Storage Tanks & System Demand

Distribution storage helps a water system balance production with changing customer demand. Tanks can also support pressure, fire flow, emergency supply, and reliable pump operation. Operators should understand how storage level, system demand, pump controls, and pressure interact throughout the day.

A storage tank that appears full does not automatically mean it is operating well. Poor turnover, excessive water age, unstable cycling, control problems, or incorrect hydraulic configuration can create both operational and water-quality problems.

Why Distribution Storage Is Used

Distribution storage can provide several functions:

• balancing normal demand;
• meeting peak demand;
• supporting fire flow;
• providing emergency supply;
• stabilizing pressure;
• reducing excessive pump cycling.

Balancing Storage

Balancing storage helps compensate for the difference between relatively steady water production and changing customer demand.

During lower-demand periods, storage may fill.

During higher-demand periods, storage may supply part of the system demand.

Peak Demand

Customer demand is rarely constant throughout the day.

Many systems experience:

• lower demand overnight;
• higher demand in the morning;
• another increase during evening use.

Storage During Peak Demand

During peak demand, water may be supplied by both:

• pumps;
• storage tanks.

This can reduce the amount of instantaneous pump capacity needed to meet short-term peaks.

Emergency Storage

Emergency storage provides water during conditions such as:

• power failure;
• pump failure;
• source interruption;
• major repair;
• temporary loss of treatment capacity.

Fire Storage

Where required, storage may provide part of the water needed for fire protection.

Fire demand can be much greater than ordinary customer demand and can cause:

• rapid tank drawdown;
• lower residual pressure;
• high main velocity;
• sediment disturbance.

Not All Stored Water Has the Same Purpose

A system may conceptually divide storage among:

• operational storage;
• emergency storage;
• fire storage.

The exact amount allocated to each purpose depends on system design and operating requirements.

Elevated Storage

Elevated tanks use water elevation to provide pressure.

The water-surface elevation determines the hydraulic grade produced by the tank.

Ground-Level Storage

Ground-level tanks and reservoirs store water but generally require pumps to supply a pressurized distribution zone.

Standpipes

A standpipe is a tall vertical storage structure.

Only the water above the minimum useful pressure elevation may provide effective pressure storage without additional pumping.

Hydropneumatic Storage

Hydropneumatic tanks use compressed air and water to help maintain pressure in smaller systems.

The compressed air provides energy as water leaves the tank.

Tank Level and Pressure

For elevated storage, tank water level directly affects system pressure.

As water level rises:

• hydraulic grade increases;
• nearby static pressure increases.

As water level falls:

• hydraulic grade decreases;
• nearby static pressure decreases.

Pressure from Elevation

A useful relationship is:

Pressure, psi = Head, ft × 0.433

Pressure Example

If the tank water surface is 140 feet above a customer:

Pressure = 140 × 0.433

Pressure ≈ 60.6 psi

This is the approximate static pressure from elevation before considering other hydraulic effects.

Tank Level Range

A tank normally operates between a lower and upper level.

The difference between those levels provides usable operating volume.

Operating Volume

For a constant-area tank:

Operating Volume = Tank Area × Change in Water Depth

Cylindrical Tank Area

For a cylindrical tank:

Area = π × Diameter² ÷ 4

Operating Volume Example

A cylindrical tank is 60 feet in diameter and the normal operating range is 8 feet.

Tank area:

Area = 3.14 × 60² ÷ 4

Area ≈ 2,826 ft²

Operating volume:

2,826 ft² × 8 ft = 22,608 ft³

Using approximately 7.48 gallons per cubic foot:

22,608 × 7.48 ≈ 169,108 gallons

The 8-foot operating range provides approximately 169,000 gallons of storage.

System Demand

System demand is the rate at which customers and other system uses withdraw water.

Demand can vary with:

• time of day;
• day of week;
• season;
• weather;
• industrial activity;
• irrigation;
• fire flow.

Average Daily Demand

Average daily demand provides a general measure of typical water use.

It does not describe short-term peaks.

Maximum-Day Demand

Maximum-day demand describes a particularly high daily demand condition.

It can be important for:

• source capacity;
• treatment capacity;
• storage planning.

Peak-Hour Demand

Peak-hour demand can be much greater than average hourly demand.

Storage can help supply this short-term peak.

