Fire Protection Engineering

Fire Water Demand

Size fire water systems for equipment cooling and deluge protection with defensible wetted area, pump capacity, and storage calculations per NFPA 15 and API 2510.

Launch Calculator Pump sizing

Application density

0.25 gpm/ft²

NFPA 15 minimum for vessel and tank cooling.

Height limit

25 ft zone

Only surfaces within 25 ft of grade are fire-exposed.

Duration

4 hours min

NFPA minimum water supply for industrial facilities.

Contents

Use this when you need to:

  • Size fire water systems for a new facility.
  • Verify existing fire water capacity is adequate.
  • Select fire pumps and storage tanks.
  • Design deluge zones for equipment protection.

1. Fire Protection Philosophy

Fire water systems in gas plants and refineries serve as a critical layer of defense against hydrocarbon fires. The primary objectives are to cool equipment exposed to fire, prevent escalation to adjacent units, and provide manual firefighting capability.

Protection Objectives

Equipment cooling

Prevent failure

Water spray keeps steel below critical temperature (~1,000°F) to prevent loss of containment.

Exposure protection

Limit escalation

Cool adjacent equipment not yet involved in fire to prevent cascading failures.

Hose streams

Manual response

Provide water for firefighters to apply manually where fixed systems do not reach.

Fire control

Area containment

Contain fire to single process area and prevent plant-wide propagation.

Design Basis Fire Scenario

The fire water system is designed for the worst-case single fire event, not all areas simultaneously. Key assumptions include:

  • Fire type: Pool fire from spilled hydrocarbons (most common in process facilities)
  • Fire zone: Largest single fire area or scenario as defined by facility layout and hazard analysis
  • Simultaneous demand: All equipment within the fire zone plus hose streams for adjacent areas
  • Duration: Minimum 4 hours of continuous water supply per NFPA
Important: The fire water system is not designed to extinguish a hydrocarbon fire. Its purpose is to cool exposed equipment and prevent escalation while process isolation and depressuring take effect.

2. NFPA Standards & Requirements

Multiple NFPA and API standards govern fire water system design for process facilities. Understanding the applicable standard for each equipment type is essential for defensible design.

Governing Standards

Standard Scope Key Requirements
NFPA 15 Water Spray Fixed Systems Application densities, system design, nozzle selection
NFPA 30 Flammable Liquids Code Storage tank protection, spacing, containment
NFPA 20 Stationary Fire Pumps Pump selection, testing, reliability requirements
API 2510 LPG Installations LPG sphere/vessel protection, water spray rates
API 2030 Water Spray Systems Application of water spray for fire protection
GPSA Ch 18 Safety & Relief Gas plant fire protection guidelines

NFPA 15 Application Densities

Application Density (gpm/ft²) Reference
Vessel/tank cooling 0.25 NFPA 15 Table 5.3.2.4
Exposure protection 0.25 NFPA 15 Table 5.3.2.4
Structural steel protection 0.10 NFPA 15 Table 5.3.2.4
Electrical transformers 0.25 NFPA 15 Table 5.3.2.4
Cable trays 0.30 NFPA 15 Table 5.3.2.4
Conveyor belts 0.25 NFPA 15 Table 5.3.2.4

25-ft height limit: Per NFPA 15, only surfaces within 25 ft above grade level are considered exposed to fire and require water spray protection. Surfaces above this elevation are generally above the expected flame height of a pool fire and do not require cooling.

01

Identify fire zones. Divide the facility into fire zones based on process areas, equipment spacing, and drainage boundaries.

02

List protected equipment. Identify all equipment within each zone requiring water spray protection.

03

Calculate wetted areas. Compute the exposed surface area for each equipment item within the 25-ft zone.

04

Sum demands. Add all cooling demands within the worst-case fire zone plus hose stream allowance.

3. Wetted Area Calculations

The wetted area is the surface area of equipment exposed to fire that requires water spray cooling. The calculation method depends on equipment geometry and orientation.

