EN 12845 Fitting and Valve Pressure Losses

In a fully calculated sprinkler system, fitting and valve losses often add a head equal to half the straight pipe loss. TS EN 12845+A2:2026 resolves these losses not with a separate resistance coefficient method but with the equivalent pipe length method: each fitting is converted into the length of straight pipe that gives the same friction loss and is added to the straight pipe length. Source: TS EN 12845+A2:2026, Clause 13.2, EN p.82-84.

The basic formula: Clause 13.2.1

Pipe friction loss calculations shall be not less than those derived from the Hazen-Williams relationship (EN p.82):

p = 6.05 × 10^5 × L × Q^1.85 / (C^1.85 × d^4.87)

• p: pressure loss in the pipe, bar
• Q: flow in the pipe, l/min
• d: mean internal diameter of the pipe, mm
• C: constant for the type and condition of the pipe (see Table 22)
• L: equivalent length of pipe and fittings, m

The definition of L is decisive here: the standard defines L not as "the length of the pipe" but as "the equivalent length of pipe and fittings". In other words, the fitting loss is not added as a separate term; it goes directly into L.

The velocity pressure loss may be neglected (EN p.83).

Table 22: C values by pipe type (EN p.83)

Note: the standard states that this list is not exhaustive.

Which fittings count: Clause 13.2.4

Friction loss in valves and in fittings where the direction of water flow changes by 45° or more is calculated using the formula in 13.2.1. The appropriate equivalent length is one of the following (EN p.83-84):

• a) as specified by the equipment supplier;
• b) where a) is not available, as taken from Table 23.

The order is binding: where supplier data exist, they are used, and the table comes into play only where no such data are available.

Where there is an elbow, tee or cross in which the direction of flow changes and there is also a change of diameter at the same point, the equivalent pipe length and the pressure loss are determined using the smaller diameter (EN p.84).

Table 23: Equivalent length of fittings and valves

The following table gives equivalent lengths in metres for straight steel pipe with a C value of 120 (EN p.84).

The figures in the top row are nominal sizes in mm.

C value correction factors (footnote to Table 23, EN p.84)

Example: a DN100 90° welded elbow in steel pipe is 1.4 m. The same fitting on a cast iron line with C = 100 enters the calculation with an equivalent length of 1.4 × 0.714 = 1.00 m. On a copper or stainless line with C = 140 it becomes 1.4 × 1.33 = 1.86 m.

The effect of equivalent length on the total loss

The following example uses only Table 23 values and addition.

Consider a DN100 steel line: 42 m of straight pipe, with 4 no. 90° welded elbows (r/d = 1.5), 1 no. standard screwed tee (flow in the branch direction), 1 no. gate valve and 1 no. swing type alarm or check valve.

On this line the fittings increase the length entering the calculation by 17.61 / 42.00 = 41.9 %, that is by roughly 42 %. Since p varies linearly with L in the relationship of Clause 13.2.1, the friction loss on this line also increases in the same proportion, by roughly 42 %.

Note: the standard gives no table of unit loss in bar. To calculate a value in bar, the mean internal diameter (d) of the pipe must be known, and that value is not the same as the nominal size; the internal diameter follows from the wall thickness of the pipe used. Clause 17.1.2 (EN p.111) requires that, where steel pipes of nominal size equal to or less than 150 mm are threaded, grooved or otherwise worked, the minimum wall thickness conform to ISO 65, and that, where the pipe ends are formed without significantly reducing the wall thickness (for example roll grooving or end preparation for welding), it conform to ISO 4200 series D. The unit loss in bar can therefore be calculated only after the pipe standard and series have been chosen.

Velocity limits: Clause 13.2.3

Under steady flow conditions in the hydraulically most favourable and most unfavourable areas of operation, the water velocity does not exceed the following (EN p.83):

• 6 m/s through any valve, flow monitoring device and/or strainer;
• 10 m/s at any other point in the system.

Calculation precision: Clause 13.2.5 and Table 24

Calculations are made using the units and precision given in Table 24 (EN p.85):

Clause 13.2.5.2 requires calculations to be balanced as follows: the algebraic sum of the pressure losses in a circuit shall be (0 ± 1) mbar; where water flows combine at a node, the calculation is balanced to ± 1 mbar; the algebraic sum of the water flows at a node shall be (0 ± 0.1) l/min.

The static pressure difference is calculated with the relationship in Clause 13.2.2 (EN p.83): p = 0.098h, where p is the static pressure difference in bar and h is the vertical distance between the two points in metres.

