watermeter selction

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A recent announcement from the

Chicago Department of Water prompt-

ed me to think about an aspect of

plumbing design that may be taken

for granted. The news is that the City

of Chicago will undertake the major

exp ense of installing water m eters in

all houses in the city. Evidently, there

are thousands of houses in Chicago

that have ne ver been m etered. The

residents have been charged for water

usage, but the charge was based on

an estimate rather than on actual use.

Someone now has figured out that the

initial water-meter installation cost will

be offset by the additional revenue.

The concern is that many engineers

who design w ater systems con nected

to municipal water supply may take

water-meter selection and sizing for

granted. Regularly, mu nicipal w ater

purveyors dictate the type, size, and

cost of the water meter to be used for

buildings. This practice has become

more prominent with the advent of

remote meter reading and digital con-

nectivity. It is very common for a

municipality to read a water meter

through the telephone system. This iseasy to do, even with yesterdays tech-

nology. Many designers have become

familiar with the size and type of water

meter that the water purveyor provides

for a certain building type. Therefore,

they can establish the amount of pres-

sure loss to apply to the pipe-distribu-

tion sizing calculations from the meter

man ufacturers p ressure loss curves.

What about installations where the

plumbing contractor is required to

provide the w ater meter based on thedesigners plans and specifications? A

water meter typically is located at the

water service entrance to a building.

Therefore, it is the first device calcu-

lated into a water-distribution system

design that inevitably results in having

the proper flow and pressure at the

farthest fixture in the building. Water

meters have other applications in

plumbing design, such as tenant sec-

ondary metering, control of the appli-

cation rate in irrigation systems, meas-

urement of water usage or loss for

process or boiler systems, and control

of the regeneration cycles for water

softeners, to name just a few.

Sizing Is CriticalMy last article, on temp ered water

systems, outlined the false concept of

matching the mixing valves inlet sizes

with the pipe sizes. The same princi-

ples ap ply to w ater-meter sizing,

although the systems dynamics are

Water-M eter Selection and SizingTim othy A.Smith, CPD

70 Plumbing System s & Design Jul/Aug 2003

DESIGNERS NOTEBOOK

Figure 1. Descriptive Specifications for Four Types of Water Meters

Disc MeterA. Positive displacement type AWWA C700-02 and/or C710-02 (cast bronze body C700-02/ plastic

body C710-02)1. (Bronze) or (plastic) body constructed with a nutating disc or oscillating piston; flanged or

threaded conn ectionsB. Application: Residential and small commercial buildi ngs with flush tank toilets or small quantity

of flush valves

C. Commentary: Available in sizes 1/2-in. through 2-in.; 1/4-gpm mini mum through 160-gpm

peak flow; available with analog/digital remote reading capability; accurate low flow range; pres-sure loss increases with flow somewhat proportionally to the recording inaccuracies.

Turbine MeterA. Turbine type, AWWA C701-02

1. Bronze body with rotating im peller (rotar) located within the measuring chamber mounted in

the center of the flow; vortex reducing flow vanes; flanged connections

B. Application: Commercial, process systems, large institutional, and facilities of potentially high

usage and high flow ratesC. Commentary: Available in sizes 2-in. through 20-in.; 2 1/2- gpm minimum through 19,000-gpm

continuous; remote reading is available; small fluctuation in accuracy throughout entire flow

range; inaccurate for flows less than 2 gpm.

Compound MeterA. Combination type, AWWA C702-01

1. Bronze body with dual measuring chambers separated by a pressure/velocity-sensiti vemechanism or plate; positive displacement chamber and turbine chamber combination; as

pressure loss through positive displacement (low-flow) chamber increases, the mechanism

or diverting plate shifts the flow to the turbine (high-flow) chamber; flanged connections

B. Application: Commercial, institutional, places of assembly, and facilities with fluctuating usageand flow rates

C. Commentary: Available in sizes 2-in. through 6-i n.; 1/4-gpm mi nimum through 1,200-gpm peak

flow; available with analog/digital remote reading capability; accurate low and high (peak) flowrange; progressing pressure loss until mid-flow range when low-flow chamber diverts to thehigh-flow chamber with lower pressure drop; can experience quick pressure-loss spike when

switching from low-flow to high-flow chambers.

