EN 12845 design parameters for class HHP3 — High Hazard Process 3: design density 12.50 mm/min, area of operation 260 m² (wet or pre-action; 325 m² for dry or alternate systems), water supply duration 90 minutes.
Which areas fall into this class?
HHP3 examples from EN 12845 Table A.3: cellulose nitrate manufacture, car and lorry tyres, and manufacture of material factor m3 foam plastics, foam rubber and foam rubber goods (excluding m4; Table B.1). See the Annex A article for the full list and edge cases.
Design parameters
| Parameter | Value |
|---|---|
| Design density (Table 3) | 12.50 mm/min |
| Area of operation — wet or pre-action (Table 3) | 260 m² |
| Area of operation — dry or alternate (Table 3) | 325 m² |
| Water supply duration (clause 8.1.1) | 90 minutes |
| Theoretical minimum sprinkler flow — wet / dry | 3250 / 4062.5 L/min |
| Theoretical minimum water volume — wet / dry | 292.5 / 365.6 m³ |
| Maximum area per sprinkler (Table 19) | 9 m² |
| Minimum sprinkler pressure (clause 13.4.4) | 0.50 bar |
The theoretical minimum values come only from density × area. The real design flow is higher because of hydraulic imbalance and minimum sprinkler pressure. EN 12845 adds no hose allowance to the sprinkler demand; if hose reels and hydrants draw from the same tank, the flow and duration required by local regulations are added to the tank volume separately.
Required pressure by K-factor
Under EN 12845 Table 19 the maximum area per sprinkler in HHP is 9 m², so 12.50 mm/min requires 112.5 L/min per sprinkler. Table 37a permits density > 10 mm/min, so K115 or K160 only. Clause 13.4.4 sets a minimum sprinkler pressure of 0.50 bar for HHP; where the calculated pressure is lower, 0.50 bar is used and the flow is recalculated:
| K-factor | Pressure for the density (bar) | Pressure to apply (bar) | Sprinkler flow (L/min) |
|---|---|---|---|
| K115 (K8.0) | 0.96 | 0.96 | 112.5 |
| K160 (K11.2) | 0.49 | 0.50 (minimum) | 113.1 |
Dry and pre-action systems
In dry systems water reaches the sprinklers late, so the area of operation is increased. For the HHP classes, EN 12845 Table 3 raises the area of operation from 260 m² to 325 m² for dry and alternate systems (pre-action systems use the wet-system area). NFPA 13 clause 19.2.3.2.6 requires a 30 percent increase in design area for dry pipe and double interlock preaction systems.
Classification is a decision; the table is a consequence. Whether an area falls into HHP3 — High Hazard Process 3 is determined by assessing its real activity, the materials stored and the fire load, and must be defended with written justification in the design report. If the classification is wrong, every number here is invalid.
Checklist
- Are the area's activity and stored materials defined in writing?
- Have parts of the same building in a different class been calculated separately?
- If there is storage, have the storage rules been applied separately?
- If hose reels and hydrants draw from the same tank, has their demand been added to the tank volume?
- Have the water supply duration and volume been verified?
- Is there any planned change in operations that could alter the class?
Frequently Asked Questions
What is the design density for HHP3?
12.50 mm/min under EN 12845 Table 3; the area of operation is 260 m² for wet or pre-action systems and 325 m² for dry or alternate systems. The water supply duration is 90 minutes (clause 8.1.1).
What is the minimum water volume for HHP3?
The theoretical minimum is 292.5 m³ for a wet system and 365.6 m³ for a dry or alternate system (density × area × duration). The real volume is higher because of hydraulic imbalance and minimum sprinkler pressure; if hose reels and hydrants draw from the same tank, their demand is added separately.
Do these values change for a dry system?
Yes. EN 12845 Table 3 increases the area of operation from 260 m² to 325 m² for dry and alternate systems. NFPA 13 clause 19.2.3.2.6 requires a 30 percent increase in design area for dry pipe and double interlock preaction systems.
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Download MEP Calc on the App StoreTS EN 12845+A2:2026 (EN 12845:2015+A2:2026) Table 3, Table 19, Table 37a, Table A.3, clauses 8.1.1 and 13.4.4 · NFPA 13 (2025) clause 19.2.3.2.6. Table values are for information; the binding text is the current edition of the standard, and separate rules apply to storage.