FM Global DS 3-0: Fundamentals of Fire Protection Hydraulics

Hydraulic calculation is the heart of sprinkler design: it shows whether the water supply can meet the system demand. FM Global DS 3-0 (Hydraulics of Fire Protection Systems, March 2010, 65 pages) compares two things at one reference point, the base of the riser (BOR): the hydraulic demand of the sprinkler system and the available water supply (Section 1.0). It covers the hydraulic method, not the physical layout of systems.

The Governing Relationships

1. Sprinkler discharge: q = K × √p (Equation 7), with q in L/min, p in bar and K in L/min/bar^0.5; conversely p = (q/K)² (Equation 4).
2. Flow from density: q = D × S × L (Equation 3), with D in mm/min and S and L the sprinkler spacings in metres.
3. Friction (Hazen-Williams): p = 6.05 × 10^5 × Q^1.85 / (C^1.85 × d^4.87) bar/m, Q in L/min, d in mm (Equation 15); total loss PF = p × L (Equation 16).
4. Elevation: P_E = 0.098 bar/m × h (0.433 psi/ft; Equation 2).
5. Velocity pressure: not used in the FM method; total pressure is the pressure at the most remote sprinkler plus friction plus elevation (2.1.2.3.1).

For liquids other than water, such as antifreeze solutions, the Darcy-Weisbach method is used instead of Hazen-Williams (2.1.2.3.6).

K-Factor and Design Density

Sprinkler flow is coverage multiplied by density. For an HC-2 area (DS 3-26 Table 2.3.1.10: 8 mm/min over 230 m²) at 11.9 m² per sprinkler (3.5 m × 3.4 m), q = 8 × 11.9 = 95.2, about 95 L/min. A K80 sprinkler then needs (95/80)² = 1.41 bar and a K115 sprinkler (95/115)² = 0.68 bar, both above the DS 3-26 minimum of 0.5 bar at the most remote sprinkler (2.3.1.11).

• FM spacing tables for nonstorage pendent and upright sprinklers cover K80 and larger (DS 2-0 Tables 2.5.2.3.1.1(a)–(c)).
• FM Approved storage sprinklers range from K160 to K480 (DS 2-0 Table 2.5.4.1.1); the K-factor to use comes from the DS 8-9 tables.
• DS 2-0 Table 2.5.1.1.2 sets minimum design pressures above 0.5 bar for some sprinklers, for example 1.4 bar for K28.0 (K400) quick-response pendent storage sprinklers.

Building the Demand Point

1. Take the design format from the occupancy data sheet: density/demand area or number of sprinklers/pressure (2.1.2.2.1).
2. Calculate the number of sprinklers in the design area and per branch line, with a shape factor of 1.2 under flat ceilings or 1.4 above 5° (Equations 10–11).
3. Locate the most hydraulically remote sprinkler and position the design area from it (2.1.2.2.4–2.1.2.2.5).
4. Work from the most remote sprinkler, calculating each sprinkler’s flow and pressure with the friction and elevation between them, and balance flows at junctions (Equation 19).
5. Add friction and elevation along the cross main and riser to reach the flow and pressure at the BOR: the demand point.

For the details and a numerical example see the DS 3-0 most demanding area article.

Supply Curve Against Demand Point

• Test: read static pressure, open the flowing devices, read residual and Pitot pressures once stable, and convert Pitot pressure to flow with Q = a × c × d² × √P_V (a = 0.666 with P in bar and d in mm; Equation 1). Avoid reducing public main pressure below 1.38 bar (2.1.1.3, 2.1.1.5).
• Graph: a simple public supply is plotted as static pressure and residual pressure at the measured flow; a fire pump with at least churn, 100% and 150% of rated flow, on N^1.85 paper (2.1.1.6.1).
• Relocation: the test result is moved from the Effective Point to the BOR by deducting friction and elevation (2.1.1.6.2). Unexpectedly low results call for a hydraulic gradient test to find partly closed valves, tuberculation or obstructions (2.1.1.7).
• Rating: hose demand is deducted from the supply curve (2.1.2.4.4). If the remaining supply meets the demand for the required duration the system is Adequate, otherwise Inadequate (2.1.2.4.5). Duration = water volume / (sprinkler flow + deductions) (Equation 30).
• Pumps: FM fire pumps deliver at least 65% of rated pressure at 150% of rated flow (DS 3-7 Section 2.4).
• Currency: flow test data for an existing supply no more than 12 months old (DS 2-0 2.6.4.6).

FM and NFPA 13 Compared

• Density and area: for nonstorage occupancies FM uses a single value per hazard category and ceiling height from DS 3-26 Table 2.3.1.10; since its 2022 edition NFPA 13 uses single-point density/area for new systems, keeping the curves only for existing systems.
• C value: both use 120 for wet black steel and 100 for dry black steel (DS 2-0 Table 2.2.1.3.3; NFPA 13-2025 Table 28.2.4.8.1).
• Velocity pressure: not used in the FM method (DS 3-0 2.1.2.3.1).
• Formulas: Hazen-Williams and q = K√p in both.

Quick Checklist

• Design taken from the occupancy data sheet (DS 3-26, DS 8-9)
• C value from DS 2-0 Table 2.2.1.3.3
• Minimum pressure at the most remote sprinkler met (0.5 bar under DS 3-26; DS 2-0 Table 2.5.1.1.2)
• Shape factor and design area placement per DS 3-0
• Hose demand deducted from the supply curve and duration checked
• Pump delivers at least 65% of rated pressure at 150% of rated flow
• Flow test data less than 12 months old

Frequently Asked Questions

What is the demand point and why does it matter?

It is the flow and pressure the sprinkler system requires at its reference point, the base of the riser in FM DS 3-0. It is what the whole calculation produces and what is compared against the water supply curve on N^1.85 paper; the system is rated Adequate only if the supply, after deductions such as hose demand, meets it for the required duration (2.1.2.4.5).

How does FM set density and area?

For nonstorage occupancies, DS 3-26 Table 2.3.1.10 gives one density and demand area per hazard category and ceiling height, for example 8 mm/min over 230 m² for HC-2 wet systems under 9 m. NFPA 13 has used single-point density/area for new systems since its 2022 edition, keeping the density/area curves for existing systems only.

How current does the water supply flow test need to be?

FM DS 2-0 2.6.4.6 calls for flow test data no more than 12 months old for an existing supply. DS 3-0 2.1.1.4 adds that the test should be planned in advance, taking account of day and night or summer and winter differences in public system operation, and 2.1.1.3 advises against reducing public main pressure below 1.38 bar.

Is velocity pressure included?

Not in the FM method. DS 3-0 2.1.2.3.1 notes that some methods use normal pressure, but FM uses a calculation method that does not include velocity pressure; total pressure is the most remote sprinkler pressure plus friction loss plus elevation pressure.

SprinkCalc — Fire Sprinkler Design Across Three Standards

SprinkCalc covers hazard classification, design density and area, K-factor selection, water demand and hydraulic calculations for NFPA 13, FM Global and BS EN 12845 in a single iOS app, and exports a professional PDF report.

Download SprinkCalc on the App Store

MEP Calc — 110+ Engineering Calculators

MEP Calc bundles 110+ engineering modules in one iOS app: 21 fire calculations plus heating, cooling, HVAC, plumbing, steam and natural gas.

Download MEP Calc on the App Store