EN 12845 Suction Pipe Design and Details

The suction pipe is the least forgiving part of a sprinkler pump set: the wrong diameter, the wrong slope or an inadequate NPSH margin translates directly into cavitation and loss of flow. TS EN 12845+A2:2026 covers this subject in Clause 10.6.2 and defines two different regimes: positive suction head and suction lift. Source: TS EN 12845+A2:2026, Clause 10.6, EN p.59-62.

First, the order of preference: Clause 10.6.1

Wherever possible, horizontal centrifugal pump sets are used and installed under positive suction head, that is (EN p.59):

• at least two thirds of the effective capacity of the suction tank shall be above the pump axis;
• the pump axis shall be not more than 2 m above the low water level of the suction tank (level X in 9.3.5).

Where this is not possible, the pump may be installed under suction lift conditions or vertical turbine pumps may be used. Suction lift and submersible pump arrangements should be avoided and used only where it is not practicable to arrange a positive suction head.

10.6.2.1 General provisions

• The pump suction is connected to a straight or tapered pipe at least two diameters long.
• The upper surface of a tapered pipe is horizontal and the total included angle does not exceed 20°.
• The suction pipework, including all valves and fittings, is designed in accordance with EN 17451 so that the NPSH available at the pump inlet, calculated at the maximum expected water temperature, exceeds the NPSH required by at least 1 m at the maximum pump set flow shown in Table 14.
• The suction pipework is laid either horizontally or with a continuous slight rise towards the pump, in order to prevent an air lock forming in the pipe. No numerical slope value is given.
• Where the pump axis is above the low water level, a foot valve is fitted (see 9.3.5).

Source: EN p.59.

Table 14: Pump pressure and flow assessment (EN p.59)

Note: in the printed text the expression in the HH row reads "Pressure and 1,4 – flow required from Table 7". Since the mark used here is a dash, it cannot be established from the printed text alone whether the factor is 1.4 times or some other operation. In design this row should be interpreted together with Table 7 and, if necessary, in consultation with the approval authority.

The numerical requirements of the two regimes

Source: EN p.59-60.

Checking the diameter against the velocity limit

The standard gives only the two velocity limits; it gives no table of maximum flow by diameter. The following table is derived from those two limits using the relationship v = 4Q/(πd²), taking the nominal diameter as the basis of calculation. In a real project the internal diameter, which depends on the pipe wall thickness, should be used; a calculation based on the nominal diameter is for preliminary screening only.

Worked check: if a maximum demand flow of 3 000 l/min is carried in DN200 under suction lift conditions, the velocity works out at 21.22 × 3 000 / 200² = 1.59 m/s and the 1.5 m/s limit of 10.6.2.3 is exceeded; DN250 is required at this flow, because in DN250 the velocity becomes 21.22 × 3 000 / 250² = 1.02 m/s. The same flow in DN200 under positive head gives 1.59 m/s and stays below the 1.8 m/s limit of 10.6.2.2. In other words, the same flow requires a different diameter depending on the suction regime.

10.6.2.4 Pump priming

Each pump set is provided with a separate automatic priming arrangement. The arrangement consists of the following (EN p.60):

• a tank located at a higher level than the pump;
• a sloping pipe connection from the tank to the delivery side of the pump;
• a check valve fitted in that connection.

Figure 6 shows two examples: example A with a low level switch, example B with a low level valve for pump starting.

The tank, the pump of the pump set and the suction pipework are kept continuously full of water, even if there is leakage from the foot valve referred to in 10.6.2.3. If the water level in the tank falls to 2/3 of the normal level, the pump shall start. Alternatively, an alarm is transmitted to a permanently attended location so that immediate action can be taken.

Annex I, Table I.1 (EN p.155) classifies this alarm as a type B (technical) alarm in the row "low level priming tank, Clause 10.6.2.4".

Table 15: Pump set priming tank capacity and pipe size (EN p.62)

10.6.2.5 Pressure maintenance pump

A pressure maintenance pump may be installed to prevent unnecessary starting of one of the main pump sets or to maintain the system pressure above the control valve sets in the case of water supplies with fluctuating pressure, such as a town main (EN p.60).

• The pressure maintenance pump is sized and arranged so that it cannot provide sufficient flow and pressure for a single open sprinkler and thereby prevent the main pump sets from starting.
• Where pressure maintenance pumps are installed with a negative suction, the suction pipework and fittings are independent of the pipework of the main pump or pumps.
• Note: some water authorities may not permit a pressure maintenance pump on systems connected to a town main.

