📅 Updated April 2026 · 🕒 10 min read · 📚 NFPA 15 (2017) · NFPA 11 (2021) · GB 50338
⚙ Quick Answer — The Two Design Calculations
1. Flow Rate Calculation
Q = A × w
Q = required flow (L/min) · A = protected area (m²) · w = application rate (L/min·m²)
2. Throw Range Requirement
R ≥ D × 1.15
R = required rated range (m) · D = monitor-to-target distance (m) · 1.15 = wind safety factor
The two most common specification errors in a fire water monitor system are selecting a monitor with too low a flow rate for the protected area, and positioning the monitor so far from the fire hazard that the rated throw range is insufficient to reach the target. Both errors are preventable with straightforward calculations — yet both are frequently made when monitors are selected from a catalogue by instinct rather than by design.
This guide walks through the complete calculation process for both flow rate and throw range, provides the application rate values from NFPA 15, NFPA 11 and GB 50338 needed to run those calculations, and ends with a worked example that takes a real petrochemical scenario from blank paper to specific monitor model selection.
In This Article
- The Three Interdependent Specifications
- Step 1 — Calculate Required Flow Rate
- Application Rates by Standard and Occupancy
- Step 2 — Determine the Required Throw Range
- Step 3 — Verify Available Working Pressure
- Worked Example — Petrochemical Vessel Cooling
- Common Sizing Mistakes to Avoid
- Frequently Asked Questions
1. The Three Interdependent Specifications
Flow rate, throw range and working pressure are the three primary specifications of any fire water monitor. They are interdependent — you cannot fix one independently of the others. Understanding how they relate is the prerequisite for correct monitor selection.
Q
Flow Rate (L/s)
Volume of water discharged per second. Determined by the fire protection design calculation — the area to be protected multiplied by the required application rate. This is the starting point of the calculation, not the result.
R
Throw Range (m)
Maximum horizontal distance the straight jet reaches at rated pressure. Determined by the site geometry — the distance from the monitor’s installation position to the furthest point it must reach in the protected area.
P
Working Pressure (MPa)
Pressure at the monitor inlet during discharge. Affects both flow rate (Q ∝ √P) and throw range (range increases with pressure up to the rated maximum). Must equal or exceed the monitor’s rated working pressure to achieve the stated Q and R.
How They Interact:
Higher flow rate requires larger nozzle bore → which requires higher pressure to achieve the same exit velocity → which is needed to maintain throw range. A monitor operating below its rated pressure will deliver less flow AND shorter range than stated in the data sheet. Always verify that the available system pressure at the monitor inlet equals or exceeds the monitor’s rated working pressure under full-flow conditions.
2. Step 1 — Calculate Required Flow Rate
The design flow rate for a fire water monitor system position is calculated by multiplying the protected surface area by the design application rate required by the applicable standard. This gives the minimum flow rate in L/min, which is then converted to L/s and compared against the monitor’s rated flow.
Flow Rate Formula
Qmin = A × w
Qmin
Minimum required flow rate at this monitor position (L/min)
A
Protected surface area to be covered by this monitor (m²)
w
Design application rate from applicable standard (L/min·m²)
The application rate w is not chosen by the designer — it is specified by the standard. The table in Section 3 below lists the values from NFPA 15, NFPA 11 and GB 50338 for common occupancy types. Once Qmin is calculated:
- 1.
Convert L/min to L/s: divide by 60. Example: 2,040 L/min ÷ 60 = 34 L/s - 2.
Select the monitor model whose rated flow rate is equal to or greater than Qmin in L/s - 3.
Apply this to each monitor position independently — different positions covering different areas or different risk classes may require different monitor models even within the same fire water monitor system
3. Application Rates by Standard and Occupancy
The following table lists the minimum design application rates from the most commonly referenced standards. Use the value that matches your project’s applicable standard and the specific equipment or hazard being protected.
