Typically, plumbing engineers are concerned with a different type of flushing. Not today … In a previously published column, the reasons for — as well as the benefits of — properly flushing underground fire water mains were discussed in detail (“Flushing of Underground Pressurized Fire Protection Looped Supply Piping,” May 2022, https://bit.ly/3oOBaVE).

In this column, we will build on the previous one by discussing how to develop robust flushing plans and review a few example procedures of how to sufficiently flush fire water mains. We explore not only the minimum code requirements for flushing but also highlight industry best practices.

Developing a Flushing Plan

Optimal execution of a flushing procedure starts with a thorough plan. Flushing plans should encompass how every segment of newly laid underground piping in an installation will be flushed. Because of this, flushing plans can be relatively straightforward or irregularly complex. 

From our experience, flushing plans are best presented on an annotated fire protection site plan or civil utility plan drawing demonstrating the sequence of operations and requirements for each flushing operation (if more than one). This plan also should demonstrate aboveground flushing outlet apparatuses and how flushing flows will be controlled. 

A flushing plan should include thoughtful consideration of the following:

1. Identification of pipe segments and the order in which they will be flushed. For straightforward installations where the entire newly laid pipe is a service lateral from an existing main to the fire protection lead-in, the identification is simple. 

However, for larger, campus-wide, looped private fire protection mains fed by multiple fire pumps, developing a plan to minimize the amount of water flowed can be crucial. 

This analysis should identify the aboveground flushing outlets through which the flushing water will flow through. The analysis also should identify which sectional valves on a piping network should be closed or opened to isolate 100% of flushing flow in a single direction to achieve the required velocities. 

A solid plan will reduce the amount of doubling back or re-flushing previously flushed pipe segments. For instance, if by flushing several pipe segments in a certain order, a subsequent flush would cause potential debris to move into an already flushed section of pipe, that already flushed section would require flushing again to clear any debris. In nearly every case, there is an optimal order of flushing sequences to minimize rework. 

Some flushing sequences may only exist to “push” any debris into a larger looped section of piping to be flushed in a subsequent operation. This is a common approach and one that minimizes rework.

2. Code-required flushing velocities and flows. This is one big area where typical underground pipe flushing procedures fall short of what is required by adopted standards. 

Proper flushing of underground piping is a function of the water velocity and duration of the water flow. Common industry practice for flushing is to attach one or two hoses at a point of connection above ground and flush the underground piping until it’s “good ‘n clear.” Unfortunately, this method does not allow the contractor to confidently document the velocity of the flushing and, therefore, confirm its compliance with the applicable standards. 

NFPA 13, Standard for the Installation of Sprinkler Systems, 2019 ed. §6.10.2.1.3 (NFPA 13 2016 ed. §10.10.2.1.3) provides the minimum required flow rates required for a pipe flow velocity of 10 feet/second. Developing this high velocity in the piping network while flushing is necessary for cleaning the pipe and lifting foreign material to an aboveground flushing point. 

Where segments of piping supply a fire pump, the minimum required flow velocity increases. NFPA 20, Standard for the Installation of Stationary Pumps for Fire Protection, 2019 ed. §14.1.1 (NFPA 20 2016 ed. §14.1.1), requires the flushing to be performed at higher flow rates resulting in approximately 15 feet/second. The fire pump suction line piping is required to be flushed at a higher velocity to be sure that stones, silt and other debris will not enter the pump or fire protection systems. 

However, it is realized the flow rates required by NFPA 20 may not be available from the municipal water supply. In these cases, NFPA 20 §14.1.1.3 allows for the flushing flow rate to be equal to or greater than 150% of the rated flow of the connected fire pump, which should be less than the tabular flow rate necessary to achieve the 15 feet/second (see Table 1). 

Where a water supply also cannot provide this lesser flow, the flushing flow rate shall be the greater of 100% of the rated flow of the connected fire pump or the maximum flow demand of the fire protection system(s).

Table 1: Required Flushing Flows of Common Pipe Sizes
 for 10 and 15 feet/second

3. Minimum flushing duration for each pipe segment. This is another big area where not only typical flushing procedures fall short, but codes and standards only provide subjective requirements. 

