Certain fire sprinkler design authorities and standards, including EN 12845, specify limits on the permissible velocity through pipes and valves in fire sprinkler systems.
EN 12845 explicitly defines velocity limits (EN 12845:2015+A1:2019 - Automatic Sprinkler Systems Design Guide, 2019). In contrast, NFPA and FM Global do not specify explicit velocity restrictions. FM Global Property Loss Prevention Data Sheet 2‑0 (FM Global Property Loss Prevention Data Sheet 2-0: Sprinkler System Design, n.d.) restricts the application of the Hazen‑Williams equation to velocities below 9.1 m/s (30 ft/s). When this threshold is exceeded, the Darcy–Weisbach equation must be used for pressure loss calculations.
The rationale for limiting velocity is that the Hazen‑Williams formula loses accuracy when applied outside its intended conditions. As an empirical equation based on observation and experimentation, its validity diminishes beyond its tested velocity range. Some authorities contend that velocity is inherently self‑limiting because friction losses increase exponentially with velocity. In practice, designers frequently increase pipe sizes to maintain acceptable pressure loss, thereby restricting excessive velocities.
Nevertheless, the Hazen‑Williams equation should not be applied outside its accepted parameters, as this may invalidate the resulting calculations.
EN 12845 specifies the following velocity limits for sprinkler systems:
- 6 m/s through valves and flow switches
- 10 m/s at any other point in the system
The velocity in a pipe can be determined using the following formula:
when:
v = velocity in the pipe in m/s
Q = flow of water in the pipe in L/min
d = internal diameter of the pipe in mm
Velocity Limits Summary Table
| Standard / Authority | Velocity Limit | Notes |
| EN 12845 | 6 m/s | Maximum velocity through valves and flow switches |
| EN 12845 | 10 m/s | Maximum velocity at any other point in the sprinkler system |
| FM Global (Data Sheet 2‑0) | 9.0 m/s | Above this velocity, Hazen‑Williams cannot be used; switch to Darcy–Weisbach. |
| NFPA | No explicit limit | Above this velocity, Hazen‑Williams cannot be used; switch to Darcy–Weisbach. |
Summary
Understanding velocity limits helps ensure accurate hydraulic calculations and reliable sprinkler performance. EN 12845 sets clear limits of 6 m/s through valves and flow switches and 10 m/s elsewhere, while NFPA places no restrictions and FM limits Hazen‑Williams use to below 9.1 m/s. Staying within these boundaries ensures your calculations remain valid and your system operates as intended.
By applying appropriate velocity limits, selecting appropriate pipe sizes, and using the correct friction loss equations, designers can ensure accurate calculations, efficient system performance, and long-term reliability of fire sprinkler installations.
