NFPA 20: Changes to the fire pump standard

Regardless of whether the 2016 edition of NFPA 20 will be applicable to the next nonresidential building project, fire protection engineers need to be aware of recent changes to the standard.

11/14/2017


This article has been peer-reviewed.Learning objectives:

  • Know the basics of NFPA 20: Standard for the Installation of Stationary Pumps for Fire Protection.
  • Understand the changes to the 2016 edition of NFPA 20, including all types of fire pumps and the electrical aspects of NFPA 20.

The 2016 edition of NFPA 20: Standard for the Installation of Stationary Pumps for Fire Protection includes new rules that specifically address fire pumps installed in high-rise buildings as well as the introduction of a multistage, multiport pump. The overall intent is to standardize fire pump design in high-rise structures to ensure an appropriate level of reliability.

Figure 1: Controllers for a centrifugal fire pump and pressure-maintenance pump are shown. All graphics courtesy: Smith Seckman Reid Inc.The fire pump is a vital part of a building’s fire suppression system. It is responsible for pulling water from some dedicated source—either an underground public water supply main or a water source, such as a well, water-storage tank, lake, or other body of water—into the building in the event of a fire. A fire pump becomes necessary when the water supply is insufficient to provide the proper water pressure for the fire suppression system to function as designed.

Fire pumps serve as critical and essential components of many water-based fire protection systems, such as sprinkler, standpipe, foam, water-spray, and water-mist systems for commercial and industrial applications. Where determined to be necessary, a fire pump installation provides for the required water pressure that is vital for the fire protection system performance.

It is imperative that pump selection be made carefully, so that it will work properly when called upon for service. In addition to being sized properly, the pump must be installed correctly and maintained regularly as required by NFPA 25: Standard for the Inspection, Testing, and Maintenance of Water-Based Fire Protection Systems. The standard pump used for fire protection service is normally a centrifugal horizontal or vertical split-case, single-stage, or multistage pump. However, the pump can also be a positive-displacement pump or vertical-shaft turbine pump.

Types of fire pumps

Centrifugal pumps are the most popular fire pumps and are classified as nonpositive-displacement pumps because they do not pump a definite amount of water with each revolution. Rather, this type of pump imparts velocity to the water and converts it to pressure within the pump itself. Nearly all modern fire apparatus use a centrifugal pump as the main fire pump. Centrifugal pumps can be mounted vertically or horizontally and are commonly driven by diesel engines or electric motors. Horizontal pumps are used in positive-pressure situations, such as the water supply coming from an elevated storage tank or the city water main. On the other hand, vertical pumps are used where there is a lack of positive pressure. This commonly occurs with reservoirs, ponds, underground storage tanks, and well systems.

Figure 2: This shows the floor plan of a typical fire pump room.Inline pumps (space savers) are installed within and supported by the suction and discharge piping. These pumps work well in compact spaces due to a reduced footprint. They are designed for easy maintenance; the motor and pump rotating assembly pulls out easily from the top without removing the pump casing from the piping.

Positive-displacement pumps move water by trapping a certain amount of the water before pushing it out through the discharge line. These pumps have an expanding cavity on the suction side and a decreasing cavity on the discharge side, allowing liquid to flow into the pump as the cavity on the suction side expands and then flow out of the discharge as the cavity collapses. These pumps fall under two major categories: piston and rotary gear, with the latter being most commonly used in fire pump applications. Positive-displacement pumps are still a necessary part of the overall pumping system on modern fire apparatus because, unlike centrifugal pumps, they can pump air. Due to this feature, positive-displacement pumps are typically installed in special-hazard fire protection systems, such as foam and water-mist systems.

Vertical-shaft turbine pumps are centrifugal pumps with one or more impellers mounted on a vertical shaft. They are unique in that the impellers are intended to be submerged in the water source. They are used for installations where the water source is below the pump impellers. These pumps have a bowl assembly, which often contains several impellers on a vertical shaft, with the discharge of each impeller directly feeding the suction of the next impeller. Impellers are mounted on a column assembly built to a specified length. A discharge head assembly holds the motor or right-angle gear drive. In a fire pump room, the discharge head assembly of a typical vertical turbine pump is the only visible part of the pump.

Figure 3: A 200-hp fire pump serves an office building and parking garage.Multistage, multiport (multiple-outlet) pumps comprise several pump assemblies in series, with outlet ports between impellers driven by a single motor. The available pressure increases with each impeller stage. These pumps are predominantly used to serve multiple zones with different pressure requirements in high-rise buildings. A single fire pump controller can be provided with multiple pressure-sensing devices so that each discharge port may be equipped with its own sensing line. Each discharge port requires its own pressure-maintenance pump, or jockey pump, and jockey pump controller.

Multistage multiport pumps

Multistage multiport pumps were introduced into NFPA 20 in the 2016 edition as an alternative to arranging separate fire pumps in series to serve high-rise buildings having system zones with widely different pressure requirements. Multistage multiport pumps operate similarly to separate pumps arranged in series, except that their stages are all driven by a single motor and there are no shutoff valves between successive stages (3.3.44.11, 4.8.2).

Multistage multiport fire pumps must have a bypass installed between the inlet and the first outlet port, and between successive outlet ports, whenever such a bypass can provide pressure-of-material value without the assistance of the bypassed impeller (4.15.4.2). This requirement echoes a similar requirement for individual fire pumps or fire pumps arranged in series. It is intended to allow the water supply to still provide flow to the fire protection system at a reduced pressure, if an impeller should fail.

Figure 4: This 3-D rendering of a fire pump installation shows the controller, jockey pump, piping, sensor lines, fire department connection, and a single fire pump.The automatic circulation relief valve for a multistage multiport fire pump must be installed between the outlet of the last stage and its discharge check valve and set below the churn pressure, or shutoff pressure, of the first port (4.12.1.3.1). The purpose of this requirement is to ensure that a limited flow of cooling water will flow through the latter stages of the pump, exiting via the circulation relief valve to maintain cooling of the impellers.

Each discharge port is required to have its own individual pressure-sensing line connected to the fire pump controller and to the pressure-maintenance pump controller (4.31.1.1).


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