systems, has been greatly simplified by employing both the McIlroy network analyzer and the digital computer. At the present time four water utilities in large metropolitan areas own McIlroy analyzers, and use them as a tool for determining the best manner of supplementing their systems in the areas presently served and of serving new areas. New pipe materials. Today there is available a wide choice of pipe materials permitting considerable latitude in designing a system specifically for the job it is to do and the conditions which will be encountered. There are several new kinds of pipe and ways to put them together.

One of these new types of pipe is ductile iron, which is cast iron in which graphite is present as spheres or spheroids as compared with flakes in gray cast iron. It has the property of permitting bending and twisting without breaking, and is recommended for use under conditions of high pressure, extreme beam and crushing loads, unusual shocks and stresses, and unstable bedding conditions, all of which contribute to breakage.

There are a number of types of plastic pipe which are produced in 4-inch to 12-inch sizes by the extrusion of thermoplastic materials, and are available in flexible, rigid, and semirigid forms. All of the types recommended for use in domestic water supplies are said to be nontoxic and to impart no taste or odor to water. They are inert and not affected by aggressive soils or electrolysis, but their use is not recommended for temperatures above 120°.

Improved construction practices.-There are also available new machines for casting concrete pipe in place by the slip-form method. One of these operates in an excavated trench and consists primarily of a traveling form with facilities for distributing and vibrating the concrete as it is poured. Concrete is supplied to the machine hopper by transit mixers. Equipment is available to construct pipe 24 to 60 inches in diameter at a maximum depth of 16 feet. Another manufacturer provides equipment for field casting of concrete in place for diameters of 12 to 30 inches. The process includes an inside form which is inflated for use and deflated for pulling forward as soon as the concrete has taken its initial set. A tamping device is provided to insure a complete fill of uniformly placed concrete. There is also available a concrete pipe casting machine, for sizes 24 to 120 inches, using aluminum interior forms, with outside forming provided by the trench. This type of cast-in-place pipe is not suited to distribution system pressure requirements, but could be used for gravity supply lines.

Laying procedures have changed greatly with the advent of the slip-type joint. Several manufacturers of cast iron pipe have pioneered the slip or push-type joint which involves the use of a rubber gasket molded to conform to ridges cast in the pipe bell (a similar development has taken place for concrete pipe). To make the joint in the field, the gasket is inserted in the bell, the spigot end is treated with a thin film of lubricant, and the spigot is then merely pushed "home." Considerable savings in time and labor are effected by this method.

Storage tanks.-Conventional concrete water storage tanks have been in use for over 50 years. In recent years, reinforced concrete water tanks have been designed and built as prestressed structures.

Prestressed tanks can be built in capacity ranges of 100,000 to 30 million gallons.

Pumping equipment.-A new method of controlling the speed of wound rotor motors for variable speed pumping has been developed, using a new device that is essentially a water rheostat. The drive motor speed is controlled by the use of a series of stainless steel plates in a two-compartment container. These resistors are wired into the rotor circuit of the wound rotor motor. The submergence of the plates is determined by the pressure at some control point in the system, and the design of the shape of these resistor plates is such that the pump motor operates at a speed which will produce a predetermined pressure level at the control point in the system.

The development of the submersible motor and its adaptation to well pump operation has led to the use of the submersible pump for booster service. Both vertical and horizontal types are in use. Some have been buried without benefit of a service vault. They permit an installation to be made completely within public rights-of-way and require only the control to be located aboveground. It can usually be mounted on the power service pole.

Instrumentation development. Most water utilities have found that as the areas they serve have expanded and commercial districts have been decentralized, the load factor has become more and more important in providing adequate water service. In order to have an accurate basis for charging customers for a high demand load, research has been underway to develop a demand meter. Some firms are now offering equipment which will register peak loads reliably with reasonable expense.

The next step is improvement of metering application to facilitate reading. The manpower required to examine registers every time a customer is billed is costly. Remote registering to avoid periodic invasion of basements is an improvement. Magnetically and cabledriven registers can be designed for positioning on the outside of the premises to be read easily and even removed, if necessary.

