From the pump flange into the tank: diameter, taper, foot valve and vortex breaker — the detail list that keeps air out.
During commissioning at a logistics warehouse, the diesel pump started, the pressure gauge climbed to 4 bar, then collapsed to 1.2 bar after six seconds with a muffled growl from the suction side. Stripping the suction pipe showed why: the installer had fitted a concentric reducer, and a fist-sized air pocket had collected at the top of the cone. The suction clause forbids exactly this error — and it is still the most common suction pipe defect we see. This article covers the pipe itself: diameter, taper, valves, foot valve and fall.
What the standard requires
EN 12845:2015+A1:2019 describes suction conditions, and the suction pipe sub-clauses define:
- General: a straight or tapered pipe connects to the pump inlet, at least two diameters long. On a tapered piece the upper face must be horizontal and the internal angle must not exceed 20°.
- Positive head: suction pipe at least DN65, velocity no more than 1.8 m/s.
- Suction lift: suction pipe at least DN80, velocity no more than 1.5 m/s, with the pump centreline no more than 3.2 m above the tank low water level.
- Priming: a separate automatic priming tank for each pump, located above the pump and connected to the pump discharge side by a sloped pipe fitted with a non-return valve.
In addition, NPSH available must be at least NPSH required plus 1 m at maximum flow.
Suction diameter: never smaller than the pump flange
The standard gives DN65 and DN80 as minimums, but a second rule governs in practice: the suction pipe must not be smaller than the pump suction flange. Most sprinkler pumps have a DN125–DN200 suction flange; the 65 and 80 mm figures are absolute floors, and the real design comes from velocity.
| Pump flow | Positive head (1.8 m/s) | Suction lift (1.5 m/s) |
|---|---|---|
| 30 L/s (108 m³/h) | DN150 | DN200 |
| 50 L/s (180 m³/h) | DN200 | DN200 |
| 80 L/s (288 m³/h) | DN250 | DN300 |
| 120 L/s (432 m³/h) | DN300 | DN350 |
Derived from Q = v × A. Once the pump is selected, the suction diameter is the largest of what the velocity limits, the pump flange and the NPSH calculation require.
Eccentric reducer: flat face always up
Where the suction pipe is larger than the pump inlet — which it usually is — the transition is made with an eccentric reducer. One face of an eccentric cone is completely flat, and that flat face is fitted facing upward, so the internal crown of the pipe stays at a constant level and no pocket forms where air can collect as the flow accelerates.
The field error: the installer fits a concentric reducer "so it's centred", or fits the eccentric upside down with the flat face beneath. Either way, air accumulates at the top of the cone for minutes and eventually turns into cavitation the moment the pump is called. By requiring the upper face of the taper to be horizontal, the standard effectively mandates an eccentric reducer.
Preventing air pockets: rising fall, no high point
The clause is explicit: the suction pipe is laid horizontally, or on a continuous slight rise toward the pump. A fall of 1/100 to 1/200 is enough. That fall means air cannot lodge in the pipe as the system fills; it escapes back toward the tank and surfaces.
The three errors we see most often:
- The pipe drops as it approaches the pump — the high point sits on the tank side and air collects there.
- An elbow creates a high point — an inverted U trapping air.
- What looks horizontal is actually laid to a fall the wrong way, because no gradient was designed and the installer improvised on site.
Walking a pump room, the quickest check is a water level or a phone inclinometer. If there is a high point, the right answer is to correct the pipe — not to fit an automatic air vent. No standard accepts a valve at the high point of a suction line.
Foot valve and vortex breaker
Where the pump centreline is above the tank low water level (suction lift), a foot valve is mandatory. It goes at the lowest point of the suction pipe, holds the prime, and works alongside the priming arrangement: the priming tank sits above the pump, the connecting pipe is sloped and fitted with a non-return valve, and the system stays permanently water-filled.
In a positive head installation a foot valve is not mandatory, though many projects fit one anyway because it makes isolating the pump for maintenance easier. Without one, fit a butterfly valve on the suction side close to the pump — where several pumps share suction, interconnection is only permitted through isolating valves.
