During the first eight years that the system was in operation, pressures in excess of 200 pounds were carried ten times, and twice pressures in excess of 250 pounds were required. A portion of the congested value district of Baltimore is protected by a separate fire main system, put in service in 1912. The fireproof pumping station contains three horizontal, twin, steam pumps having an aggregate capacity of 21,000 gallons. The pipe in the distribution system is lap-welded, open hearth steel. One hundred pounds pressure is normally carried by a small steam pump and is raised to 250 pounds in case of fire. The congested value district and important mercantile and manufacturing areas of San Francisco are protected by a separate fire main system divided into two zones. The distribution system is cast-iron, tar-coated pipe, and is normally supplied by gravity from a storage reservoir and equalizing tanks. Normal pressures are from 130 to 140 pounds and can be raised to 250 to 300 pounds by turning in the upper reservoir. The latest installation of this type, completed in 1921, protects the congested value district of the City of Boston. Pipe is tarcoated, cast-iron with specially designed, bell and spigot joints. Four pumps supply the system; two are installed in the generating station of the traction company and driven by steam turbine, and two are installed in a substation of the lighting company and driven by electric motors. They have a total capacity of 12,000 gallons.' Normal pressures range from 40 to 60 pounds and are raised to 125 pounds and higher in case of fire. In the preceding descriptions it will be noted that reference has been made to "separate fire main systems" rather than to "highpressure fire service systems." This nomenclature seems preferable because the service records of all such systems show that under normal operating conditions it is not necessary to carry excessive pressure above 200 pounds on such systems. It is the consensus of fire chiefs in each city where a separate fire main system has been installed that such systems are of inestimable value, and in many of these cities it can be stated without fear of contradiction that the systems have paid for themselves, sometimes by the service they have rendered at one combination of simultaneous fires. Such was the case during the first year that the New York separate fire main system was in service. CHAPTER XIV RELATION BETWEEN FILTERED WATER STORAGE AND FILTER CAPACITY1 The gross output of filtered water obtainable in twenty-four hours from a water purification works, with all filters operated at a "normal" rate of filtration, is often termed the daily capacity of the works. Even after deducting the volume of filtered water needed for washing and other purposes at the works, however, the net quantity will not meet an equal average daily water consumption unless a sufficient volume of stored filtered water is available to provide for long-continued seasonal periods of excessive draft. Whenever hot, dry or freezing weather occurs, the use of water will rise, the extent of departure from the average depending upon the intensity and duration of the spell and upon local conditions such as metering, adequacy of plumbing, and prevalence of lawns and gardens. Winter and summer periods of high consumption occur in ordinary years in many parts of the temperate zone. The demands at such times can be satisfied only by an excess of filter capacity or by filtered water storage over and above that required to supply continuously the average daily draft for the year. The amount of reserve necessary is surprisingly large. Lack of sufficient provision in this respect frequently constitutes the cause for unexpectedly early enlargements of purification works. Proper determination of the capacity of a proposed plant or of an enlarged existing one, capable of supplying a given annual average daily quantity of filtered water, is impossible except after consideration of the seasonal fluctuations of consumption that may be expected. Past records of daily water consumption, if of sufficiently long duration to include all local climatic variations likely to take place, may be accepted as a basis for computation. Where data 1 It should be noted that, in this discussion, the ultimate determination of total filtered water and service storage in relation to filter capacity is intricately bound up with matters of fire protection reserve, pumping equipment available, distribution system layout and relative costs. In this particular chapter only the elements of filter capacity and storage are considered. The reader must naturally consider and weigh the other elements in the problem when actual cases are under scrutiny. 1913 1914 1915 Year 1916 1917 8161 are lacking, records of communities having similar weather conditions and local characteristics should be used as a guide. The maximum Jan. Feb. Mar Apr May June July Aug Sept Oct Nov. Dec. FIG. 9. Month RELATION BETWEEN AVERAGE DAILY WATER CONSUMPTION FOR EACH MONTH AND THE AVERAGE DAILY CONSUMPTION FOR THE YEARS 1913 To 1918 INCLUSIVE long-continued period of high water consumption must be found. If there is room for doubt as to which period is the critical one, the Percent Average Consumption investigation should be pursued until all doubt is removed. Sometimes both an unusually cold winter and hot, dry summer will cause such an excessive use of water as to make necessary a consideration of the figures for both seasons. As a basis for explaining a method of determining the reserve filter bed or filtered water storage capacity needed to supply a given average daily amount of water, the records of a centrally located city, for a long hot period, from June 2 to October 31, 1913, and for a cold winter, from December 9, 1917, to February 27, 1918, will serve. These periods were actually the critical ones and were used in planning additions to the purification works of that city. It is unnecessary to tabulate here the daily water consumption figures during the above-mentioned periods, although the actual computations must be based upon them. Figure 9, showing graphically the relation between the average daily use of water for each month, in the years 1913 to 1918 inclusive, and the average daily consumption for the respective years, indicates clearly the periods to be investigated. The problem is one of calculating, with an assumed maximum filter output, the amount of filtered water required from another source, each day during the period of high draft, to make up the daily demand on the system. Results will be termed plus or minus according to whether the consumption for the day is respectively greater or less than the assumed total output of the filters. The maximum cumulative sum (algebraic) reached on any day represents the net quantity of filtered water for which storage space must be provided. This operation is similar to the "fill and draw" method of computing necessary capacities for impounding reservoirs. By varying the figure for filter output, from a minimum equalling the average daily water consumption for the year to a maximum equalling the highest demand on any day, the maximum and minimum limits, respectively, for required filtered water storage may be found. For general application it is advisable to consider an annual average daily water consumption of 1 million gallons. Actual daily draft figures for the periods of high demand investigated must be reduced to corresponding terms by dividing each by the average daily quantity used during the year. The results will be in million gallons. Table 14 will illustrate sufficiently the method of determining the volume of filtered water storage required to supply a dis tribution system with an average of 1 million gallons of water daily, assuming given capacities of filters. Filter capacities of 1 and 1.333 million gallons only are used for purposes of illustration. The computation must commence with the first day of the period showing a TABLE 14 Storage of filtered water necessary to meet an annual average daily draft of 1,000,000 gallons—winter period of 1917–1918 * Accumulated during the periods omitted. † Filtered water storage required to meet the demand of the period. greater consumption of water than can be met by filters of the assumed total capacity, and must continue until the demand is so reduced as to make certain that the maximum storage necessary has been found. For want of space, the tabulation as shown is shortened by omission of the figures for many days. While the winter of 1917-1918 is the period upon which the computation is |