Production Versus Demand

If production exceeds demand:

• tank levels tend to rise.

If demand exceeds production:

• tank levels tend to fall.

Tank Level as a Demand Indicator

A falling tank level does not necessarily indicate a problem.

It may simply mean system demand temporarily exceeds the water entering the zone.

Unexpected Tank Decline

If a tank level falls faster than expected, review:

• system demand;
• pump operation;
• main breaks;
• large hydrant use;
• valve configuration;
• level instrumentation.

Unexpected Tank Rise

If a tank fills faster than expected, review:

• lower system demand;
• increased pump flow;
• valve changes;
• level-sensor accuracy.

Tank Filling

A tank fills when the hydraulic grade of the supplying system is high enough to move water into the tank.

Tank Draining

A tank supplies water when the tank hydraulic grade is higher than the hydraulic grade in the connected system.

Flow Direction Can Reverse

The same tank connection can experience flow:

• into the tank during filling;
• out of the tank during higher demand.

Pump Controls and Storage

Pumps can be controlled using:

• tank level;
• system pressure;
• time schedules;
• SCADA logic;
• combinations of these signals.

Level-Based Pump Control

A typical control strategy may:

• start pumps when tank level reaches a lower setpoint;
• stop pumps when tank level reaches an upper setpoint.

Control Band

The difference between pump start and stop setpoints provides a control band.

A reasonable control band can reduce excessive pump cycling.

Short Cycling

Short cycling occurs when pumps start and stop too frequently.

Possible causes include:

• narrow control band;
• small effective storage volume;
• incorrect level signal;
• poorly matched pump capacity;
• control-system problems.

Why Short Cycling Is Undesirable

Frequent starts can increase:

• motor wear;
• electrical stress;
• control-equipment wear;
• pressure fluctuations.

Long Pump Runs

Long pump runs may be normal during high demand.

Unexpectedly long runs can indicate:

• increased demand;
• reduced pump output;
• leakage;
• tank-level control problems.

Altitude Valves

An altitude valve is commonly used to control filling of certain storage tanks based on water level or pressure conditions.

Its purpose can include preventing overfilling while allowing proper tank operation.

Altitude Valve Problems

Possible symptoms include:

• tank will not fill;
• tank overfills;
• unstable tank level;
• restricted flow.

Tank Overflow

A tank overflow can result from:

• failed level sensor;
• failed control valve;
• pump-control failure;
• incorrect SCADA signal;
• manual override left active.

Tank Fails to Fill

If a tank will not fill, review:

• pump discharge head;
• system demand;
• tank inlet valve;
• altitude valve;
• zone valve configuration;
• level sensor;
• available hydraulic grade.

Tank Fills but Does Not Drain

Possible causes include:

• low system demand;
• high incoming pressure;
• closed or restricted outlet;
• incorrect valve configuration;
• hydraulic grade remaining above tank level.

Tank Drains but Does Not Refill

Possible causes include:

• insufficient pump head;
• pump failure;
• high system demand;
• closed valve;
• control failure.

Storage Turnover

Turnover describes replacement of stored water with newer water.

Good turnover helps reduce:

• excessive water age;
• disinfectant decay;
• temperature increase;
• stagnation.

Poor Turnover

Poor turnover can occur when:

• tank volume is large relative to demand;
• operating range is too small;
• inlet and outlet hydraulics create short-circuiting;
• the tank rarely drains substantially.

Water Age

Storage strongly affects water age.

Long storage time can contribute to:

• lower disinfectant residual;
• higher temperature;
• biological activity;
• taste and odor;
• other water-quality changes.

Theoretical Detention Time

A simple estimate is:

Detention Time = Volume ÷ Flow

Storage Detention Example

A tank contains 1.5 million gallons and average daily flow through the tank is 0.5 MGD.

Detention Time = 1.5 MG ÷ 0.5 MGD

Detention Time = 3 days

This theoretical value indicates relatively long storage time.

Theoretical Detention Is Not Actual Water Age

Real tanks may experience:

• mixing;
• short-circuiting;
• dead zones;
• variable flow.

Mixing

Good tank mixing helps reduce zones with very different:

• water age;
• temperature;
• disinfectant residual.

Short-Circuiting

Short-circuiting occurs when incoming water travels quickly toward the outlet without effectively mixing with older stored water.

This can leave older water in other parts of the tank.

