How to select, size fire pumps
Proper fire pump sizing and selection can result in efficient and effective fire suppression systems in a cost-effective manner
- Understand the required information to evaluate, select and size a fire pump.
- Know how to specify an appropriate type and size fire pump for the expected hazard.
- The economics of fire pump selection, how to maximize the return on investment while complying with NFPA 20.
From the first committee in 1899 that created the initial standards for steam-driven fire pumps to the most recent 2019 edition of NFPA 20: Standard for the Installation of Stationary Pumps for Fire Protection, fire pumps have played a role in helping to make the built environment reasonably safer.
Among the first pieces of information that needs to be evaluated to determine if a pump is necessary is understanding the available water supply. A water supply can be derived from a variety of sources such as municipal supply, on-site water storage or even from natural bodies of water. Each of these raw sources will drive the type of fire pump that best suits the situation.
Because a fire pump can only add pressure to water flow and cannot “create” volume or flow, it is assumed that the water supply provides sufficient volume to supply a fire pump for the minimum required amount of time as required by another code or standard that drives the installation of fire sprinkler systems or standpipes or provides requirements for required water for fire flow from on-site fire hydrants.
Evaluation of a valid fire hydrant flow test should:
- By performed recently (preferably within 12 months of design or as required by the local authority having jurisdiction).
- Be adjusted to account for seasonal fluctuations in water pressure and usage.
- Take into account any low hydraulic gradient that can be established is the first step in fire pump selection.
Understanding the available water flow at any given pressure leads to determining the water supply demands of a facility. There are different types of systems that can each drive independent demands that vary based on the size of a building, construction type, number of stories and degree of hazard. For our example, it will be assumed that the low hydraulic gradient of the available water supply produced a static pressure of 40 pounds per square inch and a residual pressure of 30 psi while flowing 1,600 gallons per minute.
Fire sprinkler systems
If we take as an example a moderately sized 15,000-square-foot, single-story building at 20 feet tall of construction type II (000) per NFPA, occupied by an automotive repair shop containing parts storage that would be considered Extra Hazard Group 1 per NFPA 13: Standard for the Installation of Sprinkler Systems, with a power source that is considered reliable. There is a single fire hydrant on-site to satisfy proximity requirements to the fire department connection for the fire sprinkler system. We have to determine our demands for fire flow (NFPA 1: Fire Code Chapter 18 or International Fire Code Appendix B), fire sprinkler system and standpipe.
For our example, there is no standpipe to consider for the one-story building but will be an Extra Hazard Group 1 fire area for the fire sprinkler system. Based on NFPA 13, a density of 0.30 gpm/square foot over the most hydraulically demanding 2,500-square-foot area is required resulting in the following approximate requirements:
- Sprinkler flow: 0.30 x 2,500 = 750 gpm
- Inefficiencies or overages in design: approximately 30% of minimum flow = 225 gpm
- Hose allowance = 500 gpm
- Total = 1,475 gpm
An approximate minimum of 1,475 gpm will be the required volume to supply the sprinkler system with a residual pressure requirement based on the following known information:
- Remote sprinkler: 7 psi
- Friction loss in piping: 20 psi
- Elevation loss: 9 psi
- Backflow preventer: 8 psi
- Safety factor: 5 psi
- Total residual pressure required = 49 psi
Looking at the required fire flow for the on-site fire hydrant, assuming the requirements of NFPA 1, Chapter 18, table 18.4 for our construction type and building area, we have a fire flow requirement of 3,000 gpm at a residual pressure of 20 psi for a duration of three hours. This can be reduced by up to 75% down to a minimum 1,000 gpm at a residual pressure of 20 psi for two hours when a building is equipped with an automatic sprinkler system. The resulting fire flow is reduced to the minimum of 1,000 gpm.
Therefore, the most stringent demand is for the fire sprinkler system at 1,475 gpm at a residual pressure of 49 psi. This demand is higher than the available supply, which tells us that we do, in fact, need a fire pump to supplement the available pressure. Digging further into what to consider for a fire pump, it is worthwhile to keep in mind any future expansion or expected change in use that would merit sizing of a fire pump beyond what is needed for the known expected hazard.
One last key piece of information to obtain has to do with the power source for the pending fire pump installation. It is important to determine if the power supply is considered reliable, which can help determine what type of driver might be most efficient or if a backup power source will be required for the fire pump. NFPA 20 Section 9.3 will require an alternate source of power if the normal source is not considered reliable.
Types of fire pumps
The most common types of fire pumps are centrifugal, turbine and positive displacement style pumps arranged in either a horizontal or vertical manner. For systems that are only pumping water and not any kind of chemical additive such as foam concentrate, the centrifugal and turbine styles will be used with a turbine style pump, more commonly drawing water from a source below such as a cistern or even a raw water source such as a pond. A centrifugal type pump will have positive pressure on the suction side of the pump with the pump simply adding pressure to the incoming flow typically from a public water supply or an aboveground water storage vessel.
Continuing with the example, having a reliable municipal water supply, a centrifugal pump would likely be the most efficient pump for the supply with the known hazard.
Next, we would want to determine the style of driver that would best suit the facility. Fire pumps can be driven by electric motor, diesel engine or even steam engine, though steam is not commonly used anymore. Electric driven pumps are less expensive than a diesel driven pump and will typically take up less space for installation. Diesel driven pumps cost more than electric conversely, however they are considered a reliable power source and do not require an alternate power source.
