EN 12845 HHP High Hazard Process Design

Density and area selection for industrial process facilities, K-factor decisions, the deluge exception, and a practical comparison with NFPA Extra Hazard.

When a plastic injection moulding plant project landed on our desk, the designer had calculated it as OH3: 5.0 mm/min over 216 m² with K80 sprinklers. The material processed was PP and PS pellet, with raw material silos and the press line under one roof. The EN 12845 classification annex is clear: PP, PE and PS injection lines are not OH3 but HHP1. Density should have been 7.5 rather than 5.0 mm/min, and the area 260 rather than 216 m². The pump and pipework were resized and the project slipped six weeks. Getting the HHP classification wrong at the start is expensive.

What HHP is, and why it is separate from HHS

EN 12845 defines HHP as occupancies containing materials with a high fire load that ignite easily and spread rapidly, or that can develop an intense fire. The important distinction: HHP is a process facility, not a store. The material is worked, formed, dried or manufactured there. Stock stacked above 4 m, however combustible, falls not under HHP but under HHS, with a different design logic.

The standard divides HHP into four groups:

• HHP1 — moderate process risk (paint shops, printing works, plastic injection moulding)
• HHP2 — high process risk (paint and varnish manufacture, match works, sawmills)
• HHP3 — very high process risk (foam plastics manufacture, tyre manufacture)
• HHP4 — special risks requiring deluge (fireworks, cellulose nitrate)

The density and area table

The basic design parameters for HHP come from a single table. The values everyone doing the calculation should know:

Class	Design density	Area of operation (wet / pre-action)	Area of operation (dry / alternate)
HHP1	7.5 mm/min	260 m²	325 m²
HHP2	10.0 mm/min	260 m²	325 m²
HHP3	12.5 mm/min	260 m²	325 m²
HHP4	Deluge — outside the scope of this standard

The area is fixed: 260 m² wet or pre-action, 325 m² dry or alternate. That the area stays constant while density rises matters — in the OH classes the area grows with hazard, while in HHP the density grows. The reason is simple: an HHP fire develops intense flame from the outset, and increasing the water's penetration capacity (density) matters more than enlarging the area.

Water supply duration is 90 minutes — 1.5 times the 60 minutes required for OH.

Why HHP4 falls outside EN 12845

The note beneath the table is explicit: HHP4 hazards are typically protected by deluge systems, which are outside the scope of the standard. The reason is technical. Look at the facilities on the HHP4 list — fireworks manufacture, cellulose nitrate, tar distillation. Flame spread in those materials is faster than sprinkler activation. By the time a single 68 °C bulb has burst and the head has operated — typically 30–90 seconds — the fire has already engulfed the hall. The answer is a water spray or foam deluge system: every nozzle opens at once and the whole area is wetted within seconds.

In practice, work in an HHP4 area is calculated to EN 13565 (fixed foam systems) or NFPA 16 (foam-water deluge) rather than EN 12845. The designer usually also needs the insurer's own data sheet, because neither EN nor NFPA gives sufficient detail alone for these applications.

K-factor and sprinkler pressure selection

The pre-calculated HHP tables give K-factors and design point pressures. The field logic:

• HHP1 (7.5 mm/min) — K80 is sufficient, with a design point pressure typically between 0.7 and 2.25 bar depending on the pipe size set.
• HHP2 (10.0 mm/min) — K80 is still usable but pressure rises to 1.3–3.9 bar. With pipe losses the pump discharge climbs to 5–6 bar; moving to K115 eases the hydraulics considerably.
• HHP3 (12.5 mm/min) — K115 is the standard choice. The table gives 12.5 mm/min only on the K115 row; attempting it with K80 pushes pressure past 5 bar.

As a field rule, never go below 0.5 bar at the sprinkler — the spray pattern starts to break down. K160 appears mainly in special in-rack storage applications; it is not typical in HHP, where ceiling protection coefficients cluster around K115.

