EN 12845 Pump Suction Conditions and Pump Room Layout

Positive suction head, the NPSHa-versus-NPSHr margin, pump room fire resistance and temperature control — the root of the site errors that lead to cavitation.

Commissioning a logistics warehouse with HHS3 protection, the pump delivered 320 m³/h at its duty point and then dropped to 240 after three minutes. The gauge trembled and the casing sounded as if it were churning gravel. Water was at 12 °C, the tank was full and the calculation was clean on paper. The problem was the suction geometry: the tank outlet sat only 80 cm below the pump centreline, and as a vortex drew in air, cavitation began. The EN 12845 pump room and suction rules exist precisely to prevent this; unless the pump room and suction clauses are read together, the design is incomplete.

The positive suction head requirement

EN 12845 is direct: wherever possible, horizontal centrifugal pumps shall be used, installed with a positive suction head. That is the only preference; everything else is an exception. Two conditions apply together:

• At least two thirds of the tank's effective volume must lie above the pump centreline;
• The pump centreline may be no more than 2 m above the tank low water level.

Where that cannot be achieved, the standard permits suction lift or a vertical turbine pump, but immediately adds that suction lift and submersible arrangements should be avoided and used only where nothing else is practicable. At high risk — HHS, HHP3, HHP4 — inspectors and insurers do not accept suction lift at all, so it belongs in the design as a siting decision at the drawing stage.

NPSHa and NPSHr — the 1 m margin

The standard requires the suction pipe, valves and fittings to be sized so that NPSHa, calculated at the maximum anticipated water temperature, exceeds the pump's catalogue NPSHr by at least 1 m at maximum flow. The field calculation:

NPSHa = (P_atm - P_v)/(rho*g) + Z - h_f - h_loc

P_atm = atmospheric pressure (10.33 m at sea level)
P_v   = vapour pressure of water (0.33 m at 25 C)
Z     = positive suction head (tank level minus pump centreline, m)
h_f   = pipe friction loss, m
h_loc = local losses (foot valve, elbows, strainer), m

A worked example: 200 m³/h duty for HHS2 with DN150 suction pipe, Z = +1.5 m, 8 m of pipe, no foot valve (not required under positive head), one elbow, one valve and a suction strainer, water at 30 °C.

• Atmospheric head = 10.33 m
• Vapour pressure head at 30 °C = 0.43 m
• Z = +1.5 m
• At DN150, v = 200/(3600 × π × 0.075²) = 3.14 m/s — far too high

The standard caps velocity at 1.8 m/s under positive head. Moving to DN200 gives v = 1.77 m/s, with typical h_f of 0.8 m and h_loc of 0.6 m. NPSHa = 10.33 − 0.43 + 1.5 − 0.8 − 0.6 = 10.0 m. If the catalogue NPSHr at 200 m³/h is 4.5 m, the margin is 5.5 m — comfortably safe. Left at DN150, h_f would rise to 2.5 m, the margin would fall below 1 m and the clause would be breached.

Suction pipe rules

Parameter	Positive head	Suction lift
Minimum size	DN65	DN80
Maximum velocity	1.8 m/s	1.5 m/s
Maximum static difference	2 m (pump above)	3.2 m (pump above)
Foot valve	Not required	Mandatory
Automatic priming	No	Yes
Interconnection between pump suctions	Yes, through stop valves	Prohibited

The suction pipe is laid horizontally or on a continuous slight rise toward the pump; anything else creates an air pocket, and commissioning becomes a matter of bleeding tens of litres of air before pressure can be raised. The taper at the pump inlet has its upper face horizontal, with an included angle no greater than 20° — so air cannot collect on the crown of the cone.

Pump room fire resistance

EN 12845 requires at least 60 minutes of fire resistance for the pump room (REI 60). The order of preference:

1. A separate building (best);
2. A separate compartment attached to the sprinklered building, with direct external access;
3. A compartment inside the sprinklered building, with direct external access.

In practice a separate building is a luxury. In multi-storey industrial buildings a compartment is preferred; but where the adjoining fire load is around 1200 MJ/m² or the protected value is critical, insurer data sheets require REI 120. For insured work, specifying REI 120 from the start is the safest route. Walls, ceiling, door (EI₂ 60-S₂₀₀) and service penetrations must all be in the same rating class; a 30 minute door makes a 120 minute wall irrelevant.

