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Pump Capacities and Supply Pipe Size You have asked the following questions: “I am working on a project for a 6-story hotel with 2 stairwells. As there will be a standpipe, in each stairwell the system demand should be 750 gpm (500 for the most remote standpipe and 250 for the 2nd). The civil drawings indicate a 4-inch fire line to the building. With a pump rating of 750 gpm, NFPA 20:4.14.3.4 states that within 10 pipe diameters of the suction flange, piping shall be a minimum of 6" (see Table 4.26(a)). I have calculated the system using 4-inch to 1'0 AFF and 6 inch down the corridor to the fire pump room and it appears to work with a 110-psi pump. My question is whether this is practical or if the 4-inch underground will be too restrictive when flowing the pump at 150% (1125 gpm). The engineer has suggested using a 500 gpm pump so the piping can remain 4 inch and the 150% point will be 750 gpm, however this is the system demand. Therefore, any degradation in the pump will cause it to be unable to meet system demand. Can the 4-inch underground remain, will 6-inch be required, or is it reliant on the hydraulic calculations?”
In response to your questions, we have reviewed NFPA 20, 2016 editions as the applicable standard. Our informal interpretation is you have asked multiple questions I’ll address them below There are a couple of interrelated aspects to this question. The first is should you design to a 150 percent of the pump capacity? While NFPA 20 will allow you to design to 150 percent of the pump’s rating, they suggest you don’t exceed 140 percent. There is annex material that supports this in A.4.9 by stating, “The performance of the pump when applied at capacities over 140 percent of rated capacity can be adversely affected by the suction conditions. Application of the pump at capacities less than 90 percent of the rated capacity is not recommended.” There is also handbook commentary that suggests selecting a pump so that the system demand falls between 90 percent and 140 percent of rated capacity to ensure that the pump is not oversized and, conversely, is not operating at its maximum output. Using a pump with a rating of 750 gpm is appropriate for the situation you have described. The next question is do you have to flow 150 percent of the pump capacity? During acceptance testing, you are not required to achieve 150 percent of the pump’s flow to pass the test. NFPA 20:14.2.6.2.4 allows testing below 150 percent of the rated capacity, but it must be greater than either 100 percent of the rated flow or the maximum flow demand of the fire protection system served by the pump. Now, 14.2.6.2.4.1 states that the reduced capacity shall constitute an acceptable test, provided the pump exceeds the fire protection system design and flow rate. Additionally, NFPA 25 supports not having to test to 150 percent of the pump’s rating to prevent damage to the pump or underground supply. It states in section 8.3.3.1.1, “If available suction supplies do not allow flowing of 150 percent of the rated pump capacity, the fire pump shall be tested to the maximum allowable discharge.” So, you are not required to flow 150 percent of the pump rating. That value is simply guidance to the manufacturer on what the pump must be capable of providing. The last question is whether it is acceptable to have a 4-in. supply main especially if that main does not support flowing 150 percent of the rated flow. We’ve already determined that you do not have to reach 150 percent of the rated flow, but it’s worth saying it applies regardless of the reason. The primary question on main size is whether it is allowed by NFPA 24. It is allowed as long as the main is not serving a fire hydrant(s), is serving only fire protection systems, and the size is supported with hydraulic calculations. It is worth noting that NFPA 13 points out it can be a combined main also providing domestic water and that water shall be included in the calculations when the main is less than a 4-in. diameter. So, you can have a 4-in. main. The last aspect is the 6-in. pipe size required by NFPA 20. As you already identified, this applies only to the portion of the pipe within 10 pipe diameters of the inlet to the pump. Beyond that point, it can any size allowed by the other driving standards. In conclusion, the intent of the standards is that you use a 750-gpm pump (you could use a 500-gpm pump but you shouldn’t) and the 4-in. main is acceptable.
