Distribution Systems

The distribution system consists of booster pumps, pipes, meters, storage tanks, control valves, and fire hydrants. These distribution system components are needed to deliver treated high-quality water to users in response to widely varying rates of demand. Since all these components must work together to serve this purpose, their designs must be carefully integrated. This part of the system is often neglected because much of it is underground and out of sight, but it needs and deserves almost as much operator attention as source and treatment facilities.

If high quality treated water is allowed to flow through a poorly maintained distribution system, then the quality of the water delivered becomes questionable and possibly unsafe for consumption. The entire system must be maintained at a high level of standard to deliver high quality water, safe for human consumption. Remember, “quality” must be maintained from treatment to tap!

Compliance

The utility shall determine and document all local, state, provincial, federal, or other regulations that apply to their utility. The utility shall meet or exceed all applicable regulations.

Rationale

Customers associate health and safety issues related to drinking water directly to the utility’s ability to meet all local, state, provincial, federal, or other regulatory requirements to which the utility subscribes. It is absolutely necessary for the utility to meet or exceed all applicable regulations to gain the confidence of the consuming customers. To comply with these requirements, the utility shall establish and maintain procedures to identify and have access to the applicable legal and regulatory requirements to which the utility subscribes related to its scope and operation. The utility shall ensure that these applicable legal and regulatory requirements are taken into account in establishing, implementing, and maintaining its water quality management system.

This can be accomplished by the utility developing a legal and other regulatory requirements list and ensuring this list is periodically reviewed and updated. The utility shall also document how these requirements are adhered to in its operation.

A distribution system meeting or exceeding all applicable regulations, will be able to adequately provide a sufficient amount of safe water to all users at a pressure that will satisfy normal needs. It will also be able to provide water without undue system water loss.

Even in a metered system, not all water can be accounted for. Some may be lost because of leaks in the system, some may be discharged through hydrants as part of a flushing program, some may evaporate from storage tanks or be used fighting fires. This portion is usually referred to as unaccounted-for water.

System Components

Piping Materials

The most efficient and convenient way to transport and deliver potable water is through pipes.

Valves

Valves are a very important part of the distribution system because they regulate the flow of water, reduce pressure, provide air and vacuum relief, blow off or drain water from parts of the system and prevent backflow.

The operator should know the locations of these valves so they can be used when necessary. Each water system should have a map available with the location of the valves clearly marked. It is important that the access to the valves not be compromised by other facilities. For example, access to some valves has unknowingly been covered during a street repair process.

Most valves will need to be exercised at least on an annual basis. If not, they may become stuck and will be inoperable when really needed. This valve exercising process should be geared into a routine maintenance plan.

Valves of any size need to be operated slowly. Water hammer is caused by closing the valve too quickly. Although water hammer generates considerable noise, the real damage occurs through mechanical failure. When water is suddenly stopped, shock waves are generated, which cause large pressure increases throughout the system. These shockwaves travel quickly and can cause extensive damage, sometimes splitting pipes or blowing fittings completely off the system.

Frequently in the operation of valves, conditions cause a partial vacuum or void to occur on the downstream side of a valve. These voids will fill with low-pressure vapors from the water. When these pockets implode or collapse, they create a mechanical shock causing pockets of metal to break away from the valve surface. A noisy or vibrating valve may be an indication that cavitation is occurring, which will eventually result in leaks and a valve unsuitable for service.

Gate valves (Figure 1.1.1) are used to isolate sections of the distribution system to permit emergency repairs without interrupting service to large numbers of customers. In the gate valve, a sliding flat metal disc is moved at right angles to the direction of flow by a screw-operated stem. The disc can be taken completely out of the flow chamber—therefore, the valve provides very little resistance to flow when it is opened. Various types of rotary valves are available including the ball, butterfly, and plug valves. Usually, a 90-degree rotation movement is used with a notch, arrow or other indicator to show valve positions.

In a ball valve, the movable part is a ball with a cylindrical hole bored through it. When the ball is in one position there is a straight passage through the valve, but when it is rotated 90-degrees the flow is blocked. Ball valves should be operated either closed or opened all the way.

The butterfly valve (Figure 1.1.2) has a disc which fills the full diameter of the pipe. The disc rotates on a shaft to allow the flow of water.

Plug valves (Figure 1.1.3) are most often used extensively as corporation stops on service lines. They may have a tapered or cylindrical plug with an opening through the side which can be turned to open, restrict, or close the flow.

