UNDERGROUND FLUSHING: How Much Is Enough? Part 3
For the last few weeks, we’ve looked at why it’s important to flush fire sprinkler systems and fire pumps, and we’ve also discussed what the NFPA codes have to say about the process. This week, in the series conclusion, 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!
Other Things to Consider for Flushing
To bag or not to bag? If the discharge is to be on dirt or into a storm drain, it may be necessary to secure a burlap bag over the discharge outlet in order to demonstrate to the AHJ or owner’s representative that the piping has been flushed clear. A bag on the discharge is also useful to catch the evidence that flushing is a useful exercise. If the discharge is to be on clean concrete or asphalt, a bag may not be necessary.
How long is enough? If the 10 fps flow rate is achieved, the duration is a function of the length of the pipe being flushed. For example, a run of 1,000 feet of pipe at a rate of 10 fps, would require a minimum of 100 seconds for the first gallon of clean water that entered the pipe to reach the far end. While that may be adequate to push out the turbid water, dirt, or sand out of the pipe, a rock may be rolling along the bottom of the pipe at a slower rate. Providing for a two times safety factor on the duration has proven to be adequate in all but the most extreme circumstances. So for the 1,000 feet of pipe, 200 seconds, or nearly three minutes of flushing would be needed.
If there is a fire pump in the mix, there is a high likelihood that the pump will be needed to properly flush the underground downstream from the pump. In order to use the fire pump, it must have been acceptance tested. Acceptance tests are often performed in the morning, followed by underground flushing in the afternoon. When flushing with a fire pump, the net pressure (discharge pressure – suction pressure) can be compared to the acceptance test curve to more accurately estimate the water flow than the aforementioned method.
Where is all that water going to go? Due to the chlorine in the water, some jurisdictions (notably in California) may require that the water be captured and/or treated before it can enter their storm drains. Retention ponds may need to be pumped out and the water treated like hazardous waste. While this seems extreme, the flushing contractor is required to plan for the proper disposal of the water used for flushing.
Is there a recommended procedure for flushing? While NFPA 24 does contain annex material addressing some basic recommendations for flushing more complex underground configurations, there is no comprehensive flushing procedure contained in the code. When flushing a looped system, we recommend flushing each leg of the loop from outlets that are located remote from the water supply such that the length of the pipe for each flush is roughly equal. Sectional valves are utilized to direct the water clockwise, and then counterclockwise around the loop. Flowing outlets (ASRs) are selected such that an overlap is formed allowing one section of pipe to be flushed from both directions. Once the loop has been thoroughly flushed, the remaining ASRs and fire hydrants can be flushed at a minimum duration. One minute is adequate to flush 300 feet of pipe.
And, don’t forget about the fire department connection (FDC). If the check valve for the FDC is above grade, it can be removed to allow for flushing once the loop has been flushed. If however, the check valve is below grade in a vault, it is preferable to flush the FDC prior to the loop. The process of flipping the check valve to allow the water to flow out of the FDC will fill the vault with water. This can be avoided by connecting the FDC to a public pressure hydrant upstream of the backflow preventer with two hoses, and then opening any outlet on the system to which the FDC is connected. Any potential debris that may be contained in the FDC piping will be flushed into the loop for subsequent flushing operations to remove.
Concrete, 2X4’s, and Overalls – Oh My!
That’s right – concrete, 2X4’s, and painter’s overalls are all items that our fire protection engineers and consultants have seen come out of pipes throughout the many years that we have been providing underground flushing services to clients. We’ve also seen large rocks, a hard hat, safety glasses, nuts and bolts, 1-inch plastic coupons from domestic wet taps, the cement lining from the aging city main, and a 1-½ inch impact wrench socket, etc.
Items like these are extreme, but remember, even the smallest pieces of debris can cause catastrophic failures to your fire protection systems. That’s why properly flushing underground piping is an essential step in removing objects that could plug up sprinkler systems.
Do you think you could benefit from our underground flushing services or do you need more information about its importance? Our expert fire protection engineers can help! Contact us today!
 NFPA 24 Section 10.10.2.1.4
 NFPA 24 Section A.10.10.2.1