Instrumentation has advanced to the point where nearly any process can be observed and controlled from a central or remote location. Pressure in the line and liquid levels can be observed; pumps can be programed to keep pace with the rate of use; and valves can be opened and closed. For maximum efficiency, modern waterworks practice should take advantage of these and other automatic possibilities.

Cost will continue to be a major factor in the design and construction of water systems. Research should be continued to improve construction materials and procedures, to improve demand and pressure controls, and instrumentation.

NOTE. Since the design and construction of water distribution systems must necessarily be responsive to the problems in their maintenance, much of the above applies also to the following subject which deals with maintenance.

MAINTENANCE OF WATER DISTRIBUTION SYSTEMS

GENERAL

The distribution system of a water utility represents 60 to 75 percent of the total value of the entire facility. Maintenance and operating costs of this part of the utility are about 15 to 20 percent of total costs (excluding taxes). In the period 1951-60, mileage of mains in service has increased approximately 23 percent, customers have increased 20.6 percent but the number of customers per mile of main (3.7) has changed little.

It is estimated that 65 percent of the waterlines now being installed are of ferrous materials, with concrete mains and cement asbestos lines accounting for 5 percent and 30 percent, respectively. Transmission mains are approximately 50 percent concrete and 50 percent steel. Approximately 80 percent of the cast iron and steel being installed is cement lined. Since no water pipe was lined with cement until in the 1920's, there are in the country's water distribution systems many miles of unlined mains which are inadequately protected against corrosion. These probably amount to 10 percent of the mains in use.

THE PROBLEMS INVOLVED

Breaks and leaks.-Among the most troublesome problems in the operation and maintenance of distribution systems are main breaks and joint leaks which waste water, damage streets, other utilities, and customer property. Philadelphia has an average of 770 main breaks a year, an average of 25 per 100 miles of main. The incidence of main breaks in this city has been as high as 1,000 with 300 occurring in 1 month. New York City has an average of 330 breaks per year, 20 of these in lines 20 inches in diameter or larger. Other cities suffer similar experiences.

External forces and external corrosion are more important factors in main breaks than age of pipe and internal corrosion, although older pipe has been found more subject to breakage. Longer centrifugally cast pipe is reported more apt to break than standard 12-foot lengths of pit cast iron. The corrosive action of sulfur jointing compounds may contribute to breakage. Pressure increases contribute to breakage; for example, an increase of but 10 pounds per square inch in Philadelphia resulted in 18 breaks in a single hour.

Corrosion. It is generally assumed that cast iron pipe does not seriously deteriorate from exterior corrosion conditions. However, old pipes taken from the Philadelphia system show materially reduced wall thickness and in Winnipeg, Canada, soils have been found to be sufficiently corrosive to cause pipe failure. A 45-year study of underground corrosion by the National Bureau of Standards estimates the annual cost of protective measures and replacements due to exterior corrosion in the country's 1 million miles of gas, oil, and water lines

to be $600 million. Penetration of steel pipe by pitting has led to its replacement in many areas.

Water cannot be made entirely noncorrosive, although its corrosiveness can be lessened or mitigated. With an estimated 10 percent of the average water distribution system in unlined cast iron lines, corrosion troubles can be expected to occur since the disintegration of the original tar coatings leaves exposed metal surfaces. Such corrosion can result in red water and staining of clothes and industrial complaints. Red water difficulties are relatively easy to correct with water treatment. The problem of reduced carrying capacity resulting from tuberculation and rust deposits within water mains still eludes solution. The estimated cost of reduced water main carrying capacity due to tuberculation and deposits caused by corrosion is about $40 million a year. Mains cleaned by mechanical devices deteriorate rapidly after such cleaning in most cases, the decreases in carrying capacity sometimes amounting to 35 percent within 3 months after cleaning. The deterioration in new tar-coated pipe by corrosive waters is considerably less but has amounted to loss of 25 percent capacity within 5 years.

Deposits. Chemical deposits from the water carried in pipelines also causes loss of carrying capacity; for example, calcium carbonate laid down from an unstable, softened water and deposits caused by oxidation and precipitation of manganese.

