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Water Storage Systems

Water Storage Tanks

Types: Gravity or Elevated Ground Storage Hydropneumatic

Water Storage Tanks

Elevated Storage

Ground Storage Hydropneumatic

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Diurnal Flow Pattern

12M 12N 12M

Plant

Flow

Ave. 24 Hour Period

Average Daily Demand

Peak Demand

Empty Tank

Fill Tank

Schematic Illustrating the Benefits of a Storage Tank

Elevated Storage Tanks

Slide 6

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Elevated Tanks Stabilize distribution

system pressures

Provide water to meet peak demands

Keeps pumps from cycling and operating in efficient ranges

Fluctuating Levels in an Elevated Storage Tank

Tank Full

Well or High Service Pumps

Peak FlowTank Empty

Average Flow

Pumps are used to supplement system pressure during peak demands & refill storage tank during other times.

Advantages Offered by Elevated Tanks

Less variation in pressure Available water for fire fighting Storage to meet peak demands Allows use of lower capacity wells Cycling of well pumps is reduced Wells can be better matched to average

water demand High service pumps and the treatment plant

can operate more efficiently

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Maintenance Considerations for Elevated Water Storage Tanks

Check for Intrusion of Water Secure Tank Site from unauthorized access Clean Tank yearly recommended

(DEP 5-yr. required!) Ensure that overflow structures are working,

secure and properly drained Inspect Structures for Stability, Blockages

and Surface and Internal Corrosion.

Standpipes

Elevated Tank and Standpipe Comparison

Used to provide pressure head to the distribution system.

Large storage capacity Shallow tanks with large

diameter are preferred over deep one with small diameters.

Less pressure drop in elevated tanks.

Pressure = Water Elevation (ft) * Water Density (lb/ft ) * ( 1 ft2/ 144 in2)

Where: Water Density = 62.4 lb/ft3

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Ground Storage Tanks

Slide 13

Ground Storage

Used for storing large amounts of water. New Tanks must be Covered! Not under pressure uses transfer pumps

to pressurize or pump to elevated tank. Sometimes has aerators on top for

waters that require it.

Hydropneumatic Tanks

Must be built to ASME standards

Slide 15

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Hydropneumatic Tanks

Under 120 gallon PST 2000 tanks allowed by FDEP

Slide 16

Hydropneumatic Tanks

Combination of compressed air pressure to force water into the distribution system when pump is off

Slide 17

Types of Hydropneumatic Tanks

Conventional Floating Wafer Flexible Separators

– Flexible diaphragm– Bladder type

Floating Wafer

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Operating Considerations for Conventional Hydropneumatic Tanks

Must have air to operate correctly

Must have a pressure relief valve

Need a sight glass to observe level in tank

Must have By-pass piping for repairs

Valve for flushing Air volume control Scheduled

maintenance program

Components of Conventional Hydropneumatic System

Components Function Air Volume Control - Control amount of air in tank Relief Valve - Prevent excessive high pressure Pressure Gauge - Monitor water/air pressure Motor Controls - Control cut-in and cut out Level Controls - Regulate high/low water levels Compressor - Replenishes Air

Conventional Hydropneumatic Tank Schematic

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From Well

Overall Length

PRV

Tank Level Sight Glass

Compressor

Dia

met

er Hydropneumatic TankNew must meet ASME Stds & interior

coating must meet NSF 61 requirements

Tank Drain

Gauge

Hydropneumatic Tank Side View

Tank Bypass Line with Isolation Valve (not shown), normally closed

New Hydropneumatic Tanks as required by FDEP Rule 62-555.320(20), FAC and Recommended Standards for Water Works (a.k.a. Ten States Standards.)

Foundation

Access Manway max. size allowed by ASME Code 24” min. if possible per FDEP

ASME Name Plate

Elastomeric gasket / sheet between tank & support

Hydropneumatic Tank End View

Sizing Considerations for a Conventional Hydropneumatic Tank

Capacity of the wells and pumps should be at least ten times the average daily consumption.

Gross volume of the tank, in gallons should be at least ten times the capacity of the largest pump, rated in gpm.

Sizing must consider the need for disinfectant contact time.

Operating pressure is generally sized to operate between 40 and 60 psi

The pressure pump should not cycle more than 6 to 8 times per hour

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Air Compressor Maintenance Considerations

Regularly scheduled maintenance:

Clean or replace filters

Drain condensate frequently

NSF approved lubrication

Hydro-Tank Sanitary Deficiencies

Exterior staining does NOT always mean corrosion or possible contamination

Hydro-Tank Sanitary Deficiencies Pitting, spalling, cracks, holes, coating deterioration

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Hydro-Tank Sanitary Deficiencies

Undercoat is still intact No Pitting, spalling, cracks, holes,

coating deterioration here

Hydropneumatic Tank Failures

Metal skin under continual tension Crack rips at hundreds to thousands of miles / hour You will hear it AFTER failure occurs! You will most likely not have a chance to react

Hydropneumatic Tank Failures

This water system called about their hydropneumatic tank• They had just welded a plate over a pinhole leak• Luckily 2-weeks later when the tank blew out, no one was in the

immediate area and no extensive damage was done to other structures. • This type of repair significantly compromises the tank & shorten its life • Instead of strengthening the tank, the welds increased metal stresses

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Hydropneumatic Tank Failures