In the auto repair shop example, there is reliable power and therefore would not need to provide backup power and would choose to install an electric pump. If the power source were not reliable and we needed to provide a backup generator and an automatic transfer switch for the fire pump controller, that would likely drive the price much higher than the cost of a standalone diesel pump. If there is a need for other reasons (such as maintaining coolers and freezers at a grocery store) outside of fire protection to have backup power, then simply upsizing the generator power loads might still be cost effective in lieu of diesel.
Fire pump alignment
Next, we need to understand how much space has been allocated for the fire pump room to choose the best alignment — horizontal or vertical — for the design and layout of the fire pump. A horizontal pump will take up more space in the dedicated space, however they are more efficient at higher flow rates. Horizontal style pumps are generally easier to install than their vertical counterparts. Vertical inline fire pumps will have smaller footprint and generally are more cost effective. Vertical pumps are limited to electric driven style and will likely have a higher long-term cost for maintenance.
All of these factors are key in determining as early on as possible if a pump will be needed for a project, to allow for the proper allocation of space and ensuring all disciplines are able to provide the appropriate information in selecting a style of pump.
For our example, we will assume that there is ample planning, and space has been allocated to accommodate any style of pump that is selected. With this information and understanding that we have determined that a slightly higher than moderate flow will be required, the appropriate selection is likely to use a horizontal split-case style pump. Now it is necessary to determine the required rated flow for the pump and the minimum pressure boost to supplement the public supply.
The annex of NFPA 20 suggests that a fire pump should optimally be sized to be at 90% to 140% of the rated flow when choosing the flow, and is absolutely limited to 150% of rated flow. Section 4.10.2 and Table 4.10.2 requires a pump to have a rated capacity in accordance with the table listed, with general increments of 250 gpm greater than 500 gpm, and 50 gpm increments for flows less than 500 gpm.
In our example, we have a required flow of 1,475 gpm for our sprinkler system and hose allowance. A selection of a 1,000 gpm fire pump would put the flow at 147.5% of rated capacity and could work, but is slightly out of the recommended range. A 1,250 gpm fire pump would put the flow at 118% of rated capacity. Selecting smaller than 1,000 gpm would not meet the 150% requirement and selecting larger than 1,250 gpm would just extend the inefficiency.
In considering the pumping capacity, the larger the pump, while adequate to supply the fire sprinkler system, the more the cost of the pump, and also the supporting equipment, electrical service power requirements that drive larger wire sizes, requiring more space to install, more labor to install, etc.
To maximize efficiency and minimize cost initially, the best fit for the example will be a 1,250 gpm fire pump, which allows a capacity of up to 1,875 gpm. Selecting this pump allows for the unknowns on the front end of a project and some flexibility as design stages progress, ultimately it would be much easier to make a pump smaller later if appropriate for conditions than it would be to increase the rating.
Fire pump pressure
The next step will be to determine the amount of pressure boost necessary to increase the supplement that incoming pressure to satisfy the fire sprinkler system demands. It is necessary to take into account the pressure generation of a pump at three specific points: churn or zero flow, rated flow or 100% and 150% of rated flow. Selection of a pressure boost will be at specified at the rated flow, with consideration for the allowable pressure at churn and 150%.
NFPA 20 limits a pump to creating no more than 140% of rated boost at churn and no less than 65% of rated boost at 150% of rated flow. Pump technology has made for the ability to achieve much more efficient curves than what NFPA allows, however, to understand limitations, standard pump curves will be evaluated.
In our example, the demand is approximately 18 psi above the available public water supply. Other information to take into consideration when determining the required pressure boost:
- Minimum pressure required to satisfy demand.
- Additional safety factor.
- Additional pressure provided could help to minimize the pipe size diameter of the fire sprinkler system.
Once it is determined that a pump is required, the additional pressure boost comes at a minimal cost compared to the potential of saving on the pipe sizes. As long as the pressure boost is not so large that it pushes system pressure beyond the standard rating requirements in NFPA 13 of 175 psi, additional pressure will add flexibility to the design. For our example, we will use a pressure boost of 40 psi, which will yield a pump curve of the following:
- Churn (zero flow): 56 psi
- Rated (100% flow): 40 psi
- 150% flow: 26 psi
Adding the pump boost to the city supply will yield a combined water supply curve with the following characteristics:
- Churn: 40 psi (city) + 56 psi (pump) = 96 psi
- Rated: 73 psi
- Sprinkler demand: 67 psi
- 150%: 53 psi
The resulting water supply is now adequate to provide the required flow and pressure for fire sprinkler system resulting in an approximate pressure cushion of 18 psi when adjusted for the low hydraulic gradient. There is adequate spare pressure to work with in this design to consider smaller piping, but not so much excess pressure that over pressurization is a concern.
As we walked through the process for specifying a fire pump, the goal was to identify a code compliant option that also showed a modicum of cost effectiveness. NFPA explicitly points out that the purpose of its standards and codes is to provide reasonable degree of protection and as engineers our role should be to specify an appropriate solution for the end user that meets this reasonableness requirement while doing so in a way that doesn’t over specify and increase costs unnecessarily or underspecify and lead to potential catastrophic consequences.
The idea is to identify and subsequently specify a fire pump that is just right. Too much pumping and you are wasting resources, not enough and the results could be catastrophic, however, finding the sweet spot is much like Goldilocks finding the perfect bowl of porridge or the best bed to get a good nap. That sweet spot will provide an effective and appropriate level of safety without spending more of the precious budget than needs to be to achieve the desired outcome. NFPA 20 provides the essential guidelines to achieve this goal when armed with the necessary information to specify a reliable, cost-effective and code-compliant solution. The reliability of an appropriately designed and properly maintained fire pump will provide a more than reasonable level of fire protection and life safety for any project.