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How to Choose the Right Size Fire Pump?

How to Choose the Right Size Fire Pump?

Issue Time:2022/06/23
From the first committee that created the original standard for steam-driven fire pumps in 1899 to the latest 2019 edition of NFPA 20: Standard for the Installation of Fire Stationary Pumps, fire pumps are helping to make the built environment safer. One of the first pieces of information that needs to be assessed to determine if a pump is needed is to know the water supply available. Water supply can come from a variety of sources, such as municipal water supplies, on-site storage, or even natural water bodies. Each of these raw resources will drive the type of fire pump best suited for the situation. 

Because the fire pump can only add pressure to the water flow and cannot "create" volume or flow, it is assumed that the water supply provides sufficient volume to provide the fire pump with the minimum time required to push the fire sprinkler as required by another code or standard. Installation of sprinkler systems or risers, or provision of fire water flow for on-site hydrants. 

An evaluation of an effective hydrant flow test should:

Recently executed (preferably within 12 months of design or as required by the competent local authority). 

Make adjustments to account for seasonal fluctuations in water pressure and usage. 

Considering any low hydraulic gradients that can be established is the first step in choosing a fire pump. 

Knowing the flow of water available at any given pressure can determine the water supply needs of a facility. There are different types of systems that can drive independent demands that vary according to the size of the building, type of construction, number of floors, and level of hazard. For our example, assume that the low hydraulic gradient of the available water supply produces a static pressure of 40 psi and a residual pressure of 30 psi while flowing 1,600 gallons per minute.

Fire sprinkler system

If we take an example of a medium-sized 15,000 square foot single story building with a height of 20 feet, NFPA compliant Building Type II (000), occupied by an auto repair shop, which contains what would be considered an additional hazard group 1 the parts storage power is considered reliable according to NFPA 13: Standard for the Installation of Sprinkler Systems. There is only one fire hydrant on site to meet the proximity requirements for the fire sprinkler system to be connected to the fire department. We must determine our requirements for fire flow (NFPA 1: Fire Code Chapter 18 or International Fire Code Appendix B), fire sprinkler systems, and risers. 

For our example, a single-story building does not need to consider risers but will be an additional hazard group 1 fire area for the fire sprinkler system. According to NFPA 13, in the most hydraulically demanding 2,500sq. ft. area, a density of 0.30gpm/sq. ft. is required, resulting in the following approximate requirements:

Sprinkler flow: 0.30 x 2,500 = 750gpm

Design inefficiency or overrun: approximately 30% minimum flow = 225gpm

Hose allowance = 500gpm

Total = 1,475gpm

Based on the following known information, the minimum capacity required to provide residual pressure requirements to a sprinkler system is approximately 1,475gpm:

Remote sprinkler: 7psi

Pipe friction loss: 20psi

Altitude loss: 9psi

Backflow preventer: 8psi

Safety factor: 5psi

Total residual pressure required = 49psi

Looking at the fire flow required for hydrants on site, assuming our building type and floor area meet the requirements of NFPA 1 Chapter 18 Table 18.4, our fire flow requirement is 3,000gpm with a residual pressure of 20psi for three hours. This can be reduced by up to 75% to a minimum of 1,000gpm when the building is equipped with a sprinkler system at a residual pressure of 20psi for two hours. The resulting fire stream is reduced to a minimum of 1,000gpm. 

Therefore, the most stringent requirement for a sprinkler system is 1,475gpm with a residual pressure of 49psi. This demand is higher than the available supply, which tells us that, in fact, we do need a fire pump to supplement the available pressure. Looking further into fire pump considerations, it's worth keeping in mind any future expansion or anticipated changes in usage that would size the fire pump beyond what is required for known anticipated hazards. 

The last key piece of information to get is related to the power supply to the pending fire pump installation. It's important to determine if the power source is reliable, which helps determine what type of drive might be most efficient, or if a fire pump needs backup power. If the normal power source is deemed unreliable, then NFPA 20 section 9.3 will require a backup power source.

Types of fire pumps

The most common types of fire pumps are centrifugal, turbo, and positive displacement pumps arranged in a horizontal or vertical manner. For systems that only pump water and not any type of chemical additive (like foam concentrate), centrifugal and turbo will be used with turbopumps, more commonly from an underlying water source (like a cistern or even raw water) Pumped water sources, such as ponds. Centrifugal pumps will have positive pressure on the suction side of the pump, which will only add pressure to the inflow, usually from a public water supply or above-ground water storage. 

Continuing with this example, if a reliable municipal water supply is available, a centrifugal pump is probably the most efficient pump for a known hazardous water supply. 

Next, we want to determine the best drive style for the facility. Fire pumps can be driven by electric motors, diesel engines, or even steam engines, but steam is no longer commonly used. Electric pumps are less expensive than diesel-driven pumps and generally take up less installation space. Conversely, diesel-driven pumps cost more than electric pumps, but they are considered a reliable source of power and do not require backup power. 

In the example of an auto repair shop, where there is a reliable power source, so there is no need to provide backup power, an electric pump will be installed. If the power source is unreliable, we need a backup generator and automatic transfer switch for the fire pump controller, which can drive the price well above the cost of a standalone diesel pump. If backup power is needed for reasons other than fire protection (such as maintaining coolers and freezers at the grocery store), it may still be cost-effective to simply scale up the generator power load to replace diesel. 

The above details the importance of choosing the right size fire pump and how to choose the right one. If you want to buy a suitable fire pump, please contact us. BAOLAI is a professional custom fire protection system manufacturer. Welcome to send your needs to us, we will provide you with professional fire protection system solutions.
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