NFPA 20: Fire pump design
When designing a fire pump, there are a number of factors to keep in mind, particularly NFPA 20
- Learn how to properly size a fire pump.
- Understand the differences between various styles of fire pumps.
- Know how to design fire pump piping to meet NFPA 20 requirements.
- Appreciate cost variations between different pump styles and controller options.
NFPA 20: Standard for the Installation of Stationary Pumps for Fire Protection protects life and property by providing requirements for the installation of fire pumps to ensure that systems will work as intended to deliver adequate and reliable water supplies in a fire emergency.
A fire sprinkler system is a critical component of life safety in a building. The International Building Code grants a number of exceptions when a building is “fully sprinklered,” such as reductions in rated separations, reductions in fire hydrant flow demands, increased egress travel distances and increased building heights and areas. These exceptions are permitted with an expectation that, in the event of a fire, the sprinkler system will suppress the fire to a sufficient degree that occupants can safely evacuate the building and the growth of the fire will be controlled until the fire department arrives to fully extinguish it.
Often, the municipal water system has sufficient pressure to operate the sprinkler system. 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.
Sizing a fire pump
A fire pump’s size is dictated by the most hydraulically demanding area of the fire protection system. In many high-rise buildings, this can be the automatic fire standpipe system demand which requires 500 gallons per minute at 100 pounds per square inch at the top of the most remote standpipe, plus 250 gpm for each additional standpipe, up to a maximum of 1,000 gpm for wet systems or 1,250 gpm for dry systems.
For nonhigh-rise buildings, the most demanding area could be any number of different hazards. Though the IBC requires buildings with a highest finished floor located more than 30 feet above the lowest fire department vehicle access to be equipped with Class III standpipes or Class I if the building is fully sprinklered, NFPA 14: Standard for the Installation of Standpipe and Hose Systems allows the standpipes to be manual type with the necessary pressure provided by the fire department pumper truck through the fire department connection (2013 NFPA 14, Section 184.108.40.206), thus eliminating the standpipe demand from consideration. It is important to perform a hazard analysis of the building before attempting to size the fire pump.
For example, a new sprinkler system might be installed in a five-story medical office building with a partial basement (overall building height of 69 feet). The building construction is noncombustible, Type II-B and each floor is approximately 18,000 square feet. The basement level contains electrical rooms, general storage rooms, a small oxygen storage room (250 square feet) enclosed by a two-hour fire rating and a covered exterior loading dock.
Floors one through four are comprised of offices, exam rooms and outpatient procedure rooms. The fifth floor is a large mechanical penthouse with a roof slope of 3:12. The center core areas on levels zero through four contain elevator lobbies, public corridors and public restrooms. The building is equipped with a Class I wet manual standpipe system.
The predominate hazard classification for the overall building is that of light hazard occupancy, however, the building contains spaces that warrant higher hazard designations. While the oxygen storage room requires the highest density (0.30 gpm for extra hazard occupancy), this space is not the most hydraulically demanding. The two-hour rated enclosure provides an effective barrier to prevent fire spread outside of the room. For this reason, the calculated area need only extend to the perimeter walls of the room (NFPA 13-2013, Section 220.127.116.11).
The exterior loading dock requires the second highest density: 0.20 gpm for ordinary hazard group 2. It also requires a 30% increase to the remote area size because the system type must be dry due to exposure to freezing conditions (NFPA 13-2013, Section 18.104.22.168.5). The estimated flow demand for this area is approximately 507 gpm (0.20 gpm x 1,950 square feet = 390 gpm + 30% for sprinkler head overflow = 507 gpm). A preliminary hydraulic calculation for this area indicates a required system pressure of 65 psi.
The most hydraulically demanding area in this example is the level five mechanical room. Though the density for this remote area is only 0.15 gpm (ordinary hazard group 1), the top floor location requires additional pressure to overcome the head loss from elevation. The remote area size is increased to 1,950 square feet due to a 30% increase for slopes exceeding 2:12 (NFPA 13-2013, Section 22.214.171.124.4). The estimated flow demand for this area is approximately 380 gpm (0.15 gpm x 1,950 square feet = 292.5 gpm + 30% for sprinkler head overflow = 380 gpm). A preliminary hydraulic calculation indicates a required system pressure of 90 psi.
Once a hazard analysis and preliminary hydraulic calculations have established the fire flow and pressure required to meet the standpipe or sprinkler system demand, a review of a recent water flow test can identify if a fire pump is necessary. The water flow test used to size the fire pump is required to have been completed within the last 12 months (NFPA 20-2013, Section 126.96.36.199).
