Hydraulic calculation is the heart of sprinkler design: it shows whether the water supply, town main plus pump, can meet the system demand. FM Global DS 3-0 covers the method, the use of the K-factor, the friction, static and velocity components, and how the demand point is compared against the supply curve.
The Four Governing Relationships
- Hazen-Williams: friction loss as a function of flow, roughness coefficient and internal diameter
- Sprinkler discharge: flow from a sprinkler as a function of its K-factor and head pressure
- Static pressure: roughly 0.098 bar per metre of elevation
- Velocity pressure: generally neglected below about 6 m/s
K-Factor and Design Density
Sprinkler flow follows from the K-factor and the inlet pressure. To achieve the design density, the sprinkler coverage area multiplied by the density must equal the sprinkler flow.
Worked example: at HC-2 with 12 m² coverage and 6.1 mm/min density, the required sprinkler flow is 73.2 L/min. A K80 sprinkler needs roughly 0.84 bar to deliver that; a K115 sprinkler needs roughly 0.40 bar.
- K80: standard spray, light hazard
- K115: standard spray, ordinary hazard
- K200 and K240: CMSA storage
- K360 and K480: ESFR storage
Building the Demand Point
- Select the hydraulically most demanding sprinkler group
- List the sprinklers within the area of operation
- Calculate flow and pressure at each sprinkler, starting from the most remote
- Add friction and static head between sprinklers along the branch line
- Accumulate along the cross main to the branch connection
- Work down the riser, adding static loss
- Add friction and valve losses in the system riser
- The result is the demand point: total flow at the required pressure
Demand Point Against Supply Curve
The town main or pump curve is plotted, and the demand point must fall below it — the supply must be able to meet the demand pressure. The gap between them is the safety factor, with a minimum of roughly 0.3 to 0.5 bar recommended.
- Town main curve: derived from a hydrant flow test, giving static pressure and several flowing points
- Pump curve: the manufacturer curve, checked at churn, 100% and 150% of rated flow
- Pump plus town supply: the pump inlet pressure equals the town flowing pressure at that flow
- Twin pumps in parallel: flows add while pressure stays the same
FM and NFPA 13 Compared
- Density and area: NFPA uses a density-area curve where area rises as density falls; FM uses fixed tables by hazard class
- C factor: NFPA permits 120 for new systems, FM assumes 100
- Velocity pressure: considered in some NFPA cases, generally neglected by FM below about 6 m/s
- K-factors: the same range in both
Quick Checklist
- Friction, K-factor, static head and velocity all accounted for
- C factor correct, with C = 100 preferred by FM
- K-factor appropriate to the hazard class
- Area of operation and coverage per sprinkler correct
- Safety factor of 0.3 to 0.5 bar below the supply curve
- Pump delivers 65% of rated pressure at 150% of rated flow
- Town main flow test carried out within the last twelve months
Frequently Asked Questions
What is the demand point and why does it matter?
The demand point is the flow and pressure the system requires at its reference point, usually the pump discharge or control valve. It is the single number that the water supply must satisfy, so it is what the entire hydraulic calculation exists to produce and what the supply curve is compared against.
Why does FM use fixed density and area tables rather than a curve?
NFPA offers a density-area curve, letting the designer trade a lower density against a larger area. FM fixes the combination by hazard class, which removes that discretion. The result is more consistency between projects and, in most cases, a more conservative design.
How current does the town main flow test need to be?
Within the last twelve months as a rule. Municipal supply pressure changes with network alterations, demand growth and seasonal patterns, so a test that is several years old may no longer represent what is available. Designing on stale test data is a common cause of systems that fail their acceptance test.
Why is velocity pressure usually neglected?
Below roughly 6 m/s its contribution is small compared with friction and static head, and including it complicates the calculation for little benefit. At higher velocities it becomes significant, but sprinkler systems are normally designed below that threshold anyway for noise and erosion reasons.
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Download MEP Calc on the App StoreFM Global Property Loss Prevention Data Sheet DS 3-0 (Hydraulics of Fire Protection Systems); NFPA 13 hydraulic calculation provisions.