Piping arrangements for fire pumps
NFPA 20 provides fire protection engineers with guidance on design and installation of fire pumps and related components.
Effective fire pump installations require fire protection engineers to consider numerous components, and correctly apply a range of design and installation standards. In addition to addressing the more obvious components that comprise a fire pump installation-such as the fire pump, driver, controller, and pump room-careful attention also needs to be given to the piping leading to, from, and around the pump and the equipment associated with that piping.
While NFPA 20: Standard for the Installation of Stationary Pumps for Fire Protection serves as the principal standard addressing the sizing and installation of the associated piping, the next edition being the 2016, other codes and standards such as NFPA 13, NFPA 14, NFPA 22, NFPA 24, NFPA 25, and NFPA 291, as well as the applicable building and fire codes, also need to be reviewed and correctly applied depending on the type of fire protection systems served by the fire pump.
The piping connecting the water supply to the fire pump is referred to as suction piping. It comprises all piping, valves, and fittings that feed water to the pump's suction flange. The selection and installation of such suction pipe material is addressed by NFPA 24, which specifies the use of certain types of iron, steel, concrete, plastic, and copper. In addition, NFPA 24 addresses how the pipe and fittings are to be joined together, depth of cover if the pipe is buried,protection of the pipe from freezing and other damaging events, joint restraint, and acceptance testing including flushing and hydrostatic tests.
NFPA 20 addresses the arrangement of the suction pipe and associated devices. Generally, the suction pipe and associated devices need to be arranged in such a manner so as to minimize the likelihood of turbulent and imbalanced water flow entering the pump. Such conditions decrease overall pump performance, can result in a sudden system failure and can cause premature wear of system components.
The size of the suction pipe is influenced mostly by the fire protection system's hydraulic demand as determined in accordance with the appropriate system installation standards, such as NFPA 13 or NFPA 14, and the size of the fire pump selected. NFPA 24 provides guidance on suction pipe sizes and generally states that for any system, the pipe should be at least 6-inches in nominal diameter. Smaller pipe sizes are permitted provided hydraulic calculations verify that the pipe can supply the necessary system demand at the corresponding required pressure.
NFPA 22 provides specific guidance with regard to suction piping connecting a water tank with the fire pump. For instance, if the suction tank exceeds 100,000 gal, the size of the suction pipe must be at least 10 in. in diameter (nominal dimensions). The smaller the pipe, the faster the water flow, and therefore more turbulent flow will occur. Increasing the pipe size lowers the flow velocity and reduces the occurrence of turbulence.
NFPA 20 includes more specific provisions about suction pipe where fire pumps are installed, and specifies certain pipe sizes. The philosophy is that suction pipe be sized so that when the pump is operating at its maximum flow rate, which is 150% of its rated capacity or the maximum flow available from the water supply, the gauge pressure at the pump suction flange does not drop below -3 psi (-0.2 bar). Furthermore, the suction pipe is to be sized such that with the pump operating at 150% of its rated capacity, also referred to as pump's overload point, the velocity in that portion of the suction pipe located within 10 pipe diameters upstream of the pump suction flange does not exceed 15 ft/sec (4.57 m/sec). Pipe flows in excess of this velocity are more prone to turbulence. Where the suction pipe differs in size from the pump suction flange,reducers or increasers are permitted to be used but must be of the eccentric tapered type and installed in such a way so as to avoid air pockets.
In addition to specifying suction pipe sizes based on the rated capacity of the fire pump, NFPA 20 also addresses other system attachments that could cause turbulent or imbalanced flow into the fire pump. Where backflow preventers or check valves are being considered, they are to be located a minimum of 10 pipe diameters from the pump suction flange. If the backflow device incorporates butterfly valves, the device is to be installed at least 50-ft from the pump's suction flange. In fact, the 50-ft criterion applies to any valve, other than an outside screw and yoke gate valve, installed in the suction pipe.
Elbows and tees in the suction pipe also warrant special consideration. Such devices are to be located and positioned with respect to the orientation of their centerline plane. Where the centerline plane is parallel to a horizontal split-case fire pump shaft, the elbow or tee needs to be located a distance at least 10 pipe diameters from the suction flange of the fire pump. If the centerline plane is perpendicular to the horizontal split-case pump shaft, no limitations are placed on the location of the elbow or tee.
It is important to recognize that NFPA 20 only addresses the size of the suction pipe within 10 pipe diameters of the pump suction flange, while NFPA 22 addresses the size of the pipe connected to the tank. The provisions of NFPA 24 would apply where the requirements of NFPA 20 and NFPA 22 do not take precedence.
NFPA 20 defines discharge pipe and equipment as the pipe, valves, and fittings that extend from the pump discharge flange to the system side of the discharge control valve. Practically, any pipe,valve or fitting downstream of the fire pump's discharge control valve is no longer considered to be part of the discharge piping. Such pipe, valves, and fittings are considered part of the supply piping for the fire protection system being served by the fire pump. In the case of a sprinkler system riser, the requirements of NFPA 13 would apply from the point of the pump discharge control valve.
NFPA 20 addresses the size of the discharge pipe and associated fittings, and requires all of the above ground discharge piping to be composed of steel. In certain cases the discharge pipe is permitted to be smaller in diameter than the suction pipe because the water flow velocity is not of the same concern on the discharge side of the pump. The size of the discharge pipe has an effect on friction loss, but that effect can be accounted for though hydraulic analysis. As with suction pipe sizes, NFPA 20 specifies minimum discharge pipe diameters based on the capacity rating of the fire pump.
A control valve is to be installed on the discharge piping so that the pump can be isolated for service and repairs. Additional valves are discouraged to minimize the possibility that a valve will be inadvertently shut and not reopened-an ever-present concern with water-based fire protection systems. The control valve is permitted to be any type of valve listed for fire protection service, including a butterfly valve, because turbulence is not as critical on the discharge side of the pump.
A check valve is also to be installed on the discharge piping, between the fire pump and the discharge control valve. The discharge check valve traps the higher pressure in the fire protection system after the fire pump operation stops. The check valve also prevents other sources of water flow into the system, such as through a fire department connection, from flowing back into the fire pump.
NFPA 20 requires that the pressure rating of the discharge components, including all piping,fittings, and valves, be adequate for the maximum total discharge pressure with the pump operating at churn conditions at the pump's rated speed.
Pump bypass piping
A bypass is an arrangement of piping around the fire pump that can be used to supply water to the fire protection system should the pump fail or be taken out of service. Such bypass piping is to be sized as required for the discharge pipe.
Bypass piping is required where the water supply is considered to be of "material value" to the fire protection system without the use of the fire pump. While this is a rather subjective requirement,bypass lines are usually required where the water supply is provided by a pressurized fire service main such as municipal waterworks or private fire service main. Where the water supply for the building is from a private stand-alone fixed supply such as the suction tank, a minimum pressure due to the elevation head of the stored water in the tank is available but is not usually considered to be of material value. However, this should be verified through hydraulic analysis, and needs to be confirmed with the respective authorities having jurisdiction.
A check valve needs to be installed in the bypass piping so that the flow from the pump discharge cannot recirculate back to the pump suction. Additionally, control valves need to be installed on either side of the check valve so that the check valve can be isolated for maintenance.