A vortex breaker is not named explicitly in the standard text, but the low water level definitions and the suction intake arrangements make it a practical necessity. The typical detail is a horizontal square plate around the intake, roughly eight times the pipe diameter across, or an internal baffle plate one diameter above the tank floor. If a vortex forms, the pump draws air; even with the foot valve holding, NPSH collapses and cavitation begins.
Multiple pumps: separate lines, or interconnection with isolating valves
Under positive head, two pumps' suction pipes may be interconnected — provided each line has an isolating valve, so one pump can be maintained while the other stays in service. Under suction lift, interconnection is prohibited: each pump must have its own independent suction pipe. This rule is routinely ignored on site, with two diesel pumps hung off a single manifold in which air collects, so that neither runs properly.
The 1 m NPSH margin
NPSH available, calculated at the highest expected water temperature, must be at least NPSH required plus 1 m. That margin must remain after deducting suction pipe friction, foot valve loss, and the losses through the eccentric reducer and elbows. On the manufacturer's curve, NPSH required reaches its worst value at maximum flow — calculate at that point, not at the nominal duty.
Common field errors
- Concentric reducer: air pocket at the crown. Fix: eccentric, flat face up.
- Suction pipe smaller than the pump flange: NPSH collapses. Fix: at least flange diameter, with velocity limits observed.
- Wrong gradient: a slight fall toward the tank creates a high point. Fix: continuous rise toward the pump.
- No vortex breaker: the pump draws air with the tank half full. Fix: an anti-vortex plate at the intake.
- Missing foot valve: the prime will not hold under suction lift. Fix: foot valve at the lowest point plus an automatic priming tank.
- Two pumps on one suction manifold under suction lift: prohibited. Fix: separate suction pipe, foot valve and priming for each.
Comparison with NFPA
NFPA 20 states the vortex plate requirement and the horizontal upper face rule for eccentric reducers explicitly. There is no difference in principle between the two standards here; the difference is in detail. EN 12845 gives the DN65/DN80 minimums and the 1.8/1.5 m/s velocity limits directly, while NFPA 20 derives minimum diameter from a table indexed to rated pump flow. The results are usually close, and on projects applying both, the difference generally stays within one DN step.
Link to Turkish regulation
BYKHY accepts both EN 12845 and NFPA 13 as design references for sprinklers. Pump room detail is not specified by diameter in the regulation; it is left to the design standard. Certified foot valves, EN 12845-compatible eccentric reducers and flanged butterfly valves are all readily available in Turkey. The two items that most often attract negative findings at site acceptance are a missing vortex breaker and a concentric reducer; marking both explicitly on the drawing before contract saves hours at commissioning.
Frequently asked questions
What is the minimum suction pipe diameter?
At least DN65 with velocity no more than 1.8 m/s under positive head, and at least DN80 with velocity no more than 1.5 m/s under suction lift. It can never be smaller than the pump flange.
Which way does the flat face of an eccentric reducer point?
Upward, with the tapered face below. That keeps the internal crown of the pipe at a constant level so no air pocket forms.
When is a foot valve mandatory?
Whenever the pump centreline sits above the tank low water level. It is fitted at the lowest point of the suction pipe and works with an automatic priming tank.
Why does the suction gradient matter?
Because a high point in the pipe traps air, and that air destroys NPSH the moment the pump starts. A horizontal run or a continuous slight rise toward the pump prevents it.
What is wrong with a concentric reducer?
Centring the cone lowers the internal crown, and air collects there. Because the standard requires the upper face of the taper to be horizontal, a concentric reducer is effectively prohibited.
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Download MEP Calc on the App StoreBS EN 12845:2015+A1:2019 Fixed firefighting systems — Automatic sprinkler systems. EN 12845-2:2024 (CMSA & ESFR sprinkler systems). NFPA 13 Standard for the Installation of Sprinkler Systems. Turkish Regulation on Fire Protection of Buildings (BYKHY). FM Global Property Loss Prevention Data Sheet 2-0.