Globe valves (Figure 1.1.4) are very efficient in either flow or pressure regulation. A disc is raised or lowered onto a seat as in the common home faucet. The water flow is stopped when the disc contacts the seat. In a diaphragm valve, a flexible piece (usually rubber or leather) inside the valve’s body can be adjusted up or down using an attached stem to block or regulate the flow of water. Back-flow prevention or cross-connection control can be accomplished with the use of some valves specifically designed for that purpose.

Most valves do not provide an absolute stoppage of water. Most leak under some circumstances, many just because of age and deterioration. Leaking valves have been responsible for contamination of public water supply systems when they were used as the only separation barrier between the water system and a non-potable supply. Non-potable supplies have included private well systems, transmission mains containing untreated surface water and wells used for irrigation.

Pressure Reducing Valves

Pressure reducing valves reduce the water pressure by restricting the flow. Pressure on the downstream side of the valve regulates the amount of flow permitted through it.

This type of valve is usually of the globe design with a spring-loaded diaphragm which sets the amount of the opening. As downstream pressure is exerted against the diaphragm, the spring is compressed, moving the valve element toward the seal, thereby limiting the flow. If the downstream pressure drops, the spring will open the valve element and provide more flow.

These valves allow distribution systems to maintain pressures to the customers in the desirable 50 to 80 psi range.

Fire Hydrants

The primary reason you install and maintain fire hydrants is to provide your customers with fire protection for their homes and businesses.

If the water system is designed to provide fire protection, the water lines and the hydrant connections to the distribution system must be a minimum of six inches in diameter. In some states, the location, minimum waterline size, and spacing of hydrants is specified in State design standards. In other states, the location and number of fire hydrants is determined by a private company called the Insurance Services Office (ISO). The ISO uses a formula to calculate the needed fire flow, as well as the minimum waterline size on which to set a fire hydrant at a particular location. The formula considers factors such as type of construction material, building contents, and building density to determine the required flow. Refer to your State design standards to determine the appropriate answer on your certification test.

Flushing hydrants may be smaller than 6-inch diameter, but must not have fire-fighting compatible connections.

Storage Reservoirs

Although elevated storage is the preferred method of storage and pressure maintenance for systems of over 2,500 connections, it is recognized that local conditions may dictate the use of alternate methods.

Small water systems may have a small amount of storage in the form of a hydropneumatic tank either a standard pressure tank with an air/water interface, or a captive air tank. Other systems might have ground level concrete or steel storage tanks or elevated steel tanks which serve the system by gravity.

Bulk Water Hauling Stations

Bulk water stations provide a valuable service to those outside of the distribution system. They also may be a good source of revenue. However, it is vitally important the water system is protected against contamination from users of the bulk station.

Bulk water may be purchased and hauled for several reasons: home drinking water, livestock water, irrigation, construction, or mixing of pesticides or other chemicals. Therefore, it is critical substances from the users tank are prevented from being back-siphoned (sucked) into the public water supply system. It is also important to protect other users of the bulk station from contamination carried by the fill hose. Fill hoses may become contaminated if they are allowed to touch the ground or come into contact with solutions in the haulers tank.

Backsiphonage, siphoning of non-potable water or other substance into the water system due to negative pressure or reduced pressure in the water supply piping.

Preventing contamination of the public water supply system and fill hose can be accomplished by providing an air gap.

Preventing contamination of the public water supply system and fill hose can be accomplished by providing an air gap.

Booster Stations

A booster pump is used to increase the pressure in the mains on the discharge side of the pumps, or to supply water to an elevated or higher level storage tank. They are also often used to supply water to a service area at a higher elevation by taking water from storage to pressurize the distribution system.

Looped Systems and Dead End Mains

Water lines should be installed to loop back into another part of the distribution system. This allows circulation of water to all users. Dead ends should be avoided. The lack of movement of the water in these lines will cause stagnation and result in the growth of slimes and bacteria, and development of taste and odor problems.

All dead-end mains must be flushed at monthly intervals. Dead-end lines and other mains shall be flushed as needed if water quality complaints are received from water customers or if disinfectant residuals fall below acceptable levels.

