The first part of this series, “Underground Fire Protection Design Issues,” was published in July 2023 (https://bit.ly/3PIvbjO). It focused on identifying various exterior elements commonly overlooked in fire protection engineering design. As discussed in the column, diligent coordination efforts during the design phase can help to minimize delays and reduce the number of change orders during construction.
This column is intended to expand upon the previous one, exploring essential elements and equipment of fire protection design, such as standpipe systems, fire pumps and fire pump controllers.
Standpipe systems provide a convenient water source for fire department use within buildings and require preplanning to be designed effectively. When determining if standpipe systems are required for a new project based on the height of the building, it is important to first coordinate with the civil engineer. Ensure that the elevation used for the lowest level of fire department vehicle access is accurate, as the measurement of the building height is crucial to standpipe system design.
If the building is not a high-rise building, the standpipe system can be designed as a manual standpipe system. This is defined as a standpipe system that relies exclusively on the fire department connection to supply the system demand (NFPA 14-2019 Sections 184.108.40.206 and 220.127.116.11).
How “high” is a high-rise building? The answer varies in different states. In most states, a building is considered a high-rise if the highest occupied floor level is more than 75 feet above the lowest fire department vehicle access level. Whereas in Massachusetts, a building is considered a high-rise if the highest occupied floor is more than 70 feet above the lowest level of fire department vehicle access; that dimension is reduced to 55 feet in Michigan.
If determined that the building is a high-rise building, automatic standpipe systems are required.
During the design phase, it is important to coordinate with the local authority having jurisdiction (AHJ) to ensure the appropriate standpipe class is specified and the locations of the hose connections are approved. Be sure to understand the nozzle type for their standpipe firefighting operations, as a different type of nozzle requires a different residual pressure at the nozzle inlet.
NFPA 14 (2019 Edition), Standard for the Installation of Standpipe and Hose Systems, generally permits limiting pressure at the 2 1/2-inch hose connections to a range of 100 pounds/square inch (psi) to 175 psi. A 100 psi residual pressure at the nozzle inlet is sufficient to produce an effective fire stream for most fire departments using combination/fog nozzles and hoses up to 150 feet in length.
However, some fire departments use a gated wye supplying two separate hoses with combination nozzles, requiring more than 100 psi residual pressure at the nozzle inlet. If the AHJ anticipates using extra lengths of hoses, the additional friction losses through the hoses should be considered and calculated to ensure the minimum required pressure at the nozzle inlet can be achieved.
In a typical high-rise building equipped with a fire pump and a single-zone Class I standpipe system, pressure-reducing valves (PRVs) will most likely be required at the sprinkler floor-control valve assembly and the hose valves on the lower levels to limit the system pressure to below 175 psi. It is not unusual to find undersized express drain risers incapable of handling the designed flow rate for full-flow PRV testing.
The hindsight can be problematic and expensive as the water must instead be directed down the stairwells to the building exterior via multiple connected fire hose lengths for the test of each PRV. Interior hose valve cabinets adjacent to horizontal fire exits and other locations where they have been installed to limit the maximum travel distance between connections can be even more difficult, if not impossible, to test.
NFPA 25 (2020 Edition), Standard for the Inspection, Testing and Maintenance of Water-Based Fire Protection Systems, requires full-flow testing of PRVs every five years. This involves flowing every PRV at its designed flow rate and recording the static and residual pressure on the inlet and outlet of the valve. The test results are then compared to previous results to ensure the PRV still delivers the designed pressure and flow. It is very important to properly size the express drain riser during the design process.
In a multizone standpipe/sprinkler system, using a master PRV assembly may be cost-efficient to eliminate additional fire pump(s), PRVs, high-pressure-rated components and standpipe express drain risers from the system. Figures 1 and 2 show a typical master PRV assembly with two sets of PRV and two sets of low-flow bypass installed in series to meet the redundancy requirements in NFPA 14.
