Last week, one of our fire protection consultants went to a new construction project site in California to perform a walkthrough for a client. The walkthrough is part of our fire protection engineering construction services, which included coordination of the flushing of the underground fire service main piping that serves the building sprinkler systems.

When underground piping is installed, it is open to a variety of items that find their way into the pipe and can become obstructions, such as rocks, sticks, and dirt. During the underground flushing we performed last week, a large piece of wooden 2 x 4 was flushed out! Check out the picture above of the actual 2 X 4 that came out of the pipe.

Throughout the many years that we have been providing underground flushing services to clients, we have seen some interesting obstructions (e. g.: 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, a 1-½ inch impact wrench socket, etc.). I even once discovered a pair of painter’s overalls inside of a fire pump while performing flushing.

It is extremely important that underground piping is properly flushed, as obstructions like these can create serious problems for fire protection systems. Nuts, bolts, rocks, rags, and other items that may be drawn thru a fire pump can damage the impeller and substantially reduce the performance characteristics of the fire pump. Automatic fire sprinklers have discharge orifice diameters from less than ½-inch to less than 1-inch in diameter. If sprinklers are plugged by debris during an actual fire, the fire will be allowed to grow in size. Sprinkler systems are designed to contain or suppress a fire within a limited amount of floor area. If the fire is allowed to grow beyond the calculated area due to plugged sprinklers, the effectiveness of the fire sprinkler system is reduced and a large or even catastrophic fire loss could result.

Any underground pipe that supplies water to a fire sprinkler system is required to be thoroughly flushed in accordance with NFPA 24 Standard for the Installation of Private Fire Service Mains and Their Appurtenances prior to the connecting to the automatic fire sprinkler system. The standard for flushing is to achieve a flow rate of 10 feet per second (fps). However, if the pipe feeds a fire pump the flushing rate is increased to 15 fps. The corresponding flows in gallons per minute for the most common underground pipe sizes are provided in the following table:

Nominal Pipe Size

Flow Rate for 10 fps

Flow Rate for 15 fps



















If a 2,500 foot segment of pipe is being flushed at 10 fps, the first gallon of fresh water takes over 4 minutes to travel the length of the pipe. During our flushing procedures we add a safety factor of 2. We would flush 2,500 feet of pipe for a minimum of 9 minutes. Any segment of pipe less than 300 feet will be flushed for a minimum of 1 minute. Once the required flushing duration is achieved, we verify that the water stream is clear. If the required flushing rates cannot be achieved due to the water supply, there are contingencies to allow some lesser rates to be accepted.

Check out this short video to see an underground flushing in action. It doesn’t show the 2X4 coming out, but it shows you the amount of water that flows through the pipe and gives you an idea of how the process works!  This example is of a 50 foot section of 8 inch pipe and flowing approximately 2,000 gallons per minute based on the fire pump curve.

By Jeff Harrington, CEO and Founder of Harrington Group, Inc.