EN 12845 Minimum Sprinkler Discharge Pressure

Even where a sprinkler delivers enough water, if the pressure is low the droplet size and the spray pattern will not form as designed. That is why TS EN 12845+A2:2026 sets a pressure floor on fully calculated systems in parallel with, and independent of, the density requirement. Source: TS EN 12845+A2:2026, Clause 13.4.1 and Clause 13.4.4.

Clause 13.4.4: five values

The full text of Clause 13.4.4 (TS p.105 / EN p.102) states that, with all the sprinklers in the area of operation running, the pressure at the hydraulically most unfavourable sprinkler shall not be less than that required to provide the density specified in Clause 13.4.1, or the following, whichever is the greater:

Two points govern. First, the point of measurement is not an average point but the most unfavourable sprinkler; Clause 3.11 defines that position as the one where the water supply pressure measured at the control valve set has to be highest in order to deliver the specified design density. Second, the words "whichever is the greater" do not merge the two requirements but stack them: where the density calculation requires a higher pressure, the table value is not enough; where the density calculation gives a lower pressure, the pressure is raised to the table value.

The standard contains no general minimum pressure beyond these, such as a 2.0 bar value applying to all classes; 2.00 bar is only the row for K 80 in-rack sprinklers.

How the density is measured: Clause 13.4.1

Clause 13.4.1 (TS p.101 / EN p.98) ties the definition of density not to an average but to a local group:

• The discharge density shall be taken as the total flow in litres per minute from the four closest sprinklers divided by the area in square metres covered by those four sprinklers.
• Where fewer than four sprinklers are in open communication, the discharge density shall be taken as the lowest flow from any one sprinkler divided by the area covered by that sprinkler.
• The discharge density obtained in every area of operation containing the relevant group of four sprinklers (or over the whole protected area, whichever is the smaller) shall not be less than the design density specified in Clause 7, with every available water supply or combination of supplies.
• The area covered by each sprinkler is defined by centre lines drawn exactly midway between the adjacent sprinklers, perpendicular to the line joining the sprinklers, and by the boundary of the covered area or half the distance to the nearest sprinklers, whichever is the greater (see Figure 22).
• Where in-rack sprinklers are installed, the calculation shall take into account the simultaneous flow and pressure requirement of the ceiling/roof sprinklers and the intermediate level sprinklers.

The definition in Clause 3.19 (TS p.20 / EN p.17) is the same: the design density is determined by dividing the discharge in litres per minute of a given group of sprinklers by the area in square metres covered.

Clause 4.4.3.3 d) 8) iv) (TS p.30 / EN p.27) carries the rule through into the documentation: for fully calculated pipework, the drawings shall show the four sprinklers on which the design density is based.

The link between pressure and flow: Q = K√P

Clause 14.3 (TS p.107-108 / EN p.104-105): the flow of water from a sprinkler is calculated as Q = K√P, where Q is in litres per minute, K is the constant given in Table 37a and P is the pressure in bar. Clause 3.39 gives the same relationship in the definition of the discharge coefficient K.

Table 37a (TS p.107 / EN p.104) links the K values to the class: 57 for LH; 80 or 115 for OH; for HHP and HHS ceiling/roof sprinklers, 80, 115 or 160 at design densities of 10 mm/min and below and 115 or 160 above 10 mm/min; 80 or 115 for HHS intermediate (in-rack) sprinklers.

This relationship makes the five values of Clause 13.4.4 concrete (the figures below are arithmetic using the K values and pressures of the standard):

Two conclusions follow from the table:

• In LH the pressure floor governs. With a design density of 2.25 mm/min and a maximum coverage area of 21.0 m² (Table 19, TS p.73 / EN p.70), the required flow is 2.25 x 21.0 = 47.25 l/min and the pressure this requires is (47.25 / 57)² = 0.69 bar. The 0.70 bar floor in Clause 13.4.4 sits just above that.
• In OH the density governs. At 0.35 bar a K80 sprinkler gives only 3.94 mm/min over 12.0 m²; the 5.0 mm/min required by Table 3 calls for 60 l/min over 12.0 m², which means (60 / 80)² = 0.56 bar. In OH, therefore, 0.35 bar is in practice a lower safeguard and the density is what really governs.

