A sprinkler hydraulic calculation is carried out for the hardest point in the system: the group of sprinklers that is furthest away, highest, and least well supplied. FM Global DS 3-0 sets out how to identify that hydraulically most demanding area.
What the Most Demanding Area Is
- The area of operation that produces the highest system demand
- Usually the most remote, the highest and the least well fed
- Its extent is the design density multiplied by the area of operation
- If the required pressure is achieved there, it is achieved everywhere else
Step by Step
- Identify the most remote sprinkler, geometrically or by least available supply
- List every sprinkler within the area of operation
- Establish flow and pressure at each sprinkler from its K-factor
- Accumulate sprinkler flows, friction and static head along the branch line
- Continue along the cross main to the branch connection point
- Work down the riser to ground level
- Add losses through the system riser, control valves and check valve
- The result is the demand point: total flow at the required pressure
Worked Example
For an HC-2 office at 6.1 mm/min over 232 m² with K115 sprinklers:
- Sprinkler coverage of 12 m² gives a flow of roughly 73 L/min
- The corresponding head pressure follows from the K-factor
- The area of operation contains roughly 22 sprinklers
- Total sprinkler flow is therefore around 1,611 L/min
- Friction, static head and valve losses are then added
- The demand point lands in the region of 7 to 9 bar at that flow
Selection Criteria
- Geometrically most remote, by pipe length
- Highest, by static loss
- Least well fed, by branch friction
- Two or three candidate areas are calculated and the hardest is adopted
Comparing Against the Water Supply
- The hydrant flow test curve gives the static pressure and one or two flowing points
- The demand point must fall below that curve
- A safety margin of at least 0.3 to 0.5 bar is retained
- The pump is acceptable if it still delivers 65% of rated pressure at 150% of rated flow
Quick Checklist
- Most demanding area identified, with alternatives tested
- All sprinklers in the area of operation listed
- Friction, K-factor, static head and valve losses all included
- Demand point established as a flow and pressure pair
- Margin of 0.3 to 0.5 bar below the supply curve confirmed
Frequently Asked Questions
What makes an area hydraulically most demanding?
Three factors combine: distance from the supply, which drives friction loss; elevation, which drives static head; and how well the branch is fed. The hardest area is not always the furthest one, so two or three candidates should be calculated and the worst adopted rather than assumed.
Why must the demand point sit below the supply curve with a margin?
Because both sides of the comparison carry uncertainty: the flow test represents one moment in the life of the town main, and pipe roughness increases over time. A margin of 0.3 to 0.5 bar absorbs that drift so the system still works years after commissioning.
What is included in the demand point besides sprinkler flow?
Friction loss in branch lines, cross mains and the riser, static head from elevation, losses through the system riser, control valves and check valve, and the hose stream allowance. Omitting valve losses and the hose allowance are the two most common reasons a calculation understates the true demand.
How does the FM approach compare with NFPA 13?
The underlying hydraulics are the same, with Hazen-Williams friction loss and the sprinkler orifice relationship. The differences lie in the hazard classification, the density and area tables and the hose allowances, so the calculation method transfers but the input values must come consistently from one framework.
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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 plans and calculations chapter; EN 12845 hydraulic calculation provisions.