Domestic and Fire Protection Water Supply and Distribution Systems

Design

Piping Networks

Some of the earliest water systems in the United States were constructed using wooden water mains; however, cast iron, ductile iron, steel, and copper were the standard for these systems for much of the 20th century. Recently, advances in plastic pipe (high-density polyethylene, and PVC, for example) have brought more of this type of pipe into water utility systems. Plastics have some significant advantages in weight and cost, but there are some issues with pressure ratings, and none of these pipes have been in the ground long enough for us to really understand the longevity of these systems. While many of the plastic pipe materials appear to have good long-life characteristics, connections may have less reliability and further research is needed in the area of leaching of organic chemicals from the pipe material into the water.

When designing water distribution systems on campus, it is important to analyze the criticality of water supply to each building type. While the loss of water supply to a classroom or office building could be merely an inconvenience, the loss of water supply to laboratory or animal care facilities for even a relatively short period of time could endanger critical research. In addition, those buildings that on the surface do not appear to have critical water need may have HVAC equipment like a boiler or cooling tower that rely on makeup water. Should a water outage render those pieces of equipment inoperable, the building may no longer be habitable.

Once these critical loads have been identified, the water system can be designed (or upgraded) so that the water distribution piping network is looped and water can be fed to critical buildings from multiple sources. A critical element of a looped system is providing a sufficient number of valves so that sections of pipe can be isolated for repair and maintenance without impacting water service to multiple buildings. Typically, there should be a valve on each leg at each pipe intersection (three valves at a “T” and four valves at a cross) and a valve on every building service line. However, you can get away with fewer valves if there are several pipe intersections in close proximity with valves at each leg. The best test is to study water system maps and perform a “what if” analysis on the impacts of water main breaks or repairs. If a particular section of pipe will “take down” several buildings in the event of a failure, analyze how to retrofit valves into the system in order to minimize the impacts. Radial feed legs within the water system should be minimized due to reliability and potential water quality concerns.

In addition, it is critical to minimize “dead legs” in the water distribution system. Dead legs are created when there is a long radial feed to a seldom-used load. Fire hydrant legs are a common culprit, but old feeds to abandoned facilities are sometimes cut off at the building rather than at the main, causing a dead leg that can create water quality problems in the system. The water in these dead legs is stagnant, and fluctuations in pressure and flow can draw some of this bad water back into the water mains, causing water quality issues.

There is a downside to looped water distribution systems. Water will always follow the path of least resistance, so if there are different pipe sizes in the water distribution system, the water flow in the smaller pipes will be low. This can lead to water that has too long a residence time in the system, resulting in a degradation of water quality. In extreme cases, some of these smaller pipes can act almost as dead legs, leading to severe water quality problems. If there are a variety of pipe sizes in a looped distribution system, regular, directional flushing can help alleviate these water quality concerns.

Additional design efforts should be put into minimizing pressure drops in the system. Most utility systems provide water at 50 to 80 psig. If the water pressure is outside that range, efforts must be made to correct the situation. If the pressure is too high, a pressure-reducing valve (PRV) must be installed at the building entrance. If the pressure is too low, a booster pump and pressure tank are usually required. Note that providing a looped system will not help low water pressure problems; however, it can improve water flows in systems with marginal pressure.

Conservation versus Quality

Water conservation measures are an important aspect of a sustainable future, especially in arid areas like the western United States where the scarcity of water becomes the primary topic of conversation in drought years. However, drinking water contaminant levels are concentration-based; therefore, when more water (i.e., less conservation) is moving through a distribution system, it is more likely that the water quality will meet applicable limits. Another incentive to move large quantities of water is to prevent the water from stagnating in pipes, where it can pick up rust and other contaminants that may not represent a health threat but make the water unpleasant to taste. High water age, caused by low flow, can diminish the disinfectant concentrations, leading to possible bacteria growth, and exacerbate the formation of disinfection by products such as chloroform.

Historically, water distribution systems have been designed to provide more than adequate flow capacity for the largest demand, which is fire flow. This can result in mains and service lines that are oversized for most of their use, which is the demand in the building, resulting in stale water that may be warm or bad-tasting and potentially contain bacteria or disinfection byproducts.

Implementing conservation measures while providing healthy and pleasant-tasting water is a balancing act. We must continually reduce a building’s water demand through low-use fixtures, laboratory modifications, and other conservation measures, but still ensure healthy and pleasant water. One way to understand the impacts of water conservation measures on the distribution system is to model the system and test different scenarios. These computer-based models are time-consuming to set up, but invaluable for testing out changes in the system before implementing them. Water modeling at Colorado State University, for instance, allowed utility engineers to determine that a section of failed pipe was not required to maintain adequate flows in the system. As a result, an expensive repair was avoided and the pipe section was cut off from the system.

