Deer Run will remedy these pressure supply
problems and provide a reliable source of water for approximately
1,000 existing connections and up to 5000 total future connections.
2.
The colonia areas to the south of the City of Eagle Pass are also
experiencing major problems with water supply which will be
alleviated with the Chula Vista/El Indio one million gallon
elevated storage tank.
3-100
3.
F.
Proposed Pumping Improvements
1.
The improvements to the pumping station were calculated based
on required service plain elevations and tank overflow elevations.
The calculated static heads had 20’ of dynamic head loss added for
the resulting horsepower requirements.
2.
The main pump improvements were input as useful power. The
actual motor horsepower would be approximately 40% greater
(based on 75% pump efficiency and 95% motor efficiency.)
3.
The following pump improvements were necessary:
4.
VI.
Within the City of Eagle Pass, the small Vista Hermosa Elevated
Storage Tank is in need of major restoration and repair. It is a 30
year old tank that has outlived its economic life. In order to
provide sufficient elevated storage for the 20 year planning period,
it is more effective to replace this existing small tank with a new
one million gallon tank.
a.
Water Treatment Plant 550 hp
b.
Delete Leona Pump Station and add 165 total hp for
Hillcrest Pump Station.
c.
New pump station at Seco Mines – 130 hp.
d.
New pump station at Industrial Park Tank – 150 hp.
As shown in the schematic, the new 24” distribution lines will
pump directly from the new treatment plant and not require the
Leona Station.
FIRE PROTECTION
A.
Fire Flow Requirements
The State Board of Insurance, in grading a water distribution system for
key insurance rates, requires certain flows.
According to the TNRCC, for the current population the required total
storage is 4.4 million gallons (MG). The current storage capacity for
Eagle pass is 5.3 MG.
The required elevated storage is 100 g/p/connection or 2.2 MG. The
system currently has 4 MG of elevated storage.
3-101
The proposed plan calls for adding 2,900,000 gallons of elevated storage.
In determining future fire flow requests, the total design flow to be used is
based on fire flow added to average consumption for the maximum day.
Hydrant flow tests are conducted by the Fire Prevention and Engineering
Bureau of Texas. The peak flow requirement under their criteria would be
3,000 gpm for a three-hour duration, or a total of 504,000 gallons.
B.
Fire Flows with Existing Demands
Numerous simulations were performed to evaluate the effect of fire flow
requirements on the existing and proposed distribution system. The fire
flow requirements were placed at strategic locations either in the high or
low service plain. Three-hour fire flows were added to peak day
requirements for hours 13-15. The flow rates ranged from 1,000 gpm for
residential areas, to 3,000 gpm for industrial/commercial areas.
The results for simulations using existing peak day demands and the pump
improvements outlined in Section V (A) for EPPROP1 indicated that
3,000 gpm fire flows could be maintained in both the high and low service
plain for commercial and residential areas. It should be noted however,
that fire flows in the upper service level caused both Brown Street and
Vista Hermosa Tanks to reach very low levels.
The analysis of the existing fire demands was limited to review of the
major distribution network and the effect on storage capacity. As
evidenced in the testing by the Fire Prevention and Engineering Bureau,
some areas were unable to provide 20 psi fire flows. Due to the large
number of 6-inch networks in the system, some low pressures are to be
expected. Regular maintenance of the fire hydrants and insuring that all
valves are fully open will develop optimum fire flow.
The models were established with all the proposed water transfer mains
and pump stations. For each proposed fire flow two simulations were run,
the first without the proposed storage tanks, and the second simulation
with the proposed tanks.
1.
Low Service Plain
For an analysis of fire flows in the lower service plain, a 3,000gpm flow was imposed on Node 38. The results of the simulation
without the proposed Deer Run storage tank showed the Seco Tank
empty in the first hour and system pressures dropped below 20 psi
in the fire flow area.
3-102
The addition of the proposed 1,000,000-gallon storage tank
resulted in the maintenance of system pressures which stayed
above 20 psi even though the Seco Storage Tank was emptied.
2.
High Service Plain
For an analysis of the high service plain, fire flows were tested at
Node 13.
The results of the 3,000 gpm flow at Node 13, without the
proposed improvements, were that Vista Hermosa Tank was
emptied and system pressures for the low service plain were below
20 psi at several locations. When the proposed improvements
were added, the tanks all maintained adequate levels and system
pressures were acceptable with all pressures at least 20 psi, except
at Node 13 itself.
