Research And Insight

Fire pumps

Fire pumps are typically listed by an approval institute and are powered either by an electric motor or a diesel engine or sometimes by a steam turbine. In a fire installation there can be one or more fire pumps put into operation as a duty assist (50 %) – and standby pumps.

The fire pump delivers the water via the pipe system to the fire sprinklers to suppress the fire. The number of fire-pumps installed depends on the occupancy hazard (LH, OH or HH) and specific fire installation standard. Some known can be seen in the table to the right.

Where twin electric fire pumps are installed, there is a requirement for a secondary power source. This can be from a separate feed to the nearest electricity sub-station, or from a generator located on site. A mains changeover facility should be incorporated into the design to allow for switching to this alternate power source in the event of a mains supply failure.

The fire pump starts when the pressure in the fire sprinkler system drops below a certain set-point. If one or more fire sprinklers are exposed to heat above their design temperature, and opens, the sprinkler system pressure drops and the pressure switches gives a signal and the duty pump starts. If the duty-pump, for any reason, does not start, the standby pump will start, usually from a secondary pressure switch.

Characteristics

Fire pumps are typically made with non-corrosive internal parts to avoid clogging due to corrosion. To avoid cavitation and to have a stable system pressure, the fire pumps are most often designed specifically for fire pump purposes, with strict demands to the NPSH value and flow [Q] and head [H] curve.

The fire-pump performance is very demanding regarding the ability to avoid cavitation and have a stable and continuous decreasing Head [H] with decreasing flow [Q]. The reason for these required characteristics is reliability and performance of water distribution via the pipe system and sprinklers into the building.

Fire-pump materials
Since the fire-pumps are rarely in operation, except for testing, internal components should be made of non-corrosive materials (stainless steel, bronze) to avoid the pump clogging or seizing.

NPSH-value (Net Positive Suction Head)
Fire-pumps have low NPSH-curves to avoid cavitation, causing decreasing flow and pressure. The NPSH curves are often measured up to 16 m in order to decide the required power – but varies according to the different fire pump standards.

Q & H Curve (Flow Q and Head H curve)
Stable and continuous decreasing Q & H curves are desirable to achieve a stable system pressure, but also to have only one single point on the curve for each pressure point. In cases where fire pumps work in parallel, unstable fire pumps would cause vibrations and could cause system breakdown. Fire pump standards define their own characteristics of the Q-H curves.

Power Consumption Curve (P2)
Power consumption is measured as a function of head and flow. Power curves can be constantly increasing or with a local maximum point at the curve depending on the pump hydraulic design. Most curves have a constant rising power-curve. The power sizing for the fire-pumps vary from standard to standard but they all require adequate power reserves, either by selecting the driver size by end of the power curve or at NPSH value = 16 m for pumps with maximum flow at approximately 5 m.

Maximum permissible flow (Qmax)
Maximum flow is mainly determined to avoid cavitation of the fire-pump and to ensure having enough power reserve, but in some cases also to dictate the capacity of the stored water. For LPCB systems Qmax is determined in systems to dictate the capacity of the stored water (60 minutes on OH and 90 minutes on HH).

Grundfos supplies end-suction, horizontal split-case, vertical split-case, vertical in-line and vertical turbine fire pumps for industrial and commercial fire protection. Grundfos fire systems meet fire protection standards and fire component standards and are tested and certified by accredited laboratories and listed by authorised institutions.