Fire protection pumps: Updates to NFPA 20

Significant changes to the 2013 edition of NFPA 20: Standard for the Installation of Stationary Pumps for Fire Protection include a requirement for an alternate flow measurement means for flow meters, modifications to the water mist fire pump requirements, and limited service controller requirements.

06/25/2013


Learning objectives:

  1. Understand the basic operating characteristics of fire pumps.
  2. Understand the basic types and operating characteristics of pressure regulating valves.
  3. Learn the most significant changes in NFPA 20-2013. 

Figure 1: When the utility water pressure is inadequate to supply the fire protection system demand, a fire pump becomes the heart of the fire protection system. This is an electric fire pump installation at a large retail store. Standby electric power isThe 2013 edition of NFPA 20: Standard for the Installation of Stationary Pumps for Fire Protection has some significant changes intended to address known issues. A discussion of some issues that will likely be addressed in the 2016 edition of NFPA 20, including fuel storage and transducer reliability, is also included. Issues raised by a transducer recall occurred too late in the cycle to be resolved in the 2013 edition but will likely be addressed in the 2016 edition.

Following is a summary of some of the significant new requirements and the philosophy behind them. To help understand the changes, a discussion of fire pump discharge pressures and pressure regulating valves is included.

Dealing with pressure

Fire pump discharge pressure: Centrifugal fire pumps increase water pressure by sending water through a spinning impeller. The amount of water that goes through the impeller is a function of water discharging through an opening downstream of the fire pump. The larger the opening, the higher the flow of water. If there are no openings, water will not flow through the fire pump but will “churn” within the fire pump. The pressure added to the water is inversely related to the flow rate through the fire pump, that is, the highest pressure occurs under “churn” (no flow) conditions.

NFPA 20 allows the “churn” pressure to exceed the rated pressure by up to 40%. The churn pressure typically exceeds the rated pressure by 10% to 20% for horizontal centrifugal fire pumps. NFPA requires a fire pump to deliver a minimum of 65% of the rated pressure at 150% of rated flow. Typically the suction pressure is inversely related to the flow rate: the water supply pressure decreases as the flow rate increases. The discharge pressure is the sum of the suction pressure plus the pressure added by the fire pump (see Figure 2).

Pressure regulating valves: Spring loading and pilot operation are the two basic operating mechanisms for pressure regulating valves. Early pressure regulating valves used a spring behind a seat. The valve operating pressure setting was based on the initial tension placed on the spring. In some valves the initial tension is field adjustable, while in others it is factory set. When the force from the water pressure exceeds the force applied by the spring, the seat opens until the forces equalize. Currently this type of operating mechanism is applied to small pressure relief valves, small valves in the sensing lines of pilot operated valves, and in pressure reducing hose stations.

Pilot operated valves apply system water pressure to the back side of a diaphragm to open and close the diaphragm. A small line with a control mechanism is connected between the back side of the diaphragm and the upstream piping to apply pressure to the back side of the diaphragm. A small pilot sensing line connected to the back side of the diaphragm is piped upstream, downstream, or both, depending on the application. This sensing line is used to increase or decrease the pressure on the back side of the diaphragm.

Figure 2: This shows a pump suction supply curve, a fire pump curve for a 1500 gpm at 105 psi fire pump, and a fire pump discharge pressure curve (static 50 psi, residual 44 psi, flow 1250 gpm). Courtesy: Aon Fire Protection EngineeringPressure relief valves: Pressure relief valves are installed in a side outlet of the primary water path and regulate pressure by allowing a flow of water through the fire pump and pressure relief valve, thereby causing the fire pump to operate at a lower pressure on the fire pump curve. The flow through the pressure relief valve may be discharged to the atmosphere or returned to the fire pump suction. Older fire pump installations may use a large spring loaded pressure relief valve. These large spring loaded valves are difficult to adjust, prone to failure, and will exceed their set operating pressure by up to 25% before they fully open.

Newer fire pump installations primarily use pressure relief valves that are pilot operated with a sensing line from piping upstream of the valve, connected to the back side of the diaphragm. Under no-flow conditions, the side outlet path is normally closed by the diaphragm. If the upstream pressure is above the preset value, water is drained from the back side of the diaphragm, moving the diaphragm and increasing the size of the flow path through the pressure relief valve. Pressure relief valve issues are discussed in more detail later in this article.

