What is fire water demand in a gas plant?

Fire water demand is the total flow rate (gpm) of water required to cool equipment and protect structures during a fire event. It includes equipment cooling water (applied via deluge or spray systems at 0.25 gpm/ft2 per NFPA 15), hose stream allowances (typically 500 gpm), and a pump margin (10%). Total demand drives fire pump sizing and water storage tank capacity.

How is wetted area calculated for fire water demand?

Wetted area depends on equipment type. For horizontal vessels, it is the cylindrical shell area plus half of both heads. For vertical vessels, it is the shell area up to 25 ft above grade per NFPA 15. For atmospheric tanks, it is the shell area (no roof). For spheres, approximately 55% of the total surface area is used. Only the area below 25-30 ft above grade is considered exposed to fire.

What application density is used for deluge systems?

Per NFPA 15 Table 5.3.2.4, the minimum application density for vessel and tank cooling is 0.25 gpm/ft2. Structural steel protection uses 0.10 gpm/ft2. These densities ensure adequate film coverage to prevent steel temperature from reaching failure limits during fire exposure.

How long should fire water supply last?

NFPA standards recommend a minimum 4-hour fire water supply duration for industrial facilities. Some jurisdictions or project specifications may require 6-8 hours. The total storage volume equals total demand (gpm) multiplied by duration (minutes). A typical midstream facility may need 500,000 to 2,000,000 gallons of fire water storage.

Fire Water Demand Calculator | Deluge System Sizing (NFPA 15)

Equipment Selection

Equipment Type

Number of Items to Protect Simultaneously

items

Equipment Dimensions

Diameter

Length / Height

Width

Number of Levels

levels

Application Parameters

Application Density

gpm/ft²

NFPA 15: Vessel/tank cooling = 0.25, Structural steel = 0.10

Wetted Area Method

System Parameters

Water Application Duration

hours

NFPA minimum: 4 hours for industrial facilities

Hose Stream Allowance

gpm

Firewater Pump Margin

Water Supply Temperature

°F

Understanding Fire Water Demand

What is Fire Water Demand?
Fire water demand is the total water flow rate required to cool equipment, protect structures, and provide hose streams during a fire event in a process facility. It drives the sizing of fire pumps, water storage tanks, and distribution piping.

Typical Application Densities:
Vessel/tank cooling: 0.25 gpm/ft²
Structural steel: 0.10 gpm/ft²
Exposure protection: 0.25 gpm/ft²
Transformer: 0.25 gpm/ft²

Key Design Standards:
NFPA 15 (Water Spray Fixed Systems), NFPA 30 (Flammable Liquids), API 2510 (LPG Storage), API 2030 (Application of Water Spray), GPSA Chapter 18 (Safety & Relief).

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Results

Total Fire Water Demand

gpm

Wetted Surface Area (per item) -

Cooling Demand (per item) -

Total Cooling Demand -

Hose Stream Allowance -

Subtotal (Cooling + Hose) -

Pump Margin Applied -

Water Storage Required -

Storage (barrels) -

Storage (cubic feet) -

Fire Pump Capacity -

Estimated Pump HP -

Application Density -

Suggested Deluge Valve Size -

Formula

Q_total = (A_w × d × N + Q_hose) × (1 + M/100)

Q_total = Total fire water demand (gpm)

A_w = Wetted surface area per item (ft²)

d = Application density (gpm/ft²)

N = Number of items protected simultaneously

Q_hose = Hose stream allowance (gpm)

M = Pump margin (%)

Standards & References

NFPA 15
Standard for Water Spray Fixed Systems for Fire Protection
NFPA 30
Flammable and Combustible Liquids Code
API 2510
Design and Construction of LPG Installations
API 2030
Application of Water Spray Systems for Fire Protection
GPSA Engineering Data Book
Chapter 18: Safety, Relief, and Environmental
NFPA 20
Standard for the Installation of Stationary Pumps for Fire Protection

Engineering Notes

25-ft Rule: Per NFPA 15, only surfaces within 25 ft of grade are considered fire-exposed
Duration: NFPA requires minimum 4-hour water supply for industrial facilities
Hose Streams: Typically 500 gpm minimum for manual firefighting supplementation
Pump TDH: Estimated at 150 psi for fire water systems (elevation + friction + nozzle pressure)
Simultaneous Demand: Size for worst-case fire scenario, not all equipment simultaneously
API 2510: LPG facilities may have additional requirements beyond NFPA 15

Quick Reference — Typical Demands

10 ft × 40 ft horiz. vessel → ~375 gpm cooling
60 ft sphere → ~1,550 gpm cooling
50 ft × 40 ft tank → ~1,570 gpm cooling
Small gas plant (total) → 2,000-4,000 gpm
Large gas plant (total) → 5,000-10,000 gpm

Fire Water Demand Calculator

Equipment Selection

Equipment Dimensions

Application Parameters

System Parameters

Understanding Fire Water Demand

📚 Learn the Theory

Results

Formula

Standards & References

Engineering Notes

Quick Reference — Typical Demands

Related Calculators

Fire Relief

Relief Valve Sizing

Blowdown

Pipe Volume