Fire protection pumps: Updates to NFPA 20
Pump circulation relief valves
For pipes in areas subject to seismic activity, the clearance around the pipe passing through walls, floors, or ceilings must meet the seismic requirements referenced in NFPA 13.
Torsional vibration analysis is always needed for diesel-engine driven vertical turbine pumps. Harmonics that can develop at specific rpm can cause the driver to self-destruct if not accounted for. Requirements to clarify the torsional vibration analysis were included in the 2103 edition of NFPA 20.
The committee’s experience is that flow meters are highly inaccurate, and a requirement for an alternate means to measure the flow after it has passed through the flow meter has been added to NFPA 20. The alternate measurement allows verification of the accuracy of the flow meter. In most cases this will require a pump test header downstream of the flow meter. Figures showing the correct location of a pump test header were modified in NFPA 20. While many flow meters cannot be adjusted, correction factors may be established. Where the accuracy of the flow meter cannot be established, it should be replaced.
The U.S. Consumer Product Safety Commission recall of approximately 25,000 Gems model 3100 pressure transducers affected approximately 15,000 fire pumps. The issue was not picked up in time for action in the 2013 edition of NFPA 20 but will likely be reviewed in the 2016 edition. While the recall was limited to one model of one manufacturer, it does point out a potential issue that could be monitored in a fire pump controller.
Mercoid switches (no longer used in modern controllers), the primary pressure actuation method prior to pressure transducers, also are subject to failure. However, the interaction between a controller and a mercoid switch is limited to open or closed contacts, which does not allow the controller to detect any issues with the mercoid. Transducers are interactive, providing a continuous voltage reading to the controller. It is possible for the controller to monitor these voltage readings for abnormalities. This capability that was not available for mercoid switches will form the basis for the discussion for the 2016 edition of NFPA 20.
Consideration was given to processing a tentative interim amendment (TIA) to address this issue. However, the NFPA 20 committee believed the current issue was being addressed through the recall mechanism and any modifications to NFPA 20 would not meet the emergency nature needed to issue a TIA because it would only affect future controllers without improving the current situation.
Low-suction pressure throttling valve
Any device that restricts the flow of water is discouraged in a fire pump installation. Some jurisdictions have requirements to limit the pump suction pressure to a minimum pressure (typically 20 psi) through the use of a low-suction throttling valve (also referred to as a pump suction control valve). These valves are installed downstream of the fire pump but monitor pressure upstream of the fire pump, and will partially close as necessary to maintain a preset minimum suction pressure.
Because these valves limit the minimum suction pressure, the design must maintain the suction pressure at the required flow rate (the higher of 100% rated flow or system demand) above the preset minimum.
There is a significant pressure loss through these valves that must be accounted for in determining the available flow and pressure downstream of the valve. A low-suction throttling valve should be wide open as long as the pressure remains above the minimum suction pressure, and the friction loss can be calculated using the Cv value of the valve in the formula:
P = (Q/Cv)2
- P = pressure loss across the valve
- Q = Flow through the valve in gpm
- Cv = Friction loss factor as determined by the manufacturer in gpm/psi0.5
NFPA 20 does not permit the installation of pressure-reducing valves (also referred to as pressure-control valves) within the fire pump suction or discharge piping. Piping before the suction or after the discharge control valve is not within the jurisdiction of NFPA 20. The suction components consist of all pipe, valves, and fittings from the pump suction flange to the connection to the public or private water service main, storage tank, or reservoir that feeds water to the pump. The discharge components consist of pipe, valves, and fittings extending from the pump discharge flange to the system side of the discharge valve.
NFPA 14: Standard for the Installation of Standpipe and Hose Systems does permit the use of pressure control valves to control the pressure on vertically zoned standpipe systems. However, meeting the NFPA 14 requirement that the failure of a single pressure control valve does not overpressurize more than two hose valves requires a series arrangement of pressure control valves that must accommodate both low and high flow rates. A full flow test method also must be provided for the valves. NFPA 20 does not prohibit using the pump test header to test these valves.. This is a complex arrangement, and the use of pressure reducing valves should be thoroughly understood before using this design approach.
Figure 5 shows a testing arrangement for a single pressure reducing valve installed downstream of the fire pump discharge control valve.
Gayle Pennel is a project manager with Aon Fire Protection Engineering. His expertise lies in fire protection systems and water supplies. He is currently chairman of the NFPA 20 committee and serves on the NFPA 25 committee. He has designed fire protection systems for super high-rise and large exhibition centers, as well as industrial sites. Pennel has consulted on pressure surge, corrosion, and other fire protection failure issues and has successfully presented fire protection design alternatives to state and local authorities.