Reading the diesel pump clause on site: power margin, fuel, cooling, exhaust and the 15-second rule.
Taking over a logistics warehouse pump room, the inspector said "the diesel's running" and we looked at our watches. The pressure gauge had settled 22 seconds after the manual test. We wrote "non-compliant" on the acceptance record. EN 12845 is explicit: a diesel pump must be fully operational within 15 seconds of the start of the starting sequence. This article reads the diesel clause from a site perspective.
The only reason a diesel pump exists in a fire installation is electrical independence. The standard drills that in: automatic starting and running of the pump set must not depend on any energy source other than the engine itself and its batteries. Even if the mains fails, the generator does not pick up, and nobody reaches the control room, the diesel must start on its own.
Engine selection and power margin
The engine must be capable of running continuously at full load at site altitude and temperature, with continuous power output declared to ISO 3046 — the "prime power" line in a supplier catalogue. In field practice, the selected engine should cover roughly 110 % of the calculated pump shaft power. That margin absorbs hard starting on a cold morning, engine ageing, and parasitic loads from exhaust and accessories. Without it, an engine that passes at the nominal point still throws delay alarms.
The engine must hold speed within ± 5 % through its governor and must be able to start at a pump room temperature of at least 5 °C. Any mechanical device that could prevent automatic starting — a compression release or decompression lever — must return to the starting position; otherwise an engine that runs on test may not run in a real alarm.
Cooling: four options
The standard defines four cooling topologies; types b and c are the most common on site.
| Type | Description | Typical use |
|---|---|---|
| a | Sprinkler pump water direct to the engine jacket, with visible open discharge | Older installations |
| b | Heat exchanger: sprinkler water as primary, engine water in a closed circuit, engine-driven auxiliary pump | The modern standard choice |
| c | Air-cooled radiator with a multi-belt fan and closed circuit | External or roof-level pump rooms |
| d | Direct air cooling with a multi-belt fan | Small engines |
Two things not to miss on a heat exchanger installation: the discharge must run to open drain so the operator can see the cooling flow, and where the auxiliary pump is belt driven it must have multiple belts. The standard requires that if half the belts break, the remainder can still drive the pump or fan. We once saw a single-belt installation; the belt broke and the engine hit a high water temperature alarm within 90 seconds.
A radiator (type c) makes sense in a roof-level pump room. Using sprinkler water as the primary coolant degrades heat exchanger performance in hard-water areas. Closed-circuit capacity must match the supplier's figure exactly.
Fuel tank: at least 6 hours
The core of the calculation:
| Hazard | Full-load running time |
|---|---|
| LH | 3 hours |
| OH | 4 hours |
| HHP / HHS | 6 hours |
On high-hazard storage projects we always size the fuel tank for 6 hours; if an OH warehouse is later converted to HHS, the tank falls short. In practice, take the engine's declared full-load fuel consumption in L/h, multiply by six, then add a 10 % margin and round up to the nearest standard tank.
The tank must be welded steel. Where there is more than one engine, each needs its own tank and its own supply line. If you see one tank serving two engines, write it up. The tank sits above the engine fuel feed pump but not directly above the engine, with a robust level gauge.
On the supply pipework: valves close to the tank, indicating and locked open. Soldered joints are prohibited; metallic pipe is mandatory. The supply is taken at least 20 mm above the tank bottom so sediment does not reach the pump, with a drain valve of at least 20 mm at the bottom. The vent is routed outside the building where possible.
Exhaust: stainless, insulated, discharging outside
The exhaust requirements come down to four rules:
- A silencer is mandatory, and total back pressure must not exceed the supplier's limit — verify with a gauge.
- Where the exhaust terminates above the engine, a drip leg is required so condensate cannot run back into it.
- The exhaust discharges outside, arranged so that gases cannot re-enter the pump room.
- The pipe is insulated and mounted so that it cannot become an ignition source.
In field practice we specify at least 50 mm of stainless steel (typically 304L); carbon steel exhausts soften within a few years and condensate perforates them. Silencer attenuation should be high; in a real alarm the pump room goes above 75 dB, and the alarm acoustics have to carry over it.