Leak Detection Program

Leak detection programs are an effective way to reduce operating and maintenance costs. If leaks can be detected when they are small, the system may save many dollars, hours of work and possible property damage. Leaks not only waste water, but may create an environment around the pipe which increases corrosion. Once corrosion develops pinhole leaks in the pipe, contamination can be drawn into the system when the pressure in the pipe is reduced.

Leaks are not easy to locate. Of course, the most obvious method is to look for wet spots on the ground. However, in some soils the leak may never get to the surface. In some soils, the leak may surface several hundred feet from the actual point of discharge from the pipe.

Specialized listening devices are sometimes used to locate a leak. There are also commercial leak detection companies which serve systems needing leak detection assistance.

If the water system is completely metered, a water balance between the amount produced and the amount billed to customers can sometimes indicate when losses are occurring.

System Pressure and Basic Hydraulics

System Pressure

A distribution system must not only provide an adequate quantity of water, it also must be designed to provide sufficient pressure to satisfy normal customer requirements. All water mains are sized to maintain a normal working pressure of at least 35 pounds per square inch (psi). However, pressures in the system under normal operating conditions should range between 40 and 60 psi.

The very minimum pressure under combined fire and drinking water flow conditions, is 20 psi. Any pressures less than that are unacceptable because they create conditions for potential back-siphonage and contamination of the public water supply system.

Backpressure, flow of non-potable water or other substance into the water system from connections that are higher pressure than the water system

Backsiphonage, siphoning of non-potable water or other substance into the water system due to negative pressure or reduced pressure in the water supply piping.

Pressures above 100 psi should be avoided to minimize leakage, reduce water consumption, and prevent excessive wear on water-using appliances. When the pressures are this high in a residential setting, pressure reducing valves should be installed on individual homes or on an isolated portion of the distribution system to limit the pressure to less than 100 psi.

Hydraulics

To better understand what is happening to the water in a distribution system, it is helpful to review some of the fundamental principles of hydraulics. Hydraulics is the study of liquid in motion and under pressure.

Construction and Repair

Although the water distribution system operator may not be involved in the design and construction of the water system, it is important to understand a few basic facts about this process. Being aware of some of these fundamental concerns will also assist in making sound decisions when system repairs are needed.

As a general rule, distribution systems should be designed to require the minimum level of operation and maintenance that is commensurate with satisfactory delivery (quantity and quality) of water to the users. This requires those who oversee the operation of a public water distribution system to give ample consideration to the reliability of processes and equipment, to anticipate the types of failures that are likely to occur, and to make provisions for dealing with them with as little disruption in service as is possible. Failure to anticipate and make adequate provisions for dealing with failures is perhaps the most common shortcoming of the typical water system.

In order, to help the operators of a water distribution system cope with problems that may arise, it is common practice to provide a system operation manual and for the supplier to furnish an operation and maintenance (O&M) manual for each piece of equipment. In some states, a system operation manual is not optional, but a requirement. These documents should provide instructions for operating the system under various scenarios (e.g., normal, peak demand, and minimum demand periods as well as various types of emergency situations), preventative and routine maintenance procedures, and troubleshooting. If the operation and maintenance (O&M) manual is developed as the system is being designed, rather than after final plans are prepared, as is often the case, many pitfalls can be avoided. In developing the O&M manual, it is important to remember that many public water systems are operated by fairly nontechnical, personnel. Thus, unambiguous, clearly explicit instructions should be given.

It is important for a public water distribution system to maintain a minimum of three (3) copies of these manuals. One should be located in a convenient place for the operators, this copy is usually called a “field” copy. Another copy should be available for management reference, this copy is normally called an “office” copy. And one complete copy for the archives. These days, for convenience and to save space, digital copies of these manuals can substitute for print copies and be maintained as management and archives copies.

Construction and Minimum Separation Distances

Parallel Installation With Sewer Pipes – Water mains should never be laid in the same trench with sewer pipes. The potential for contamination is too great. State regulations require at least a 10-foot separation between these two systems.

Preventing Contamination During Construction

During the construction process it is almost impossible to prevent some contaminating materials from entering the pipe system. It is extremely important to keep this contamination to a minimum, so the system can easily be flushed and disinfected before being put back into service. All pipes should be carefully stored and handled to prevent damage and the possibility of contamination. After it is in the trench, the end of the pipe should be securely plugged each evening before the crew leaves the site. This will prevent small animals from entering and will also be a safety measure to protect children who may play in the area.