Propeller MeterA. Propeller type, AWWA C704-02

1. Iron or fabricated steel body and thermoplastic impeller with deep impeller vanes, curved

and tapered (similar to a propeller) for reduced resistancy; mounted in open pipe with wafer

or lug connectionsB. Application: Process or industrial facilities, well-pump di scharge, pumping stations or sewage

treatment plants where flow rates are very high and constant

C. Commentary: Available in sizes 2-in. through 72-in.; 30-gpm mi nimum through 50,000-gpm

continuous; unable to accurately register low and intermittent flows; broader band of inaccuracy;exceptional for high, continuous flow applications.


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more forgiving of an improperly sized

water meter than an improperly sized

three-way m ixing valve. Rem em ber,

using the one-size, one-type-fits-all

philosophy is really asking for trouble.

Improperly sized water meters can

affect the operation of the buildings

water distribution system and plumb-

ing fixtures, which also can affect the

health of the buildings occupants.

Undersized water meters can cause

excessive pressure loss, reduced flow,

and noise. High-pressure losses

through meters at peak flow rates can

result in pressure surges, water ham-

mering, and the inability to maintain

proper residual pressure at the fixtures

or equipment. High-pressure loss

through a meter also corresponds

with a high-flow velocity and thus

results in more noise. Improperly

sized meters can cause negative pres-

sures in the piping system and lead to

contamination of the potable water

supply by backflow or

back siphonage. In addi-

tion to being unecon omi-

cal, oversized water

meters typically do not

accurately measure mini-

mal flow rates. Larger

meters are more expen-

sive than smaller meters.

Municipalities u sually

assess meter and tap fees

according to size, so

oversizing can become

expensive for a buildings

owner.

When selecting and

specifying a domestic

water meter for a build-

ings service entrance, you

must understand the

bu ildings size, fun ction ,

fixture types, usage occu-

pancy, and peak popula-

tion. These factors will

allow you to determine

the minimum and m axi-

mum flow rates and

enable you to select the

proper water meter, based

on pressure loss, record-

ing accuracy, and capabili-

ty of accommodating the

projected flow variations.The ASPE Data Books pro-

vide good resources for

calculating the buildings

water usage. Remember to

always specify tapered fit-

tings when reducing or

increasing from the meter

size to calculated pipe

sizes. The alternative

reducing/increasing

method using p ipe bushings can p ro-

duce three to ten times the pressureloss of tapered fittings, depending on

size.

M eter TypesThe American Water Works

Association (AWWA) has pro mu lgated

widely recognized and accepted stan-

dards for the construction of water

me ters. All water m eters man ufactured

Jul/Aug 2003 Plumbing Systems & Design 71

DESIGNERS NOTEBOOK

continued on page 72


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for domestic water service use in the

United States are manufactured in

acco rdan ce with AWWA stand ards.

These standards set forth construction

criteria, physical construction, internal

comp onen ts, dimensions, and testing

criteria to help ensure uniformity

across the industry. All water-meter

man ufacturers that me et the AWWA

standards can provide the test results

for their products. The designer

should maintain a water meter prod-

uct binder with pressure-loss curves

and accuracy graphs, produced in

acco rdan ce w ith AWWA stand ards for

each meter type and size.

Figure 1 lists descriptive specifica-

tions for various types ofwater meters utilized for

domestic water applications.

Table 1, Table 2, and

Table 3 are reprints from

AWWA Standard M22, as

reprinted in ASPE Data Book

Volume 2, Chapter 5. These

tables will provide additional

guidelines for selecting and

sizing water meters, but the manufac-

turers of water meters can supply test

curves of each meter type and sizethat better define actual performance

and characteristics. See Figure 2 for

typical performance and accuracy

graphs.

M eter SelectionThe first step in selecting a meter is

to establish the purpose for metering

the service. If the meter is to be used

for billing purposes, accuracy is

critical. Process applications may only

require approximate flow recording.

Positive d isplaceme nt m eters are the

best selection for small commercial or

institutional facilities or residen ces.

They have excellent accuracy at low

flows and, if sized properly, can accu-

rately measure the peak flow rates.

