8.3 Water Storage Tanks: Gravity, Suction & Pressure Tanks

8.3 Water Storage Tanks: Gravity, Suction & Pressure Tanks

When municipal water supplies lack sufficient volume, flow rate, or pressure to meet building fire protection demands, dedicated water storage tanks are installed. Governed by NFPA 22 (Standard for Water Tanks for Private Fire Protection) for construction and NFPA 25 (Standard for the Inspection, Testing, and Maintenance of Water-Based Fire Protection Systems, Chapter 9) for maintenance, these vessels supply water to automatic sprinkler systems, standpipes, and fire pumps.

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Tank Types & Operational Principles

Water storage tanks are categorized into three primary structural and operational configurations based on how pressure and energy are imparted to the water.

 GRAVITY TANK                      SUCTION TANK                     PRESSURE TANK
 +-----------------------+        +-----------------------+        +-----------------------+
 | Elevated Structural   |        | Ground / Below Grade  |        | Sealed Hydro-Pneumatic|
 | Tower (Head Pressure) |        | Dedicated Fire Reserve|        | 2/3 Water + 1/3 Air   |
 | P = 0.433 psi/ft      |        | Anti-Vortex Suction   |        | 75+ psi Compressed Air|
 | Natural Elevation     |        | Feeds Fire Pump       |        | Instant High Pressure |
 +-----------------------+        +-----------------------+        +-----------------------+

1. Gravity Tanks

Elevated steel, wooden, or concrete tanks supported by structural towers, building roofs, or elevated terrain.

• Operating Principle: Supplies pressure via static elevation head. Water exerts 0.433 psi per foot of vertical height. $\text{Static Pressure (psi)} = \text{Height (ft)} \times 0.433 \text{ psi/ft}$ Example: A gravity tank bottom elevated 120 feet above grade provides a static pressure of $120 \times 0.433 = 51.96 \text{ psi}$ at ground level.
• Application: Used in high-density urban areas or industrial facilities to provide reliable gravity-fed supply independent of electrical power.

2. Suction Tanks

Large flat-bottom ground-level or sub-surface storage reservoirs constructed from bolted steel, welded steel, reinforced concrete, or fiberglass.

• Operating Principle: Provides a large volume of water (e.g., 50,000 to 500,000+ gallons) at zero or minimal head pressure. Requires a dedicated fire pump to draft water from the tank and boost pressure into the fire system.
• Application: High-hazard warehouses, rural facilities, and high-rise buildings lacking municipal high-volume mains.

3. Pressure Tanks

Sealed cylindrical steel pressure vessels storing water under compressed air.

• Operating Principle: Contains a combination of water and compressed air. When a fire sprinkler opens, compressed air expands and forces water out of the tank at high pressure without relying on pumps.
• Application: Provides immediate initial water supply for high-hazard areas, deluge systems, or small isolated hazard zones.

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Pressure Tank Pneumatic Physics & Calculations

NFPA 22 establishes strict volume and pressure relationships for fire protection pressure tanks to guarantee that water is completely discharged before air enters system piping.

Standard Volume Ratio

Unless designed for specialized high-pressure conditions, pressure tanks must be maintained at:

• 2/3 Water Volume
• 1/3 Compressed Air Volume

 +-----------------------------------+
 |    1/3 COMPRESSED AIR SPACE       |  <-- Air Pressure (Pa)
 |===================================|  <-- High/Low Water Line
 |                                   |
 |          2/3 WATER VOLUME         |  <-- Water Supply Reserve
 |                                   |
 +-----------------------------------+

Air Pressure Calculation Formula

As water is discharged from the tank into opening sprinklers, the compressed air expands. Based on Boyle's Law ($P_1 V_1 = P_2 V_2$), the initial operating air pressure ($P_a$) must be high enough so that when the final drop of water leaves the tank, the residual air pressure matches or exceeds the minimum pressure ($P_r$) required at the highest, hydraulically most remote sprinkler.

$P_a = \frac{P_r + 14.7}{1 - V_w} - 14.7$

Where:

• $P_a$ = Initial gauge air pressure in tank (psi)
• $P_r$ = Minimum required residual pressure at highest sprinkler + head loss to tank base (psi)
• $V_w$ = Fraction of total tank volume occupied by water (standard = $2/3 = 0.667$)
• $14.7$ = Atmospheric pressure (psi)

Example: If the remote sprinkler requires 30 psi residual pressure ($P_r = 30$) and the tank maintains a 2/3 water fraction ($V_w = 0.667$):

$P_a = \frac{30 + 14.7}{1 - 0.667} - 14.7 = \frac{44.7}{0.333} - 14.7 = 134.23 - 14.7 = 119.53 \text{ psi}$

The baseline minimum air pressure for standard pressure tanks is 75 psi.

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Critical Tank Appurtenances & Controls

Tank appurtenances maintain water supply integrity and prevent system failure during draft.

1. Anti-Vortex Plate: A heavy steel plate (minimum 24 in. $\times$ 24 in. or twice the suction pipe diameter) positioned horizontally 6 inches above the tank suction outlet. It breaks up fluid swirl, preventing air vortices from forming and introducing air into fire pump suctions during high GPM pumping.
2. Automatic Fill Valves: Sized to completely refill the fire storage tank within 8 hours per NFPA 22 requirements.
3. Overflow Piping: Sized equal to or larger than the fill line capacity, discharging to a drain location visible to facility personnel.
4. Water Level Supervisory Switches: High and low water level sensors that signal a supervisory alarm when water strays 3 inches or more from normal capacity.
5. Air Pressure Switches: On pressure tanks, signals high/low supervisory alarms at a 10 psi variance from normal air pressure.

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Freeze Protection & Heating Requirements

In cold climates, water tanks subject to freezing must be equipped with heating systems to prevent ice formation. Ice accumulation creates severe hazards: falling ice can collapse tank legs, and interior ice layers can seal suction intakes or damage internal piping.

Heating Criteria (NFPA 22)

• Minimum Water Temperature: Tank water temperature must be maintained at or above 42°F (5.6°C) at all times.
• Low-Temperature Alarm: A supervisory alarm must activate if water temperature drops to 40°F (4.4°C).
• Heating Systems: Steam coils, circulating hot water heat exchangers, electric immersion heaters, or direct gas heaters.

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NFPA 25 Inspection & Testing Frequencies