System Repairs

Care must be taken when making repairs to avoid contamination if possible. The section of the distribution system to be repaired should be isolated as quickly as possible. This is especially necessary if extensive damage is occurring because of washouts or flooding. Some water system personnel leave a small positive pressure on the line to be repaired so all water will flow out of the pipe and the contaminated water will not flow back in. This generally requires sufficient pumps to keep the water out of the hole and keep it from surrounding the pipe while the repair is being made. Customers in the area which will be without water should be notified as quickly as possible and requested to not open their taps until notified by the system operator. Notification before the water is shut off is best so the customers can make sufficient preparations.

Preventive Maintenance Program and Recordkeeping

Even though much of the distribution system cannot be seen, regularly scheduled maintenance and good recordkeeping must be part of an operator’s activities.

Again, if high quality treated water is allowed to flow through a poorly maintained distribution system, then the quality of the water delivered becomes questionable and possibly unsafe for consumption. The entire system must be maintained at a high level of standard to deliver high quality water, safe for human consumption. Remember, “quality” must be maintained from treatment to tap!

Recordkeeping

The distribution system’s operating records must be organized. Monitoring and operating records must be kept on file or stored electronically and must be accessible for review during inspections and be available upon official request. All paper and electronic files should be backed up. Loss of records is not an acceptable excuse during an inspection or upon official request. When there is a change or update to the system, the files should also be updated and note the date of last update. Outdated records is not an acceptable excuse during an inspection or upon official request.

All distribution systems shall maintain a record of operations. In addition, the following records, specific to a distribution system, have set specific retention requirements.

The following records shall be retained for at least two years:

The amount of chemicals used
The volume of water treated and distributed
The date, location, and nature of water quality, pressure, or outage complaints received by the system and the results of any subsequent complaint investigation
Dates that dead-end-mains were flushed
Dates that storage tanks and other facilities were cleaned
Maintenance records for water system equipment and facilities.

The following records shall be retained for at least three years:

Copies of notices of violation and any resulting corrective actions
Copies of any public notice issued by the water system
Disinfectant residual monitoring results from the distribution system
Calibration records for laboratory equipment, flowmeters, rate-of-flow controllers, on-line turbidimeters, and on-line disinfectant residual analyzers
Records of backflow prevention device programs
Free and total chlorine, monochloramine, ammonia, nitrite, and nitrate monitoring results if chloramines are used in the water system

The following records shall be retained for five years after they are no longer in effect:

Records concerning a variance or exemption granted to the system
Results of microbiological analyses
Results of inspections for all water storage and pressure maintenance facilities
Documentation of compliance with state approved corrective action plan and schedules
Documentation of the reason for an invalidated fecal indicator source sample and documentation of a total coliform-positive sample collected at a location with conditions that could cause such positive samples in a distribution system
Notification to wholesale system(s) of a distribution coliform-positive sample for consecutive systems using groundwater
Consumer Confidence Report compliance documentation
Records of the lowest daily residual disinfectant concentration and records of the date and duration of any failure to maintain the State approved minimum specified disinfectant residual for a period of more than four hours for groundwater systems providing 4-logtreatment
Records of any repeat sample

The following records shall be retained for at least ten years:

Monthly Operating Reports and any supporting documentation
Results of chemical analyses
Written reports, summaries, or communications relating to sanitary surveys of the system conducted by the system itself, by a private consultant, or by the State shall be kept for a period not less than ten years after completion of the survey involved
Customer Service Inspections
Initial Distribution System Evaluation (IDSE) plan, report, approval letters, and other compliance documentation required (relating to Stage 2 Disinfection Byproducts (TTHM and HAA5));
Notification of any modifications to an IDSE report
Sample Siting Plans

A distribution system shall maintain records relating to lead and copper requirements for no less than 12 years.

A distribution system shall maintain records relating to special studies and pilot projects, special monitoring, and other system-specific matters as directed by the State.

A distribution system shall submit routine reports and any additional documentation that the State may require to determine compliance.

The reports must be completed in ink, typed, or computer-printed and must be signed, at minimum, by a licensed operator.

All distribution systems must maintain the following records for as long as they are applicable to the system:

An emergency preparedness plan approved by the State and a copy of the approval letter
All required operating and maintenance records for auxiliary power equipment, including periodic testing of the auxiliary power equipment underload and any associated automatic switch over equipment
Copies of the manufacturer’s specifications for all generators that are part of the approved emergency preparedness plan

The operator should keep a file with a distribution system map showing the location of all valves and valve boxes, hydrants, and pipe with pipe diameter indicated.