NFPA 13, 2019 ed. §6.10.2.1.2 (NFPA 13 2016 ed. §10.10.2.1.2) only requires the flushing operation to continue until water flow is verified to be clear of debris. While this is not a very clear direction, we have developed recommended guidance for your consideration. 

Imagine a slug of water moving through a pipe network. As that slug of water flows through the piping at a given velocity, it will begin to take with it any residual debris of a certain size existing in the piping. The higher the velocity of the slug of water, the larger the debris it can transport. This is the rationale behind the NFPA 13 and NFPA 20 flushing velocity requirements. With the realization that it is all the water moving through the pipe that is scouring the debris, one can see how this is an effective method.

If water is flowing at 10 feet/second through a 3,000-foot-long water line, it will take five minutes for the first gallon of water to move through the entire pipe section. Through performing many dozens of system flushes over time, a 2x safety factor has been observed to be adequate to verify the debris is flushed out of a piping network. Therefore, for a 3,000-foot-long water line, a reasonable target flushing duration of 10 minutes would be used. 

Only a handful of times (out of many hundreds) have we observed debris discharged beyond our target duration. In these rare instances, the amount of debris was substantial, and flushing operations were continued for an additional target duration after the last object was discharged. 

4. Flushing apparatus considerations. While this is typically in the court of the sprinkler and underground contractor, it is important to think through how the water will be flushed out of the system. 

When flushed through a sprinkler lead-in, the flow rates required can generate an incredible amount of reactionary force that, if unchecked, can create a significant safety hazard. Typical flushing assemblies are either fabricated to attach directly to a lead-in stub-up or to the end of a fire sprinkler manifold and are routed out the exterior through the nearest door. 

It is common for restraint of the discharge end of the flushing apparatus to be provided with heavy site work machinery, such as a fork truck, to provide a reaction force to the resulting jet force that can be caused by flushing high flows out of 6-inch and 8-inch piping. In many other cases, a tee fitting is placed on the flushing end to diffuse the flushing water and its resulting forces (see Figure 1 above).

Regardless of the configuration used, extreme care should be taken to reduce the potential for an unsafe condition when configuring temporary flushing apparatuses. 

Field Flushing Procedures: Determining the Available Water Supply

Once a robust flushing plan is developed and reviewed with all parties involved, flushing procedures can be performed. Flushing operations should involve the fire protection engineer of record, general contractor, civil utilities contractor, fire sprinkler contractor, landscape contractor and site paving contractor.

When designing a fire protection system, NFPA 13 requires a hydrant flow test to be performed to identify the available water volume, pressure and corresponding hydraulic performance of a given water supply. 

This same procedure for determining the available water supply early in design should be used to determine the available water supply via a day-of water flow test performed in accordance with NFPA 291, Recommended Practice for Water Flow Testing and Marking of Hydrants. The resulting data should be used to approximate the water flow of each flushing operation. 

A high-level summary of a flow test is as follows:

• First, place a static/residual (S/R) gauge near the beginning of the piping network and record the static pressure. Preferably this gauge can be placed on the inlet side of the backflow preventer so you can see the fluctuations in the public water supply. Otherwise, this is typically placed on a fire hydrant.

• Then, you can measure the flow from the nearest downstream private fire hydrant on the piping network with a pitot tube while recording the residual pressure indicated on the S/R gauge.

• On logarithmic graph paper (N1.85 scale), graph the static pressure at zero flow and the residual pressure at the observed flow. Drawing a linear line between the two pressures and extrapolating to 0 psi provides a hydraulic curve.

Once the available water supply is known, the residual pressures required to achieve the required flows from the test plan can be evaluated. 

While in communication with an observer at the residual gauge, throttle the control valve at the above- ground flushing outlet until the S/R gauge provides the corresponding residual pressure reading for the required flow. If able to monitor a pressure gauge at the backflow preventer, this will allow verification that the pressure on the public water main does not drop below the jurisdiction’s minimum allowable pressure (often 20 psi). 

If the pressure begins to drop near the minimum allowable pressure, one of the backflow preventer isolation control valves can be throttled accordingly.

Only once this pressure is reached should you start timing the flushing duration determined by the test plan. During the flushing operation, observe the flushing outlet for any sizable debris that has cleared from the piping network. Once the flushing duration has elapsed and there have not been any recent observations of sizeable debris, close the control valve. Flushing of the segment of piping is complete. 