| Standard | Protected Equipment / Hazard | Application Rate (L/min·m²) |
Notes |
|---|---|---|---|
| NFPA 15 — Water Spray Fixed Systems | |||
| NFPA 15 §7.4 | Exposed ordinary hazard equipment (vessels, piping, structures) | 10.2 | Most common rate for process unit equipment cooling |
| NFPA 15 §7.4 | Electrical transformers — oil-filled | 10.2 | Applied to wetted transformer surface area |
| NFPA 15 §7.5 | Cooling exposure — adjacent equipment protection | 10.2 | 60 min minimum duration for cooling exposures |
| NFPA 15 §7.7 | LPG / pressurised flammable liquid vessels (BLEVE risk) | 10.2 | Applied to the wetted surface of vessel + 1 m above liquid level |
| NFPA 11 — Foam Systems (Foam-Water Monitors) | |||
| NFPA 11 §11.6 | Flammable liquid storage tanks — AFFF foam-water monitors | 6.5 | Applied to liquid surface area; 65 min foam concentrate supply required |
| NFPA 11 §11.6 | Flammable liquid tanks — protein or FFFP foam monitors | 8.1 | Higher rate required for less effective foam types |
| NFPA 11 §12 | Loading racks — flammable liquid spill fires | 6.5 | Applied to the impounded spill area (bund or containment zone) |
| GB 50338 — Fixed Monitor Systems (China) | |||
| GB 50338 §4 | Petrochemical / flammable liquid facilities | 40 L/s min per monitor* | GB 50338 specifies minimum monitor flow rates by facility type, not application rates — check §4 table for the specific category |
| GB 50338 §4 | Warehouse / general industrial | 30 L/s min per monitor* | Any point in the protected area must be reachable by ≥2 monitors simultaneously |
| GB 50338 §5 | All categories — design duration | 60 min | Water supply must sustain all monitors simultaneously for 60 minutes minimum |
* GB 50338 uses minimum monitor flow rates rather than application rates. Always consult the current edition of the standard and confirm with the design authority for your project category.
4. Step 2 — Determine the Required Throw Range
The throw range requirement is set by the site geometry — specifically, the distance from the monitor’s planned installation position to the furthest point it must reach within the protected area. This distance is measured horizontally on the site plan.
Required Rated Range Formula
Rrated ≥ Dmax × SF
Rrated
Minimum rated throw range of the selected monitor (m)
Dmax
Maximum horizontal distance from monitor to furthest protected point (m) — measured on site plan
SF (Safety Factor)
1.15 for sheltered sites · 1.20–1.25 for exposed coastal / offshore / windy sites
The safety factor accounts for wind effects on the stream — a crosswind deflects the stream sideways, reducing the effective horizontal reach. Throw range data in product data sheets is measured in still air. At exposed sites with frequent crosswinds, the actual working range is reduced, so the monitor’s rated range must exceed the required distance by the safety factor margin.
⚠️ Range is NOT constant
Throw range is stated at the rated working pressure. If the actual pressure at the monitor inlet is lower than rated — due to friction losses in the pipework — the throw range will be shorter than stated. A monitor specified for 70 m range at 1.0 MPa will not achieve 70 m if the actual inlet pressure is only 0.8 MPa.
📐 Elevation angle affects horizontal reach
The rated throw range is the maximum horizontal distance at the optimal elevation angle (typically 30–35° for most monitors). At very high elevation angles (50–60°) the horizontal reach is significantly shorter, even though the stream travels further through the air. If the monitor must aim steeply upward to reach a vessel top, the effective horizontal range is reduced.
📏 Measure Dmax correctly
Dmax is the horizontal distance from the monitor nozzle position to the furthest point in the protected area that must receive water. For an outdoor vessel, this is typically the far side of the vessel at the widest point. For a tank bund, it is the distance from the monitor to the far bund wall plus the width of the tank.
5. Step 3 — Verify Available Working Pressure
Once the required flow rate and minimum throw range are established, the selected monitor must be verified to achieve both at the pressure available at the monitor inlet. The available inlet pressure is the fire pump discharge pressure minus all friction losses between the pump and the monitor inlet.
Establish the fire pump rated discharge pressure
From the fire pump data sheet — the rated pressure at the pump’s rated flow condition. Note: pump pressure decreases as flow rate increases; use the pressure at the design flow rate, not the shut-off pressure.
Calculate friction losses in the distribution pipework
Sum the friction losses through the ring main, branch pipes, fittings, isolation valve and monitor base from pump to monitor inlet. Use the Hazen-Williams or Darcy-Weisbach equation for each pipe segment. For a preliminary check, estimate pipe friction loss at 0.01–0.02 MPa per 10 m for DN100 pipe at 40–60 L/s.
Account for elevation difference
Add 0.01 MPa pressure loss per metre of elevation gain between the pump and the monitor. If the monitor nozzle is 3 m above the pump discharge, add 0.03 MPa to the friction loss total.
Verify residual pressure ≥ monitor rated working pressure
Residual pressure = pump pressure − friction losses − elevation losses. This must equal or exceed the monitor’s rated working pressure (typically 0.8–1.0 MPa). If not, either increase the pump discharge pressure, upsize the pipework to reduce friction, or select a monitor with a lower rated pressure that still meets the flow requirement.
6. Worked Example — Petrochemical Vessel Cooling
Scenario
A horizontal pressure vessel at a petrochemical plant has an exposed surface area of 280 m². The vessel is 12 m long and 5 m in diameter. The monitor must be positioned 45 m from the near side of the vessel. The site is partially sheltered. Applicable standard: NFPA 15. Available fire pump outlet pressure: 1.3 MPa. Estimated pipework friction loss to monitor position: 0.25 MPa.
