FM Global DS 2-89: Sprinkler Pipe Friction Loss Tables (Hazen-Williams)

Friction loss is the foundation of every sprinkler hydraulic calculation. FM Global DS 2-89 (Pipe Friction Loss Tables, May 1973, Revised April 2026, 127 pages) is a set of tables prepared with the Hazen-Williams formula, giving friction loss in psi per foot: Table 1 steel pipe at C=100, Table 2 steel pipe at C=120, Tables 3–8 cement-lined cast iron at C=140, Tables 9–11 unlined cast iron at C=100 and Tables 12–13 asbestos cement at C=140. Tables 14–15 give multipliers for other C values, Table 16 equivalent lengths of fittings, Table 19 losses through FM Approved alarm check valves and Table 20 multipliers for plastic underground pipe. Steel wall thickness is Schedule 40 for 3/4 to 6 in. and Schedule 30 for 8 and 12 in. (p. 2).

The Hazen-Williams Formula (DS 3-0 Equation 15)

DS 2-89 does not state the formula in its text; it appears in DS 3-0 (Hydraulics of Fire Protection Systems, March 2010, pp. 25–26):

p = 4.52 × Q^1.85 / (C^1.85 × d^4.87): psi/ft; Q in gpm, d internal diameter in inches

p = 6.05 × 10^5 × Q^1.85 / (C^1.85 × d^4.87): bar/m; Q in L/min, d internal diameter in mm

• Q: flow rate
• C: the dimensionless Hazen-Williams roughness coefficient
• d: actual internal diameter

Total loss is PF = p × L (Equation 16), where L includes fitting equivalent lengths corrected to the C value of the pipe. Note the powers: diameter has a far stronger effect than flow, so increasing pipe size is the most effective way to reduce friction loss.

Which C Value?

DS 3-0 notes that DS 2-89 Table 1 (C=100) can be used for dry-type systems and Table 2 (C=120) for wet-type systems. Current defaults are in DS 2-0 Table 2.2.1.3.3 (October 2021, Interim Revision April 2026), applying unless the occupancy-specific data sheet says otherwise:

Pipe and system	C	Source
Wet system, black steel	120	DS 2-0 Table 2.2.1.3.3; DS 2-89 Table 2
Dry / preaction, black steel	100 (120 with nitrogen)	DS 2-0 Table 2.2.1.3.3; DS 2-89 Table 1
Dry / preaction, internally galvanized	120	DS 2-0 Table 2.2.1.3.3
Polymer enhanced steel	140	DS 2-0 Table 2.2.1.3.3
Plastic (wet system)	150	DS 2-0 Table 2.2.1.3.3
Cement-lined cast iron (underground)	140	DS 2-89 Tables 3–8
Unlined cast iron	100	DS 2-89 Tables 9–11
Asbestos cement	140	DS 2-89 Tables 12–13

In other words, FM uses C=120 by default for wet steel systems; C=100 is specific to dry and preaction black steel. NFPA 13-2025 Table 28.2.4.8.1 draws the same distinction (wet black steel 120, dry 100).

Reading the Tables: DN100 Steel Pipe, C=100

The internal diameter of 4 in. Schedule 40 pipe is 4.026 in. (102.3 mm; DS 2-89 Table 18). Values from the formula agree with DS 2-89 Table 1; for example, Table 1 (p. 8) gives 0.039 psi/ft at 300 gpm, about 0.88 bar per 100 m.

Flow (L/min)	Friction (bar/100 m), C=100	Friction (bar/100 m), C=120
500	0.19	0.14
1000	0.70	0.50
2000	2.52	1.80
3000	5.34	3.81
5000	13.7	9.80

The C=120 column is the C=100 value multiplied by 0.714 from DS 2-89 Table 14. For 3000 L/min through 50 m of DN100 steel at C=100: 5.34 × 0.5 = 2.67 bar.

For thin-wall pipe, DS 2-89 gives conversion factors (p. 2): 0.072 psi/ft for 500 gpm in 4 in. Schedule 40 at C=120 becomes 0.055 psi/ft in 4 in. thin-wall pipe with the 0.760 factor.

Equivalent Lengths for Fittings (DS 2-89 Table 16)

Table 16 gives equivalent lengths in feet for C=120. For 4 in. (DN100):

• Standard screwed elbow: 10 ft (3.0 m)
• 45° elbow: 4 ft (1.2 m)
• 90° flanged or medium-sweep elbow: 8 ft (2.4 m)
• 90° long-radius flanged elbow: 6 ft (1.8 m)
• Standard tee, flow turned 90°: 20 ft (6.1 m)
• Gate valve, fully open: 2 ft (0.6 m)
• Swing check, alarm check and dry pipe valve: 33 ft (10.1 m)
• Butterfly valve, fully open: 12 ft (3.7 m)

Friction loss for the straight run through a tee is ignored (Table 16 Note 1). For C values other than 120, correct the equivalent lengths with the Table 15 multipliers; for C=100 the factor is 0.714 (DS 3-0 Equation 17: × (C/120)^1.85). Total loss = (actual length + corrected equivalent length) × unit friction loss.

FM and NFPA 13 Compared

• Formula: both use Hazen-Williams
• 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)
• Units: the DS 2-89 tables are in US units only (gpm, psi/ft); metric calculations use the metric form of the DS 3-0 formula
• Underground: both use C=140 for cement-lined cast iron

Quick Checklist

• C value chosen from DS 2-0 Table 2.2.1.3.3 for system type and pipe material
• 120 for wet black steel, 100 for dry black steel (120 with nitrogen)
• Cement-lined underground at C=140
• Nonmetallic pipe only under DS 2-0 Section 2.4.1.3
• Equivalent lengths from Table 16, corrected for the C value
• Friction, elevation and fitting losses accumulated to the most remote sprinkler

Frequently Asked Questions

What C coefficient should I use for a steel sprinkler system?

Under FM DS 2-0 Table 2.2.1.3.3, unless the occupancy-specific data sheet says otherwise, use C=120 for black steel in wet systems and C=100 for black steel in dry and preaction systems (120 where nitrogen is used). DS 3-0 likewise points to the C=100 table of DS 2-89 for dry systems and the C=120 table for wet systems, and NFPA 13-2025 Table 28.2.4.8.1 gives the same values.

Why does a lower C factor produce a higher calculated demand?

C represents smoothness, so a lower value means more friction loss per metre. For the same flow, friction loss at C=100 is 1/0.714, about 1.4 times, the loss at C=120 (DS 2-89 Table 14), so a dry system at C=100 needs a higher pressure at the base of the riser than a wet system of the same layout.

How much difference do fittings make?

More than people expect. Take 50 m of DN100 steel at C=100 with eight standard elbows: the Table 16 value of 10 ft (3.05 m) corrected by 0.714 gives about 17.4 m of equivalent length. At 3000 L/min the total loss is 67.4 m × 5.34 bar/100 m, about 3.60 bar; leaving out the elbows gives 2.67 bar, an error of about 26%.

Why does pipe diameter matter so much more than length?

Because friction loss varies with diameter to the power 4.87 in the Hazen-Williams formula (DS 3-0 Equation 15), while it varies with length linearly. Going up one pipe size reduces friction loss dramatically, which is usually far more effective than shortening a run, and it is the first lever to reach for when a calculation fails.

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