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F ire pumps are critical turbomachines. They probably save more lives and prevent more damage than any other machinery.
Fire pumps are nearly always centrifugal pumps with capacities from around 20 m3/h to about 2,000 m3/h. The requirements for fire pumps are briefly noted in fire codes, such as NFPA 20. But this may not be sufficient for the specification of high-performance, reliable fire pumps at an optimum price.
It is worth noting that the performance and reliability of a fire water pumping system are often investigated in many safety and risk studies, as well as by investors and insurance providers. As much as 40% of insurance deficiency rating points can be related to the fire pump system.
Despite their importance and fire code requirements, as much as 10% of all fire pump systems fail to provide satisfactory service during fires or fire drill exercises.
Passive fire protection measures for power plants and industrial facilities ensure there are sufficient clearances, that protective barriers are installed, and that the smallest amount of hazardous materials, processes and equipment are employed. Active fire protection systems, on the other hand, detect and apply fire extinguishing measures. The fire pump is a key element of active fire protection.
Fire pump demands are normally calculated on the maximum rate of water required for the worst single fire situation. Computerized simulations often play a critical role in incident modeling, fire-fighting method verification and estimation of system capacity.
During a fire-fighting scenario, additional pressure should be maintained in all remote units and critical systems to ensure that an adequate water stream can be maintained for all applicable equipment.
Treated water is always preferred for all onshore plants over other options, such as sea water, brackish water or untreated water. However, it is best to use corrosion-resistant materials or coatings for the system in case untreated water or sea water is used as a secondary source of water during a major fire. In such a case, the system should be flushed with treated water after the incident.
Centrifugal pumps with a relatively flat characteristic performance curve (head vs. flow) are generally selected for fire pumps. The head should rise continuously from rated point to shutoff point, with only a relatively small increase of head (perhaps a 10% - 15% rise of head from the rated point to the shutoff point).
These pumps can provide a steady, stable flow of water at a relatively uniform pressure over a wide range of flows. Check valves should be provided at the discharge and suction. The rated pressure is between 4 - 30 Barg. Single-impeller pumps (for below about 11 Barg) and multi-impeller pumps (for high-pressure systems) are typically used.
The differential pressure of a pump is proportional to the square of rotating speed and the square of impeller diameter. A discharge pressure of 10-to-11 Barg can be obtained by a relatively large single-impeller pump (with a suitable speed).
Fire pumps should be able to operate in parallel, although there are associated challenges such as overheating. When operated in parallel, the pump with lowest head may operate, at times, at a reduced flow.
In this situation, this pump could end up working far from its Best Efficiency Point (BEP), which can result in damage. Even with identical pumps, one pump that has worked more hours, has a minor defect or runs at a slightly lower speed, could be subjected to reduced flow. Therefore, it is advisable that each pump work as the main fire pump in rotation to achieve an even wear pattern between identical pumps.
It should be noted that there are various ways to monitor pumps in parallel operation to make sure they function within a reliable range. For example, it is possible to monitor pump head/flow or differential temperature. This is a good way to estimate operational issues, such as reduced flow, poor operation or overheating.