Why are in-rack sprinklers treated separately?

Clause 7.2.3.4 (TS p.40 / EN p.37): in-rack sprinklers and their associated ceiling sprinklers shall always be fully calculated (see Clause 13.1). The NOTE to the clause states that the minimum pressure at any operating sprinkler is 2.0 bar and refers to Clause 13.4.4.

The physical reason is written in Clause 12.5.1 (TS p.81 / EN p.78): water from sprinklers operating at intermediate levels shall be able to penetrate into the stored goods. The same clause sets the flue spaces (longitudinal at least 0.15 m, transverse at least 0.10 m) and the clearance between the deflector and the top of the stack (flat spray 0.10 m, others 0.15 m) for the same purpose.

Annex G raises this floor further in certain cases: Table G.6 (TS p.153 / EN p.150) requires in-rack sprinklers in rack storage of all plastic pallets other than timber/cellulosic and solid platform HDPE to be selected with K = 115 and a minimum operating pressure of 3 bar. At 3 bar the flow of a K115 sprinkler is 115 x √3 = 199.2 l/min.

Where is the pressure defined on pre-calculated systems?

Clause 13.4 governs fully calculated systems. On pre-calculated systems the pressure is defined not at the sprinkler but at the design point and at the control valve set. The NOTE to Clause 7.1 (TS p.38 / EN p.35): on pre-calculated systems the design criteria are achieved by applying the water supply and pipework requirements specified elsewhere in the standard (see Clauses 7.3, 9.3.2.2 and Clause 10.7).

• LH and OH, Table 6 (Clause 7.3.1, TS p.42 / EN p.39): at the control valve set, 2.2 + ps bar at 225 l/min for LH; 1.0 + ps bar at 375 l/min and 0.7 + ps bar at a maximum demand flow rate of 540 l/min for OH1 wet; 1.4 + ps bar at 725 l/min and 1.0 + ps bar at 1 000 l/min for OH1 dry/OH2 wet; 1.7 + ps bar at 1 100 l/min and 1.4 + ps bar at 1 350 l/min for OH2 dry/OH3 wet; 2.0 + ps bar at 1 800 l/min and 1.5 + ps bar at 2 100 l/min for OH3 dry/OH4 wet.
• HHP and HHS, Table 7 (Clause 7.3.2.1, TS p.43 / EN p.40): the pressure pd at the highest design point is given according to the design density, the pipe table used and the area per sprinkler. For example, in the Table 7 (1) block (Table 32 and 33 diameters, K80), pd is 1.80 bar at 6 m² per sprinkler and 3.90 bar at 9 m² for 10.0 mm/min.

Clause 7.3.2.1 describes the total: the working pressure requirement at the control valve set is the sum of the pressure at the design point, the pressure equivalent of the height difference between the control valve set and the highest sprinkler downstream of the design point, and the pressure loss in the pipework from the control valve set to the design point.

Clause 13.3.4.2 (TS p.92 / EN p.89) is a limit that is frequently misapplied: on pre-calculated OH systems, the pipe diameters between the design point in the most remote area and the control valve set are calculated such that the friction loss at a flow of 1 000 l/min does not exceed 0.5 bar. That is a pipe sizing limit; it is not a safety margin to be added to the pump pressure on a fully calculated system.

Precision and documentation of the calculation

• Table 24 (Clause 13.2.5.1, TS p.88 / EN p.85): pressure in mbar and pressure loss in mbar/m are calculated to a precision of 1.0; flow in l/min to a precision of 1.0; water application density in mm/min to a precision of 0.1.
• Clause 13.2.5.2: the algebraic sum of the pressure losses around a loop shall be (0 ± 1) mbar; the calculation at a node where water flows combine shall be balanced to ± 1 mbar; and the algebraic sum of the water flows at a node shall be (0 ± 0.1) l/min.
• Clause 4.4.3.3 e) (TS p.30 / EN p.27): for each operating sprinkler, the sprinkler node or reference number, the nominal K-factor (see EN 12259-1), the flow through the sprinkler in litres per minute and the inlet pressure at the sprinkler or group of sprinklers in bar shall be given.
• Clause 18.2.3.2 (TS p.119 / EN p.116): the durable plate fixed to the riser next to each control valve set shall state the design requirements (area of operation and discharge density) for each hazard class area within the installation, together with the pressure-flow requirement at the flow test facility for the most unfavourable and the most favourable areas of operation.