Dead Zones

Dead zones are areas with limited water movement.

They can contribute to:

• high local water age;
• low residual;
• sediment accumulation.

Inlet and Outlet Configuration

Tank mixing and turnover can be influenced by:

• inlet location;
• outlet location;
• flow rate;
• tank geometry.

Separate Inlet and Outlet

Some tanks use separate inlet and outlet piping to improve circulation.

Common Inlet and Outlet

Other tanks use one common pipe for both filling and draining.

Actual mixing performance depends on hydraulic conditions.

Active Mixing

Some storage facilities use active mixing equipment to improve circulation and reduce stratification or stagnant zones.

Temperature Stratification

Stored water can develop temperature differences, particularly during warm weather.

Thermal differences can affect:

• mixing;
• water age distribution;
• disinfectant decay.

Disinfectant Residual in Storage

Residual generally decreases with time.

Long storage duration can therefore produce lower outlet residual than inlet residual.

Residual Comparison

Useful monitoring may compare:

• tank inlet residual;
• tank outlet residual;
• tank interior or representative storage sample where applicable.

Sediment Accumulation

Sediment can accumulate in storage facilities over time.

Possible material includes:

• iron deposits;
• manganese deposits;
• corrosion products;
• treatment carryover.

Sediment and Water Quality

Sediment can contribute to:

• discoloration;
• turbidity;
• disinfectant demand;
• customer complaints.

Tank Inspection

Routine inspection should evaluate:

• roof condition;
• hatches;
• vents and screens;
• overflow protection;
• structural condition;
• sediment;
• security.

Overflow Protection

Tank overflow openings should be protected from:

• animals;
• insects;
• contaminants;
• unauthorized access.

Tank Venting

Storage tanks need proper venting so air can enter or leave as water level changes.

Vent openings should also be protected against contamination.

Tank Level Measurement

Level may be measured using:

• pressure transducers;
• ultrasonic instruments;
• radar;
• float systems;
• other level devices.

Verify Level Instruments

An incorrect level signal can cause:

• pump cycling;
• tank overflow;
• failure to refill;
• incorrect operator response.

SCADA and Storage

SCADA can display:

• tank level;
• inlet or outlet flow;
• pump status;
• pressure;
• alarms.

Do Not Trust One Signal Blindly

If tank behavior does not match the displayed level, verify:

• field level;
• instrument calibration;
• signal scaling;
• communication status.

Demand Patterns and Tank Curves

A tank-level graph often reflects daily system demand.

A normal pattern may show:

• tank filling during low demand;
• tank drawdown during peak demand;
• repeating daily cycles.

Changing Tank Pattern

If the normal level pattern changes, possible causes include:

• different customer demand;
• pump performance change;
• large leak;
• valve configuration change;
• control problem.

Tank Level and Leakage

Unexpected overnight tank drawdown can provide evidence of:

• system leakage;
• unusual continuous demand;
• unauthorized use.

Minimum Night Flow

Low-demand nighttime periods are useful for evaluating abnormal system flow because ordinary customer demand is often reduced.

Demand Forecasting

Operators can use historical patterns to anticipate:

• morning peaks;
• seasonal irrigation;
• high-temperature demand;
• special events.

Storage and Pump Efficiency

Storage can allow pumps to operate for longer periods near efficient operating conditions instead of constantly matching every small demand change.

Too Much Storage Can Create Water-Quality Problems

Large storage volume is not automatically beneficial.

Excessive storage relative to actual demand can increase:

• water age;
• residual decay;
• stagnation.

Too Little Storage Can Create Operational Problems

Insufficient storage can contribute to:

• rapid tank drawdown;
• frequent pump starts;
• limited peak-demand reserve;
• reduced emergency flexibility.

Storage and Pressure Zones

Each pressure zone must have an appropriate hydraulic source of pressure.

This may be:

• elevated storage;
• booster pumping;
• pressure-control connection to another zone.

Zone Isolation

Incorrect valve configuration can accidentally:

• isolate storage from a zone;
• connect incompatible pressure zones;
• change tank filling behavior.

Example: Tank Level Falling During Normal Demand

Review:

• pump flow;
• system demand;
• tank outlet flow;
• main breaks;
• valve position.

Example: Tank Level Falling Overnight

Because demand should normally be lower, review:

• large leaks;
• unexpected industrial use;
• hydrant use;
• pump shutdown;
• incorrect level reading.