Flushing Examples: Municipal Water Supply

For example, assume we are assigned to the flushing of a 1,000-foot service lateral of 10-inch piping, which feeds a bank of automatic fire sprinkler risers. Using the procedure above, a day-of flow test was performed prior to flushing. A static pressure of 80 psi was recorded, and a residual pressure of 60 psi was recorded while a hydrant flowed 1,875 gallons/minute (gpm). 

As shown in Figure 2, plotting these points linearly on an N1.85 scale graph and extrapolating the line to 0 psi provides a theoretical understanding of the flows available at any pressure along the curve. This hydraulic curve is what you will use to determine the target residual pressure for the required flow to flush the system properly. 

 As we know from developing our flushing plan, NFPA 13 2019 ed., Table 6.10.2.1.3 requires 2,440 gpm to flush a 10-inch pipe at 10 feet/second properly. Analyzing our day-of hydrant flow test curve, we see that we would achieve a flow of 2,440 gpm at approximately 48 psi of residual pressure. This becomes our target pressure monitored during the flushing operation. 

To determine the duration, we know that for a velocity of 10 feet/second through 1,000 feet of pipe, the first gallon will travel through the 1,000 feet of pipe after
1 minute and 40 seconds. Applying a 2x safety factor would establish a flushing duration of 3 minutes and 20 seconds, which we would round up to the nearest whole minute for a target flushing duration of 4 minutes. 

Now, if, in this example, the 1,000 feet of 10-inch piping supplies a 2,000-gpm fire pump, that changes some things. We would now be evaluating this as a fire pump suction line and would be required to flush the pipe in accordance with NFPA 20. 

However, per NFPA 20 (2019 ed.) Table 14.1.1.1, to achieve a velocity of 15 feet/second, we would need to flow 3,670 gpm, which would cause the residual pressure in the water main to drop below 20 psi, commonly the minimum allowable residual pressure by most
jurisdictions. 

Because of this, the plan would be to flush a flow rate equal to 150% the rated flow of the connected fire pump. For a 2,000 gpm fire pump, this equals 3,000 gpm. Revisiting our day of flow test curve, we observe that for a required flow of 3,000 gpm, our target residual pressure would be 32 psi, and we would establish it as the target pressure for the flushing operation. 

Considerations for Flushing Private Fire Mains

When a fire pump is provided and supplies a private fire main, the process is similar. However, instead of performing a day-of hydrant flow test to identify the available water supply, the hydraulic curve generated during the fire pump final acceptance test curve is used. Additionally, instead of observing an S/R gauge on a hydrant, the fire pump discharge gauge is used to monitor the target pressure and the flow is regulated at the fire pump discharge control valve. 

Unless another fire pump is located downstream of the fire pump, providing pressure to the private fire main, the piping downstream of the fire pump will only require flushing at a flow velocity of 10 feet/second. The required flows are typically achieved using the fire pump without any issues. 

The challenge accompanying private fire mains typically revolves around flushing a looped system. Using a private fire main for a large warehouse as an example, we would observe a looped main serving multiple fire water lead-ins to the building as well as laterals to fire hydrants. 

An ideal flushing approach is to perform a clockwise (CW) loop flush followed by a counterclockwise (CCW) loop flush, overlapping the previous flushed segment by one lead-in. The selected lead-in for the CW flush is typically the most remote lead-in from the fire pump, allowing for a significant portion of the fire loop to be flushed in a single flushing operation that might last anywhere from 7 to 14 minutes, depending on the length of the CW loop section. 

Additionally, following up with the CCW allows for the remaining portion of the loop to be flushed in another operation lasting between 7 to 14 minutes, depending on length. 

Proper application of underground flushing requirements and engineering judgment have resulted in fruitful flushing operations numerous times. Developing a plan to facilitate the required flow rates for appropriate durations will lead to successful system flushing and mitigation of potential system impairments down the road. 

Zach Ataiyan, PE, is a registered professional engineer (fire protection). He has been working in the fire protection engineering industry since 2014 and is a lead fire protection engineer at the Harrington Group. James W. Tuten, SET, has been working in the fire protection industry since 1988. Tuten is a certified engineering technician, holding NICET Level IV certification in sprinkler systems.