D.
Fire Flow Summary
In summary of the fire flow analysis, it appears that the proposed storage
tanks, pipelines and pump station will be required to meet future demands
and extra storage will be required to provide capacity to meet fire flow
requests.
The proposed elevated storage tanks for the middle service plain are
required to prevent fire flows with the current peak demands from
emptying the existing elevated tanks.
Currently, the only elevated storage in the Deer Run service is 100,000
gallons at Seco Mines. This is not sufficient reserve for the required fire
flow at the peak day. The proposed 1,000,000-gallon tank for the service
level should be an adequate solution to supply requirements for the service
level area, especially for the north sector. As development continues in
the south limits of the city, the proposed additional storage in the area will
be required to meet the minimum TNRCC requirement of 200 gallons per
connection. Approximately 5,000 connections are projected by the year
2025, with a required storage capacity of 1,000,000 gallons. With the two
new tanks of 500,000-gallons each combined with retaining one-200,000
gallon standpipe, the El Indio area will comply with TNRCC
requirements.
One further suggestion is made at this point concerning recommended
improvements. It would increase the system flexibility to respond to the
large changes in demand which result from fire flows if remote control
valves and bypass lines were installed throughout the system. With
controlled bypass lines, water could be diverted from the high service
3-103
plain storage facilities to the low service plain during temporary
emergencies.
VII.
Future System Demands
The existing model includes demand for Eagle Pass based on current water
system pumping average and peak day, plus the average and peak historical
demands for El Indio Water Supply Corporation.
Existing model demands were increased for the proposed analysis of growth for
2010 and 2025.
The increase in flow ultimately at 2025 included new demands of 10 MGD peak
day and 5.9 MG average day flow. These were distributed based on the projected
land use map.
The year 2025 average flow increase was distributed with (a) 620 gpm at Seco
Mines, (b) 620 gpm at Deer Run, (d) 330 gpm at North Loop, (d) 330 gpm at
Vista Hermosa, (e) 620 gpm in El Indio Northwest, (f) 620 gpm at El Indio
Northeast, (g) 300 gpm at Rosita Valley and (h) 400 gpm for the Kickapoo Area.
The above corresponds to total increase of 1.8 MG to the north, 1.4 MG to the
east and 2.7 MG to the South, for a total of 5.9 MG as referenced above.
For calculation of average demands, the demands were divided by the 1.66
peaking factor.
The 2010 demands were based on the existing demands plus 2.4 MG for average
day flows. The peak day for 2010 was based on existing plus 4 MG. The
increases were distributed in the same percentages as the 2025 scenario discussed
above.
VIII. Proposed System Model Results
A.
Overview
The proposed simulation looked at average and peak day demands on the
system based on the development periods 2010 and 2025. The phased
improvements can be summarized as follows.
By 2010 the proposed system improvements will include all system
improvements except a) 18” outer loop main, b) 24” distribution main
from Hillcrest to College Hills and continuing out to the proposed outer
loop.
3-104
The model for 2010 looked at average and peak day demands. Similarly,
the completed system for 2025 was modeled for average and peak day
flows.
B.
Results for Year 2010
Based on the planned improvements and projected flow increases, the
EPANET model for the existing system was revised.
Review of output for an average day demand showed system pressures
from 35 to 99 psi and tank levels maintained full. This assures
maintenance of adequate fire flow and pressures except in areas with small
local distribution lines.
Looking at the peak flow day analysis shows the system maintaining
adequate system pressures from 34 to 97 psi and tank levels fluctuating by
5’ on average except at El Indio, where 10 fluctuations were seen. This
again assures good fire flow supply.
C.
Results for Year 2025
1.
Average
The output for the full phased improvements in 2025 showed the
following key points for average flow.
System pressures were good and tank levels were maintained
throughout the day. This would assure adequate pressure for any
fire flow demand and maintain residual pressures in the system.
System pressure ranged from 35 to 93 psi.
2.
Peak
During simulation of the peak day demand for 2025 the model
showed that tank levels dropped during the morning high demand
and again in late afternoon, but tank levels recovered quickly.
System pressures ranged from 33 to 90 psi.
Summary
The final system as projected for 2025 is an ideal looped system.
Combined with the El Indio System improvements, the increase in
pumping capacity, distribution lines and elevated storage meet both
average and peak demands with fire reserves.
3-105