Field note: the most common interpretation error on site is reading the minimum pressure as if it were an average across the system; the sentence in Clause 13.4.4 is tied to a single sprinkler, the most unfavourable one, and the hydraulic calculation is verified at that node.

Frequently Asked Questions

What is the minimum sprinkler discharge pressure in EN 12845?

Clause 13.4.4 (TS p.105 / EN p.102) gives five values: 0.70 bar in LH; 0.35 bar in OH; 0.50 bar in HHP and HHS excluding in-rack sprinklers; 1.00 bar for K 115 in-rack sprinklers; and 2.00 bar for K 80 in-rack sprinklers. This pressure is measured at the hydraulically most unfavourable sprinkler with all the sprinklers in the area of operation running.

Does the minimum pressure still apply where the density is achieved?

Yes. The text of Clause 13.4.4 states that the pressure shall not be less than that required to provide the density specified in Clause 13.4.1, or the listed value, whichever is the greater. The two requirements are met together; one does not replace the other.

Is the design density calculated as an average?

No. Clause 13.4.1 (TS p.101 / EN p.98): the discharge density is taken as the total flow from the four closest sprinklers divided by the area covered by those four sprinklers. Where fewer than four sprinklers are in open communication, it is taken as the lowest flow from any one sprinkler divided by the area covered by that sprinkler. In every area of operation containing the relevant group of four sprinklers, this value shall not be less than the design density in Clause 7.

Why is a higher pressure required at in-rack sprinklers?

Clause 13.4.4 lists in-rack sprinklers as separate items: 1.00 bar for K 115 and 2.00 bar for K 80. The NOTE to Clause 7.2.3.4 also cites 2.0 bar as the minimum pressure at any operating sprinkler and refers to 13.4.4. In the same part, Clause 12.5.1 requires that water from sprinklers operating at intermediate levels be able to penetrate into the stored goods.

Where is this pressure checked on pre-calculated systems?

On pre-calculated systems the pressure is defined not at the sprinkler but at the design point and at the control valve set. Table 6 (TS p.42 / EN p.39) gives the pressure at the control valve set for LH and OH, and Table 7 (TS p.43 / EN p.40) gives the pressure pd at the highest design point for HHP and HHS. The NOTE to Clause 7.1 states that on pre-calculated systems the design criteria are achieved by applying the water supply and pipework requirements.

References

• TS EN 12845+A2:2026 Clause 3.11, Clause 3.19 (TS p.19-20 / EN p.16-17), Clause 3.39 (TS p.21 / EN p.18)
• TS EN 12845+A2:2026 Clause 4.4.3.3 (TS p.30 / EN p.27)
• TS EN 12845+A2:2026 Clause 7.1 and its NOTE (TS p.38 / EN p.35)
• TS EN 12845+A2:2026 Clause 7.2.3.4 and its NOTE (TS p.40 / EN p.37)
• TS EN 12845+A2:2026 Clause 7.3.1, Table 6 (TS p.42 / EN p.39), Clause 7.3.2.1, Table 7 (TS p.43 / EN p.40)
• TS EN 12845+A2:2026 Clause 12.2, Table 19 (TS p.73 / EN p.70), Clause 12.5.1 (TS p.81 / EN p.78)
• TS EN 12845+A2:2026 Clause 13.2.5.1, Table 24, Clause 13.2.5.2 (TS p.88 / EN p.85)
• TS EN 12845+A2:2026 Clause 13.3.4.2 (TS p.92 / EN p.89)
• TS EN 12845+A2:2026 Clause 13.4.1, Figure 22 (TS p.101-102 / EN p.98-99)
• TS EN 12845+A2:2026 Clause 13.4.4 (TS p.105 / EN p.102)
• TS EN 12845+A2:2026 Clause 14.3, Table 37a (TS p.107-108 / EN p.104-105)
• TS EN 12845+A2:2026 Clause 18.2.3.2 (TS p.119 / EN p.116)
• TS EN 12845+A2:2026 Table G.6 (TS p.153 / EN p.150)

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