Compound meters (see Figure 3) are

an excellent choice for large commer-

cial or institution al facilities b ecau se

they have the ability to accurately

accommodate low flows and high

flow s through their multiple-measur-

ing chamber design. Refer to Figure 2

for performance curves for a typical

comp ound -type meter. Turbine and

propeller meters arebetter suited for contin-

uou s, higher-flow app li-

cations and are inaccu-

rate at low flows.

Turbine and propeller

meters are not recom-

mended for commer-

cial, institutional, or res-

idential buildings

because the flow rates are constantly

fluctuating and minimum flow rates

may be as low as 1/22 gpm.After the meter type is selected, a

size needs to be determined. The size

is established by comparing the mini-

mum and peak flow rates of the facili-

ty against the minimum and maximum

flow capabilities of the meter size.

The flow rate will correspond to a

pressure loss, which factors into the

pipe-sizing calculations. The greater

the pressure loss at the meter, the less

pressure is available for the systems

friction loss. Thus, larger pipe sizes

are required downstream of the

undersized meter. If a backflow pre-

venter is required o n the service (typi-

cally consuming 510 psi), it is neces-

sary to be more conservative with the

water-meter pressure-loss selection.

It is recommended that the calculat-

ed peak flow rate for the facility not

exceed 80% of the maximum capacity

of the meter. This recommendation

needs to be evaluated closely when

low incoming residual pressures are

prevalent or backflow preventers are

required. It is not uncommon to

design for a maximum of 23 psipressure loss through the water meter

where municipal water pressures are

weak.

StrainersWater meters have internal mechani-

cal components that move with the

flow of water, so it stands to reason

that debris and sediment will have an

adverse effect on the meters opera-

tion. It is recommended that an in-line

strainer on the meters inlet be speci-

fied to collect debris and sediment

and prevent them from entering the

meter body. Strainers are available

that are designed specifically for use

in conjunction with water meters.

These strainers are designed and engi-

neered to provide low pressure losses.

They also have removable screens

and accessible covers to facilitate

cleaning and maintenance. Strainer

72 Plumbing System s & Design Jul/Aug 2003

DESIGNERS NOTEBOOK

Figure 2. Typical Accuracy and Performance Graphs

Courtesy of Badger Meter, Inc.

If the meter is

to be used for

bill ing purposes,

accuracy

is critical.


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maintenan ce is critical becau se a

clogged or dirty strainer impedes flow

and increases pressure losses.

InstallationThe installer needs to strictly follow

the ma nu facturers installation instruc-

tions. Improper installation may affect

the meters operation, capability, and

accuracy. Meters should be located in

an accessible location and protected

from damage. Meters are mechanical

devices that need pe riodic testingand repair. Make sure there is amp le

room to repair or remove the unit .

Envision what i t would take to work

on the unit. Refrain from installing

meters in crawl spaces, under stairs,

or in confined spaces.

Some areas still allow the installa-

tion of water meters in exterior pits

or vaults . I do not recommend this

installation. It makes the meter hard

to m aintain and service, and exterior

pits and vaults can flood. In some

cases, submerging the water meters

causes rapid de terioration and corro-

sion to the unit. Also, there is a

remote possibility that the contami-

nated water in the pit can leech into

the potable water supply.

It is recommended that a floor drain

be installed adjacent to the watermeter, especially for larger meters. The

floor drain can reduce or eliminate

water damage to the building caused

by meter repairs, replacement, or test-

ing. Larger meters are tested in place,

so it is imperative to have somewhere

to discharge the test water.

The bottom line is meter accuracy

and pressure loss. Accuracy is critical

to the purveyor of water because

meters that inaccurately measure the

supp ly result in lost revenue . Pressure

loss is critical to designers because

they need to ensure that their engi-

neered systems operate properly.

Jul/Aug 2003 Plumbing Systems & Design 73

DESIGNERS NOTEBOOK

Figure 3. Three Types of Water Meters

Compound M eter

Disc M eter

Turbine Meter

Courtesy of Badger Meter, Inc.

Tim Smith is vice presi-dent, partner, and found-

ing member of Metro

Design Associates, Inc., in

Schaumburg, IL. He has

more than 23 years expe-

rience covering all aspects of consulting

engineering, specializing in plumbing, fire

protection, and civil engineering. His e-

mail address is [email protected].


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