Step 1 — Calculate Required Flow Rate
From NFPA 15 §7.4: application rate for ordinary hazard equipment = 10.2 L/min·m²
Qmin = 280 m² × 10.2 L/min·m² = 2,856 L/min = 47.6 L/s
The monitor must discharge at least 47.6 L/s. Round up to the next standard model: select PS10/50W (50 L/s) as the minimum compliant model, or PS10/60W (60 L/s) for margin.
Step 2 — Determine Required Throw Range
Dmax = distance to near side + vessel length = 45 m + 12 m = 57 m to the far end of the vessel
Apply safety factor for partially sheltered site (SF = 1.15):
Rrated ≥ 57 m × 1.15 = ≥ 65.6 m → specify ≥70 m rated range
The PS10/50W has a rated range of ≥70 m — this satisfies the requirement. ✓
Step 3 — Verify Available Working Pressure
Monitor nozzle is approximately 1.5 m above pump discharge → elevation loss = 0.015 MPa
Residual pressure = 1.3 − 0.25 − 0.015 = 1.035 MPa
PS10/50W rated working pressure = 1.0 MPa. Available: 1.035 MPa ≥ 1.0 MPa. ✓ Pressure is adequate.
Result — Monitor Selection
Specify: PS10/50W Turbine-Worm Water Monitor (50 L/s, ≥70 m, 1.0 MPa, worm-gear)
Flow: 50 L/s ≥ 47.6 L/s required ✓ | Range: ≥70 m ≥ 65.6 m required ✓ | Pressure: 1.035 MPa ≥ 1.0 MPa ✓
Note: worm-gear specified because flow rate ≥40 L/s — handle operation not recommended at this flow. On PZ100-1.6 or PZ150-1.6 base depending on ring main pipe size.
7. Common Sizing Mistakes to Avoid
| Common Mistake | Why It’s a Problem | How to Avoid It |
|---|---|---|
| Selecting monitor by “typical” rather than calculated flow | A PS10/50W “feels right” for a large vessel. Actual calculation may require 60 or 80 L/s for the surface area involved | Always calculate Q = A × w first. Never start with a monitor model and work backwards |
| Not applying a wind safety factor to the range requirement | A monitor with a rated range of 70 m may only achieve 55–60 m effective reach in a crosswind — insufficient to cover the far side of the target | Apply SF ≥ 1.15 for all outdoor installations. Increase to 1.25 for coastal, offshore or open-plain sites |
| Assuming pump pressure equals monitor inlet pressure | Friction losses in the ring main and branch pipes can reduce the inlet pressure by 0.2–0.5 MPa — taking the monitor below its rated pressure and reducing both flow and range | Always calculate friction losses and verify residual pressure at the monitor inlet ≥ rated working pressure |
| Ignoring simultaneous operation demand | NFPA 15 and GB 50338 require simultaneous operation of multiple monitors. Sizing only for one monitor at a time means the pump and ring main are undersized when multiple monitors activate | Design the ring main and pump for the worst-case simultaneous operating scenario — typically all monitors that can reach any single fire target activated at once |
| Specifying water monitors for Class B liquid fire positions | Water cannot suppress a burning liquid surface — water-only monitors at tank farm positions are a design error under NFPA 11 | Use foam-water monitors (PL Series) wherever the primary fire scenario involves burning hydrocarbons on a liquid surface |
| Undersizing the pipe base inlet diameter | Specifying a DN100 base for a PS10/80W (80 L/s) creates a high-velocity restriction at the base inlet, causing excessive friction loss and reducing monitor inlet pressure below rated | For monitors ≥60 L/s, specify DN150 base inlet. Match base inlet diameter to the ring main pipe diameter at that branch connection |
Frequently Asked Questions
Related Products & Resources
Need Help with Fire Water Monitor System Sizing?
CA-FIRE can assist with monitor selection for your project’s calculated flow rate, required throw range and available system pressure. Provide your protected area dimensions, applicable standard and available pump pressure — we will confirm the correct model and base specification.
📞 +86 134-0071-5622 · 💬 WhatsApp +86 181-5036-2095 · 🌐 ca-fire.com
Authoritative Sources & Standards
- NFPA 15: Standard for Water Spray Fixed Systems for Fire Protection (2017) — National Fire Protection Association
- NFPA 11: Standard for Low-, Medium-, and High-Expansion Foam (2021) — National Fire Protection Association
- API 2030: Application of Fixed Water Spray Systems for Fire Protection in the Petroleum Industry — American Petroleum Institute
- GB 50338: Code for Design of Fixed Fire Monitor Extinguishing System — Ministry of Housing and Urban-Rural Development, China