Air filter
One sentence in the standard: the engine air intake is fitted with a suitable filter. On site that means a dry-type production filter on the periodic maintenance list. In dusty environments — cement works, flour mills — a pre-filter extends engine life considerably. That is not a standard requirement but common sense.
Automatic starting and the 15-second rule
The automatic starting sequence makes six attempts. Each cranks for 5–10 seconds, with pauses of no more than 10 seconds between them. The system resets itself and operates independently of mains power, alternating automatically between the two batteries after each attempt.
The pump must be at full flow within 15 seconds of the start of the sequence. Horizontal pumps are directly driven, with no coupling or gearbox in between.
The only manual intervention the user has is stopping; engine monitoring devices — high water temperature, low oil pressure and so on — must never stop the engine. Panel programming that "protects the engine from itself" is not acceptable. At site inspection we want to see that behaviour on the programming screen.
Emergency manual starting is provided by a covered button independent of the controls, and a separate manual test button allows periodic testing without breaking the seal on the emergency start.
Batteries, chargers and alarms
Two separate battery sets, used for no other purpose. Either vented nickel-cadmium prismatic cells (EN 60623) or lead-acid (EN 50342-1/-2), 12 V nominal minimum. Each battery has an independent, permanently connected, fully automatic constant-potential charger; one can be removed while the other continues to serve. Lead-acid charging floats at 2.25 ± 0.05 V per cell.
Batteries sit on stands as close as practical to the starter to minimise voltage drop, and out of the path of any oil, fuel or coolant drips.
Four alarms are indicated locally and at a continuously supervised location:
- The automatic start disable switch has been operated.
- The engine failed to start after six attempts.
- The pump is running.
- Diesel controller fault.
Site acceptance test
Cut the fuel, engage automatic starting. Six cycles, each cranking at least 5 s, no more than 15 s, with at least 10 s of rest. At the end a fail-to-start alarm must appear automatically. Restore the fuel; pressing the manual test button must start the engine. Taking over a facility without that test on the acceptance record is a mistake in principle.
Common field errors
- Pump sets delivered with a single battery, purely on cost.
- Exhaust terminating close to the pump room ventilation louvre, so gases return.
- Fuel tank mounted above the engine — explicitly prohibited.
- A single-belt auxiliary cooling pump, breaching the half-the-belts rule.
- No manual test button, so testing requires breaking the emergency start cover.
- No user stop facility on the diesel panel — we have seen operators forced to cut power to stop an engine.
- Soldered joints on the fuel line.
Comparison with NFPA
NFPA 20 allows 20 seconds for a diesel pump to come on line; EN 12845 is stricter at 15. NFPA 20 sizes the fuel tank at 1 gallon per HP-hour plus 5 % expansion; EN 12845 fixes it by hazard class at 3, 4 or 6 hours. NFPA 20 requires dual ECM control on each engine; EN 12845 sets no such requirement, but the two-battery rule follows the same philosophy.
Turkish context
BYKHY treats EN 12845 as a referenced standard, and TS EN 12845 is the official text applied in Turkey. In climates where the pump room can fall to 5 °C — central and eastern Anatolia — watch the engine start temperature limit; an unheated roof-level pump room cannot start a frozen engine. Fuel quality must meet the supplier's specification, and the seasonal changeover between summer and winter diesel should not be delayed.
Frequently asked questions
How quickly must a diesel engine come on line?
Fully operational within 15 seconds of the start of the starting sequence. Automatic starting makes six attempts of 5–10 s each, with pauses of no more than 10 s.
How many hours of fuel are required?
Three hours for light hazard, four for ordinary hazard and six for high hazard process and storage, at full load. The tank is welded steel, above the engine fuel feed pump but never directly above the engine.
Can a diesel pump depend on mains power?
No. Automatic starting and running must not depend on any energy source other than the engine and its batteries.
What may stop the engine?
Only a manual command. Monitoring devices cannot stop it — high temperature and low oil pressure raise signals only.
What is the most common exhaust error?
Terminating the exhaust close to the pump room ventilation louvre. The standard requires the arrangement to prevent gases re-entering the pump room.
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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.