Valves, strainers and fittings on the suction pipe

Clause 10.5 (EN p.58):

• A stop valve is fitted in the pump suction pipe unless the maximum water level is below the pump.
• A check valve and a stop valve are fitted in the delivery pipe of each pump.
• For booster pumps, a bypass is installed around the pumps; the bypass contains a check valve and two stop valves, all three of the same size as the main header.
• The tapered pipe fitted at the pump outlet expands at an angle not exceeding 20° in the direction of flow. Valves on the delivery side are fitted after the tapered pipe.
• Arrangements are made to vent all cavities in the pump casing, unless the pump is self venting by the arrangement of the pump branches.
• Arrangements are made to ensure a continuous flow of water sufficient to prevent overheating while the pump is running against a closed valve. This flow is taken into account in the system hydraulic calculation and in the selection of the pump. The outlet is clearly visible and, where there is more than one pump, the outlets are separate.
• The connection points on the pump for the inlet and outlet pressure gauges are readily accessible.

Clause 9.3.6 (EN p.51):

• For pump sets under suction lift conditions, a strainer is fitted in the pump suction pipe upstream of the foot valve. The strainer is fitted so that it can be cleaned without the need to drain the tank.
• For open tanks feeding pumps under positive head conditions, a strainer is fitted in the suction pipe outside the tank. A stop valve is placed between the tank and the strainer.
• The cross sectional area of the strainers is at least 1.5 times the nominal area of the pipe and they do not permit the passage of objects larger than 5 mm in diameter.

Clause 20.3.4.5 (EN p.122) requires, in the annual routine, that pump set suction strainers, the settling chamber and screens be inspected at least annually and cleaned where necessary.

Frequently Asked Questions

What is the velocity limit in an EN 12845 suction pipe?

TS EN 12845+A2:2026 Clause 10.6.2.2 (EN p.59): under positive head conditions the suction pipe size shall not be less than 65 mm and a velocity of 1.8 m/s shall not be exceeded with the pump running at its maximum demand flow. Clause 10.6.2.3 (EN p.60): under suction lift conditions the size shall not be less than 80 mm and a velocity of 1.5 m/s shall not be exceeded at the maximum demand flow. In addition, on suction lift the height from the low water level to the pump axis does not exceed 3.2 m.

What does EN 12845 require for NPSH?

Clause 10.6.2.1 (EN p.59): the suction pipework, including all valves and fittings, is designed in accordance with EN 17451 so that the NPSH available at the pump inlet, calculated at the maximum expected water temperature, exceeds the NPSH required by at least 1 m at the maximum pump set flow shown in Table 14.

May suction pipes be interconnected?

Conditionally yes under positive head, and no on suction lift. Clause 10.6.2.2 (EN p.60): where there is more than one pump set, the suction pipes may be interconnected only if they are provided with stop valves which allow one to continue to operate while the other is removed for maintenance; the connections are sized for the required flow. Clause 10.6.2.3 (EN p.60): on suction lift, where there is more than one pump set, the suction pipes shall not be interconnected.

What capacity shall the priming tank have?

Table 15 (Clause 10.6.2.4, EN p.62): for LH the minimum tank capacity is 100 litres and the minimum priming pipe size 25 mm; for OH, HHP and HHS, 500 litres and 50 mm. Clause 10.6.2.4 (EN p.60) requires each pump set to be provided with a separate automatic priming arrangement, the tank to be at a higher level than the pump, a sloping pipe connection to run from the tank to the delivery side of the pump, and a check valve to be fitted in that connection.

What shall the slope of the suction pipe be?

Clause 10.6.2.1 (EN p.59): the suction pipework is laid either horizontally or with a continuous slight rise towards the pump, in order to avoid the possibility of an air lock forming in the pipe. The standard gives no numerical slope percentage or ratio here.

References

• TS EN 12845+A2:2026 Clause 9.3.6 (EN p.51)
• TS EN 12845+A2:2026 Clause 10.5 (EN p.58)
• TS EN 12845+A2:2026 Clauses 10.6.1, 10.6.2.1, Table 14 (EN p.59)
• TS EN 12845+A2:2026 Clauses 10.6.2.2, 10.6.2.3, 10.6.2.4, 10.6.2.5 (EN p.59-60)
• TS EN 12845+A2:2026 Figure 6 (EN p.61)
• TS EN 12845+A2:2026 Table 15 (EN p.62)
• TS EN 12845+A2:2026 Clause 20.3.4.5 (EN p.122)
• TS EN 12845+A2:2026 Annex I, Table I.1 (EN p.155)

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