Horizontal Vessel

Wetted area (horizontal cylinder): A_w = pi x D x L + 2 x (pi/4) x D^2 x 0.5 Where: A_w = Wetted area (ft^2) D = Vessel outside diameter (ft) L = Vessel length, tan-to-tan (ft) Shell area = pi x D x L Head area (2 half-heads) = pi x D^2 / 4 If D > 25 ft, reduce proportionally for height cutoff.

Vertical Vessel

Wetted area (vertical cylinder): A_w = pi x D x min(H, 25) + pi x D^2 / 4 Where: H = Vessel height (ft), capped at 25 ft above grade Bottom head included: pi x D^2 / 4

Atmospheric Storage Tank

Wetted area (atmospheric tank): A_w = pi x D x min(H, 25) Where: D = Tank diameter (ft) H = Tank shell height (ft), capped at 25 ft Shell area only - roof is not included.

Sphere

Wetted area (sphere): Standard method: A_w = 0.55 x pi x D^2 API 2510 method: A_w = 0.55 x pi x D^2 Where: D = Sphere diameter (ft) 55% accounts for area below and near equator exposed to pool fire radiation.

Pipe Rack

Wetted area (pipe rack): A_w = L x W x N_levels x C_f Where: L = Rack length (ft) W = Rack width (ft) N_levels = Number of pipe levels C_f = Coverage factor (typically 0.50)
Coverage factor: Pipe racks are not solid surfaces. The coverage factor (typically 0.50) accounts for the void space between pipes, conduit, and structural members. This factor should be adjusted based on actual pipe density.

Example: Horizontal Vessel

Given: 8 ft diameter x 40 ft long horizontal vessel, below 25 ft grade

Step 1: Shell area

Shell = pi x 8 x 40 = 1,005 ft^2

Step 2: Head area (2 half-heads)

Heads = 2 x (pi/4) x 8^2 x 0.5 = 50.3 ft^2

Step 3: Total wetted area

A_w = 1,005 + 50.3 = 1,055 ft^2

Step 4: Water demand

Q = 1,055 x 0.25 = 264 gpm per vessel

4. Deluge System Design

Deluge systems are open-head water spray systems that activate simultaneously when a fire is detected. They provide immediate, uniform water coverage over the protected equipment.

System Components

Deluge valve

System control

Normally closed valve that opens on fire signal, releasing water to all spray nozzles simultaneously.

Spray nozzles

Water distribution

Open-type nozzles sized for required density. Typical K-factors: 5.6, 8.0, 11.2, 14.0.

Detection

Fire sensing

Fusible links, heat detectors, or flame detectors trigger the deluge valve.

Piping

Distribution

Dry piping from deluge valve to nozzles. Sized for hydraulic balance and velocity limits.

Deluge Valve Sizing

Flow Range (gpm) Valve Size Typical Application
≤ 100 2" Small vessels, instrument air compressors
100 - 250 3" Medium vessels, heat exchangers
250 - 500 4" Large vessels, small tanks
500 - 1,000 6" Large tanks, multiple vessels
1,000 - 2,000 8" Spheres, large equipment groups
2,000 - 3,500 10" Large spheres, tank farms
> 3,500 12" or multiple Multiple zones required

Nozzle Selection

Spray nozzles are selected to provide uniform coverage at the required application density. Key considerations:

  • Spray angle: 120-180 degrees for full coverage
  • K-factor: Relates flow to pressure: Q = K x sqrt(P)
  • Spacing: Maximum 10 ft between nozzles for vessels; 12 ft for structural steel
  • Minimum pressure: 30-35 psi at most remote nozzle for adequate spray pattern
  • Orientation: Nozzles aimed at equipment surface, not at air space
Hydraulic design: Deluge systems are hydraulically calculated (not pipe schedule). The most remote nozzles must receive adequate pressure (typically 30 psi minimum) to maintain the required application density across the entire protected area.

5. Fire Pump Selection

Fire pumps must deliver the total fire water demand at adequate pressure to the most hydraulically remote point in the system. NFPA 20 governs pump selection, testing, and reliability requirements.

