Key Source Meter Sites for Proper Flow Balancing

Water audits are most commonly conducted to track treated drinking water in transit through retail distribution systems. Separate water audits can also be conducted on wholesale transmission systems carrying untreated (raw) water or treated water; or discrete pressure zones or DMAs inside of a retail distribution system. Table 6.1 lists system configuration locations where metering is typically employed. In this publication, the water audit process is discussed in terms of the retail distribution system and the metering sites given below are those encountered in a typical retail distribution network. Figure 6.1 illustrates a basic retail distribution system configuration for the

All water sources should include flowmeters that are technologically current, accurate, reliable, well maintained and—ideally—continuously monitored by a Supervisory Control and Data Acquisition (SCADA) System or similar monitoring system.

The water supplied value is calculated as a composite value that includes as components the primary untreated and/or treated water meters, meters registering water going into and out of tanks, basins and reservoirs, and meters measuring water across pressure zones or district metered areas.

Location

Function

Water source (untreated water)

Measure withdrawal or abstraction of water from rivers, lakes, wells, or other raw water sources

Treatment plant or works

Process metering at water treatment plants; metering may exist at the influent, effluent, and/or locations intermediate in the process

Distribution system input volume

Water supplied at the entry point of water distribution systems; either at treatment plant, treated water reservoir, or well effluent locations.

Distribution system pressure zones

Zonal metering into portions of the distribution system being supplied different pressure. Also includes metering at major distribution facilities such as booster pumping stations, tanks, and reservoirs.

District metered areas (DMAs)

Discrete areas of several hundred to several thousand properties used to analyze the daily diurnal flow variation and infer leakage rates from minimum hour flow rates

Customers

Consumption meters at the point of end use

Bulk supply

Import/export meters to measure bulk purchases or sales

Miscellaneous

Capture use of water from fire hydrants, tank trucks, or other intermittent use

Table 6.1 Typical Source Meter Locations in Drinking Water Supply Systems

Table 6.1 Typical Source Meter Locations in Drinking Water Supply Systems

County Water Company water treatment plant Water exported to

County Water Company water treatment plant Water exported to

Industrial water meter outdoors in meter pit (in warm climates, residential water meters are also in outdoor meter pits)

Figure 6.1 Typical retail water distribution system configuration. (Source: Ref. 1.)

fictitious County Water Company. As shown, source meters should exist at the point where the treated drinking water leaves the water treatment plant, shown as metering location (Mj). At this point the water quality has improved from untreated to potable quality and the water has been pressurized for conveyance in the distribution system; hence the monetary value of the water is greatest at this location. Source meters should also be included at any points of imported (M2) or exported water supply (M3). Finally, water meters should be included to measure flow entering or leaving tanks or reservoirs, and crossing pressure zones and DMAs.

Volumes of water purchased and imported from a neighboring supplier should be added to the composite metered values to obtain the water supplied value. The source meter (M2) in Fig. 6.1 registers water purchased from a neighboring water utility by County Water Company. Interconnections between water utilities should always be metered. Such meters should be carefully maintained and monitored since the metered data provides the basis for billing large water volumes. Both the water utility supplying the water and the system purchasing the water have a strong motivation to keep this bulk measurement accurate since significant costs are at stake for each water utility.

Any water volumes sold and exported outside of the distribution system to a neighboring water utility should be monitored and adjusted with the same scrutiny given to imported water, for the same revenue implications exist. The source meter (M3) in Fig. 6.1 registers water sold and exported out of the County Water Company grid.

Flows at storage facilities should be balanced for the water audit period. If source meters are located upstream of reservoirs and storage tanks, then stored water must be accounted for in the water audit. Generally, water flowing out of storage is replaced; as the "replacement" water flows from the source into storage, it is measured as supply into the system. If the reservoirs have more water at the end of the audit period than at the beginning, then the increased storage is measured by the source meters but not delivered to consumers. Such increases in storage should be subtracted from the metered supply. Conversely, if there is a net reduction in storage, then the decreased amount of stored water should be added to the metered supply. Table 6.2 shows how to figure the change in storage volume using data for County Water Company. Remember, decreases in storage are added to the supply; storage increases are subtracted from the supply. In this case, the net reservoir and tank storage was a drop in storage volume so the adjustment of 0.83 million gal should be added to the value of water supplied.

