Hydraulic calculations are a practice within the fire safety industry of determining the flow of liquids through a medium (usually a piping network) to ensure that fires can be adequately controlled.
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Purpose
Hydraulic calculations are often required to prove the flow of water (or water mixed with additives like firefighting foam concentrate) through piping networks for the purpose of suppressing or extinguishing a fire. The full hydraulic calculation procedure is defined in the applicable reference model codes such as published by NFPA (National Fire Protection Association), or EN 12845 Fixed firefighting system - Automatic sprinkler systems - Design, installation and maintenance.
Hydraulic calculations provide a verifiable analysis of the 3 primary components of a fire suppression system:
1) The water delivery requirements to suppress a possible fire
2) The available water supply
3) The network of piping that will deliver this water in the event of a fire.
Water Delivery Requirements
Requirements for the density of water discharge are usually specified by an applicable model code such as NFPA 13, NFPA 15, EN 12845, BS 9251, NFPA 750 CP 52, ASIB, and AS2118.1. Other possible international and insurance underwriter fire design standards may apply to projects.
The probable intensity and extent of a possible fire inside the building is determined by factors including the building use, the building height, the items expected to be stored or processed, and the arrangement in which these items are stored. These variables are compared to tables and values expressed in the model codes. In turn, these tables and values in the model codes are based primarily on decades worth of fire tests but may also be based on fire growth modeling.
Available Water Supply
The water available is often determined by means of a water flow test (opening a fire hydrant and recording the water pressures and gallons flow per minute). Some municipal water jurisdictions may provide their own estimate of available water supplies.
In locations where a municipal connection is not possible or practical, the needed piping network may draw water from an open (lake, pond, river) or closed (underground, aboveground, elevated tank) water source.
Where the water supply is drawn from a static supply like an underground tank or a pond, hydraulic calculations will also determine what pressure must be added to deliver the water. This pressure is typically applied by means of a fire pump or pressurized water storage tank.
System Piping Network
Suppression system piping networks are usually arranged in one of 3 configurations: Tree, Loop, or Grid. Tree systems may be imagined as a network of piping starting with a larger trunk and branching out to progressively smaller pipes with fire sprinklers or other devices. Looped systems may have a larger pipe that runs throughout a building and ties back to itself near the start, with smaller branches reaching out from this 'loop'. A gridded system may be imagined as similar to the lines on a gridiron football field, where the sidelines would be 2 parallel larger 'main' pipes and the yardage lines would be smaller 'branch' lines connecting the two sidelines. Gridded systems provide multiple paths for the water to travel to any point in the system. This arrangement can be very effective at reducing friction pressure losses in the system.
Most design standards require application of the Hazen-Williams method for determining frictional pressure losses through the piping network as water passes through it. Tree and Loop systems are simple enough that the hydraulic calculations could be performed by hand. Because hydraulic calculations for gridded systems require an iterative process to balance the water flow through all possible water paths, these calculations are most often performed by computer software. Today most calculations on all types of piping networks are performed by computer software. The sizes of network components can be more readily modified and recalculated on a computer than through a manual process.
The 2013 NFPA 13 Handbook includes a supplement that describes some of the application theory and processes applied when performing hydraulic calculations.