Fire Hydrants

The single most important thing about having fire hydrants is that they advertise “Fire Protection!” Communities expect that at a moment’s notice, day or night, in any weather, the hydrant will supply sufficient water to extinguish a fire.

Like any other piece of equipment, if you don’t operate and maintain your fire hydrants properly, they won’t work when you need them most.

Because of the many different brands and models, always refer to the manufacturer’s literature for maintenance items and repair procedures. The manufacturer, or its representative, also can tell you what tools are necessary to perform repairs, such as removing the main valve stem or maintaining other parts of the hydrant. Hydrant repair requires specialized tools. Do not start a project without them.

Most manufacturers package repair parts in “repair kits,” including all of the parts necessary to make a certain repair. If you are located close to a distributor who sells and stocks parts for your hydrants, you may want to stock very little yourself. In case you aren’t lucky enough to live close to a distributor, you at least should have, traffic repair kits, if you have hydrants with this breakaway feature, a seals and O-ring kit or two, and a couple of seats.

When possible, it is always better to have the manufacturer’s representative advise you as to the correct stock of repair parts, grease, oil, or anything else you may need. A particular repair item that most of us never seem to have is an extension kit, which is used to raise a hydrant after the ground around it has been raised. No one would consider a hydrant accessible if its nozzles were resting on the ground.

Each hydrant manufacturer has its own design for almost every operation. The more brands, series, and designs you have in your system, the more complicated your operation and maintenance (O&M) program will become, and the more repair parts and specialized tools you will need to stock. So keep it simple, find a style, brand, or model you like and stick with it.

Opening a hydrant too quickly could create negative pressure and set up a dangerous backflow situation. Closing a hydrant too quickly can cause water hammer, which is very destructive to lines and equipment.

Do not use a pipe wrench to operate a fire hydrant! In my years of experience, I have seen many experienced crews use a pipe wrench to operate a fire hydrant. The teeth of a pipe wrench will distort the operating nut until it is eventually rounded off. Always use a hydrant wrench!

A good O&M program is only as good as the records you keep. Start by assigning a number to each hydrant. And, if you have a distribution map of your system, note the location of each hydrant by number. In addition, the record for each hydrant should include all pertinent information, such as location, make and model, size and number of nozzles, thread type, flow rates, size and material of the main line, and maintenance history.

The American Water Works Association (AWWA) recommends all hydrants be inspected regularly, at least once a year.

A program of distribution system flushing should be established. A variety of impurities, such as sand, iron and manganese from the well source can build up in the system promote bacterial growth creating cloudy water, or taste and odor problems at the tap resulting in customer complaints. They can also obstruct the proper operation and shorten the life expectancy of meters, valves and other system components. Iron buildup, manganese and turbidity issues can be corrected by flushing programs.

A system flush in the spring and again in the fall will help to keep sediments and slimes loosened and washed away.

The hydrant flushing program can be combined with an inspection of the hydrants.

Valves

To avoid mechanical failure, design a practical valve maintenance program, which can result in cost savings for the water system. A job that appears to be mammoth becomes less daunting if the operator implements a systematic maintenance schedule. Operation and maintenance procedures for various types of valves are included in the manufacturer’s operation manuals and in the appropriate product standards. Valve records are essential for planning, operating, and verifying the system’s integrity. The valve record should contain information about valve condition, testing, and maintenance required.

Valves should be inspected and operated annually. Valves are more apt to suffer from lack of use than from overuse. Annual exercise of valves will keep them in operable condition.

Valves should be exercised full open to full closed. Records should indicate whether the valve is right or left hand and whether the valve is normally closed or open. The condition of the packing and nut should be noted. A follow-up program of correction of valve deficiencies should be carried out and these corrections or replacements noted in the valve file and on maintenance maps. Valve boxes are often located in streets and they should be checked annually to see that they are not damaged, filled with debris, or paved-over.

Storage Tanks

Storage tanks should be drawn down at least once a year for a complete inspection, inside and out.

The information in this site is intended solely for the personal non-commercial use of the user who accepts full responsibility for its use. While we have taken every precaution to ensure that the content of this site is both current and accurate, errors can occur.