Diesel engine power for fire pumps: This is the key issue when determining whether a diesel engine is truly suitable for driving a main pump. In this response, we review the criteria set forth in UNI EN 12845 and explain how to apply them in practice.*
Table of Contents
Where to start with UNI EN 12845 (sections 10.1 and 10.9.1)
The UNI EN 12845 standard addresses the issue of the power required by the engine, clarifying in section 10.1 that the engine power must be calculated by considering the points on the pump’ s power characteristic curve (this applies to both diesel and electric engines).
For diesel engines, compliance with section 10.9.1 is also required; this section specifies a particular method for delivering the required power.
Power Characteristic Curve: What You Really Need to Look For
The criterion is not to “choose an engine based on gut feeling” or to look at just a single value: the operational reference is the power curve.
In other words, the points on the curve that represent the pump’s power consumption levels are the ones that determine how much power the motor driving the pump (diesel or electric) must be capable of delivering.
This step is essential because it shifts the focus from the “rated power” to where and how the pump draws power during operation.
Diesel engine: power requirement according to the “NA curve”
To be consistent with the requirements for diesel engines, the engine must deliver the required power according to the NA curve, specified as:
- Continuous power with 10% overload capacity
- DIN 6271 – ISO 3046-1
This is the performance profile in question: it’s not enough to “reach it once”; rather, the goal is to maintain a continuous power output in line with the curve shown.
Case 1: Pumps with a power curve without overload (e.g., VTP)
In the case of pumps with a power characteristic curve without overload, the criterion is clear: the diesel engine must have a power rating higher than that required by the pump, specifically:
- greater than the maximum power consumption at the peak of the power curve
In the original text, this case is attributed—for clarity’s sake—to VTPs. The practical point is that the “peak” of the curve serves as the benchmark for the test: if the engine does not reach that maximum, it is not adequate.
Case 2: Pumps with an increasing performance curve (e.g., single-impeller centrifugal pumps)
In the case of pumps with an increasing power characteristic curve, the comparison is not based on the “peak” in a general sense, but on the condition under which the pump requires a specific NPSHr to operate.
The specified requirement is that the diesel engine must have a power output of:
- greater than the power consumed by the pump when it requires an NPSHr of 16 m
In the original text, this is associated—to clarify—with single-stage centrifugal pumps. Here, the operational concept is as follows: identify that operating point (linked to the specified NPSHr) and verify that the motor power is greater than the power consumption under those conditions.
Continuity at full load and installation altitude (ISO 3046)
In addition to the “greater than…” requirement, there is a performance requirement: the engine must be capable of delivering that power:
- continuously at full load
- at the installation level
- in accordance with ISO 3046
This part is often what makes the verification more a matter of “documentation” than of intuition: it is not enough to identify the correct point on the curve; one must also verify that the motor is certified to deliver that power continuously under the conditions related to the installation altitude, as specified.
Quick Verification Checklist (in line with the criteria mentioned)
If you need to check the power of a diesel fire pump engine in an orderly manner, you can follow this sequence:
- Refer to section 10.1 and identify the points on the power curve to be considered.
- For diesel engines, verify that the criterion is consistent with the NA curve (10% continuous overload capacity, DIN 6271 – ISO 3046-1).
- If the pump does not have an overload protection feature, compare it to the maximum power at the peak of the curve.
- If the pump has increasing head, compare it to the head when the pump requires an NPSHr of 16 m.
- Confirm that the power can be supplied continuously at full load and at the installation altitude, in accordance with ISO 3046.
Conclusion
UNI EN 12845 guides the selection of motor power based on the power characteristic curve (section 10.1) and, for diesel engines, requires consistency with the NA curve (section 10.9.1), distinguishing between pumps without overload (peak of the curve) and pumps with increasing power (a condition linked to NPSHr 16 m). The final requirement is the ability to deliver this power continuously at full load and at the installation elevation in accordance with ISO 3046.