Reporting: Clause 4.4.3

The following are given for each hydraulically significant pipe (Clause 4.4.3.3 f, EN p.27):

1. the pipe node or other reference number;
2. the nominal size in mm;
3. the Hazen-Williams constant;
4. the flow in l/min;
5. the velocity in m/s;
6. the length in m;
7. the number, types and equivalent length in m of fittings and components;
8. the change in static head in m;
9. the inlet and outlet pressures in bar;
10. the friction loss in bar;
11. an indication of the direction of flow.

In other words, the provision to "list the fitting equivalent lengths" is not in Clause 13 but in Clause 4.4.3, which governs documentation.

The only special line for which fitting loss is separately regulated

Clause 16.1.3 (EN p.110) covers the pipe to the water motor alarm separately: the pipe is 20 mm in diameter and of galvanized steel or non ferrous metallic material. The equivalent pipe length between the alarm valve and the water motor is not more than 25 m, assuming an equivalent length of 2 m for each change of direction. A stop valve located within the premises is fitted in the pipe, and permanent drainage is provided through an orifice not exceeding 3 mm in diameter. The orifice plate may be integral with a pipe fitting and is made of stainless steel or non ferrous material.

This line is the only place where the standard gives an equivalent length outside Table 23.

Frequently Asked Questions

Which fittings are taken into account in EN 12845?

TS EN 12845+A2:2026 Clause 13.2.4 (EN p.83): friction loss in valves and in fittings where the direction of water flow changes by 45° or more is calculated using the formula specified in 13.2.1. The appropriate equivalent length is either a) as specified by the equipment supplier, or b) where a) is not available, as taken from Table 23. Fittings where the change of direction is less than 45° are not included in this calculation.

Which diameter is used where both direction and diameter change at the same point?

Clause 13.2.4 (EN p.84): for an elbow, tee or cross where the direction of flow changes and there is also a change of diameter at the same point, the equivalent pipe length and the pressure loss are determined using the smaller diameter.

For which C value is Table 23 given, and what is done for other pipes?

Table 23 (EN p.84) gives equivalent lengths for straight steel pipe with a C value of 120. The footnote to the table states that equivalent lengths for other C values may be converted with the following factors: 0.714 for C=100; 0.85 for C=110; 1.00 for C=120; 1.16 for C=130; 1.33 for C=140. The C values by pipe type are in Table 22 (EN p.83): cast iron 100, ductile iron 110, mild steel 120, galvanized steel 120, sprayed cement 130, cement lined cast iron 130, stainless steel 140, copper 140, glass reinforced plastic 140.

With which formula is friction loss calculated?

Clause 13.2.1 (EN p.82): pipe friction loss calculations shall be not less than those derived from the Hazen-Williams relationship: p = 6.05 × 10^5 × L × Q^1.85 / (C^1.85 × d^4.87). Here p is the pressure loss in bar, Q the flow in l/min, d the mean internal diameter of the pipe in mm, C a constant for the type and condition of the pipe, and L the equivalent length of pipe and fittings in metres. The velocity pressure loss may be neglected.

How are fitting equivalent lengths shown in the hydraulic report?

Clause 4.4.3.3 f) 7) (EN p.27): for each hydraulically significant pipe, the number, types and equivalent length in metres of fittings and components are reported. The same list also requires the pipe node number, the nominal size in mm, the Hazen-Williams constant, the flow in l/min, the velocity in m/s, the length in metres, the change in static head in metres, the inlet and outlet pressures in bar, the friction loss in bar, and an indication of the direction of flow.

References

• TS EN 12845+A2:2026 Clause 4.4.3.3 f) (EN p.27)
• TS EN 12845+A2:2026 Clauses 13.1, 13.2.1 (EN p.82)
• TS EN 12845+A2:2026 Table 22, Clauses 13.2.2, 13.2.3, 13.2.4 (EN p.83)
• TS EN 12845+A2:2026 Table 23 and its footnote (EN p.84)
• TS EN 12845+A2:2026 Clause 13.2.5.1, Table 24, Clause 13.2.5.2 (EN p.85)
• TS EN 12845+A2:2026 Clause 16.1.3 (EN p.110)
• TS EN 12845+A2:2026 Clause 17.1.2 (EN p.111)

SprinkCalc — Fire Sprinkler Design Across Three Standards

SprinkCalc covers hazard classification, design density and area, K-factor selection, water demand and hydraulic calculations for NFPA 13, FM Global and BS EN 12845 in a single iOS app, and exports a professional PDF report.

Download SprinkCalc on the App Store

MEP Calc — 86+ Engineering Calculators

MEP Calc bundles 86+ engineering modules in one iOS app: 21 fire calculations plus heating, cooling, HVAC, plumbing, steam and natural gas.

Download MEP Calc on the App Store