Freezing.-Extreme cold weather always results in some freezing of water mains and services. The problems involved are less than formerly was the case as newer mains are laid to greater depths. The magnitude the problem can attain is shown by the experience of Knoxville, Tenn., in February 1958, when in 8 days the water department. handled 2,034 complaints of frozen service lines, frozen pipes on private property, and frozen meters. There were also 78 breaks in main lines.

Maintenance of pressure and storage. All water systems seek to maintain adequate pressure, generally between 50 and 80 pounds per square inch. The National Board of Fire Underwriters allows some credit for residual pressures in excess of 60 pounds per square inch and in some cases 75 pounds per square inch. System pressures should preferably be below 100 pounds per square inch.

Maintaining uniform pressure is a difficult problem because of the wide range in water requirements during high peak demand periods. Lawn watering, air conditioning, refrigeration, industrial demands, and human habits are among the factors which create peak demands on water systems.

Combined effects of various water uses increase storage requirements by 10 to 25 percent. Maximum hourly draft rates expressed as percent of the maximum day rate for certain uses are: Lawn sprinkling, 37 to 70 percent; air conditioning, 22 to 32 percent; and combined uses, 48 to 127 percent. The problem is to provide storage which will handle peak demands within reasonable cost bounds. A rule-of-thumb quantity to eliminate severe hourly peaks is 15 to 20 percent of the average daily consumption, with 20 percent being favored.

Cross connections.-Cross connections with industrial and domestic supplies continue to be a problem of distribution system management.

The potential hazards from cross connections may be grouped into three main classes: (1) Pollution of the general distribution system as a result of improper construction or faulty operation; (2) pollution by backflow connections or through interconnections with plumbing fixtures, waste water disposal systems, or equipment conveying or containing harmful liquids or nonpotable water or water subject to pollution; and (3) pollution through cross connections with other supplies.

Valves.-Gate valves offer another problem of distribution system maintenance and many operators feel that this problem can only be solved by the design of an entirely new type of valve. Corrosion and deposition of material on the bottom and the valve seats cause the wedges to wear. A large part of distribution maintenance expense (which can be as high as $725 per year per mile of mains) is chargeable to valve maintenance and repair.

TRADITIONAL PRACTICES

Flushing programs and corrosion control.-Most smaller and mediumsize water systems flush thoroughly their distribution systems once or twice a year. In larger systems this is not practical and flushing is limited to area flushing as required. Active corrosion can result from flushing if the velocities result in removing the protective deposits which seal the mains.

Leakage surveys. Most water departments keep a rather close check on "unaccounted for water." Hydrants and lines are checked with leak-detecting instruments which electronically intensify the sound of flowing water. At least one corporation conducts professional system leak surveys which have produced considerable savings.

An "unaccounted for" figure of more than 15 percent should be cause for concern and any sudden increase in the percentage should signal a search for leaks. A system that accounts for 90 percent of water input is considered a tight system.

Cross-connection studies.-A great deal of work and expense goes into the protection of public water supplies and all can be undone if the distribution system is not continuously protected. Waterborne diseases have been virtually eliminated in this country by the practice of good water supply sanitation. The possibility that practice can yet be improved is exemplified by the few outbreaks continuing to occur as the result of inadequate protection in distribution systems.

Responsibility for programs to control or prevent cross connections usually rests with the owner of the property served and with local or State agencies having jurisdiction over building and residential plumbing, seldom by the water purveyor. However, inasmuch as the large majority of city water systems are publicly owned, overall jurisdiction as in a Government agency or head of government such as city, county, or State.

Practice has leaned toward connections in a manner permitting no possibility of backflow or siphonage, e.g., no direct connection. How

1 The division of responsibility for water supply protection is recognized in the Public Health Service drinking water standards by the following statement, "* ** responsibility for the conditions in the water supply system shall be considered to be held by: (1) The water purveyor from the source of supply to the connection to the customer's service piping; and (2) the owner of the property served and the municipal, county, or other authority having legal jurisdiction from the point of connection to the customer's service piping to the free-flowing outlet of the ultimate consumer."