Introduction
Fire water demand calculation is a critical aspect of fire protection engineering in process plants, oil & gas facilities, and polyolefin plants. A well-designed fire water system ensures adequate water availability for firefighting operations, minimizing fire-related risks. This article provides an in-depth analysis of fire water demand calculations with references to OISD (Oil Industry Safety Directorate), NFPA (National Fire Protection Association), API, IS, and other international standards.
1. Governing Standards for Fire Water Demand Calculation
Several international and national standards provide guidelines for fire water demand calculation. The key standards include:
1.1 Indian Standards
- OISD-116: Fire Protection Facilities for Petroleum Refineries and Oil/Gas Processing Plants
- OISD-117: Fire Protection Facilities for Petroleum Depots, Terminals, Pipeline Installations & Lube Oil Installations
- OISD-118: Layouts for Oil and Gas Installations
- IS 15105: Design and Installation of Fixed Automatic Sprinkler Fire Extinguishing System
- IS 15325: Design and Installation of Fixed Automatic High & Medium Velocity Water Spray System
1.2 NFPA Standards
- NFPA 13: Standard for the Installation of Sprinkler Systems
- NFPA 15: Standard for Water Spray Fixed Systems for Fire Protection
- NFPA 20: Standard for the Installation of Stationary Pumps for Fire Protection
- NFPA 24: Standard for the Installation of Private Fire Service Mains and Their Appurtenances
- NFPA 30: Flammable and Combustible Liquids Code
- NFPA 25: Standard for the Inspection, Testing, and Maintenance of Water-Based Fire Protection Systems
1.3 API and Other International Standards
- API 2030: Application of Fixed Water Spray Systems for Fire Protection in Petroleum Industry
- API 2001: Fire Protection in Refineries
- ISO 13702: Control and Mitigation of Fires and Explosions on Offshore Production Installations
- BS 5306: Fire Extinguishing Installations and Equipment on Premises
- OSHA 29 CFR 1910.159: Automatic Sprinkler Systems
2. Fire Water Demand Calculation Methodology
The fire water demand is calculated based on the worst-case fire scenario. The methodology includes:
2.1 Identifying Fire Hazard Scenarios
- Pool fires
- Jet fires
- Flash fires
- Fireball and BLEVE scenarios
2.2 Determining Fire Water Flow Rate
Fire water flow rate depends on:
- Type of Fire Protection System:
- Sprinklers
- Hydrants and monitors
- Fixed spray systems
- Design Fire Scenarios
- Cooling Water Requirement for Exposed Equipment
The flow rate is calculated using:
- NFPA 15 Formula: Q=0.25×D2×PQ = 0.25 \times D^2 \times P Where:
QQ = Water flow rate (gpm)
DD = Diameter of the tank (ft)
PP = Protection level factor - OISD-116 Guidelines:
- 10.210.2 LPM/m² for floating roof tanks
- 8.28.2 LPM/m² for cone roof tanks
- 12.212.2 LPM/m² for sphere tanks
2.3 Calculating Fire Water Storage Requirement
Storage volume should be sufficient for a minimum of 4 hours of continuous fire-fighting operations. The total fire water storage is:
Vstorage=Qhydrants+Qmonitors+Qfixed systemsV_{storage} = Q_{hydrants} + Q_{monitors} + Q_{fixed\, systems}
Where:
- QhydrantsQ_{hydrants} = Hydrant and monitor demand
- QmonitorsQ_{monitors} = Fixed monitor demand
- Qfixed systemsQ_{fixed\, systems} = Fixed spray/sprinkler system demand
2.4 Fire Water Pumping Capacity
NFPA 20 specifies minimum fire pump capacities based on hazard classification:
- Light Hazard: 500 gpm (1892 LPM)
- Ordinary Hazard: 1000 gpm (3785 LPM)
- High Hazard: 3000 gpm (11355 LPM)
OISD-117 specifies two diesel-driven and one electrically driven fire pump, each capable of handling the highest fire water demand scenario.
3. Fire Water Distribution System Design
3.1 Piping Network Design
- OISD-118: Underground fire water mains should have looped networks for reliability.
- Minimum Pipe Size:
- 200 mm (8 inches) for main header
- 150 mm (6 inches) for branch lines
- Hydraulic Calculation: Ensuring minimum residual pressure of 7 kg/cm² at the farthest hydrant.
3.2 Fire Water Monitor and Hydrant Spacing
- NFPA 15 & OISD-117:
- Hydrants at 30m intervals around process units
- Monitors at 60m intervals for tank farms
3.3 Water Spray and Sprinkler Design
- NFPA 13 & 15 specify density requirements:
- 4.1 LPM/m² for pump stations
- 6.1 LPM/m² for process areas
- 10.2 LPM/m² for tank cooling
3.4 Fire Water System Reliability
- Dual pumping sources (diesel and electric)
- Dedicated fire water storage tanks
- Automatic pressure maintenance systems
4. Case Study: Fire Water Demand Calculation for a Refinery
Assumptions:
- Largest fire scenario: Crude oil storage tank fire
- Tank Diameter: 30 m
- Fire water application rate (OISD-116): 8.2 LPM/m²
Calculation: Q=π×(30/2)2×8.2Q = \pi \times (30/2)^2 \times 8.2
Q=5770 LPMQ = 5770 \text{ LPM}
For 4-hour duration:
V=5770×60×4=1,385,000 litersV = 5770 \times 60 \times 4 = 1,385,000 \text{ liters}
Additional hydrants, monitors, and hose reel demands increase total fire water requirement to 2 million liters.
5. Conclusion
Fire water demand calculation is a vital aspect of fire safety in the process industry. Adherence to standards like OISD, NFPA, API, and IS ensures the availability of adequate fire protection resources.