Hydraulic calculation

Hydraulic calculation is the fire safety practice of calculating the flow of liquids through a medium (usually a piping network) to ensure that fires could be extinguished.

Purpose

Hydraulic calculations are required to prove the flow of water (or water mixed with chemical additive) through piping networks for the purpose of controlling or extinguishing a fire. The hydraulic calculation procedure is defined in the reference model codes as published by NFPA (National Fire Protection Association),^[1] EN 12845 Fixed firefighting system - Automatic sprinkler systems - Design, installation and maintenance ^[2] and other international fire design standards.

The calculations prove that the water available (usually from a city water main, elevated storage tank, or fire pump) is strong enough (has enough pressure), and plentiful enough (has enough gallons / volume) to fight the worst case fire expected in a given building (based on the building use, the building height, and the items expected to be legally stored).

Basis of calculation

Calculations are based on the worst expected fire, located in the geometrically farthest point from the water source (based on the path the extinguishing water is required to travel to get to the fire).

Analysis of the worst expected fire is based on the use of the building and areas. The hazard rating of various areas is defined by National Fire Protection Association (NFPA) Codes. Areas include:

• Light Hazard (offices, toilets, and similar areas of light combustibles and light fuel loading)
• Ordinary Hazard (car parking, stores, restaurants)
• Extra Hazard (flammable chemical use, heavy manufacturing, plastics)
• Storage (flammable items stored in solid piles, on shelves, or on racks to a significant height).

The analysis of hazard gives a design density required to control a fire, which has been derived from years of fire tests conducted by insurance companies and other testing agencies. The design density is described by two variables that must work together to achieve fire control:

Water flowfrom the sprinkler head (how heavy the rainfall of water from open fire sprinklers)

Total area (the expected size of the fire before it will not continue to grow)

The shortened expression of a common design density for a Light Hazard office is .1/1500, which is fully expressed as,
" 0.1 GPM per square foot is required to fall from the fire sprinklers onto the fire over the most remote 1,500 square feet (140 m²) of area, which is the maximum expected size of a fire in this Light Hazard building area."
A common density required for a warehouse type "big box" store that has higher flammability items stored on racks to twenty feet high is .6/2000. Note that the density of water to fall per square foot is six times heavier than an office, and the expected fire size is larger.

Storage warehouses commonly use a newer technology type fire sprinkler, ESFR (early suppression fast response), which have discharge requirements not based on design densities, and which are designed to extinguish a fire before the arrival of the fire department.

The water available is verified by means of a water flow test (opening a fire hydrant and recording the water pressures and gallons flow per minute).

References

External links

• National Fire Protection Association
• Canute LLP - Fire Sprinkler Hydraulic Calculation Software
• M.E.P.CAD, Inc. - Fire Sprinkler Hydraulic Calculation Software