Operations and Maintenance

Operating Plan

A campus that has the legal responsibility for its own distribution system should have a written operating plan that defines routine and non-routine tasks and events and who should be notified during such tasks or events. At a minimum, the operating plan should describe the following tasks and events:

• Main breaks
• Pressure loss
• Violations
• Complaints
• Water Quality issues
• Cross connection incidents
• Routine sampling
• Flushing
• Valve and hydrant operation
• New pipe connections and disinfection
• Leak detection
• Pressure fluctuations.

The operating plan should describe whether each task or event is an emergency or a non-emergency, and what steps to follow. Some states offer guidance or have regulatory requirements that dictate the notification and actions required in each circumstance.

System Mapping

The importance of accurate distribution system maps cannot be overemphasized. Accurate maps provide

• Effective distribution system operation and maintenance
• Minimization of mistakes (e.g., closing a service at an incorrectly shown location)
• An effective planning tool for replacement and rehabilitation programs
• Identification of water quality problem areas
• A basis for modeling

Water System mapping showing domestic water distribution lines, fire lines, pipe sizes, and fittings.

The following topics should be considered as the system maps are developed or upgraded:

• Valve locations: These are important in times of emergency, such as main breaks. They are also important for planning unidirectional flushing, system replacement, and upgrades. The direction of valve operations should also be noted on the map (e.g., “open counterclockwise”).
• Main locations, size, and material: A pipe’s size and material can affect flow and pressure characteristics at the delivery points. Accurate size and material information helps troubleshoot flow and pressure problems.
• Age of the pipe: Pipe age is helpful in water quality and flow modeling and helps guide replacement and repair programs. It also provides necessary information to help you comply with water quality testing regulations of contaminants such as lead and copper.
• Locations and dates of breaks: This information is needed for system replacement and rehabilitation planning.
• Locations and dates of complaints: This information is needed for system evaluation, replacement, and upgrade planning.

Mapping can be performed using AutoCAD software, GIS software or both. If both are used it is important to have a process for establishing one type of software as the master, with the other being updated promptly after changes are made to the master.

Symbols for valves, fire hydrants, meters and backflow preventers can be standardized using symbol libraries provided with the software, or developing user-specific symbols. Pipe and fitting symbols are less universal today than they were a decade ago, therefore it is helpful to provide a legend for the map users. Typically, each valve is assigned an identifier and shown on the map at its actual location relative to buildings and other permanent objects. Other information about the valve, such as direction of closure and GPS coordinates, might be embedded in the map at the valve location.

Methods of locating valves in the field vary depending on the resources available to the user. A user might query the map electronically at his or her computer to determine relevant distance to the valve from fixed objects, mark these on a to-scale printout and take it to the field as an aid in finding the valve. Utility locating equipment and a metal detector can help pinpoint a valve box lid that has been buried. GPS units are helpful if the valve’s coordinates are known.

Mapping is an ongoing process and relies on field measurements and observations to develop accurate and to-scale maps that are useful electronically as well as in hardcopy format. Maps must be continually and promptly updated to show changes reflected on construction as builts. It is helpful to have a copy of the distribution system on your smartphone, tablet, or laptop for use in the field.

Leak Detection

Water leaks often account for a substantial portion of lost revenue in water utility systems. If you have a campus that either treats its own water, or purchases water at the campus boundaries and maintains a distribution system, water leaks are a direct revenue loss to the water system. For example, if 5 percent of the water in the Colorado State University water distribution system were lost to leaks, it would be at a cost of over $100,000 per year.

A wide variety of strategies and technologies can be used to pinpoint leaks. These include water balance comparing submeter to master meter usage, field observations, and a variety of listening devices. The effectiveness of these strategies is related to the pipe material in your system, depth of installation, soil type, and so on. Often a preventive maintenance (PM) program of actively monitoring water mains to find and repair leaks can pay for itself with the water saved. If resources are not available, or if the system is not owned by the university, a PM leak detection program may not be cost-effective. However, in order to preserve and protect the integrity of their system, water utilities often provide this service for little or no cost.

A simple and cost-effective leak detection unit consists of electronic ground microphones. These are listening devices that can be used at valves, hydrants or along a water main alignment. Ambient noise such as traffic can interfere with the use of this type of equipment, therefore it is best to attempt to identify leaks using this equipment when ambient noise levels are low, typically very early in the morning or at night.

The following are suggested key steps for leak detection using this type of equipment:

1. Select an appropriate listening device for the location where you want to begin listening – elephant foot for asphalt or concrete surface, probe rod for landscaped area, or magnet for fire hydrant.
2. Attach the listening device to the amplifier module and put headphones on
3. Listen for leakage noise and observe the instrument’s display
4. Move to next logical location and repeat the process
5. Triangulate to identify the leak location

If a leak is not located with certainty using this equipment, then subsequent steps might include isolating the suspect segment of pipe and rechecking to see if noise has disappeared. It is important to attempt to differentiate between normal water movement in the pipe and a leak. One way to differentiate between usage and a leak is to cease water movement in the pipe (for example by shutting off the water at the nearest building). If a leak is difficult to locate, then correlator leak detection equipment might be needed. Correlators can be purchased or rented, or a firm specializing in leak detection retained to conduct the investigation.

