Reliable, code-compliant fire protection systems for commercial buildings begin with the consulting engineer or designer meeting the overall project objectives and selecting the necessary systems for the project. He or she must be knowledgeable in each component of the system and understand how each component will affect the selection of another, and how they will all work together.
Without question, the fire pump is one of the most critical pieces of equipment inside a commercial building. Standards governing fire pumps are most often included in broader guidelines for automatic sprinkler systems. NFPA 20, The Standard for the Installation of Stationary Pumps for Fire Protection, is the most widely recognized standard in the world, issued by the U.S.-based National Fire Protection Association and prepared by NFPA’s Technical Committee on Fire Pumps.
NFPA 20 is considered the most stringent of all the leading codes with its many redundancies and safety measures — designed to account for all conceivable scenarios in order to ensure maximum protection. NFPA 20 addresses the entire fire pump room — the pump, driver, controller, as well as water and fuel supplies, valves and other ancillary equipment necessary — to ensure each of the individual components will perform as a whole if the system is activated.
Xylem’s AC Fire Pump brand provides hands-on fire pump training for municipal fire department inspectors as well as education for engineers on proper system design in accordance with NFPA 20. Below are three frequently asked questions about fire protection system design.
Q: Do piping design requirements apply to all types of fire pumps?
A: It’s a common misconception that the pipe requirements in section 18.104.22.168 apply to all pumps. In fact, the code applies only to double-suction centrifugal pumps such as the one pictured here, and not end suction or inline type pumps which are single suction.
A-C Fire Pump 8100 Series double-suction pump
The distinction between double- and single-suction pumps in regard to the code is important for engineers to consider based on the size of the pump room. A compact footprint in the pump room means more salable square footage in the building, a prime objective in the design of modern commercial buildings.
The code, however, is very specific in regard to the length of pipe required before a double-suction impeller pump. The code states that no elbows and tees with a centerline plane parallel to a horizontal split-case pump shaft are permitted, unless the distance between the pump suction intake and the elbow and tee is greater than 10 times the suction pipe diameter. Elbows and tees with a centerline plane perpendicular to the horizontal split-case pump shaft are permitted at any location in the pump suction intake.
The drawing below depicts the right and wrong ways to pipe a fire pump.
Source: NFPA 20 Annex A
Q: What role does a derate factor play in selecting a diesel engine drive?
A: Diesel engines are paired with pumps in fire protection systems around the world, often in areas where electric power is unreliable or in rural areas. However, in selecting a diesel engine, engineers cannot only rely on the horsepower rating listed on the engine — engines must be derated for altitude and/or ambient temperature. Standard engines are designed to operate below 3,300 feet. In higher altitudes where the air is thinner or in areas with higher ambient temperatures, there is not enough combustible air for the engine and it does not produce the horsepower the pump requires to meet its performance curve. In a demand situation, it’s imperative there is sufficient horsepower to drive the fire pump.
Below is an example of a temperature derate curve. Some fire pump driver suppliers, such as Clarke, provide an online derate calculator to assist engineers in selecting the proper components for fire protection systems.
Q: Are there NFPA guidelines on how to properly size a jockey pump?
A: A low-flow, high-pressure jockey pump is usually required in a fire protection system, particularly one with automatically controlled (pressure actuated) pumps. There are several situations in which a jockey pump would come into play, the primary one associated with losses or leaks in a sprinkler system not due to a fire.
In that scenario, logic — and safety — dictates the use of a pressure maintenance or jockey pump. Here’s why: If the main fire pump is triggered, say it’s a large 200 horsepower pump, an alarm would sense the flow in the main pump, triggering the building’s fire alarm and forcing the evacuation of the building. In fact, NFPA 20 prohibits primary and standby fire pumps to be used as pressure maintenance pumps, except as permitted in NFPA 20 22.214.171.124.
The rule of thumb in sizing a jockey pump is 1 percent of flow and 10 percent over the design pressure. Section 4.26 of NFPA 20 says pressure maintenance pumps should:
- Be sized to replenish the fire protection system due to allowable leakage and normal drops in pressure
- Have rated capacities not less than any normal leakage rate
- Have discharge pressure sufficient to maintain the desired fire protection system pressure
Improper sizing of a jockey pump can have serious consequences. If the pump is sized larger than it should be according to code, and the pressure settings on the controller are then set for a larger pressure drop, in a demand situation the jockey pump is then being used to put out a fire instead of the main fire pump that is tied to the building alarm. One of the possible consequences of the main pump not being activated is that the building occupants are not alerted to the fire. If the fire spreads, life safety could be jeopardized.
The diagram below shows jockey pumps in relation to primary fire pumps.
Advancements in technology, evolving construction practices and new products in the marketplace drive changes to NFPA 20 and other model codes. In the case of fire protection systems, it is critical for engineers to keep abreast of these changes and seek out training and education opportunities in order to safeguard life and property in all commercial buildings.
Brian Buscher is a global marketing manager for Xylem’s AC Fire Pump brand with expertise in centrifugal pump systems for fire suppression in industrial and commercial facilities. He is a principal member of the NFPA 20 Technical Committee on Fire Pumps. Any comments/opinions made are the author’s own and not those of NFPA.