Typical HHP facilities

The classification annex is what you will consult most often. A summary of the examples you need:

HHP1	HHP2	HHP3	HHP4
Solvent paint application shops Printing works Linoleum manufacture Resin, gum and turpentine manufacture Plastic injection (PP/PE/PS) Plastics factories (excluding foam) Carpet factories (including unexpanded plastic) Particle board manufacture Shoe factories (including plastics)	Paint, varnish and lacquer manufacture Tar distillation Wax and paraffin production Match factories Paper machine halls Carpet factories (including foam plastic) Sawmills Bus and empty lorry depots	Cellulose nitrate manufacture Car and truck tyre manufacture Foam plastics and foam rubber manufacture	Fireworks manufacture (protected by deluge)

A frequent point of confusion: tanneries. They do not appear in the annex, but treating them as HHP2 is correct, because chrome processing and solvent-oil drying cabinets push the fire load above the OH limit. The same applies to composite resin and fibre processing — for hybrid facilities not listed in the standard, run the material factor methodology from the classification annex and select the appropriate HHP group.

Where the HHP area is smaller than the design area

Not all of a facility need be HHP. Suppose an 8000 m² metal assembly hall (OH2) contains an 800 m² paint line. Where the HHP area is smaller than the area of operation (260 m²), the flow may be reduced proportionally — but the design point pressure stays the same. So the whole 800 m² is protected as HHP1, but the hydraulic calculation takes a proportional flow from the 260 m² basis. Where the HHP area adjoins an OH area, the total flow is the sum of both, with the HHP pressure still the reference point.

For an HHP area containing fewer than 48 sprinklers, the tabulated flow and pressure must be provided at the highest sprinkler at the entry point to the HHP area. That detail is commonly missed — relaxing the pressure because "it's a small area with few sprinklers" means the area is not protected.

Common project errors

• Confusing process with storage: writing HHP2 for "plastics storage". Storage is HHS; stacks above 4 m come from the storage table, not the process one.
• Downgrading a solvent paint line to OH3: the annex does say that high fire load areas within an OH1 or OH2 space should be treated as OH3 — but solvent paint application is already HHP1, not OH3.
• Choosing K80 for HHP3: the calculation closes, but achieving a 4–5 bar design point requires 8–10 bar at the pump; pipe class rises and cost escalates. Start with K115.
• Taking a 60-minute water supply duration: HHP requires at least 90 minutes. A typical HHP1 flow of 2300 L/min means 207 m³; reusing an OH tank will not do.
• Trying to solve HHP4 with sprinklers: the standard says it is out of scope, the insurer will not approve it, and flame spread means it would not work on site. Deluge or foam — there is no other option.

Comparison with NFPA Extra Hazard

NFPA 13 splits Extra Hazard into Group 1 (moderate) and Group 2 (high). The approximate mapping:

EN 12845	Density × area	Nearest NFPA 13 equivalent	NFPA typical density/area
HHP1	7.5 mm/min × 260 m²	Extra Hazard Group 1	about 6.1 mm/min × 232 m²
HHP2	10.0 mm/min × 260 m²	Between EH Group 1 and 2	about 8.1 mm/min × 232 m²
HHP3	12.5 mm/min × 260 m²	Extra Hazard Group 2	about 10.2 mm/min × 232 m²
HHP4	Deluge	NFPA 16 foam-water deluge	Project specific

The two standards work on similar logic, but EN 12845 applies a higher density over a smaller area while NFPA applies a slightly lower density over a larger one. In practice an EN 12845 calculation demands roughly 15–20 % more pump pressure and a higher instantaneous flow than the NFPA equivalent. Where an international client requires EN 12845, calculating to EN from the outset is faster and less error-prone than converting from NFPA.

Application in Turkey

BYKHY ties sprinkler design to TS EN 12845, so HHP classification is not academic but a regulatory requirement. Even so, "let's call it OH3, the calculation will pass" still appears on site. Over the last couple of years, metropolitan fire authorities have begun asking for classification verified against the annex — so a plastic injection plant submitted as OH3 comes back. Including the classification table extract in the submission avoids the cost of a revision cycle.

Frequently Asked Questions

What separates HHP from HHS?

HHP is a process occupancy where material is worked, formed or manufactured. Storage above 4 m, however combustible, is HHS and follows a different design table.

Why is the area fixed at 260 m² across HHP classes?

Because an HHP fire develops intense flame from the outset. Increasing water penetration through density matters more than enlarging the design area.

Why is HHP4 outside the standard?

Because flame spread in materials like fireworks and cellulose nitrate outruns sprinkler activation. Deluge or foam systems designed to EN 13565 or NFPA 16 are required.

What water supply duration applies?

90 minutes — 1.5 times the OH figure. A typical HHP1 flow of 2300 L/min therefore means about 207 m³ of stored water.

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