A fire pump is any type of purpose-driven pump used within a fire protection system. It can be driven by diesel engines, electric motors or even steam and is used to provide increases in water pressure to meet the design requirements of a fire protection system. Fire pumps do not create a water supply. Instead, they create pressure from an existing water supply, such as a city supply line or a storage tank, by converting mechanical energy into hydraulic energy.
A fire pump is required when the available water source does not have adequate pressure. When a sprinkler system relies on a fire pump, the performance of the system is dependent on the pressure created by the pump. Because of the critical importance of the fire pump, careful consideration should be employed when selecting and designing a fire pump.
Objectives:
The basic functional purpose of a fire pump is to boost the available water pressure to the demand requirement at the appropriate flow demand.
Calculating standpipe system pressure
Two types of structure calculations for pressure are available. One is for high-rise structures (buildings greater than 75 feet in height, measured from the lowest level of fire department vehicle access to the floor of the highest occupiable story) and one is for non-high-rise structures. This is a factor because any high rise requires a pressure of 100 pounds per square inch (psi) at the top of the structure while flowing the rated gallons per minute (gpm) of the fire pump. This discussion concentrates on high rises because the pressure calculations for most non-high-rise buildings are determined using software specifically designed for fire sprinkler hydraulic calculations
Calculating pump gpm
To calculate pump gpm, two sizing methods are available, the standpipe method and the sprinkler area calculation. In a fully sprinklered structure with standpipes, NFPA 14 (2010): Standard for the Installation of Standpipes and Hose Systems says that the first standpipe requires 500 gpm and each additional standpipe requires 250 gpm, up to a maximum of 1,000 gpm.
Selecting the pump
Once you have calculated the gpm and psi requirements for the pump, you need to determine the type of pump that works best for the job. The three most widely used pumps are horizontal split case, inline and vertical turbine.
Horizontal split case pumps are also called double-suction fire pumps because the water pathways direct water to both sides of the impeller. They are the most common type of fire pump on the market, partly because of the ratings available in this style of pump, typically 250 through 5,000 gpm. This was the first type of pump used for fire protection systems.
Inline fire pumps offer several benefits:
- Their size and design offer space savings.
- They offer the ability to increase the ratings allowed by NFPA 20 from a maximum of 499 gpm to 750 gpm, to today’s unlimited rating. (The largest currently available is 1,500 gpm.)
- They offer a low cost of installation because they don’t require a base plate that needs grouting.
Vertical turbine pumps are used in situations where the water supply is below the suction flange of the fire pump, because NFPA 20 requires a positive suction pressure to a fire pump.
The other item that needs to be determined is the type of drive: diesel or electric. Once that is determined, you can find the appropriate pump model and horsepower in a manufacturer’s catalogue. I don’t recommend using pump curves to select fire pumps, as every selection must be UL approved, which might lead to picking the wrong horsepower for a particular selection.
One other note on fire pump selection is that selecting pumps that have a higher rpm is not necessarily a misstep, because fire pumps only run once a week for a limited amount of time, so the length of life will be about the same for a 1,750-rpm pump as for a 3,500-rpm pump.
Power Supply
If a generator is going to be used as a secondary power supply, the fire pump will require a transfer switch, which must be dedicated to the fire pump. A typical design would be to use a combination controller and transfer switch in a cabinet to avoid the need for additional requirements laid out in NFPA 20. A reduced voltage start also should be considered when connecting to a generator to potentially reduce the size of the generator.
Code issues
Some of the code requirements for fire pumps needed to consider into pump selection and system design.
- Horizontal elbows or tees upstream of a fire pump must be 10 pipe diameters from the suction flange on a split-case fire pump.
- Pumps must maintain a positive suction pressure at the suction flange.
- Electrical feeds to fire pump controllers must have a two-hour fire rating.
- Fire pumps can’t be used as pressure-maintenance pumps.
- Variable-speed pumps are allowed by the code.
- Fire pumps need to be installed in a 2-hour rated room.
