Friction loss is the foundation of every sprinkler hydraulic calculation. Given pipe diameter, flow, material and length, the pressure drop per metre follows from the Hazen-Williams relationship. FM Global DS 2-89 provides an extensive set of tables to make that calculation practical, covering steel, cast iron, copper and CPVC. This article explains how to read them, how to choose the C coefficient, and how the calculation flows.
The Hazen-Williams Relationship
Friction loss depends on flow, pipe roughness and internal diameter. In SI form the loss per metre is proportional to flow raised to the power 1.85, divided by the C coefficient raised to 1.85 and the diameter raised to 4.87.
- Q: flow rate
- C: the dimensionless roughness coefficient
- d: internal diameter
Note the powers: flow has a strong effect, but diameter has a far stronger one. Increasing pipe size is by far the most effective way to reduce friction loss.
Choosing the C Coefficient
| Material | C factor | Notes |
|---|---|---|
| New black steel (sch 40) | 120 | Clean, at first fill |
| Steel wet pipe, 20 years and older | 100 | The common field assumption |
| Steel dry pipe, 20 years and older | 100 | Conservative FM assumption |
| Galvanised steel | 100–120 | Depending on age and internal condition |
| Cement-lined cast iron | 140 | Underground mains |
| CPVC sprinkler pipe | 150 | Where FM Approved |
| Copper | 150 | Small diameters |
FM Global typically requires steel wet pipe systems to be designed at C = 100, a conservative choice that remains valid as the pipework ages.
Equivalent Lengths for Fittings
- 90 degree elbow (standard): around 25 pipe diameters
- 90 degree elbow (long radius): around 17 diameters
- 45 degree elbow: around 12 diameters
- Tee, straight through: around 10 diameters
- Tee, flow to branch: around 50 diameters
- Gate valve, fully open: around 5 diameters
- Swing check valve: around 70 diameters
Total friction loss is the actual pipe length plus the equivalent lengths, multiplied by the unit loss from the tables.
FM and NFPA 13 Compared
- Formula: identical — both use Hazen-Williams
- C factor: NFPA 13 permits C = 120 for new systems; FM assumes C = 100
- Underground: both accept C = 140 for cement-lined pipe
Quick Checklist
- C factor chosen for the pipe material and age
- Steel wet pipe at C = 100 for FM work
- Cement-lined underground at C = 140
- CPVC only where FM Approved and in permitted areas
- Equivalent lengths added for every elbow, tee and valve
- Total friction, static head and branch losses accumulated to the most remote sprinkler
Frequently Asked Questions
What C coefficient should I use for a steel sprinkler system?
FM Global generally requires C = 100 for steel wet pipe systems, which reflects the roughness the pipework will actually have after years in service rather than its condition on the day of commissioning. NFPA 13 permits C = 120 for new systems, so the same pipework can produce different calculated demands under the two frameworks.
Why does a lower C factor produce a higher calculated demand?
C represents smoothness, so a lower value means more friction loss per metre. Designing at C = 100 rather than 120 therefore produces a higher required pressure at the demand point, which is exactly the intent: the system should still work when the pipe has aged rather than only when it is new.
How much difference do fittings make?
Considerably more than people expect. A tee with flow into the branch is worth around 50 pipe diameters of straight pipe, and a swing check valve around 70. On a compact system with many fittings, equivalent lengths can rival the actual pipe length, which is why omitting them is one of the most common calculation errors.
Why does pipe diameter matter so much more than length?
Because friction loss varies with diameter to roughly the power of 4.87, 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 or reducing flow, and it is the first lever to reach for when a calculation fails.
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Download MEP Calc on the App StoreFM Global Property Loss Prevention Data Sheet DS 2-89 (Pipe Friction Loss Tables); NFPA 13 hydraulic calculation provisions.