In the example scenario, the water flow test indicates pressures of 54 psi static, 48 psi residual, flowing at 940 gpm. When the required outside hose demand is added to the system flow demand (380 gpm + 250 hose = 630 gpm) and plotted on a graph, the available city water pressure is approximately 49 psi when flowing at 630 gpm.
Typically, a minimum safety factor of 10 psi is required. To meet the demand, the fire pump size should be at least 400 gpm rated at 51 psi (100 psi – 49 psi city pressure = 51 psi). Fire pumps are typically sized by pressure range, therefore a 400 gpm pump with a revolutions per minute speed of 3,550 can deliver a rated pressure from 40 to 56 psi without increasing the size of the pump. Because there is no cost difference between the rated pressure of 51 and 56 psi, and high pressure is not a concern, the 400 gpm pump rated at 56 psi is acceptable. Fire pump pressures will be explored in further detail later.
For exceptionally tall buildings, more than one fire pump may be necessary to deliver the pressure required to the higher floors. NFPA 20 permits a maximum of three pumps to operate in series (NFPA 20-2013, Section 188.8.131.52).
Fire pumps cannot operate in parallel because the discharge check valve is forced closed when the pressure on the outlet side of the valve is higher than that on the inlet side. For this reason, it is not possible to add a parallel fire pump to boost the pressure and/or flow to a system.
Selecting a fire pump
Selection of the fire pump depends on the building infrastructure and available space. The most common choices for fire pump drivers are electric motors and diesel engines. Electric motors requiring high horsepower are commonly run on 460 volt or higher, three-phase power. Steam turbines are also an option, but are fairly uncommon.
In buildings that are not equipped with enough power to supply an electric motor, a diesel fire pump may be utilized. A fuel storage tank with the capacity to hold 1 gallon of fuel per horsepower plus an additional volume to provide room for thermal expansion is required. A dike must be provided beneath the fuel storage tank to contain any potential fuel spills. Often, a pressure–relief valve is required on the discharge side of the pump to relieve excess pressure in the event the engine revs out of control or if a combination of suction pressure and pump pressure rise above a certain threshold. The diesel motor exhaust must be routed through a muffler to the outside.
A diesel fire pump must be located in a separate enclosure or in a room with direct access to the exterior. The enclosure size is substantially larger than normally required for an electric fire pump because of the stored fuel and batteries necessary to provide a backup power source. Diesel fire pumps are more expensive to install and maintain because of the large number of mechanical parts, which can be prone to failure.
In buildings where the electrical capacity is not a concern, an electric driver is the preferred choice. Electric motors are more compact, require fewer mechanical parts and produce fewer negative environmental impacts.
Though NFPA 20 provides guidelines for various types of pumps (centrifugal, vertical shaft turbine, positive displacement and multistage multiport), centrifugal fire pumps are — including horizontal split case and vertical in-line — the most common among commercial buildings and thus highlighted in this example. Vertical in–line pumps are generally more compact, with a smaller footprint. While horizontal split case pumps must be mounted on a concrete housekeeping pad, vertical in-line pumps can instead be mounted on pipe stand supports. For these reasons, vertical in–line pumps are often a preferred choice for replacements or retrofits.
The impeller rotation in a vertical in-line pump is less susceptible to mechanical damage from water turbulence, allowing for more flexibility in the piping arrangement on the suction side of the pump. Horizontal split case pumps are only permitted to have elbows and tees installed perpendicular to the pump when the fitting is located at least 10 pipe size diameters from the suction flange (NFPA 20-2013, Sections 184.108.40.206.1 to 220.127.116.11.3). These requirements are not applicable to vertical in-line styles.
The impeller on a horizontal split-case pump is located in a separate casing in front of the motor, allowing for easy access if maintenance is required. On a vertical in–line pump, the impeller is beneath the motor, requiring the entire motor be raised and/or removed to access the impeller. For this reason, it is recommended that a hoist beam or another means of lifting is provided for vertical inline pumps greater than 30 horsepower.
Fire pump pressures
The total head of a fire pump is the energy imparted to the liquid as it passes through the pump, usually expressed in psi. For fire pumps such as horizontal split-case and vertical in-line centrifugal pumps that are required to operate under net positive suction head, the total head of a fire pump is calculated by adding the suction head (city pressure) to the discharge head. The discharge head of the pump varies along a performance curve that is determined by three limiting points: the shut-off, the rating and the overload.