Be sure to coordinate with the PRV manufacturer on the minimum required pressure differential between the inlet and outlet of the PRV and its flow range. Some PRVs require the inlet pressure to be at least 10 psi higher than the set outlet pressure, while others require at least a 20 psi pressure differential. Coordinate with the PRV manufacturer to ensure the required pressure differential is accounted for and the desired pressure at the high-pressure zone master PRV outlet can be achieved.
It’s our experience that it can be difficult to maintain a large PRV in an open position when testing the water flow switch on a sprinkler system with the flow equivalent to a single sprinkler. Implementing the low-flow bypass lines with smaller PRVs in Figures 1 and 2 resolves this issue and prevents potential water hammer to the sprinkler system caused by constant opening and closing of the PRV. In addition, it allows the jockey pump to operate more effectively to maintain the system pressure.
It is vitally important to work with the local AHJ and understand the pumping capabilities of the fire department apparatus when designing a multizone standpipe/sprinkler system. If the pumping capability is not sufficient to supply the high zone(s) due to elevation, an auxiliary means of supply acceptable to the AHJ is required per NFPA 14.
An auxiliary means of supply can be a dedicated water storage tank on the roof level supplying the high zone with or without additional pumping equipment. Early coordination with the AHJ and planning for the auxiliary means of supply during the design phase can prevent the headache of adding it during construction.
The need for a fire pump should be decided early in the design process based on the maximum system demand and the available water supply. Depending on the site conditions and application, the selected fire pump type and desired location can vary. A dedicated fire pump room is required to house a fire pump and related equipment.
Ideally, a fire pump room is located on grade with direct access from the exterior of the building. However, in crowded cities, it is not unusual to locate the fire pump room belowgrade and provide indirect access through a fire-rated enclosed passageway from a fire-rated enclosed exit stairway.
Since the fire department usually responds to the fire pump room early in a fire event, the location of, and access to, the fire pump room should be carefully preplanned with the local fire department as required by NFPA 20 (2019 Edition), Standard for the Installation of Stationary Pumps for Fire Protection.
For a high-rise building assigned to Seismic Design Category C, D, E or F, an automatic secondary on-site water supply having a capacity not less than the maximum sprinkler system demand, including hose streams, is required by the International Building Code, 2018 Edition. The secondary on-site water supply does not include a secondary connection to the primary water supply, as a failure of the primary water supply would jeopardize the functional reliability of the fire protection system.
Work closely with the local water department to comprehensively understand the municipal water infrastructure. If the required independent secondary on-site water supply is unavailable, an on-site water storage tank should be provided.
A fire pump room design relies on seamless collaboration with the design team. All trades must understand that the fire pump room is a dedicated space only for the fire pump and related equipment. It is important to coordinate the fire pump room layout with the architect to ensure sufficient space for the fire pump and related equipment.
A 3D detailed sketch of a fire pump room layout often helps determine if the fire pump and related equipment are arranged with adequate space and clearances for operation and maintenance. A fire pump controller should never be obstructed by any equipment or located in the back corner of a fire pump room where personnel entering the room must pass the pump to reach the controller.
When a vertical turbine fire pump is designed to draw from an underground concrete cistern, assembly and installation should be carefully preplanned. A fire-rated roof hatch at least 4 feet by 4 feet in size is usually designed on the roof of the fire pump room for hoisting the fire pump in place from above.
Where a diesel-engine-driven fire pump, or an electric-motor-driven fire pump with a variable-frequency drive (VFD), is provided, a main pressure relief valve on the fire pump discharge is required as a means of controlling pressure in the event of an overspeed condition on the diesel fire pump or the VFD failure. Although the pressure-relief drain (PRD) typically spills to grade directly, in urban areas, the PRD may have to spill to some drainage system, such as a storm drainage tank.
Depending on the fire pump size, the maximum flow rate through the PRD can exceed 2,000 gallons/minute. While it is realized that the sustained high flow through the PRD is a rare event for a properly installed and well-maintained fire pump, it is still important to communicate this high flow rate to the civil engineer sizing the storm drainage tank to ensure the potential flow rate from the PRD is included in the design volume of the tank.
