Sunday, February 9, 2020

Fire Pump Rating (Size) Selection

Fire pump sizing is not like commercial pump sizing. We don't care about efficiency, and you order by pumps in only specific sizes. This article touches upon some items to consider when picking a fire pump.

In general, the first step is determining your system demand point. Discussion of how exactly you determine this is beyond the scope of this article and has a lot of nuance depending upon your site-specific needs. However, for simplicity let's assume that you have a dry-system in an attic with a demand point of 305 gpm (2535 sq ft x 0.10 gpm/sq ft x 1.20 overflow/imbalance factor).

Rated Flow (gpm) Sizing

The first item you must specify is the pump flow rate. Per NFPA 20 (2013 edition) table 4.8.2 pumps are only allowed to be listed with the following flow rates in gpm:

  • 25, 50, 100, 150, 200, 250, 300, 400, 450, 500, 750, 1000, 1250, 1500, 2000, 2500, 3000, 3500, 4000, 4500, 5000
So with our example demand of 305 gpm, would you go with a 300 or 400 gpm rated fire pump?

While manufacturer's pump curves go way beyond the ratings above, NFPA 20 section 4.8.1 only allows you to use a fire pump out to 150% of its rated capacity. That means that a 300 gpm pump can be run out to 400 gpm (300 x 1.5) and could easily meet the example demand flow rate. One could even use a 250 gpm pump with the corresponding max flow rate of 375 gpm (see the bottom of this article for additional discussion).

The other common condition is a standpipe. Assume you have two stair-towers and total demand of 750 gpm. Should you run a 500 gpm out 150% to 750 gpm? In our opinion that is probably a poor choice since you are so close to the 150% limitation.

So what is the harm in just selecting the next pump size up (i.e. 400 gpm for our 305 gpm example demand)? First, can you water supply handle flowing at least the 100% and/or 150% flow rate? While technically NFPA 20 allows you complete testing at less than 150%, if you can avoid this situation it would be better.

Second, there may be some hidden cost. For example going from a 400 to 500 gpm pump also means you go from 4-inch suction to 6-inch suction (see our NFPA 20 pump pipe size look-up app at https://anvil-fire.com/apps_pump_sizing.html). In addition, you are generally probably increasing the horsepower and therefore the overall cost of the equipment. Or even causing an electrical transformer to be upgraded.

Note that FM Global only allows you to run the pump out to 140% of rated pressure.

Rated Pressure (psi) Sizing

The second item you must specify is the rated pressure you want. Manufacturers must list each pressure rating in order to maintain listing with UL and FM Global. On occasion you will find some holes in approved pressure ratings, but AC Fire has a long history and therefore an amazingly wide range of rated pumps to accommodate almost any demand.

While not directly impacting your selection, it should be noted that NFPA limits manufacturers to how "steep" their curves can be in regard to pressure. See the curve below for the specifics. These limitations are a non-issue when you buy a listed pump as they have already had to comply with the limitations.

However, you should pay careful attention to the churn pressure (sometimes also called shut-off or dead-head pressure). When a fire pump is not flowing any water it produces the highest discharge pressure. In general most fire sprinkler components are listed to 175 psi.

Let's take the example of you have 50 psi static pressure from the city + our 100 psi rated pressure for a total 150 psi when flowing the 100% rated gpm. Sounds good, right? Maybe not if your churn pressure is 126 psi which is not unthinkable depending upon the specific pump selection (50 psi static + 126 psi churn = 176 psi). Always provide the max city pressure to your pump rep and look at the pump specific curve to ensure no surprises in the field.

In addition, make sure you check how much pressure you have at your desired flow rate from the city water supply. You can use the formula below or just use the quick calculator at https://anvil-fire.com/apps_log_graph.html





Check out AC Fire's online pump selector at https://www.selectacfirepump.com/ and play around with some selections, or reach out to us at sales@anvil-fire.com for support on your projects in the NorthWest.


A quick reference chart for the 150% and 140% flows of the various pump ratings.

Rated Capacity150%140%
2537.535.0
507570
100150140
150225210
200300280
250375350
300450420
400600560
450675630
500750700
7501,1251,050
1,0001,5001,400
1,2501,8751,750
1,5002,2502,100
2,0003,0002,800
2,5003,7503,500
3,0004,5004,200
3,5005,2504,900
4,0006,0005,600
4,5006,7506,300
5,0007,5007,000