The shut-off represents the maximum allowable total head pressure when the pump is operating at zero flow; this is sometimes also referred to as the churn pressure. The rating is the listed pressure and flow that the pump should produce when operating at 100% of pump capacity. The total head pressure should not be less than 65% of the rated total head when the pump is operating at 150% of rated flow capacity, this is the overload point. System flow demands that exceed the overload point can expose the pump to possible cavitation and damage.
A fire pump performance curve has an allowable operating range not to exceed 140% of the rated pressure of the pump. Consider the previous example of a 400 gpm pump rated at 56 psi. This pump will produce 400 gpm at 56 psi when operating at 100% of pump capacity. It also can produce a maximum volume of 600 gpm at 36 psi when operating at 65% of pump capacity. The available volume and pressure vary along the pump curve.
Referring back to the medical building example, the loading dock required an estimated 507 gpm at 65 psi. From the pump curve in Figure 3, the pump will deliver approximately 47 psi when flowing 507 gpm. When this discharge pressure is combined with the city supply (47 + 48 psi = 95 psi), it is evident that the selected pump can easily satisfy the hydraulic demand for the loading dock dry system.
A fire pump’s churn pressure is the amount of pressure generated when the pump is operating at zero flow. The churn pressure is combined with the static water pressure from the connected source, resulting in a combined static pressure for which all components must be rated. As an example, a churn pressure rating of 126% will produce 71 psi of static discharge pressure from the aforementioned pump. When the churn pressure is combined with the static city pressure, the total amount of static pressure expected on the discharge side of the pump is 122 psi (71 psi discharge pressure + 51 static city pressure = 122 psi).
If the static pressure exceeds 175 psi (the pressure rating for standard sprinkler components and maximum pressure allowed for fire hose valve connections), pressure–reducing valves may be required unless all components of the system are rated for high pressure. It is important to include the pump churn rating in the factors to consider when weighing all of the options to make a proper pump selection.
The cost of a fire pump is largely based on the horsepower rating of the pump and the type of controller. Vertical inline pumps are usually more cost effective when compared to horizontal split-case pumps in smaller sizes (less than 1,000 to 1,250 gpm ratings). It is recommended to consult a local fire pump representative to compare the horsepower ratings between horizontal split-case and vertical in-line pumps, as the horsepower rating can drive up costs related to controls and electrical connections.
NFPA 20 requires that a fire pump be supplied by a continually available power source, usually identified as an uninterrupted power source (NFPA 20-2013, Section 9.1.5 and 9.2.1). In many cases, this requirement necessitates that a backup generator be provided as a secondary source in the event of a power failure, in which case the fire pump controller must be equipped with an automatic transfer switch. An ATS is an option on a fire pump controller that must be specified; a controller does not come normally equipped with an ATS.
The least costly type of fire pump controller is an “across-the-line” direct–voltage controller without an ATS. This is the default controller that will usually be supplied unless a different style has been specified. Many electrical engineers prefer “soft start” reduced-voltage controllers instead, because these controllers reduce the immediate power draw on the backup generator by slowly ramping up the voltage, allowing for a reduction in generator size.
Consult with the electrical engineer to discuss the pros and cons of the different controller styles. The cost savings to the overall project may be greater by selecting the more expensive soft start controller to reduce the size of the generator.
Fire pump design
An outside screw and yoke gate valve must be installed in the suction pipe to provide a means of isolation from the incoming supply line (NFPA 20-2013, Section 18.104.22.168). This is the only device that is explicitly permitted to be installed in the suction line within 50 feet of the pump suction flange, though NFPA 20 does provide allowances for other equipment, which may be required by the authority having jurisdiction or by other sections of the standard. These valves must be electrically supervised through the fire alarm system.
Where the local AHJ and/or municipal water department requires a backflow preventer to be installed in the fire pump suction line, it must be located a minimum distance of 10 times the pipe size diameter from the pump suction flange (NFPA 20-2013, Section 4.27.3). This distance requirement is specific to backflow preventers equipped with outside screw and yoke gate valves. If a backflow preventer is equipped with butterfly valves, the minimum distance to the suction flange is increased to 50 feet (NFPA 20-2013, Section 22.214.171.124). This increased distance is provided to allow for dissipation of air bubbles that may form as water passes across the center disk of a fully open butterfly valve. Other nontraditional methods of backflow prevention, such as break tanks, are not addressed within the purview of this article.