Ventilation of a fire pump room is extremely important. Ventilating a diesel-engine-driven fire pump requires larger building openings than an electric-motor-driven fire pump, as the room requires much more air for combustion and heat removal. The maximum room temperature for a diesel fire pump is 120 F. It is ideal to provide the air supply ventilator on the opposite wall of the air discharge ventilator so the air sweeping through the engine carries away the radiated heat more effectively.
If a sufficient incoming air supply is not provided, the engine will “suffocate” and be unable to operate at its rated horsepower. Therefore, combustion airflow and airflow for engine-radiated heat removal should be calculated based on the engine selection and the maximum design temperature rise inside the fire pump room.
It is important to provide the calculated ventilation airflow rate to the mechanical engineer to ensure the fire pump room exhaust fan and ventilation intake louver are properly sized. When motor-operated dampers are used in the air supply or discharge paths, the sequence of operation needs to be coordinated with the electrical engineer and mechanical engineer to ensure a fail-safe design.
Upon power or signal failure, the spring-return actuator should drive the dampers to a predetermined safe position, which is an open position. NFPA 20 requires motor-operated dampers to spring-operate to an open position and motored closed. They should be signaled to open before the engine start signal so they are not opening against the static pressure created by the engine drawing combustion air (see Figure 3).
In climate zones where freezing temperatures are possible, the fuel tank for a diesel fire pump must be located inside the fire pump room in accordance with NFPA 20 requirements. When it comes to selecting the construction of the fuel tank, even though NFPA 20 allows the use of a single-wall tank, it is our opinion that a UL-listed, double-wall fuel tank with leak detection should always be the first choice.
As diesel fuel is considered a Class II combustible liquid, NFPA 20 requires secondary containment to be sized for the capacity single-wall fuel tank plus the overhead sprinkler system flow for the duration of the hazard. A double-wall fuel tank has an outer layer surrounding the inner container, automatically containing leaked fuel oil before it can enter the atmosphere.
When a fuel leakage switch is provided in the interstitial space between the two tank layers, a signal is to be annunciated by the engine controller. Figure 4 shows the installation of a fuel leakage switch on the low point of a pitched double-wall fuel tank. It is our experience that this arrangement of secondary containment is not required by most AHJs.
A properly sized floor drain should be provided in the fire pump room. Floors must be pitched toward the floor drain to ensure any water is directed away from critical equipment in the room. Coordinate with the plumbing engineer if a diesel-engine-driven fire pump is designed, as oil/water separators might be required in some municipalities.
When sprinkler system risers are located inside the fire pump room, the sprinkler drain riser must either spill to grade or spill to a storm drainage system. The remaining water trapped in the drainage system can be discharged to the floor drain in the fire pump room as the anticipated flow rate is low. However, a floor drain is not required to be sized to handle a flow rate of a main drain test.
Fire Pump Controllers
Of course, electric-motor-driven fire pumps require a higher level of coordination with the electrical engineer as compared to diesel-engine-driven fire pumps. Pump controllers for electric-motor-driven fire pumps have several methods used to start the motor, including:
These different starting methods can majorly impact the electrical service design. For example, the across-the-line starting method is like a light switch; when initiated, the full electrical load is imparted on the incoming service, which will fail if not properly designed. It is very important for the fire protection engineer to coordinate with the electrical engineer to ensure the electrical service is appropriate for the type of pump controller (starting method).
Remember, this coordination may be more than the fire protection engineer telling the electrical engineer what starting method was selected; the fire protection engineer may change to another starting method based on input from the electrical engineer.
This column and its predecessor focus on various elements commonly overlooked in fire protection engineering design. Minimizing delays, the number of change orders and cost overruns during construction is the focus of high-quality coordination efforts during the design phase.
Ying Zou, PE, LEED AP BD+C, is a senior fire protection engineer with the Harrington Group. She is a licensed professional fire protection engineer in three states and has more than eight years of experience. Ying is a Society of Fire Protection Engineers and the National Fire Protection Association member. She received a master’s degree in fire protection engineering from Worcester Polytechnic Institute.