Reservoir

Start Volume, gal

End Volume, gal

Change in Volume, gal

Apple Hill

32,350

36,270

+3,920

Cedar Ridge

278,100

240,600

-37,500

Monument Road

978,400

318,400

-660,000

Davis

187,300

55,300

-132,000

Total change in reservoir storage

-825,580

Volume in million gal

-0.83

Table 6.2 Changes in Reservoir Storage for County Water Company

Component

Volume, million gal

1

Volume from own sources (treated water)

3480.76

2

Adjustment: source meter error

+136.89

3

Adjustment: changes in reservoir and tank storages (±)

+0.83

4

Other adjustments (specify)

0

5

Total adjustments = lines 2+3+4

+137.72

6

Volume from own sources (adjusted) = Lines 1 ±5

3618.48

7

Volume of water imported (adjusted)

783.68

8

System input volume = volume from own sources + water imported

4402.16

9

Volume of water exported (adjusted)

0

10

Water supplied = system input volume - water exported

4402.16

Table 6.3 Water Balance Calculations to Determine the Value of Water Supplied for County Water Company

Table 6.3 Water Balance Calculations to Determine the Value of Water Supplied for County Water Company

Table 6.3 shows data for County Water Company and a series of tabulations that the water auditor should follow to arrive at the proper value of water supplied for a retail water distribution system. The procedure to obtain the data included in Table 6.3 is discussed below.

Compile the Volume of Water from Own Sources

Identify all water sources that are owned or managed by the water utility to supply water into the distribution system. Such sources can include raw water that is treated adjacent to sources such as wells, rivers, streams, lakes, reservoirs, or aqueduct turnouts. However, most water audits are performed on the potable water distribution system so that the "source" is often the location where treated water enters the distribution system; which is often the effluent of the water treatment plant. All volumes from such sources should be metered, with routine meter testing and upkeep conducted so that volumes of water taken from the sources are registered accurately. Data should be available on a daily, weekly, or monthly basis to compile into an annual volume of water supplied from each source. Meter information can be kept in a table similar to Table 6.4.

In this example, County Water Company withdraws water from three sources: an aqueduct, a well field, and an interconnection (city intertie) with a neighboring water utility. Table 6.5 is a summary of water withdrawn from these sources for the year of 2006, illustrating how source meter and flow data can be arranged and adjusted for the water audit period. The data listed is based upon uncorrected meter registrations. In this example, water withdrawn from the aqueduct and well field is presumed to be untreated water. For the simplicity's sake, it is assumed that the volumes of water for these two sources shown in Table 6.5 are the same volumes delivered to the water distribution system after the water undergoes treatment. This simplistic assumption often

Water from Own Sources

Water Imported

Source 1

Source 2

Source 3

Characteristics

Aqueduct Turnout 41

Well Field

City Intertie

Type of measuring device

Venturi

Propeller

Venturi

Identification number (may

be serial number)

0000278-A

8759

OC-16

Frequency of reading

Daily

Weekly

Daily

Type of recording register

Dial

Dial

Builder type M

Units registers indicate

100,000 gal

gallons

Cubic feet

Multiplier (if any)

1.0

1.0

100.0

Date of installation

1974

1990

1978

Size of conduit

24 in

10 in

11.5 in

Frequency of testing

Annual

2 years

4 months

Date of last calibration

4/1/2006

8/21/2006

1/15/2006

Table 6.4 Source Water Measuring Devices for County Water Company

Subtotal

Source 3

Total for all

Own

City Intertie

sources 1,

2006 By

Source 1

Source 2

Sources

(water

2, and 3

Month

Turnout 41

Well Field

(unadjusted)

imported)