NFPA 20 also provides an exception for a pressure–sensing line connection to the suction line when the AHJ requires a low–suction throttling valve to maintain positive pressure on the suction piping (NFPA 20-2013, Section 126.96.36.199). The low–suction throttling valve is installed on the discharge side of the pump before the discharge check valve.
On the discharge side of the pump, a check valve and an indicating control valve are required. The control valve must be installed after the check valve (NFPA 20-2013, Section 4.15.7). If the fire pump is equipped with a flowmeter bypass, the bypass connection to the discharge pipe should be between the check valve and control valve. Where fire pumps are installed in a series, butterfly valves are not permitted to be installed between the pumps.
A fire pump bypass is required on all fire pumps where the suction supply is of sufficient pressure to be of material value without the pump (NFPA 20-2013, Section 4.14.4). The bypass must be at least as large as the discharge pipe and should be equipped with a check valve installed between two normally open control valves oriented in a manner to prevent backflow to the suction side of the pump. The bypass line should be connected before the outside screw and yoke on the suction side and after the control valve on the discharge side of the pump.
Every fire pump must be equipped with a metering device or fixed nozzles to accommodate pump testing. This equipment must be capable of water flow not less than 175% of rated pump capacity (NFPA 20-2013, Section 188.8.131.52). When the metering device is installed in a loop arrangement for fire pump flow testing, an alternate means of measuring the flow must also be provided.
A flowmeter bypass is preferred in some municipalities as part of a water conservation effort. The flowmeter bypass allows routine tests to be performed without discharging water to the environment. The bypass line is equipped with a Venturi flowmeter located between two normally closed butterfly valves. To achieve proper performance of the flowmeter, manufacturer–specified minimum distances must be maintained between the flowmeter and the adjacent normally closed butterfly valves. The flowmeter bypass must be connected after the outside screw and yoke on the suction side and between the check valve and the control valve on the discharge side of the pump.
The minimum pipe diameter and number of outlets required for a fire pump test header is dictated by the flow capacity of the pump. These minimum requirements are outlined in NFPA 20 (NFPA 20-2013, Table 4.26(a)). When the pipe between the test header and the pump discharge flange exceeds 15 linear feet, the pipe diameter must be increased to the next size up.
When transitional fittings are required to reduce or increase the pipe diameter at the pump flange, care should be taken to select the proper reducing fitting. On the suction side of the pump, the flanged reducer must be the eccentric tapered type, installed in a manner to avoid air pockets. The reducer on the discharge side of the pump should be the concentric type.
The fire department connection should tie into the system on the discharge side of the pump. When an FDC is located upstream of a fire pump, the result can be high velocities that increase water turbulence and expose the fire pump to damaging conditions. Many fire pumps have maximum suction pressure ratings that can be exceeded by the pressures distributed through the FDC.
Fire pump enclosure
Lastly, when determining a location for a new fire pump enclosure, it is important to consider service accessibility and proximity to the building exterior. A fire pump room should be located on an exterior wall adjacent to the fire lane and above the floodplain. If the enclosure must be located inside, it shall be accessible by a passageway with a fire rating equal to that of the fire pump enclosure. NFPA 20 requires the fire pump room to have a minimum two-hour fire rating when located in a high-rise building. The fire rating can be reduced to a one-hour rating when the fire pump enclosure is located in a fully sprinkled, nonhigh-rise building.
The enclosure should be large enough to provide adequate clearance for installation and maintenance of the fire pump and related components. A good rule of thumb is to provide at least 12 inches of clearance behind the fire pump and a minimum distance of 12 inches from the edges of the entire fire pump assembly, piping and valves to the walls. If the room consists of multiple sprinkler and/or standpipe risers, a minimum clear distance of 12 inches between risers should be maintained to allow for easy access to equipment. An approach clearance of at least 3 feet should be maintained in front of the fire pump and related equipment. Minimum clearances in accordance with NFPA 70 must be maintained around energized electrical equipment.
The fire pump room is intended solely for fire protection equipment and is not to be shared by other mechanical trades. This rule is applicable to all equipment that is nonessential to the operation of the fire pump except equipment related to domestic water distribution. NFPA 20 provides an exception for domestic water equipment to be located within the fire pump room.
There are many factors to consider when designing a fire pump. NFPA 20 contains valuable requirements, which should be strictly followed to ensure that the fire pump will perform as intended, should it ever be needed.