3.3 Storage Facilities (Ground, Elevated, Hydropneumatic)

3.3 Storage Facilities (Ground, Elevated, Hydropneumatic)

Quick Answer: Storage tanks equalize demand (peak-hour draw is met from storage, not just the source), provide fire storage during emergencies, and help maintain pressure. Three families dominate: ground storage tanks, elevated tanks (standpipes, composite, pedestal), and hydropneumatic tanks. The exam tests how each maintains pressure, the concern of water age, and the vents, overflows, and altitude valves that keep them safe.

Why Storage Exists

A treatment plant or well can produce at a relatively steady rate, but demand swings by hour and season. Storage lets the source run near its efficient output while a tank fills during low demand and drains during peaks. It also carries fire flow — a few thousand gpm that may exceed what the source can deliver for hours — and buffers pressure. Without storage, every peak would need a bigger source and bigger pumps.

Ground Storage Tanks

A ground storage tank sits at or below grade and is atmospheric — water is not under pressure in the tank. They are built of concrete (often prestressed or circular) or steel (welded or bolted, sometimes with an interior coating for potable water). Because the tank is at ground level, the water leaving it has only the elevation head of the tank itself; a booster pump sends it into the distribution system at the right pressure.

Ground tanks are common at treatment-plant clearwells and as in-system storage. They can hold a large volume cheaply. Their weakness for pressure is that they don't add head — they need a booster or an elevation advantage to push water into the grid.

Elevated Tanks

An elevated tank stores water above the system so the elevation itself provides pressure. Recall 1 psi = 2.31 ft of head: a tank 100 ft above a customer's service line gives about 43 psi before friction. Elevated tanks come in several styles:

• Standpipe — a tall vertical cylinder, partly ground-supported, partly elevated storage. Lower portion is fire/peaking reserve; upper portion provides pressure.
• Composite tank — a steel tank on a concrete pedestal; common modern style for large elevated storage.
• Pedestal (spheroid) — a single-pedestal tank with a spheroidal or ellipsoidal bowl, used for moderate-volume elevated storage.

Elevated tanks fill during low demand (often at night when the pumps run against low system pressure) and drain during peaks. The water level in an elevated tank roughly tracks demand — high level means low demand; falling level means rising demand. That makes them natural equalizers.

Hydropneumatic Tanks

A hydropneumatic tank holds water and compressed air in the same steel vessel. The air cushion provides pressure: as water enters, the air compresses; as water leaves, the air expands and pushes the water out. A small air compressor maintains the air charge, and an air-to-water interface (sometimes a bladder or diaphragm) keeps the air from dissolving into the water. Hydropneumatic tanks are common in small systems and rural or booster setups where building an elevated tank is not economical.

Their advantage: pressure comes from the air cushion, not elevation, so they can sit on the ground. Their limits: storage volume is small (mostly pressure, not reserve), and they need compressor and air-charge maintenance. They are not used for large fire-flow reserve.

Water Age, Turnover, and Stratification

Stored water ages. The longer it sits, the more disinfectant residual decays and the more likely nitrification is in chloraminated systems (ammonia-oxidizing bacteria convert ammonia to nitrite and nitrate). Two problems show up:

• Stratification — water layers by temperature, so the tank holds old water while only the top (or bottom) exchanges. Baffles and proper inlet/outlet placement force water to move through the whole volume.
• Short-circuiting — inlet and outlet too close together, so fresh water zips in and out without mixing.

Operators aim for turnover — a meaningful fraction of the volume exchanged each day. Stagnant tanks are drained, flushed, or taken offline. Mixing systems (mechanical mixers or pulse mixers) help maintain turnover in large tanks.

Vents, Overflow, and Access

Every tank needs vents sized so that filling and draining don't build vacuum or pressure that could collapse or over-pressurize the tank. Vents are screened and sometimes hooded to keep birds and insects out. The overflow pipe discharges above grade with a screen and an air gap so contaminated surface water cannot back up into the tank. Access hatches are locked, gasketed, and curbed so rainwater doesn't enter. Cathodic protection protects buried steel components from corrosion.

Altitude Valves

An altitude valve is installed on the inlet of an elevated or ground tank. It opens when system pressure drops (the tank is below full) and closes when the tank reaches the set level, so the tank does not overfill. A single-direction altitude valve lets flow in only; a two-way altitude valve also lets the tank feed back out into the system during peaks. Failure of an altitude valve shows up as an overflowing tank or a tank that will not fill.

Maintenance and Inspection

AWWA tank standards (such as AWWA D100 for welded steel and D110 for prestressed concrete) cover construction; AWWA C652 covers disinfection after construction or cleaning. Typical maintenance:

• Periodic washout or cleaning to remove sediment.
• Interior and exterior inspection every 3–5 years (often by a certified tank inspector).
• Coating repair; lead-paint abatement on older tanks.
• Cathodic-protection check.
• Overflow and vent verification.
• Disinfection and bacteriological sampling before return to service.