High Volume Flushing – Understanding the Challenges of this New AWWA Standard Requirement
For years little attention has been paid in waterline projects to what happens with water after it is flushed from the system. In the past, when constructing a waterline system up to 24-inch diameter, you would place calcium hypochlorite in the line during construction and then fill the line as slow as possible to keep the calcium in the proper place within the line, to chlorinate the system. After your cooking time you would just discharge the water. If high volume flushing was required, no problem; without the requirement for dechlorination you just open the main and let it rip, after you have super chlorinated the system. This was typical of all chlorination methods, until dechlorination became part of the process.The current standard defines four methods of chlorination, three methods that have been used for years and a new Spray Method for large diameter pipe. The three existing methods have changed with an increase in flushing rates to prepare systems for chlorination, and in most cases the need to dechlorinate all discharges no matter the chlorine residual level. The spray method requires a completely different procedure for large diameter pipe.
Protecting Your Source Supply
Each method talks about the need for protection of the existing or source system form the new system. This is accomplished with a complete separation of the two systems until the new system has passed all tests, then the final connection can be made. The most common construction method is the source system is tapped with a tapping valve and sleeve to provide final water supply to the new system. The tapping valve serves as the isolation valve between the two systems, until the new system is approved. A 2” (typical size) service connection is installed and connected to the new system within the first 10 feet of the new system.
For over the last twenty-plus years the standard has provided the option of backflow protection. It also states that you should have a physical separation between the active system and the new water main. Required water for pressure testing, flushing and for chlorination should be supplied through a temporary by-pass connection. What this means is you cannot fill your new system by opening your tapping (or Isolation) valve without a complete separation and a backflow preventer between the two; your best solution in preventing a major problem. The reason being if you experience a water main break in the existing system you could get a backflow of super chlorinated water into your source system.
With the new changes to project specifications arising from updated AWWA Standards for field chlorination & dechlorination, the procedures for how you complete your flushing need to be updated. With the increased flushing rates the old method of loading the pipeline during construction is longer be feasible, as there is only one device currently in the marketplace that will dechlorinate super chlorinated water at flows meeting the 3 feet per second flushing rate. That device is the 3M H2O Neutralizer. AWWA took that into consideration when updating the standard. By flushing with potable water any of the devices in the marketplace will perform dechlorination of potable water at the higher flow rates. When completing waterline projects, system flushing and dechlorination of discharge water are now a standard step in finishing the project.
The requirement of the new AWWA C-651-14 Standard for Disinfecting Water Mains means you now need to change the steps for building your systems. After you have completed your pipe installation, it’s time to fill the system and pressure test. By pressure testing at this time you are only using potable water, so your chlorine residual level will not create environmental problems. After all tests have passed, it’s time to flush, as the standard calls for flushing before chlorination to clear dirt and debris from the system first. High volume flushing at the rate of 3 feet per second, per the chlorination standard, is now a requirement for all new water main installations; a significant increase from the old standard.
What the New Standard Requires
Dechlorination and high-volume flushing is the reason the standard (AWWA C-651-14) states in section 4.2.2 Flushing that “Potable water shall be used for disinfection, hydrostatic pressure testing, and flushing.” When high volume flushing is required, your construction steps should be to first build the system (protecting the system from dirt, debris & water infiltration while installing the line), then fill the system with potable water and pressure test. If you encounter a leak at this point you would not risk discharging super chlorinated water.
You must now flush the system of dirt and debris using just potable water, during which any dechlorination equipment will work in neutralizing chlorine residual, before cooking the system with chlorine. After you have pressure tested and flushed, you must chlorinate your system using a chemical injector like the H2O Neutralizer to introduce needed chemical to complete the chlorination process. The H2O Neutralizer can chlorinate using either calcium hypochlorite in solution or sodium hypochlorite.
The standard calls for 25 PPM at the start of your cooking time, however, most specifications in the past have called for 50 PPM. The reason being that in the past the chlorine was placed in the line during construction, so you had to have higher chlorine levels, to compensate for dirt and debris that got into the line during construction, to ensure that you would pass your purity test; and as most contractors will tell you “the more the better.” That is until you need to dechlorinate that super chlorinated discharge water.
This is the problem. Discharging super chlorinated water into the environment without dechlorination is no longer permitted. So high-volume flushing with potable water before chlorination, to clear dirt and debris from the line, will allow you to chlorinate to levels recommended by the standard and no more. This will mean cheaper dechlorination chemical cost, as you no longer have to overload the system to compensate for dirt and debris left behind from construction. High-volume flushing allows you to clear that debris from the system before cooking it.
You will only need to change one thing in your steps of construction during the installation of the line, stop placing calcium hypochlorite in the line during installation. Everything else will be the same; build the system, fill the system and pressure test, flush and then chlorinate the system. The only catch is you now need to flush at high volume, 3 feet per second, before chlorinating. By doing this with potable water any dechlorination equipment should work.