Temperature 4–25 °C and ventilation

The standard sets the lower limits: 4 °C for electric motor pumps and 10 °C for diesel (for oil viscosity and battery performance). No upper limit is given for the room, but water temperature is capped at 40 °C. In practice, above 25 °C the electrical panel, the batteries and the diesel ECU start causing trouble, so the working field band is 4–25 °C. Where there is a diesel pump, adequate ventilation is required: run the exhaust outside, fit a fresh air louvre and verify by calculation (a rule of thumb being roughly 0.15 m³/s of fresh air per BHP). In freezing climates the fans are thermostatically controlled, set to a lower limit of 8–10 °C.

Field error: pump low, tank high — vortex risk

The reasoning that "the tank is above the pump, gravity feeds it, so NPSH is guaranteed" produces surprises at commissioning. Even where the tank outlet is 5–6 m above the pump centreline, if the clearance dimensions between the pipe and the tank floor and wall are not satisfied, a vortex forms at the outlet. The typical site picture:

• Tank outlet flange only 50 mm from the tank floor;
• Only 200 mm between the outlet and the nearest wall;
• No anti-vortex plate;
• Fine at duty point, but once a second pump runs in parallel and flow rises by half, inlet velocity exceeds 1.5 m/s and air is drawn in.

The solution is geometry, not hardware: raise the suction outlet about half a diameter above the tank floor, weld a horizontal plate at least two diameters across above it, and set the fill level so that the water surface is at least 600 mm above the outlet flange. Building a sump and using the "with sump" clearance figures is the most robust approach.

Comparison with NFPA

NFPA 20 sets very similar pump room rules: two hours of fire resistance (equivalent to REI 120), a 4–40 °C operating range (broader than EN's practical 4–25 °C), positive suction preferred and suction lift to be avoided. On the NPSH margin, NFPA 20 has no explicit 1 m rule, though it does require that components such as valves or vortex plates not be removed before NPSH testing. The EN 12845 1 m margin is numerically the stricter, and it is the figure that ends the argument on European projects.

Turkish context

BYKHY describes the fire pump room generally as a fire-separated compartment without giving figures; in practice EN 12845 or NFPA 20 governs. In GRP panel tanks, the distance between the outlet flange and the floor is left to the manufacturer's catalogue, and placing the flange very close to the floor is a common error. Raising the outlet 250–300 mm and adding a vortex breaker both preserves the warranty and satisfies the standard. In a diesel pump room, the fuel tank must sit above the engine fuel pump. Running the exhaust out of the room and mounting the silencer on an external wall is mandatory for both noise and temperature.

Site check list

• Is two thirds of the tank's effective volume above the pump centreline?
• Is the pump centreline no more than 2 m above the low water level?
• Is NPSHa minus NPSHr at least 1 m, calculated at maximum temperature?
• Is the suction pipe within the DN65/DN80 minimum and the 1.8/1.5 m/s velocity limits?
• Is the taper 20° with a horizontal upper face, and the suction pipe horizontal or rising to the pump?
• Is the pump room REI 60, or REI 120 where insured?
• Is the fire door EI₂ 60-S₂₀₀ with firestopped penetrations?
• Is the temperature within 4–25 °C, with a 10 °C lower limit for diesel?
• Is there a vortex breaker and a half-diameter rise at the tank outlet?
• Has the sump geometry been verified against the clearance table?

Frequently asked questions

What is the minimum fire resistance for a pump room?

At least 60 minutes (REI 60). Insurer specifications generally require REI 120, and specifying 120 with door, wall and firestopping all in the same class is the safest approach.

Which water temperature should be used for NPSHa?

The maximum anticipated water temperature. In an above-ground tank in summer, 30–35 °C is realistic; at that temperature vapour pressure rises and NPSHa loses 0.2–0.3 m. Be conservative.

Why is there a vortex risk when the tank is above the pump?

Because if the clearance dimensions at the tank outlet are not respected, rapid suction sets up natural rotation and an air core opens at the inlet. NPSHr is then exceeded and cavitation follows. Raise the outlet half a diameter, add a vortex breaker plate and use a sump geometry.

What is the minimum pump room temperature?

4 °C for electric pumps and 10 °C for diesel. No upper limit is stated, but 25 °C is the practical ceiling for electronics and batteries.

What is the field difference between suction lift and positive head?

Positive head means two thirds of the tank's effective volume sits above the pump centreline with no negative static difference. Suction lift means the pump is above the tank, requiring a foot valve and automatic priming. EN 12845 says to avoid suction lift, and at high risk insurers will not accept it.

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