(unadjusted)

January

0

130.34

130.34

104.27

234.61

February

0

195.51

195.51

65.17

260.68

March

130.83

130.34

261.17

0

261.17

April

160.18

260.68

420.86

0

420.86

May

326.53

97.76

424.29

0

424.29

0

81.46

June

368.62

368.62

450.08

0

84.72

July

372.64

0

372.64

89.61

457.36

August

400.89

32.59

400.89

32.59

490.50

September

360.72

32.59

393.31

97.76

425.90

October

160.18

0

192.77

130.34

290.53

November

160.18

0

160.18

97.76

290.52

December

160.18

160.18

257.94

Annual total

2600.95

879.81

3480.76

783.68

4264.44

Daily average,

11.68

million gal/day

Table 6.5 Total Water Supply in Million Gallons for County Water Company (Uncorrected)

Table 6.5 Total Water Supply in Million Gallons for County Water Company (Uncorrected)

does not hold true in reality, as a portion of the water passing through a treatment process is lost due to plant infrastructure leakage and process uses such as backwashing of filters, chemical mixing, and maintenance activities such as flushing. Metering water at the source of withdrawal is essential and many regulatory agencies require this in order to track water resource utilization. However, it is recommended to also meter treated water at the location when it leaves the water treatment plant; particularly if the water treatment plant is distant from the water source.

Once a volume is established for each source for the year, the measured amounts should be reviewed and corrected for known systematic or random errors that may exist in the metering data. Figures for the total water supply, based on readings from source meters and measuring devices, are raw data. The raw data must be adjusted for a number of potential factors, including

• Meter inaccuracies (see Table 6.6)

• Changes in reservoir and storage levels (see Table 6.2)

• Any other adjustments such as losses that occur before water reaches the distribution system. One example would be losses incurred during the treatment process (filter backwashing, etc.) if the source meter is located upstream from the water treatment plant. None are included in the example data shown in Table 6.3 so a volume of zero is entered on line four of this table.

The tabulations shown in Table 6.3 arrive at a corrected value of water supplied of 4,402.16 million gal for the water audit period. This data takes into account the registered yearly volumes for three source meters (two of own sources and one imported supply), a correction for meter error on "Source 1" and the storage adjustment. This is a simplified example that includes only a few sources. It is recognized that many water utilities withdraw water from many sources, purchase/sell water at multiple interconnection points, and have many tanks and pressure zones. It is incumbent upon utility managers operating supply systems with such complex configurations to be meticulous in identifying the key source metering locations, establishing and maintaining source meters,

Source

Yearly Total: Uncorrected Metered Volume (UMV)*

Meter

Accuracy

percent

Meter Error Calculation UMV/MA+ - UMV

Meter Error

Adjusted Metered Volume^

Turnout 41

2600.95

95

(2600.95/0.95) -2600.95

+136.89

2737.84

Well Field

879.81

100

(879.81/1.00) -879.81

+0.0

879.81

+136.89

f A percentage, written as a decimal (95 percent = 0.95) taken from meter testing performed regularly. t The corrected meter volume for sources 1 and 2 is 2,737.84 + 879.81 = 3617.65 million gal; note that this is 136.89 million gal greater than the raw total supply given for these sources in Table 6.5.

f A percentage, written as a decimal (95 percent = 0.95) taken from meter testing performed regularly. t The corrected meter volume for sources 1 and 2 is 2,737.84 + 879.81 = 3617.65 million gal; note that this is 136.89 million gal greater than the raw total supply given for these sources in Table 6.5.

Table 6.6 Volume of Water from Own Sources in Million Gallons for County Water Company—Adjusted for Source Meter Error and creating spreadsheets or databases that properly balance flows such that an accurate value of water supplied is attained and made available to the water audit each year.

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