A water audit should be conducted periodically (for example annually) to estimate the amount of water potentially lost to leaks. The water audit should include actual or estimated volumes of the following consumed water:

• Metered
• Unmetered but estimated
• Fire hydrant flushing
• Construction use
• Loss during main break(s)

You can arrive at an estimate of losses due to leaks when you compare these consumption volumes (after accounting for storage) to the volume of water supplied at the master meter(s), If the amount of losses due to leaks exceeds ten percent of your annual consumption then it is appropriate to initiate a rigorous leak detection evaluation of the distribution system. Keep in mind that inaccuracies in the water audit can be introduced due to water theft and meter inaccuracy; both of these should be considered and addressed if possible.

The AWWA provides a water loss tool on its website:

http://www.awwa.org/resources-tools/water-knowledge/water-loss-control.aspx

Flushing

Rust, tuberculation (deposits of corrosion products), sediment deposition, and slime growth are aspects of pipe deterioration and stagnant water that can result in poor water quality. Periodic water main flushing using system hydrants helps to remove these impurities and improve flow. The frequency of flushing may vary from three times per year to once every three years, depending on the system’s age, configuration, and condition. Areas of the distribution system with long dead-legs or low water usage may require more frequent flushing than other areas.

View a video of a hydrant being flushed.

System-wide flushing should be “unidirectional” to the extent possible, where the water is flushed from one end of the system to the other. This helps to increase water velocity in each main and helps to prevent moving “dirty” water through or past segments previously flushed. Unidirectional flushing is accomplished by closing select valves to force water along a chosen main in a specified direction. During flushing, the goal is to achieve a velocity of 5 feet per second (at a minimum, the velocity should exceed 2.5 feet per second) in each main. Each hydrant should be flushed until chlorine and turbidity are within specified parameters. The chlorine and turbidity limits may depend on the supplier’s normal disinfection rates and system age. An example of the flushing goals follows:

• Chlorine: at least 0.5 mg/L
• Turbidity: below 3 NTU (nephelometric turbidity units)
• If parameters are not achieved, flush for no more than 15 minutes per hydrant.

Left: Sampling a fire hydrant during unidirectional flushing. Note diffuser and safety cones.
Right: Measuring pressure during unidirectional flushing.

A system-wide flushing effort may take several weeks and requires planning and coordination. The following activities should be considered for a comprehensive flushing program:

• Coordination with the wholesale supplier: Unidirectional flushing can dislodge materials in the mains that may appear in customers’ water in another location. If flushing with the wholesale supplier is not concurrent, it is important to notify the wholesale water purveyor so that it can be prepared to respond to any complaints.
• Notification to all system users: This may take the form of a mailing or campus-wide e-mail. A sample notification paragraph is shown in Exhibit A.
• Special notification and coordination with sensitive users such as hospitals and dialysis patients.
• Preparation of equipment for each crew, including diffusers, safety cones, dechlorination equipment, sampling kits, and valve keys.
• Compliance with any dechlorination requirements: These may be imposed at the state level by the environmental agency having jurisdiction over surface water discharges. Requirements may vary from no action required to dechlorination at every fire hydrant.
• Mapping of each area with valve closures planned out in advance.

Once the flushing program has been planned, each flushing crew should follow the same procedures, which may include the following:

1. Plan the days flushing with an effort to minimize customer disruption.
2. Notify any “critical” customers that may need to turn off sensitive equipment.
3. Close requisite valves slowly to avoid water hammer.
4. Mark closed valves on map as they are closed.
5. Set up at hydrant with pedestrian, bike path, and traffic protection (cones, flaggers, etc., as appropriate).
6. Open fire hydrant for flushing slowly, to avoid water hammer. View video.
7. During flushing, watch for flooding or erosion.
8. Sample at start and every five minutes. Observe quality periodically (Styrofoam cup works best because white cup allows for easy visual inspection for particulates).
9. Document flushing information.
10. Cease flushing if system pressures drop below 20 psi or clogged catch basins cause flooding or erosion.
11. When the goals are met, close fire hydrant slowly to avoid water hammer.
12. Open the closed valve and mark each on the map (or erase “closed” marks made earlier) as it is opened.

Checking water quality. Sample and check visually and by testing every five minutes.

View video on chlorine testing of water sample.

A sample flushing documentation sheet is included as Exhibit B. One sheet per hydrant being flushed should be used.

During flushing, care should be exercised in the operation of fire hydrants. To avoid water hammer, the hydrant should be opened slowly; likewise, when closing a hydrant, the last several turns should be very slow. One instance of rapid fire hydrant closure at a university resulted in water hammer causing a five-foot longitudinal break in a 40-year-old cast iron water main that was otherwise in excellent condition. A dry barrel hydrant should always be opened fully, not partially, so that it does not saturate or erode the dry well at the base and drains properly when closed.