Pump Capacity

Fire pump capacity: Q_pump = Q_cooling + Q_hose + Q_margin Where: Q_cooling = Total deluge/spray demand (gpm) Q_hose = Hose stream allowance (gpm) Q_margin = Pump margin, typically 10% Pump horsepower: HP = Q x TDH / (3960 x eta) Where: Q = Flow rate (gpm) TDH = Total dynamic head (ft of water) eta = Pump efficiency (typically 0.70) 1 psi = 2.31 ft H2O

Total Dynamic Head (TDH)

TDH is the total pressure the pump must overcome, including:

Component Typical Range (psi) Notes
Nozzle pressure 30 - 75 Minimum 30 psi for adequate spray pattern
Friction loss (piping) 20 - 50 Depends on pipe size, length, and flow rate
Elevation head 10 - 50 Height from pump to highest nozzle
Fitting/valve losses 10 - 25 Valves, elbows, tees in distribution
Typical Total TDH 125 - 175 150 psi is common design basis

Pump Types for Fire Service

Electric motor driven

Primary pump

Centrifugal pump with electric motor. Requires reliable power supply or emergency generator.

Diesel engine driven

Backup pump

Independent of electrical supply. Auto-starts on pressure drop in fire main. NFPA 20 requires weekly test runs.

Jockey pump

Pressure maintenance

Small pump to maintain system pressure and compensate for minor leaks. Prevents unnecessary main pump starts.

Redundancy requirement: NFPA 20 and most facility standards require at least two fire pumps, each capable of delivering 100% of the design demand. Typically one electric-driven and one diesel-driven pump are provided for maximum reliability.

Example: Pump Sizing

Given: Total demand = 3,000 gpm, TDH = 150 psi

TDH in ft = 150 x 2.31 = 346.5 ft HP = 3,000 x 346.5 / (3,960 x 0.70) HP = 1,039,500 / 2,772 HP = 375 HP Select: 400 HP rated fire pump (next standard size above calculated)

6. Water Storage Design

Fire water storage must hold sufficient volume to sustain the design fire water demand for the required duration without replenishment.

Storage Volume Calculation

Required storage volume: V = Q_total x t x 60 Where: V = Storage volume (gallons) Q_total = Total fire water demand (gpm) t = Duration (hours) 60 = minutes per hour Conversions: 1 barrel = 42 gallons 1 ft^3 = 7.48052 gallons

Typical Storage Requirements

Facility Type Typical Demand (gpm) Duration (hrs) Storage (gallons)
Small compressor station 1,000 - 2,000 4 240,000 - 480,000
Gas processing plant 3,000 - 5,000 4 720,000 - 1,200,000
NGL fractionation plant 4,000 - 8,000 4 960,000 - 1,920,000
LPG storage facility 3,000 - 6,000 4 - 6 720,000 - 2,160,000
Refinery unit 5,000 - 15,000 4 - 8 1,200,000 - 7,200,000

Storage Tank Design Considerations

  • Tank type: Atmospheric storage (API 650), typically ground-level or elevated
  • Minimum volume: Dedicated fire water volume must not be shared with process water
  • Replenishment: Makeup water system to refill after fire event (not credited for initial supply)
  • Location: Outside process areas, protected from fire exposure, accessible to fire pumps
  • Freeze protection: In cold climates, heating or recirculation may be required
  • Water quality: Treated to prevent biological growth, corrosion, and nozzle plugging

Fire Water Ring Main

Fire water is typically distributed through a ring main (looped piping) around the facility. The ring main ensures water can reach any point from two directions, providing redundancy if a section is damaged. Ring main sizing is typically 10-12 inch diameter for gas plants, with 6-8 inch lateral branches to individual deluge zones.

References:
• NFPA 24 - Installation of Private Fire Service Mains
• API 2510 - Section 7.4 - Water Supply Distribution

References

  • NFPA 15 - Standard for Water Spray Fixed Systems for Fire Protection
  • NFPA 20 - Standard for the Installation of Stationary Pumps for Fire Protection
  • NFPA 22 - Standard for Water Tanks for Private Fire Protection
  • NFPA 24 - Standard for the Installation of Private Fire Service Mains
  • 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

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