Example: Tank Never Reaches Upper Setpoint

Possible causes include:

• pump cannot overcome required head;
• system demand remains too high;
• partially closed valve;
• control-valve restriction;
• pump wear.

Example: Tank Reaches Upper Setpoint Too Quickly

Review:

• low system demand;
• excessive pump flow;
• incorrect level calibration;
• changed valve configuration.

Example: Frequent Pump Starts

Review:

• control band;
• effective tank volume;
• level-sensor stability;
• pump capacity;
• demand variability.

Example: Low Residual at Tank Outlet

Review:

• tank turnover;
• water age;
• temperature;
• inlet residual;
• mixing;
• sediment condition.

Example: Pressure Falls as Tank Drains

This is expected in an elevated-storage system because decreasing water-surface elevation lowers hydraulic grade.

Example: Pressure Changes but Tank Level Does Not

Review other hydraulic causes such as:

• pump operation;
• system demand;
• valve position;
• PRV operation;
• main break.

Common Storage and Demand Mistakes

• Assuming a full tank always means good operation.
• Ignoring tank turnover and water age.
• Confusing storage volume with pressure-producing elevation.
• Ignoring daily demand patterns.
• Using too narrow a pump-control band.
• Ignoring short cycling.
• Ignoring level-instrument accuracy.
• Ignoring valve configuration when a tank will not fill.
• Ignoring water-quality effects of excessive storage.
• Looking at tank level without reviewing pump flow and system demand.

A Practical Tank Operation Review

1. Review tank level trend.
2. Review inlet and outlet flow where available.
3. Review pump operation.
4. Review system demand.
5. Review pressure in the connected zone.
6. Review control setpoints.
7. Review turnover and water age.
8. Review disinfectant residual.
9. Review level-instrument accuracy.
10. Document abnormal behavior.

A Practical Tank-Will-Not-Fill Review

1. Verify the tank-level measurement.
2. Review pump status and discharge head.
3. Review system demand.
4. Review tank inlet and zone valves.
5. Review altitude or control valve operation.
6. Check for hydraulic restrictions.
7. Compare available hydraulic grade with tank elevation.
8. Correct the confirmed cause.

A Practical High-Water-Age Review

1. Review tank volume and average flow.
2. Calculate approximate theoretical detention time.
3. Review normal operating range.
4. Review tank turnover.
5. Review mixing.
6. Review inlet and outlet residual.
7. Review temperature.
8. Adjust tank operation according to approved procedures.

A Practical Demand Review

1. Compare current flow with historical demand.
2. Review tank-level response.
3. Review production and booster flows.
4. Review weather and seasonal demand.
5. Check for hydrant use or unusual customers.
6. Check for leaks if demand is unexplained.
7. Compare nighttime flow with normal minimum demand.
8. Document significant changes.

What to Remember for the Exam

• Distribution storage balances changing customer demand with water production.
• Storage can provide operational, emergency, and fire-flow capacity.
• Elevated storage provides pressure through water-surface elevation.
• Pressure from water head can be estimated using psi = feet of head × 0.433.
• Tank volume and tank elevation are different concepts: volume provides stored water, while elevation provides hydraulic pressure.
• When demand exceeds production, tank level tends to fall.
• When production exceeds demand, tank level tends to rise.
• Peak-hour demand can be much greater than average demand.
• Storage helps reduce the need for pumps to match every short-term demand change directly.
• Pump start and stop setpoints should provide enough control range to avoid excessive cycling.
• Short cycling increases motor, electrical, and control-equipment wear.
• A tank that will not fill may have pump, valve, control, demand, or hydraulic-grade problems.
• Poor turnover increases water age and can reduce disinfectant residual.
• Theoretical detention time can be estimated as volume divided by flow.
• Actual water age may differ from theoretical detention time because of mixing, short-circuiting, and dead zones.
• Tank mixing affects residual, temperature, and water-age distribution.
• Storage facilities should be protected with properly maintained vents, hatches, overflow structures, and security.
• Tank-level instruments and SCADA signals should be verified when behavior does not match displayed data.
• Tank-level trends can provide useful information about demand, leakage, pump performance, and control problems.
• Good storage operation balances hydraulic reliability, pump operation, emergency reserve, pressure, turnover, and water quality.