Pressure reducing valves: Pressure reducing valves are installed in the primary water path and regulate the downstream pressure. Pressure reducing hose valves may use a spring mechanism for controlling downstream water pressure. The inlet and outlet pressures must be known to set the initial spring tension.

Pilot operated pressure reducing valves (also referred to as pressure control valves) have a sensing line connected downstream of the valve to the back side of the diaphragm. Under no-flow conditions, the primary water path is normally closed by the diaphragm. If the downstream pressure is above the preset pressure, water pressure is applied to the back side of the diaphragm, decreasing the size of the flow path through the pressure reducing valve.

Low suction throttling (pump suction control) valves: Low suction throttling valves are installed in the primary water path to prevent the suction pressure from falling below a preset pressure. These valves are pilot operated and have a sensing line connected from the fire pump suction to the back side of the diaphragm. Under normal conditions the primary water path is open. If the fire pump suction decreases to or below the preset value, water pressure is applied to the back side of the diaphragm, decreasing the size of the flow path through the valve.

Tank fill: Tank fill valves are installed in the primary water path to control the flow of water into a tank. These valves are pilot operated and can be controlled by a solenoid valve or a pressure sensing line connected from the tank to the back side of the diaphragm. Under normal conditions the primary water path is closed by the diaphragm. In a solenoid operated valve, opening a solenoid relieves pressure from the back side of the diaphragm, moving the diaphragm and causing the primary water path to open. If the tank fill valve is pressure operated, low pressure on the sensing line relieves pressure from the back side of the diaphragm, moving the diaphragm and causing the primary water path to open.

Multiple function valves: By using multiple sensing lines, a pilot operated valve can be trimmed to perform multiple functions. As an example, adding a sensing line connected upstream of the valve can allow a tank fill to prevent the upstream pressure from falling below a preset pressure.

The scope of NFPA 20 does not include pumps installed under NFPA 13D: Standard for the Installation of Sprinkler Systems in One- and Two-Family Dwellings and Manufactured Homes. NFPA 13D is limited to the design and installation of automatic sprinkler systems for protection against the fire hazards in one- and two-family dwellings and manufactured homes. Although the scope statement in NFPA 20 does not specifically exclude these pumps, NFPA 13D does not require listed pumps and references NFPA 20 only as a reference standard. One of the goals of NFPA 13D is to encourage installation of sprinkler systems in dwellings by limiting the cost. This issue becomes more complex when localities allow NFPA 13D to be applied to buildings that are not one- or two-family dwellings or manufactured homes. When applied to apartment buildings or other (typically residential) applications that must comply with NFPA 25, correlating components needed to perform the inspection, testing, and maintenance required by NFPA 25 may not be provided.

Controllers

The use of limited service controllers has been debated for multiple code cycles. They were initially introduced into NFPA 20 as a means to provide controllers for lower horsepower motors, and to provide a low-cost alternative to minimize the cost of retrofitting schools with sprinkler systems. Since their original introduction, many things have changed. Full-service controllers are now available for low horsepower motors, and the cost differential between full-service and limited service controllers has decreased significantly (the manufacturing cost differential was reported to be as low as $100). Another concern is the use of limited service controllers for applications which were not contemplated by NFPA 20. These non-contemplated applications include foam pumps in aircraft hangars and other applications where high value and/or high life safety risks are involved and the increased reliability of a full service controller is needed. Previous proposals to change the requirements on limited service controllers have varied from eliminating limited service controllers altogether to removing the 30 hp limitation. Also, no statistical analysis has been conducted to evaluate the reliability of limited service controllers.

There are two main issues with the thermal magnetic breaker that was allowed in limited service controllers prior to the 2013 edition of NFPA 20. The first concern is that the allowable trip time of 200 seconds allowed for a thermal magnetic breaker will cause burnout of a locked motor. When this occurs, the limited service controller may be reset by someone without realizing that the motor is non-operational. The second concern is the extended time it takes to reset a limited service controller because the thermal magnetic breaker must cool down before it will reset. In case of a fire, time delay can be critical. A limited survey in the Chicago area of pump motor replacements revealed that motors with 25 hp or less were replaced at a significantly higher frequency per installed base than higher horsepower motors.

The deadlock in limited service controllers was resolved by a “compromise” to change the breaker requirement on limited service controllers to match full-service controllers. This will likely further reduce the price differential.


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