UNDERGROUND FLUSHING: How Much Is Enough? Part 2

Last week we took a look at how flushing a fire sprinkler system or fire pump can help mitigate the threat of catastrophic system failures. If you missed it, you can find it here. This week, we are exploring what NFPA codes have to say about flushing.

Where Can You Find Information in the Codes?

Fire protection underground piping is required to be installed in accordance with NFPA 24 the Standard for the Installation of Private Fire Service Mains and Their Appurtenances. The Underground Requirements chapter (Ch. 10) has been incorporated into NFPA 13 the Standard for the Installation of Sprinkler Systems for convenient reference. Additional underground requirements can be found in NFPA 20 the Standard for the Installation of Stationary Pumps for Fire Protection. The minimum acceptable flushing water flow rate is established based on how much water is actually available. The best of the three acceptable conditions is when enough water is available to reach a velocity of 10 feet per second (fps). If that is not achievable, flow all the water that is available from the water purveyor without dropping the pressure in the city supply to their limitation, usually no lower than 20 psi. The least amount of water flow that can be accepted is just more than the most demanding fire sprinkler demand flow rate, including any required hose stream allowances. However, if there is a fire pump in the system, the flushing rate for the pump supply piping is required to meet 15 fps and the flow rate needs to be at least 150 percent of the rated capacity of the fire pump. The following table is compiled from NFPA 13/NFPA 24 and NFPA 20 requirements:

REQUIRED FLUSHING RATES (gpm)
Pipe Size	10 fpsi	15 fpsii
6	880	1,360
8	1,560	2,350
10	2,440	3,670
12	3,520	5,290
i. Required flow rate for Private Fire Service Mains (NFPA 24 Table 10.10.2.1.3). ii. Required flow rate for Fire Pump Supply Piping (NFPA 20 Table 14.1.1.1).

Ideally, an outlet(s) can be opened on the system to achieve the desired flow rates. For smaller diameter pipe (6 or 8 inch) one or two 2-1/2 inch diameter discharge outlets (e.g., hydrant butt, pipe end or hose end) may be able to flow enough water. For larger pipes (10, 12 inch, or larger) several smaller outlets or one like-size outlet may be necessary. For a fire pump supply, the best option is to flow the same size outlet through a short segment of pipe to a safe discharge point.

Regardless of how the water is flowed, the discharge must be measured in order to be able to attest that the required flow rate is achieved. A direct reading with a flow meter would be best, but is likely cost prohibitive. An indirect measurement, via a same day water flow test,[1] is an acceptable method:

• Position a (Static/Residual) gauge on a convenient hydrant or backflow preventer test cock, upstream of the underground piping to be flushed.
• Flow a hydrant downstream of the above-referenced gauge and measure the water flow with a pitot tube.
• Graph the resulting water supply curve on N=1.85 graph paper.
• Compare the residual pressure during subsequent flushing operations to the water supply graph to estimate the water flow.
• The flushing discharge can also be calculated from the flow test data by using the formula provided in Section 4.10.1.2 of NFPA 291.

Often, on a new construction site, the paving has not been completed when the flushing is attempted and the flushing contractor will often be directed to discharge the water into a storm drain or pond. Hoses or pipes may be needed to direct the water to a safe discharge point. The length of the hoses or pipes, as well as the diameter, will have an effect on the water discharge rate that can be achieved.

When limited to flushing through hydrant outlets, multiple hydrants may need to be opened to reach the required flow rates. When flushing through automatic sprinkler run-ins (ASRs), the sprinkler riser diameter should be large enough for a short discharge pipe. One pipe size larger will likely be necessary if a hose is used to direct the water flow to a safe discharge location.

For the flushing of ASRs, the best option is to hard pipe the riser or riser header out the nearest man or roll-up door opening. Adding a tee fitting on the end of the pipe will counteract most of the force that is generated by the flowing water. When longer runs of a larger diameter are needed, a hard suction hose has been a go-to solution for many contractors. The use of a backhoe, forklift, or other heavy equipment is strongly advised to secure the flowing end of the pipe or hose and prevent damage to material or personnel.

Next week, in the conclusion to this series, we’ll review a few other important things to keep in mind while flushing fire protection systems, as well as some surprising examples of items our fire protection engineers have seen come out of sprinkler pipes during this process!