A baker’s dozen
13 tips for better fire pump design
By Samuel S. Dannaway, PE,
President, S.S. Dannaway Associates, Inc., Honolulu, Hawaii
Fire pump systems are often needed to give our fire protection systems that much needed boost in pressure. It goes without saying that it is critical for the fire pump system supporting fire protection systems to be extremely reliable. Compliance with NFPA 20 Standard for the Installation of Stationary Fire Pump Systems, 2010 being the current edition, is essential to assuring this reliability. The fire pump system is a collection of mechanical, electrical and electronic components, which through design must be given the best possible chance of reliably operating in the event it is called to duty. Here are some pointers, many of which result from lessons learned through experience with pump design, which hopefully will help others in their designs.
1. Protect the pump. The fire pump system must be located in a separate building or in a room, which is separated from the remainder of the building. Rooms containing fire pumps in buildings must be separated by fire-resistant construction. In high-rise buildings, the fire resistance rating must be two hours. In other than high-rise buildings, non-sprinklered rooms must have a two-hour fire resistance rating and sprinklered rooms must have a one-hour fire resistance rating. It is recommended that the room be dedicated to the fire pump system. NFPA 20 does not state directly that it must be in a dedicated space, but the annex notes indicate that it should not share space with equipment that represents a potential fire hazard such as boilers. Pumps should be accessible during a fire emergency so consideration should be given to providing exterior access to the room. And do not forget to provide emergency lighting. Oh yes, and while we are at it, let’s make sure that our fire pump room is protected with automatic sprinklers.
2. Flooded suction. Ensure that the fire pump suction pump is sized such that with the fire pump operating at 150% of rated capacity, the gauge pressure at the fire pump suction inlet is at least 0 psig or more. If there are multiple fire pumps, and system demand requires multiple pump operation, then the suction inlet gauge also must read at least 0 psig with all required pumps running at 150%-rated capacity. There is an allowance for pump(s) taking their supply from a suction tank in that the pressure is allowed to drop to -3.0 psig at the lowest expected tank level after maximum system demand at maximum required duration has been used.
3. Suction piping. To avoid potential for cavitation, arrange the pump suction to avoid elbows and tees within 10 pipe diameters of the fire pump suction flange. It is acceptable to have an elbow with a centerline plane perpendicular to the horizontal split-case pump shaft, i.e. an elbow turning up or down into the pump, but not right or left into the pump. This is one of the most common errors I see in pump room layout. Also make sure an eccentric reducer is used at the pump suction flange if its diameter is less than the suction pipe.
4. Controller sensing lines. Provide a separate sensing line for each fire pump/controller and for the jockey pump/controller. Ensure that the sensing lines connect into the pump discharge between the discharge check valve and discharge controller valve. Also, provide two check valves with direction of flow toward the pump with the appropriate sized hole drilled in the clapper and located at least five feet apart.
5. Out-on-the-curve. Select a pump so that the maximum system demand is in the range of 90 to 140% of pump rated capacity. NFPA 20 does not specifically prohibit using a fire pump to meet flow demands in the range of 140 to 150% of rated capacity but advises against it. Factory Mutual (see FM Data Sheet 3-7 Fire Protection Pumps) does not permit going beyond 140%.
6. Design with testing in mind. Make it as easy and as safe as possible to test the fire pump system using the test header. Ideally, a test header should be located so UL playpipes can be directly attached to the tester header hose valves and oriented so large flows can occur without causing damage. Do not, like some have done, locate the test header inside the pump room. A poorly positioned test header practically guarantees it will never be used.
7. Provide adequate drainage for the pump room. Provide drains for water drips from piping at centrifugal pump pack at impeller bowls, discharge of circulation relief valves on electrical motor driven pumps, and heat exchanger cooling water discharge for diesel engine-driven fire pumps.
8. Supervision. Make sure the fire pump is properly supervised at a constantly attended location. Signals for pump running alarms and various pump/driver/controller trouble conditions must be identified to responsible personnel without delay.
9. Check your fire pump system against the sizes listed in NFPA 20 Table 5.25(a) or (b) Summary of Centrifugal Fire Pump Data just to ensure you have correctly sized suction and discharge piping, relief valve and relief valve discharge piping, flow meter, hose header size, and number of hose valves. Also, note that these are minimum sizes and quantities, and may require adjustment for long runs of piping.
10. Ensure electric motor driven fire pumps are provided with reliable power. Unless the normal source of power is considered reliable an alternate source of power must be available. The annex of NFPA 20 provides guidance regarding power supply reliability.
11. Provide a proper foundation. The base plate for the pump and driver must be firmly mounted to a reinforced concrete pad that is an integral part of a solid concrete floor. The base plate should be fully grouted in place after initial alignment.
12. Know your right from your left. When designing a pump room for a diesel engine driven fire pump the handedness of the pump must be considered. Most diesel engine driven fire pumps are right-handed, i.e., the pump rotates clockwise when viewed from the driver end. Designing a pump room for the right-hand drive diesel will help avoid redesign or change orders if a left-hand drive engine meeting your requirements is not available.
13. Give your pump some room. It is very hard to maintain a fire pump system when one is required to climb over and under piping to access important equipment. Stand up to the architect and get a reasonable space. Make sure there is proper safety clearance in front of fire pump controllers per the NEC.
That concludes my baker’s dozen. The list is by no means complete, note that NFPA 20 contains 113 pages, but I hope it will help serve as a memory jogger. I would be interested in hearing from any readers that have other tips for fire pump design that can be shared in a future article.
Samuel S. Dannaway, PE, is a registered fire protection engineer and mechanical engineer with bachelors and masters degrees from the University of Maryland Department of Fire Protection Engineering. He is past president and a Fellow of the Society of Fire Protection Engineers. He is president of S. S. Dannaway Associates, Inc., a 15-person fire protection engineering consulting firm with offices in Honolulu, Hawaii and Guam.