Now the bigger issue, how can you get water flow at 3 feet per second?
What is the proper Service Tap Size?
With the addition of high-volume flushing prior to chlorination you now must contend with setting up your water supply to allow water flow of 3 feet per second. Table 3 of the AWWA C-651 standard has a chart that lists the GPM required for pipe sizes through 16”. This chart lists the required outlets to achieve 3.0 ft/sec at 40 psi residual pressure. Residual pressure is different from static pressure, it is pressure that your system can maintain with open discharges.
Remember, if you must dechlorinate and you have multiple outlets, you will need a device at each outlet to neutralize chemical in that water discharge. However, that will not be your only challenge. Your water source must be of proper size to give you the required water flow; plus, all connections between the existing system and the new system should be separated by a backflow prevention device
The Importance of a Properly Sized Backflow Preventer
Your next issue is the need for and the flow range of a backflow preventer. While it is required that you employ a backflow preventer to keep chlorinated water from backing up into your water supply, backflow preventers have some restrictions in their range of performance that need to be taken into consideration for high volume flushing. A 2” BF preventer will only allow water flow of 160 GPM, while a 4” device is limited to 500 GPM and 6” devices are rated at 1000 GPM.
To reach the 3 feet per second of water flow required by the standard means that you must match the size of line you are treating with an equally sized back flow preventer. In most field situations the standard equipment for supply line size is two inches. At line sizes of more than 4 inches a single 2” backflow preventer will not provide the GPM of flow to reach the 3 feet per second flow requirement of the standard. Not to mention that larger backflow devices are expensive, big and heavy, making them difficult to handle in field applications. And in field use they must be recertified for every project they are used on, making larger sizes of BFP impractical in field applications.
Given the limitations of flow you can achieve with a backflow preventer some contractors have constructed dual backflow (header system) device setups, splitting the water flow between two devices in parallel to achieve a flow rate of 3 feet per second. But there are limits to the size line you can treat with this type of arrangement. This type set up will deliver 320 GPM of flow, enough to treat lines up to 6 inches in size.
If your water source is 2 inches and you use a 2” backflow device your maximum flow is 160 GPM; to meet the 3.0 ft/sec requirement for a 6” water line you will need 260 GPM to hit 3.0 ft/sec. So even with the a 2“-dual device setup you will not be able meet the 3.0 ft/sec required in line sizes of 8 inches and larger.
If your system cannot provide the proper flow, then go with the alternative procedure discussed in the Standard.
The Alternative to Flushing at 3 Feet per Second
We have had a few contractors call us with the following question. They state; “I have been told that we must flush at 3.0 ft/sec and I’m being told that we must bring in additional equipment (i.e. pumps) so we can get to 3.0 ft/sec.” My answer to them is that is not possible if you don’t have the system capacity to reach that flow rate, however the standard allows an alternative to this requirement in section 4.4.2 Preliminary Flushing.
In talking about flow rates in table 3 needed to produce a velocity of 3.0 ft/sec it is stated; “Where such flow rates are not possible, flushing at the maximum expected flow rate for the line for 2-3 volumes may be acceptable.” When system capacity cannot provide the required flow rate of 3.0 ft/sec there really is no other way to complete the job than to flush 2 – 3 turns of water. It helps if your crews understand this during construction so that extra precautions can be taken to insure the pipe stays clean.
What to Do
Times are changing, so maybe it is time to change your installation method. By increasing your service line connection to increase your water flow and switching to the H2O Neutralizer for both your chlorination and dechlorination requirements, you will insure your projects go smoothly every time. The H2O Neutralizer with a full vacuum draws in just under one-gallon per minute of solution so, with a safety factor, you can safely neutralize chlorine residual levels up to 200 PPM at flows up to 1,200 GPM.
The chlorination standard takes into account that most other types of equipment CANNOT handle high flows with high chlorine residual levels. The H2O Neutralizer is the only device that can handle high flows with high chlorine residual levels and is the only device you can use for both chlorination and dechlorination.
And with the introduction of our new Generation 5 Locking Ball-Valve Design setup and operation is easier than ever before. Our new nine-stage locking ball-valve allows you to adjust main barrel water flow while the device is in operation. No need to disconnect the device to change internal orifice rings, like in our previous generation designs. Just setup the device, establish water flow and adjust to the needed flow rate to achieve necessary vacuum in the lateral bypass venturi, to establish proper rate of chemical injection.
If you would like to purchase a copy of the AWWA standard for chlorination C-651 Disinfecting Water Mains or the dechlorination standard C-655 Field Dechlorination give us a call. We would be happy to assist you in obtaining your own copy of each standard, because it’s always best to know what is required before you start work in the field.
Give us a call at 877-889-8482 today and we will discuss your situation and how the Neutralizer will assure your projects end well.